Patent Application
20240295037
This invention relates to a photoelectrode. Specifically, this invention relates to a photoelectrode including a homojunction of metal oxide semiconductor films.
Solar energy has been an attractive energy source because it is clean, widely spreading, easy to achieve, and inexpensive. The past decades have witnessed the rapid development of solar energy by photoelectrolysis of water. The Photoelectrochemical (PEC) water-splitting technique harvesting solar energy directly from the Sun provides an ideal way to achieve hydrogen (H2) with minimal environmental impact. In the PEC process, the semiconductor-based photoelectrode is excited by the sunlight and can generate electron-hole pairs, the separation of which provides electrons, facilitating the oxygen evolution reaction (OER) to occur at the photoanode and the hydrogen evolution reaction (HER) to occur at the photocathode. Among all the investigated semiconductor materials, bismuth vanadium (BiVO4) has been widely used as a promising photoanode for PEC water splitting due to its advantages of relatively narrow bandgap (˜2.4 eV), the appropriate band edge position, relatively high chemical stability, non-toxicity and low cost.
Despite all the attractive advantages, BiVO4 photoanodes still suffer from some drawbacks, such as long electrical conductivity, long carrier diffuse length and low charge mobility. The theoretical maximum photocurrent density of BiVO4 under standard Air-Mass 1.5 Global (AM 1.5 G) solar light illumination is ˜7.5 mA/cm2. But in practical applications, the achieved photocurrent density is much lower than that. Various strategies have been applied to BiVO4 photoanode to optimize its PEC performance, including regulating the composition by element doping, forming the homojunction, constructing the p-n heterojunction, modification of the surface with a cocatalyst. For the homojunction construction strategy, various kinds of dopants are incorporated into the pristine BiVO4 to regulate its electronic structure and then form the homojunction structures with the other modified/pristine BiVO4 component. However, excess heteroatoms incorporated in the host semiconductor always leave some drawbacks, such as introducing the recombination center, forming non-conductive impurities and reducing the photostability, etc. Besides, previous methods for fabricating metal oxide thin film homojunction adopted heteroatoms incorporated, such as Mo or W, to improve the original drawbacks. But the high price of chemicals of heteroatoms increases the cost and the dopant of heteroatoms adds one step to the processes and further enhances the technical difficulty. These hinder their industrial-scale and public applications.
Thus, it is of great significance to construct BiVO4 homojunction structures without introducing the heteroatoms to realize its practical PEC applications in the future.
An embodiment of the present invention relates to a photoanode including a double-layer homojunction, where the double-layer homojunction includes two layers of a metal oxide semiconductor film.
An embodiment of the present invention also relates to a method for fabricating a photoelectrode including a double-layer homojunction, including the steps of:
An embodiment of the present invention also relates to a photoelectrode including a plurality of homojunctions, where at least one; or each, homojunction includes two metal oxide semiconductor films. In such an embodiment, each metal oxide semiconductor film is formed via electrodeposition in an alkaline electrolyte or an acidic electrolyte.
Without intending to be limited by theory it is believed that the present invention may provide a novel photoelectrode including a double-layer homojunction of metal oxide semiconductor films without heteroatoms incorporated. The metal oxide semiconductor films such as bismuth vanadate (BiVO4) films are uniform in large size with rich oxygen vacancies. The Bi precursor can be electrodeposited on various substrates under atmospheric pressure and air atmosphere without incorporating heteroatoms. The electrolytes for electrodeposition are acidic or alkaline with controllable pHs. The electrodeposited substrate is transferred to the muffle furnace for thermal evaporation with V precursor. The thickness and size of the films can be easily controlled by changing electrodeposition parameters. The double-layer homojunction is stable in aqueous electrolytes and prolonged mechanical cycling. This invention is expected to lead to advanced applications of metal oxide films in PEC cells, hydrogen producers, solar cells, and photocatalysis. The fabricating method of the photoelectrode is facile and noncomplex by the deposition of thin films with controlled surface morphology, thickness, and size. The materials are also economical and non-toxic.
The BiVO4 double-layer homojunction can be used as a safer and cheaper material in photo-driven devices, hydrogen producers, and solar cells. The BiVO4 double-layer homojunction is comparatively economical, hence can replace the costly III-V compounds, polymers, and valuable fossil materials for various applications. The BiVO4 double-layer homojunction can also be employed as photoelectrodes for H2 production via photoelectrochemical (PEC) water splitting under solar light, which can provide pivotal reactor materials for hydrogen producers and solar cells.
The figures herein are for illustrative purposes only and are not necessarily drawn to scale.
Unless otherwise specifically provided, all tests herein are conducted at standard conditions which include a room and testing temperature of about 25° C., sea level (1 atm.) pressure, pH 7 if not specified, and all measurements are made in metric units. Furthermore, all percentages, ratios, etc. herein are by weight, unless specifically indicated otherwise. It is understood that unless otherwise specifically noted, the materials compounds, chemicals, etc. described herein are typically commodity items and/or industry-standard items available from a variety of suppliers worldwide.
Without intending to be limited by theory, it is believed that an acidic electrolyte herein means the pH of electrolyte is below 7, more specifically, within about 0.1 to less than 7 (where a pH of 7 is excluded). In contrast, an alkaline electrolyte herein means the pH of the electrolyte is above 7, more specifically, within about more than 7 to about 14 (where a pH of 7 is excluded). A pH of 7 is considered to be neutral.
An embodiment of the present invention relates to a photoanode including a double-layer homojunction, where the double-layer homojunction includes two layers of a metal oxide semiconductor film; or a first layer of a metal oxide semiconductor film and a second layer of a metal oxide semiconductor film. In an embodiment herein, the metal oxide semiconductor film contains a BiVO4 film; or a BiVO4 film containing only BiVO4. In an embodiment herein, the BiVO4 film is a pure BiVO4 film without a (typical) metal heteroatom. Without intending to be limited by theory, it is believed that the homojunction structure herein possesses a built-in electric field that can facilitate the electron-hole separation at the BVOac/BVOal interface which can improve the PEC performance. Furthermore, it is believed that without the need for incorporating (typical) metal heteroatoms, the photoanode of BiVO4 homojunction is cheaper and easier to manufacture.
In an embodiment herein, one of the two layers of the metal oxide semiconductor films of the photoanode is formed via electrodeposition in an acidic electrolyte.
In an embodiment herein, one of the two layers of the metal oxide semiconductor film of the photoanode is formed via electrodeposition in an alkaline electrolyte. The films deposited in acidic or alkaline electrolytes have different electronic structures and different amounts of oxygen vacancies respectively so that the built-in electric field can be constructed.
In an embodiment herein, one of the two layers of metal oxide semiconductor film is formed via electrodeposition in an acidic electrolyte, and the other layer of metal oxide semiconductor films is formed via electrodeposition in an alkaline electrolyte.
In an embodiment herein, the double-layer homojunction comprises a first layer and a second layer, wherein the first layer of the metal oxide semiconductor film is formed via electrodeposition in an acidic electrolyte, and the second layer of the metal oxide semiconductor film is formed via electrodeposition in an alkaline electrolyte. Without intending to be limited by theory, it is believed that a homojunction is formed by two layers adhesive to each other. The second layer is deposited directly and closely onto the first layer. A homojunction is sandwiched with two layers of metal oxide semiconductor films. Two layers synthesized by different processes may possesses different oxygen vacancies. The one with more oxygen vacancies can serve as surface electron-trapping sites to promote charge separation and increase the electron conductivity of the BiVO4 photoanode.
In an embodiment herein, the second layer of the metal oxide semiconductor film contains more oxygen vacancies than the first layer of the metal oxide semiconductor film. Without intending to be limited by theory, it is believed that the different concentrations of oxygen vacancies show that the single films have different electronic structures. The combined homojunction also has a built-in electric field which can facilitate the electron-hole separation at the BVOac/BVOal interface.
In an embodiment herein, the first layer of the metal oxide semiconductor film is electrodeposited in the acidic electrolyte for from about 0.5 minutes to about 120 minutes, or about 25 minutes, and the second layer is electrodeposited in the alkaline electrolyte for from about 10 seconds to about 100 seconds, or about 40 seconds.
Another embodiment herein of the present invention relates to a method for fabricating the photoelectrode, where the substrate is selected from the group of FTO glass, ITO glass, graphitic carbon film and metals. Without intending to be limited by theory it is believed that all conductive substrates are applicable to the present invention. The conductive substrates contain conductive materials.
In an embodiment herein, the step (c) of the method occurs in an acidic electrolyte, and the step (g) of the method occurs in an alkaline electrolyte.
In an embodiment herein, the pH of the acidic electrolyte is about 0.1-7, or about 2. It is believed that the film deposited in an acidic electrolyte (pH=2) has fewer oxygen vacancies than that deposited in an alkaline electrolyte.
In an embodiment herein, the pH of the alkaline electrolyte is about 7-14, or about 13. It is believed that the film deposited in an alkaline electrolyte (pH=13) has more oxygen vacancies than that deposited in an acidic electrolyte.
In an embodiment herein, the step (c) of the method occurs for about 0.5 minutes to about 120 minutes, or about 25 minutes, and the step (g) of the method occurs for about 10 seconds to about 100 seconds, or about 40 seconds.
In an embodiment herein, the thermal evaporating in steps (e) and (h) occurs in a muffle furnace at a temperature of about 300-600° C., or about 500° C. Without intending to be limited by theory, it is believed that, at this annealing temperature, the obtained BiVO4 layer is monoclinic scheelite which can get better PEC performance.
In an embodiment herein, the temperature is controlled at a heating rate of about 0.5-30° C./min, or about 2-3° C./min. It is believed that the temperature-controlling rate of about 2° C./min is optimized for the formation of monoclinic scheelite BiVO4.
In an embodiment herein, the thermal evaporating in steps (e) and (h) takes about 0.1-5 hours, or about 2-3 hours. The reaction time of about 2 hours is optimized for the formation of monoclinic scheelite BiVO4.
Without intending to be limited by theory it is believed that the present invention provides a novel double-layer homojunction fabrication method without heteroatoms incorporated. The oxygen vacancies rich, large size, and uniform metal oxide semiconductor thin films such as bismuth vanadate (BiVO4) films can be synthesized on a substrate under atmospheric pressure and air atmosphere. The precursor materials may be preferably salts and oxides with the Bi precursor. The solvent materials may be acidic or alkaline electrolytes with a controllable pH. The electrodeposition techniques can be applied to synthesize thin films using the precursors. The electrodeposited substrate will be transferred to the muffle furnace for thermal evaporation with V precursor and to design bonding and defects between the precursors of Bi and V. The thickness and size of the films can be easily controlled by changing electrodeposition parameters.
The BiVO4 single-layer film and the BiVO4 double-layer homojunction may be formed by electrodeposition and followed by thermal evaporation. The micro flower-like BiVO4 arrays with sufficient oxygen vacancies may be obtained with this method. In particular, it is believed that the BiVO4 single-layer film electrodeposited in the alkaline (pH=13) electrolyte (denoted as BVOal) possesses more oxygen vacancies than that of the single-layer electrodeposited in the acidic (pH=2) electrolyte (denoted as BVOac). Furthermore, it is believed that an n-n+ type-II homojunction photoanode constructed in the present invention consists of BVOac as the first layer and BVOal as the second layer (denoted as BVOac-BVOal homojunction). The BiVO4 single-layer film electrodeposited in an acidic electrolyte for 25 mins (denoted as BVOac-25) shows a photocurrent density of 1.2 mA/cm2 at 1.23 V versus research hydrogen electrode (RHE) in 0.1 M potassium borate (K2B4O7·4H2O) buffer with 0.1 M sodium sulfite (Na2SO3) as a hole scavenger (pH=9.44) under 1 sunlight illumination (AM 1.5 G, 100 mW/cm2). And the BiVO4 single-layer film that is electrodeposited in the alkaline electrolyte for 40 s (denoted as BVOal-40) has a photocurrent density of 1.6 mA/cm2 at 1.23 V vs. RHE under AM 1.5 G illumination. Specifically, the BVOac-BVOal homojunction may exhibit a photocurrent density of 3.6 mA/cm2 at 1.23 V vs. RHE under AM 1.5 G illumination and shows a significant improvement in PEC performance compared to the BVOac-25 and BVOal-40 single-layer films. An embodiment of the invention herein therefore provides a convenient way to construct the highly efficient BiVO4 photoanode without introducing any heteroatoms. It is therefore believed that this cost-effective method provides more possibilities for BiVO4-based photoanode for its practical PEC application.
Turning to the figures,
In this invention, acetone is purchased from Anaqua Global International Inc., Limited (Cleveland, OH, USA). Isopropanol is provided by Dieckmann Chemical Industry Company Ltd (Shenzhen, China). Sodium hydroxide (NaOH) is purchased from Acros Organics Company (Geel, Belgium or New Jersey, USA). Nitric acid (HNO3) is obtained from VWR International Company Ltd (Shanghai, China). 2,3-dihydroxybernsteinsaeure, vanadium (IV)oxy acetylacetonate and glacial acetic acid are purchased from Aladdin Biochemical Technology Company, Ltd (Shanghai, China). Bismuth (III) nitrate pentahydrate (Bi(NO)3·5H2O), sodium sulfate (Na2SO4) and sodium sulfite (Na2SO3) are purchased from Sigma-Aldrich LLC (Shanghai, China). Potassium borate (K2B4O7·4H2O) is bought from Sinopharm Chemical Reagent Co., Ltd (Shanghai, China). All the chemical reagents were analytical grade and used without any further purification.
All the BiVO4 films in this work are first fabricated via an electrodeposition method and followed by a thermal evaporation process. The electrodeposition of the bismuth precursor on the substrate occurs via a three-electrode cell system at a constant potential in acidic or alkaline electrolytes, respectively. Afterwards, a thermal evaporation method is applied to the vanadium precursor, allowing it to fully react with the bismuth precursor on the substrate. The BiVO4 thin film forms gradually during this annealing process, as illustrated in
Bismuth precursor films are firstly electrodeposited in the acidic and alkaline electrolytes, respectively. FTO glasses (NSG, 2.2 mm, 7 Ω sq−1) are ultrasonically cleaned by a mixture of deionized (DI) water, acetone and isopropanol (1:1:1 vol. %) for 15 mins, then ultrasonically washed by DI water for another 15 mins and dried in the furnace for further use.
An acidic electrolyte for BVOac: 0.6 g 2,3-dihydroxybernsteinsaeure (99.5%) is dissolved in a mixture of 30 mL DI water, 5 mL glacial acetic acid (99.8%) and 1 mL 1 M NaOH (97%) solution. The pH value of the mixture is exactly tuned to 2 by adding 0.5 M HNO3 (69%). Then, 4.336 g Bi(NO)3·5H2O (98%) is added to the mixture with continuous magnetic stirring at 50° C. for 1 h. The acidic electrolyte is ready for use after being purged by nitrogen flow for 15 min.
An alkaline electrolyte for BVOal: 1.352 g 2,3-dihydroxybernsteinsaeure and 4.366 g Bi(NO)3·5H2O are added into 30 mL DI water. The pH value of the mixture is regulated to 13 by adding 1 M NaOH and the white suspension changed into transparent during this process.
The electrodeposition method is carried out in a three-electrode cell system to prepare the bismuth precursor films. The three electrodes (working electrode: FTO substrate, counter electrode: platinum foil, reference electrode: Ag/AgCl electrode) are distributed in an equilateral triangle with a side length of 1.8 cm and are immersed in the electrolyte. The external bias is set to 2.6 V vs. RHE for the acidic electrolyte and 2.3 V vs. RHE for the alkaline electrolyte. The electrodeposition in the acidic electrolyte may take from about 0.5 minutes to about 120 minutes; or from about 1 minute to about 90 minutes; or from about 5 minutes to about 60 minutes; or from about 10 minutes to about 40 minutes; or about 25 minutes. The electrodeposition in the alkaline electrolyte may take from about 10 seconds to about 100 seconds; or from about 20 seconds to about 70 seconds; or from about 30 seconds to about 50 seconds; or about 40 seconds.
70 mg vanadium(IV)oxy acetylacetonate (vanadium precursor) is uniformly spread in the bottom of a square corundum crucible. The bismuth precursor films are then placed face down to cover the crucible. The square corundum crucible is moved to the muffle furnace and annealed at about 300-600° C. (heating rate: about 0.5-30° C./min) in the air for about 0.1-5 h. In a preferred example, the square corundum crucible is moved to the muffle furnace and annealed at 500° C. In a preferred example, the heating rate is about 2-3° C./min. In a more preferred example, the heating rate is about 2° C./min. In a preferred example, the annealing in the air lasts for about 2-3 h. During the annealing process, the as-coated bismuth precursor converted into BiVO4 gradually. Finally, the BiVO4 photoanodes are immersed in 1M NaOH solution for 15 mins to wash away the residual V2O5 on the surface.
Two types of BiVO4 homojunctions that consist of BVOal-40 and BVOac-25 single-layer films are constructed. The one employs a BVOal-40 as the first layer and BVOac-25 as the second layer is denoted as BVOal-40-BcVOac-25. Another one that employs BVOac-25 as the first layer and BVOal-40 as the second layer is denoted as BVOac-25-BVOal-40. BVOal-40-BVOac-25: The bismuth precursor films is firstly electrodeposited in an alkaline electrolyte (pH=13) for 40 seconds followed by the thermal evaporation of vanadium precursor at 500° C. (heating rate: 2° C./min) in the air for 2 h to form the first layer—BVOal-40. Then it is electrodeposited again in acidic (pH=2) electrolyte for 25 mins followed by the thermal evaporation of vanadium precursor at 500° C. (heating rate: 2° C./min) in the air for 2 h to construct the second layer—BVOac-25.
BVOac-25-BVOal-40: The Bismuth precursor films are electrodeposited for 25 mins in acidic (pH=2) electrolyte firstly followed by the thermal evaporation of vanadium precursor at 500° C. (heating rate: 2° C./min) in the air for 2 h to form the first layer—BVOac-25. Then it is electrodeposited again in an alkaline electrolyte (pH=13) for 40 seconds followed by the thermal evaporation of vanadium precursor at 500° C. (heating rate: 2° C./min) in the air for 2 h to construct the second layer—BVOal-40.
The BiVO4 films deposited in acidic or alkaline electrolytes have different electronic structures. After combining the two films together, an n-n+ type II homojunction is achieved. The architecture can facilitate the electron-hole separation at the BVOal-BcVOac or BVOac-BVOal interface, by introducing a built-in electric field.
The observation of the film morphology was carried out on a scanning electron microscope (SEM, Zeiss Sigma 300). The crystalline structure is confirmed by X-ray diffractometer (XRD, Rigaku smartlab 9 kw) with Cu Kα (k=1.54051 Å) irradiation.
From the top-view SEM images (
The chemical composition is achieved using X-ray photoelectron spectroscopy (XPS, Thermo SCIENTIFIC K-Alpha). As shown in
Oxygen vacancies, a kind of defect as well as an essential kind of dopants, have been reported that they can improve the PEC performance of BiVO4. Oxygen vacancies, as a critical class of dopants, could serve as surface electron-trapping sites to promote charge separation and increase the electron conductivity of the BiVO4 photoanode. In order to investigate the surface composition and the amount of oxygen vacancies on the BiVO4 photoanodes, XPS spectra are obtained from both BiVO4 single-layer films and the BVOac-BVOal homojunction. In order to analyze the impact of the oxygen vacancies on the PEC performance, the high-resolution O 1s core level XPS spectrum is achieved.
As shown in
With respect to single-layer films, BVOal-40 owns more oxygen vacancies than BVOac-25. More impressively, the content of OV for the BVOac-BVOal homojunction is the highest (30.80 at. %), revealing it possesses the most oxygen vacancies on its film surface.
The combined homojunction has more oxygen vacancies which can serve as surface electron-trapping sites to promote charge separation and increase the electron conductivity of the BiVO4 photoanode. The combined homojunction also has a built-in electric field which can facilitate the electron-hole separation at the BVOac/BVOal interface. All these contribute to the improvement of the performance of homojunction.
In
Those V4+ species bonded with OL cause a negative shift of the OL peak as shown in
Moreover, the formation and the energy band schematic of the BVOac-BVOal homojunction are demonstrated in
It is well known that the thickness of the photoanode is a trade-off between ensuring sufficient light absorption and efficient charge transportation. Thus, in order to optimize the thickness of the double-layer BiVO4 homojunction, the photoelectrochemical (PEC) performance of the BiVO4 homojunctions of different thicknesses is investigated.
The PEC properties of embodiments of BiVO4 photoanodes are evaluated in a standard three-electrode system by the electrochemical station (CHI 760E, Shanghai Chenhua Limited, Shanghai, China). Before the measurement, all the samples are cut into identical pieces and then sealed with epoxy resin to avoid current leakage. The active area of the films is kept constant at 0.5 cm2 to contact and react with the electrolyte. The three-electrode systems (BiVO4 photoanode as the working electrode, platinum foil as the counter electrode, Ag/AgCl electrode as the reference electrode) are immersed into the electrolyte (pH=9.41) consisting of 0.1 M potassium borate electrolyte (K2B4O7·4H2O, 99.5%). And another type of electrolyte (pH=9.44) is 0.1M K2B4O7·4H2O electrolyte added with 0.1 M Na2SO3 (99%) as the hole scavenger. The applied potential vs Ag/AgCl is converted to the reversible hydrogen electrode (RHE) scale according to the Nernst equation:
where E°Ag/AgCl=0.197 V at 25° C.
A 300 W Xe lamp (NewBet HSX-F300) equipped with AM 1.5 G filter is used as the Sun simulator. The power density of the illumination is calibrated to be 100 mW/cm2. Transient photocurrent density (i-t) is measured by applying a bias of 1.23 V vs. RHE under chopped illumination (on: 20 s, off: 20 s). Linear sweep voltammetry (LSV) is recorded by sweeping the potential from negative to positive direction with a scan rate of 10 mV/s.
Different homojunctions, BVOac-15-BVOal-25, BVOac-15-BVOal-40, BVOac-15-BVOal-55, BVOac-25-BVOal-25, BVOac-25-BVOal-40 and BVOac-25-BVOal-55 are prepared. Similar to the preparation described in Example 2 of BVOac-25-BVOal-40, BVOac-15-BVOal-25 homojunction is prepared by the first deposition in an acidic electrolyte for 15 mins and followed by the second deposition in an alkaline electrolyte for 25 seconds. BVOac-15-BVOal-40 homojunction is prepared by the first deposition in an acidic electrolyte for 15 mins and followed by the second deposition in an alkaline electrolyte for 40 seconds. BVOac-15-BVOal-55 homojunction is prepared by the first deposition in an acidic electrolyte for 15 mins and followed by the second deposition in an alkaline electrolyte for 55 seconds. BVOac-25-BVOal-25 homojunction is prepared by the first deposition in an acidic electrolyte for 25 mins and followed by the second deposition in an alkaline electrolyte for 25 seconds. BVOac-25-BVOal-55 homojunction is prepared by the first deposition in an acidic electrolyte for 25 mins and followed by the second deposition in an alkaline electrolyte for 55 seconds.
As shown in
BVOac-BVOal Homojunction Shows Improved PEC Performance
The thickness of the BVOac-BVOal homojunction is optimized firstly (
BVOac-BVOal Homojunction Shows Long-Term Stability Under Illumination During the PEC Process
In order to understand the underlying mechanism of how the BVOac-BVOal homojunction enhances the PEC performance, its optical property is investigated by the Ultraviolet-Visible (UV-vis) absorption spectroscopy as shown in
To understand the interfacial kinetics during the PEC process, electrochemical impedance spectroscopy (EIS) curves of the BiVO4 films are measured at the open circuit potential under AM 1.5G illumination. Therefore, an equivalent circuit model (inserted in
It should be understood that the above only illustrates and describes examples whereby the present invention may be carried out, and that modifications and/or alterations may be made thereto without departing from the spirit of the invention.
It should also be understood that certain features of the invention, which are, for clarity, described in the context of separate embodiments, may also be provided in combination in a single embodiment. Conversely, various features of the invention which are, for brevity, described in the context of a single embodiment, may also be provided separately, or in any suitable subcombination.
All references specifically cited herein are hereby incorporated by reference in their entireties. However, the citation or incorporation of such a reference is not necessarily an admission as to its appropriateness, citability, and/or availability as prior art to/against the present invention.
