Enhancement of photocatalytic water dissociation efficiency of naul semimetals by magnetic fields

Abstract

The present disclosure relates to enhancing photocatalytic water dissociation efficiency of Weel semimetals by magnetic fields. In a preferred embodiment of the present disclosure, the Weyl semimetal is selected from the group consisting of NbP, TaP, NbAs, and TaAs.

Description

[0001] This application is a divisional application of Chinese invention patent application No. 201980016975.4 (the corresponding international application number is PCT / EP2019 / 052874), whose priority date is February 14, 2018 and whose invention name is "Enhancing the photocatalytic water dissociation efficiency of Weyl semimetals by magnetic field". Technical Field

[0002] The present disclosure belongs to the technical field of photocatalytic water dissociation. More specifically, the present disclosure relates to a method for enhancing the catalytic activity of Weyl semimetals suitable for use as catalysts in photocatalytic water dissociation reactions for hydrogen production. Background Art

[0003] The Background Description includes information that is helpful in understanding the present invention. It is not an admission that any of the information provided herein is prior art or relevant to the presently claimed invention, or that any publication explicitly or implicitly referenced is prior art.

[0004] The global energy challenge is universal and it is inevitable to turn to alternative sustainable energy sources, especially the sun. Since solar energy is only available during the day, it would be very advantageous to store the energy in a proper way. Storage in the form of hydrogen would be a very attractive way to achieve this goal. The ideal method for hydrogen production is to use water as a hydrogen source and solar energy for conversion. Therefore, photocatalytic water dissociation, which is a light-induced conversion reaction of water into hydrogen and oxygen, has attracted attention as one of the most promising methods for hydrogen production.

[0005] Efficient water dissociation using photocatalysts is a huge challenge for researchers. In addition, water dissociation using renewable forms of energy may be the ultimate solution to environmental and energy problems. However, the development of highly active photocatalysts for water dissociation has long been hindered by the difficulty of thermodynamic uphill reactions. The focus on improving materials by designing suitable structures, increasing surface area, and using co-catalysts are among the most common techniques. Current strategies for catalyst design focus on increasing the number and activity of local catalytic sites, such as the edge sites of molybdenum disulfide (MoS2) in the hydrogen evolution reaction (HER).

[0006] EP 2 407 419 A1 describes a solution in which the oxygen evolution photocatalyst and the hydrogen evolution photocatalyst are separated by an electrically conductive separator layer.

[0007] US2012 / 0145532A1 discloses the use of broad spectrum excitation of noble metal core / semiconductor shell hybrid nanoparticles for unassisted photocatalytic dissociation of water. The metal core / semiconductor shell composite nanoparticles comprise a noble metal (e.g., Au, Ag, Pt, Pd or a noble metal alloy) core coated with a wide bandgap semiconductor photocatalyst (e.g., TiO2, ZnS, Nb2O5) transparent to photoexcitation in the visible and near infrared (NIR) spectral ranges, which is consistent with the plasmon absorption band of the metal core.

[0008] US 2015 / 0010463 A1 relates to a photocatalyst for producing hydrogen from water using visible light radiation, comprising nanocrystalline cobalt(II) oxide nanoparticles.

[0009] H. Li, C. Tsai, AL Koh, L. Cai, AW Contryman, AF Bragapane, J. Zhao, HS Han, HC Manoharan, F. Abild-Pedersen, JK Norskov, X. Zheng (Nat. Mater. 2016, 15, 48); D. Voiry, M. Salehi, R. Silva, T. Fujita, M. Chen, T. Asefa, VB Shenoy, G. Eda, M. Chhowalla, (Nano Lett. 2013, 13, 6222) and U. Maitra, U. Gupta, M. De, R. Datta, A. Govindaraj, CNR Rao (Angew. Chem. Int. Ed. 2013, 52, 13057) reported that MoS2 nanoparticles exhibited high efficiency in catalyzing photochemical and electrochemical HER in photocatalysts for hydrogen production.

[0010] C.Tsai, K.Chan, J.K F. Abild-Pedersen (Surf. Sci. 2015, 640, 133) found that metallic transition metal dichalcogenides (TMDs) are excellent candidates for HER.

[0011] Adv, Mater. 2017, 1606202, "Weyl semimetals as Hydrogen Evolution Cataysts" describes transition metal single pnictides NbP, TaP, NbAs and TaAs as catalysts for HER. These single pnictides are topological Weyl semimetals.

[0012] All publications herein are incorporated by reference to the same extent as if each individual publication or patent application was specifically and individually indicated to be incorporated by reference. When a definition or use of a term in an incorporated reference is inconsistent or contrary to the definition of that term provided herein, the definition of that term provided herein applies and the definition of that term in the reference does not apply.

[0013] The grouping of alternative elements or embodiments of the invention disclosed herein should not be construed as limiting. Each group member may be individually referred to and claimed, or may be combined with other members of the group or other elements that occur herein. For reasons of convenience and / or patentability, one or more members in a group may be included in the group or deleted from the group.

[0014] Purpose of the Invention

[0015] An object of the present disclosure is to improve the efficiency of the hydrogen evolution reaction (HER).

[0016] Another object of the present disclosure is to improve the catalytic efficiency of catalysts used for HER.

[0017] Another object of the present disclosure is to improve the catalytic efficiency of transition metal monophosphides NbP, TaP, NbAs and TaAs as HER catalysts.

[0018] Yet another object of the present disclosure is to provide a method for treating Weyl semimetals to enhance their photocatalytic water dissociation efficiency. Summary of the invention

[0019] The inventors have found that the catalytic efficiency of the Weyl semimetal can be improved by subjecting the Weyl semimetal to an external magnetic field. Therefore, one aspect of the present disclosure provides a method for improving the catalytic activity of the Weyl semimetal by subjecting the Weyl semimetal to an external magnetic field (>0T). In a preferred embodiment of this aspect, the catalytic activity of the Weyl semimetal can be enhanced by exposing at least a portion of the Weyl semimetal to an external magnetic field having a magnetic field strength greater than 0.1T. The catalytic activity of the Weyl semimetal can be enhanced by exposing at least a portion of the Weyl semimetal to an external magnetic field of>0.3T.

[0020] In a preferred embodiment of the present disclosure, the Weyl semimetal is selected from the group consisting of NbP, TaP, NbAs and TaAs.

[0021] Another aspect of the present disclosure relates to the use of a Weyl semimetal having enhanced catalytic activity obtained by a method according to an embodiment of the present disclosure in photocatalytic production of hydrogen from water.

[0022] Various objects, features, aspects and advantages of the present subject matter will become more apparent from the following detailed description of preferred embodiments and the accompanying drawings, wherein like reference numerals represent like parts. BRIEF DESCRIPTION OF THE DRAWINGS

[0023] The accompanying drawings are included to provide a further understanding of the disclosure and are incorporated in and constitute a part of this specification. The drawings illustrate exemplary embodiments of the disclosure and together with the description serve to explain the principles of the disclosure.

[0024] Figure 1 A comparison of the HER catalytic activities of MoS2 and NbP with (denoted by "m") and without an external magnetic field is shown.

[0025] Figure 2 Comprising two curves “a” and “b”, a comparison of the HER activity in the Weyl semimetals NbP, TaP, NbAs and TaAs is shown with an applied magnetic field (curve a) and without a magnetic field (curve b).

[0026] Figure 3 Schematic diagram of a Weyl semimetal used to catalyze dye-sensitized hydrogen evolution.

[0027] Figure 4 An exemplary experimental setup for producing hydrogen by photocatalytic water dissociation according to an embodiment of the present disclosure is shown.

[0028] Figure 5 The catalytic activity of the Weyl semimetal TaP as an electrocatalyst for HER in the presence or absence of a magnetic field is shown. DETAILED DESCRIPTION

[0029] The following is a detailed description of an embodiment of the present disclosure. The embodiments are described in detail in order to clearly convey the present disclosure. However, the amount of detail provided is not intended to limit the expected variations of the embodiments; on the contrary, it is intended to cover all modifications, equivalents, and alternatives that fall within the spirit and scope of the present disclosure defined by the appended claims.

[0030] Unless the context requires otherwise, throughout the following specification, the word "comprise" and variations thereof (such as "include" and "contain") should be interpreted in an open, inclusive sense, i.e., "including but not limited to".

[0031] Throughout the specification, reference to "one embodiment" or "an embodiment" means a particular feature, structure, or characteristic described in connection with the embodiment in at least one embodiment. Thus, the phrases "in one embodiment" or "in an embodiment" appearing in various places throughout the specification are not necessarily all referring to the same embodiment. Furthermore, in one or more embodiments, the particular features, structures, or characteristics may be combined in any suitable manner.

[0032] As used in the description herein and throughout the claims that follow, the meanings of "a," "an," and "the" include plural references unless the context clearly dictates otherwise. Further, as used in the description herein, the meaning of "in" includes "in" and "on," unless the context clearly dictates otherwise.

[0033] Recitation of ranges of values ​​herein are merely intended to serve as a shorthand method of referring individually to each separate value falling within the range. Unless otherwise indicated herein, each separate value is incorporated into the specification as if it were individually recited herein.

[0034] All methods described herein can be performed in an appropriate order unless otherwise indicated herein or otherwise clearly contradictory to the context. The use of any and all examples or exemplary language (e.g., "such as") provided relative to certain embodiments herein is intended only to better illustrate the present invention, without limiting the scope of the present invention otherwise claimed. Any language in the specification should not be interpreted as representing any unrequired element essential to the implementation of the present invention.

[0035] The titles and abstracts provided herein are for convenience only and do not interpret the scope or meaning of the embodiments.

[0036] Various terms are used herein. If a term used in a claim is not defined below, it should be given the broadest definition persons in the pertinent art have given that term as reflected in printed publications and issued patents at the time of filing.

[0037] Weyl semimetals are topological insulators. A prerequisite for Weyl semimetals is band inversion. Many compounds containing heavy metals show such band inversion, more commonly known as the inert pair effect. In the relativistic band structure, band crossing is prohibited, so that new band gaps open in topological insulators and surface states with Dirac cone electronic structure appear. Weyl semimetals appear at the boundary between topological insulators and ordinary insulators. In Weyl semimetals, paired Dirac cones are formed in the bulk of the material, where the number of Dirac cone pairs depends on the detailed symmetry of the specific metal or compound. Weyl semimetals also exhibit unusual surface states with open Fermi arcs.

[0038] NbP, TaP, NbAs and TaAs are semimetals with Weyl nodes located near the Fermi level, with a total of 12 pairs of Weyl nodes in the first Brillouin zone.

[0039] In HER, sunlight is absorbed by a photon-harvesting system, such as eosin Y (EY). In the presence of a sacrificial agent, such as triethanolamine (TEAO), the generated excited electrons can be transferred to the Weyl semimetal surface, resulting in charge separation, allowing H + Although the above-mentioned Weyl semimetals (in themselves) are already good HER catalysts, it is surprisingly found that the catalytic HER efficiency of each of these Weyl semimetals can be even further improved in an external magnetic field greater than 0 T. In a magnetic field of 0.3 T, the catalytic efficiency (as μmol·g -1 The activity of NbP is increased by at least 20%, preferably by at least 40%, more preferably by at least 50%, and even up to about 70% for some Weyl semimetals after 3 hours of HER near a 0.3 T permanent magnet. Figure 2 The activity of some Weyl semimetals in the presence (and absence) of an external magnetic field is shown in FIG.

[0040] "In an external magnetic field" means in the vicinity of a permanent magnet or an electromagnet, such that the catalytic material is subjected to a magnetic field of >0 T, preferably >0.1 T, more preferably ≥0.3 T; of course, this means that the applied external magnetic field is stronger than the Earth's magnetic field (which is between about 20 and 50 μT). Although at least a portion of the surface of the catalytic material should be exposed to the magnetic field, it is preferred that the entire catalytic material, i.e., at least 70%, more preferably at least 90%, of the catalytic material is exposed to a magnetic field of >0 T, preferably >0.1 T, more preferably >0.3 T.

[0041] Among photocatalysts for hydrogen production, MoS2, a conventional topologically trivial catalyst, has been reported to have high efficiency in catalyzing photochemical as well as electrochemical HER. However, when exposed to an external magnetic field, the catalytic behavior of MoS2 hardly changes (see Figure 1 ). The magnetic field only affects the Weyl semimetal. In the absence and presence of a magnetic field (0.3 T, 6 hours), MoS2 shows a temperature of about 625 μmol·g -1 Using TaP as an electrocatalyst, the catalytic efficiency was increased by 34% using a magnetic field of 113Oe.

[0042] The manufacture of Weyl semimetals may include providing metal (Ta, Nb) powders and arsenic powders in equal molar ratios; heating the mixed powders to form a polycrystalline binary 1:1 pnictide. The polycrystalline pnictide and a transfer agent (e.g., SnI4 or TeI4) are sealed at a first end of a closed ampoule; and the closed ampoule is heated to about 800°C at the first end and to about 1000°C at the second end of the ampoule to generate a temperature gradient along the length of the closed ampoule, thereby growing the pnictide Weyl semimetal crystals by a chemical vapor transfer (CVT) reaction. The corresponding phosphorus-containing compounds may be prepared by solid-state reactions at temperatures ranging from 800°C to 1100°C.

[0043] Example

[0044] The present invention is explained in more detail below with reference to examples. The disclosure is further explained in the form of the following examples. However, it should be understood that the aforementioned examples are only exemplary and should not be regarded as limiting the scope of the invention. Those skilled in the art will be aware of various changes and modifications to the disclosed embodiments. Such changes and modifications can be made without departing from the scope of the present invention.

[0045] TaAs manufacturing

[0046] 3.536 g of tantalum powder (purity: 99.98%) and 1.464 g of arsenic powder (purity: 99.999%) were weighed to obtain 5 g (1:1 molar ratio) of the mixture. The mixture was filled in a silicon oxide tube. The evacuated tube (pressure <1 Pa) was then sealed and heated at 600°C in an electric furnace for 36 hours. Thereafter, the temperature was slowly increased from 600°C to 1050°C over 50 hours and maintained at 1050°C for 36 hours.

[0047] 300 mg of the obtained polycrystalline TaAs and 60 mg of SnI4 were placed at the bottom of a silica ampoule. The silica ampoule was then evacuated and circulated with argon (99.999%), and then evacuated again (pressure <1 Pa) and sealed. The closed ampoule was then placed in a three-zone furnace. The end of the closed ampoule containing polycrystalline TaAs was in the first zone, and the opposite end of the ampoule was in the third zone. The second (middle) zone was located between the first zone and the third zone. The first zone containing polycrystalline TaAs was set to 800°C, and the middle zone was set to 1000°C. The CVT reaction time was 170 hours. TaAs single crystals were formed from the middle of the silica ampoule to the end that did not contain polycrystalline TaAs.

[0048] The single crystals were crushed using a pestle and mortar and separated using a 20 μm sieve.

[0049] Manufacturing of NbAs

[0050] NbAs is produced from a stoichiometric mixture of the elements niobium and arsenic in the same manner as described above for TaAs.

[0051] Manufacturing of NbP, TaP and MoS2

[0052] NbP, TaP and MoS2 are grown from a stoichiometric mixture of elements by solid phase reaction (same as TaAs but without CVT).

[0053] All compounds were characterized by XRD.

[0054] HER experimental setup

[0055] Experimental setup Figure 4 As shown, it is an assembly consisting of a halogen lamp (1), a reaction vessel (2) and a pair of permanent magnets (3). The halogen lamp (1) is aligned with the glass reaction vessel (2) and the magnet (3) is placed below the reaction vessel (2). The precipitated H2 is collected from the head space (4) of the vessel (2) every 1 hour and analyzed using a thermal conductivity detector in a Perkin Elmer 580C gas chromatograph.

[0056] Experimental details

[0057] In a glass container, disperse 3-4 mg of the catalyst in 50 mL of water containing triethanolamine (TEAO). Place the glass container containing the sample directly on a NdFeB magnet (see Figure 4 , two bar magnets bonded to each other, B ~ 3000 Gauss). Before starting the reaction, Eosin Y dye was added to the mixture.

[0058] After experiments performed in the presence of a magnetic field, control reactions were performed in the absence of a magnetic field. All measurements were performed at room temperature (about 23° C.) and repeated three times for reproducibility.

[0059] Figure 1 A comparison of the HER catalytic activities of MoS2 and NbP with (denoted by "m") and without an external magnetic field is shown. When an external magnetic field is applied, the activity of topologically nontrivial NbP increases significantly (NbP(m)), while the activity of topologically trivial MoS2 does not change much (MoS2(m) and MoS2).

[0060] Figure 2 A comparison of the HER activity in the Weyl semimetals NbP, TaP, NbAs and TaAs is shown:

[0061] (curve a) with applied magnetic field, and

[0062] (Curve b) No magnetic field.

[0063] In an external magnetic field of 0.3 T, the activities of NbP, TaP, TaAs, and NbAs (after 3 h) increased from 420, 220, 180, and 390 μmol·g -1 Increased to 720, 320, 280 and 450 μmol·g -1 .

[0064] Figure 3 Schematic diagram of a Weyl semimetal used to catalyze dye-sensitized hydrogen evolution. When light shines on Eosin Y, it becomes excited, and in the presence of a sacrificial agent, triethanolamine (TEAO), the dye transfers electrons to the surface of the Weyl semimetal, resulting in charge separation and thus the conversion of H2O (H2O) into H2O. + ) is reduced to hydrogen.

[0065] Figure 5 The catalytic performance of TaP as a HER catalyst is shown. A potential of -0.36 V relative to the reversible hydrogen electrode (RHE) was used in the measurement. When no magnetic field was applied, a current of 0.129 mA was observed. When a magnetic field of 113 Oe was applied, the current increased to 0.172 mA. This corresponds to a 34% efficiency improvement.

Claims

1. A photocatalyst comprising a magnetized Weyl semimetal in a magnetic field greater than 0 T, wherein the magnetized Weyl semimetal exhibits an enhanced photocatalytic efficiency for a photocatalyzed hydrogen evolution reaction (HER) compared to the Weyl semimetal in the absence of the magnetic field.

2. The photocatalyst comprising a magnetized Weyl semimetal according to claim 1, wherein the photocatalytic efficiency is increased by at least 20% compared to the catalytic efficiency when no magnetic field is applied. 3 . The photocatalyst comprising a magnetized Weyl semimetal according to claim 1 , wherein the Weyl semimetal is selected from one or more compounds of NbP, TaP, NbAs and TaAs.

4. A method for improving the catalytic efficiency of a photocatalyst comprising a Weyl semimetal as claimed in claim 1, the method comprising subjecting the Weyl semimetal to an external magnetic field greater than 0 T. 5 . The method according to claim 4 , wherein the Weyl semimetal is selected from one or more compounds of NbP, TaP, NbAs and TaAs.

6. The method of claim 4, wherein the photocatalytic efficiency in the hydrogen evolution reaction is increased by at least 20%. The method according to claim 4 , wherein the external magnetic field is greater than 0 T to 0.3 T.

8. The photocatalyst comprising a magnetized Weyl semimetal according to claim 1, wherein the photocatalytic efficiency of the magnetized Weyl semimetal is increased by at least 40% in a magnetic field of 0.3 T compared to the catalytic efficiency when no magnetic field is applied.

9. A photocatalytic hydrogen evolution reaction (HER) device comprising (i) an aqueous dispersion of a magnetized Weyl semimetal contained in a HER reaction vessel; (ii) a pair of permanent magnets disposed below the HER reaction vessel; and (iii) a headspace collection port.

10. The photocatalytic HER device according to claim 9, wherein the HER reaction vessel is a glass vessel and a magnetic field is generated from a magnet consisting of the pair of permanent magnets disposed below the HER reaction vessel.

11. A hydrogen evolution reaction (HER) catalyst comprising a magnetized Weyl semimetal, said magnetized Weyl semimetal exhibiting an enhanced HER photocatalytic efficiency compared to said Weyl semimetal in the absence of said magnetic field, The magnetic field is greater than 0T to 0.3T.

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