NiS / CdS heterojunction material and preparation method and application thereof

Abstract

The invention discloses a NiS / CdS heterojunction material as well as a preparation method and application thereof. The preparation method comprises the following steps: dispersing hexagonal CdS with exposed (001) crystal face, a soluble nickel source and a soluble sulfur source in ethylene glycol according to a molar ratio of CdS: Ni: S of 1: (0.5-1.5): (0.5-1.5), and reacting at a constant temperature of 180 + / -10 DEG C to obtain the NiS / CdS heterojunction material. According to the preparation method, in a single mild system, phase change-morphology cooperative regulation and control of NiS are realized in situ on a (001) crystal face exposed by hexagonal-phase CdS only through time regulation and control: gradient phase change of NiS from amorphous-NiS to alpha-NiS (hexagonal phase) to beta-NiS (rhombohedral phase) is realized for the first time, and multi-dimensional morphology evolution (quantum dot-amorphous state-crystal face exposed type-block-long rod type) is accompanied. Particularly, when the reaction time is 18-24 hours, the preparation method can be used for successfully preparing the NiS / CdS heterojunction material in which the alpha-NiS and beta-NiS double crystal phases coexist. In addition, the nanowire NiS / CdS heterojunction material with NiS extending in the axial direction of the CdS nanowire can be prepared through the preparation method, and a foundation is laid for promoting the nanowire NiS / CdS heterojunction material to be practical from the concept.

Description

TECHNICAL FIELD

[0001] The application relates to the technical field of heterojunction material preparation, in particular to a NiS / CdS heterojunction material and a preparation method and application thereof. BACKGROUND

[0002] As a core means of realizing green energy conversion such as water splitting and CO2 reduction under solar drive, the efficiency and stability of photocatalysis are highly dependent on the electronic structure and interface dynamics of the catalytic material. Cadmium sulfide (CdS) is considered as an ideal candidate for hydrogen production by water splitting due to its suitable visible light absorption (2.4 eV) and strong reduction band position. However, the intrinsic fast recombination of photo-generated carriers, insufficient surface reaction sites and serious photocorrosion of CdS seriously restrict its practical application. Although traditional modification strategies (such as noble metal loading and heterojunction construction) can partially alleviate the above problems, the performance improvement is often bottlenecked due to the non-directional charge transport and lattice mismatch defects of the static composite interface.

[0003] In recent years, nickel sulfide (NiS) has been widely used as a cocatalyst for CdS to promote electron extraction and surface reaction due to its metal-like conductivity and multi-valence characteristics. However, existing researches mainly focus on the mechanical compounding of single crystal phase (such as alpha-NiS or beta-NiS) and CdS, which has significant limitations, including the following three aspects.

[0004] Single crystal phase limitation: alpha-NiS (hexagonal phase) and beta-NiS (rhombohedral phase) have different band structures and catalytic activities, but traditional synthesis methods are difficult to realize dynamic regulation of crystal phase, resulting in limited optimization space of interface electronic structure; Static composite interface defects: mechanical mixing or random deposition of heterojunction often accompanies high density of lattice distortion and Schottky barrier, hindering efficient migration of carriers; Non-equilibrium phase transformation dynamics missing: existing processes cannot trigger in-situ crystal phase transformation during the growth of composite materials, resulting in ineffective utilization of interface stress release and lattice matching mechanism.

[0005] In addition, the alpha-NiS / CdS broadens the light absorption to 650 nm, the hydrogen production rate is 15 times that of pure CdS, has good corrosion resistance, but poor stability; the beta-NiS / CdS mainly absorbs 400-550 nm light, the hydrogen production rate is 39 times that of pure CdS, has weaker corrosion resistance but higher stability. In summary, the coexistence of alpha-NiS and beta-NiS bimorphs is expected to make breakthroughs in spectral response, corrosion resistance and multiple reaction sites. However, the coexistence of alpha-NiS and beta-NiS bimorphs faces many technical difficulties, resulting in no technical reports on the coexistence of alpha-NiS and beta-NiS bimorphs: (1) Crystal phase compatibility problem: The lattice parameters of alpha-NiS (hexagonal close-packed) and beta-NiS (trigonal system) differ significantly (alpha phase c / a = 1.633, beta phase c / a = 1.622), and direct compounding easily leads to interface stress accumulation, thereby reducing the structural stability; (2) Band matching barrier: The energy band of alpha-NiS and beta-NiS is obviously offset (ΔEc = 0.3 eV, ΔEv = 0.2 eV), which requires accurate regulation of the CdS interface to match the double-channel charge transfer; (3) Synthesis controllability problem: Alpha-NiS needs low-temperature hydrothermal synthesis (<180℃), while beta-NiS needs high-temperature phase transition (>300℃), the temperature window of bimorph coexistence is narrow, which easily leads to incomplete phase transition or impurity generation. SUMMARY

[0006] The applicant found that during the preparation of the NiS / CdS heterojunction material under the hydrothermal condition of 180℃, the phase transition of the product can be regulated by time to realize the continuous evolution process of "NiS quantum dots → amorphous NiS → alpha-NiS crystal surface exposure type grain → alpha-NiS + beta-NiS mixed phase block → long rod type beta-NiS" on the (001) crystal surface of the hexagonal phase CdS, covering the complete growth path from zero-dimensional quantum dots to stable crystal phase. In particular, when the reaction is carried out for 18-24h, the alpha-NiS and beta-NiS bimorphs coexist in the crystal group formed by NiS on the (001) end surface of the hexagonal phase CdS, which realizes the coexistence of alpha-NiS and beta-NiS bimorphs for the first time. Based on this, the application provides a preparation method of the NiS / CdS heterojunction material, the NiS / CdS heterojunction material prepared by the method and the application of the NiS / CdS heterojunction material in the field of photocatalysis and semiconductors.

[0007] The technical scheme provided by the application is as follows: In a first aspect, the application provides a preparation method of a NiS / CdS heterojunction material, comprising: The hexagonal phase CdS exposing (001) crystal surface, the soluble nickel source and the soluble sulfur source are dispersed in ethylene glycol according to the molar ratio of CdS:Ni:S = 1:(0.5-1.5):(0.5-1.5), and constant temperature reaction is carried out at 180±10℃ to obtain the NiS / CdS heterojunction material.

[0008] In some embodiments of the first aspect of the present application, the reaction time is 18-24 hours, and a first NiS / CdS heterojunction material is obtained.

[0009] In some embodiments of the first aspect of the present application, the reaction time is greater than 24 hours, and a second NiS / CdS heterojunction material is obtained.

[0010] In some embodiments of the first aspect of the present application, the reaction time is no more than 3 hours, and a third NiS / CdS heterojunction material is obtained.

[0011] In some embodiments of the first aspect of the present application, the reaction time is 3-12 hours, and a fourth NiS / CdS heterojunction material is obtained.

[0012] In some embodiments of the first aspect of the present application, the hexagonal CdS is a hexagonal CdS nanowire; and / or, the end surface of the hexagonal CdS nanowire exposes a (001) crystal surface; and / or, the aspect ratio of the hexagonal CdS nanowire is (50-100):1.

[0013] In some embodiments of the first aspect of the present application, the preparation method further comprises a preparation step of hexagonal CdS, and the preparation step of hexagonal CdS comprises: a soluble diethyldithiocarbamate and a soluble cadmium salt are co-dissolved in water at a molar ratio of diethyldithiocarbamate:Cd = 1:(1-1.2) to generate cadmium diethyldithiocarbamate precipitate; the cadmium diethyldithiocarbamate is dispersed in ethylenediamine and reacted at a constant temperature of 180±10℃ for 10 hours or more to obtain the hexagonal CdS nanowire.

[0014] In some embodiments of the first aspect of the present application, the soluble nickel source is nickel nitrate hexahydrate; and / or, the soluble sulfur source is thiourea.

[0015] In a second aspect, the present application provides a NiS / CdS heterojunction material prepared by the preparation method of the NiS / CdS heterojunction material.

[0016] In a third aspect, the present application provides an application of the NiS / CdS heterojunction material in the field of photocatalysis and semiconductors.

[0017] In some embodiments of the third aspect of the present application, the NiS / CdS heterojunction material is used as a catalyst for photocatalytic hydrogen production.

[0018] Compared with the prior art, the present application has the following advantages: 1. The present application controls the crystal phase structure (α-NiS, β-NiS and mixed phase thereof), size (quantum dot to nanoparticle) and interface composition of the heterojunction by precise control of time variables in a single preparation process, thereby breaking through the limitations of traditional methods which rely on complex conditions (such as different precursors, temperature or surfactants).

[0019] 2. The present application overcomes the problem of coexistence of α-NiS and β-NiS in the prior art, and successfully prepares a NiS / CdS heterojunction material coexisting with α-NiS and β-NiS, thereby laying a foundation for the study of α-NiS and β-NiS.

[0020] 3. The present application successfully prepares a nanowire type NiS / CdS heterojunction material in which NiS extends along the axis of CdS nanowires, thereby laying a foundation for promoting the nanowire type NiS / CdS heterojunction material from a concept to practicality.

[0021] 4. The method of the present application is simple, low in cost and can adjust the band gap of the obtained NiS / CdS heterojunction material. BRIEF DESCRIPTION OF DRAWINGS

[0022] In order to more clearly illustrate the technical solutions in the embodiments of the present application, the drawings needed in the embodiment description will be briefly introduced as follows. Obviously, the drawings in the following description are only some embodiments of the present application, and other drawings can be obtained by those skilled in the art without creative labor on the basis of these drawings.

[0023] Figure 1 TEM images of CdS nanowires and NiS / CdS heterojunction materials; wherein: (a) represents the CdS nanowires obtained in step two of the embodiment; (b) represents the NiS / CdS heterojunction material (referred to as 3h-NiS / CdS) formed when the reaction time is 3h in step three; (c) represents the NiS / CdS heterojunction material (referred to as 6h-NiS / CdS) formed when the reaction time is 6h in step three; (d) represents the NiS / CdS heterojunction material (referred to as 12h-NiS / CdS) formed when the reaction time is 12h in step three; (e) represents the NiS / CdS heterojunction material (referred to as 18h-NiS / CdS) formed when the reaction time is 18h in step three; (f) represents the NiS / CdS heterojunction material (referred to as 24h-NiS / CdS) formed when the reaction time is 24h in step three.

[0024] Figure 2High resolution transmission electron microscopy (HRTEM) image of the heterojunction facet of 12h-NiS / CdS; wherein: (a) represents the HRTEM image of the heterojunction facet of 12h-NiS / CdS; (b) represents the enlarged image of the HRTEM image of the CdS facet in (a); (c) represents the fast Fourier transform image of (b); (d) represents the enlarged image of the HRTEM image of the a-NiS facet in (a); (e) represents the fast Fourier transform image of (d).

[0025] Figure 3 High resolution transmission electron microscopy (HRTEM) image of the heterojunction facet of 24h-NiS / CdS; wherein: (a) represents the HRTEM image of the heterojunction facet of 24h-NiS / CdS; (b) represents the enlarged image of the HRTEM image of the CdS and β-NiS heterojunction facet in (a); (c) represents the fast Fourier transform image of (b).

[0026] Figure 4 X-ray diffraction (XRD) spectra of NiS on CdS nanomaterials and different NiS / CdS heterojunction materials; wherein: (a) represents the X-ray diffraction (XRD) spectra of the nanoheterojunction formed at different stages of the solvothermal synthesis of CdS nanomaterials and NiS nanocrystals; (b) represents the enlarged image of the X-ray diffraction (XRD) spectra in (a) at 30º~36º.

[0027] Figure 5 X-ray photoelectron spectroscopy (XPS) spectra and chemical state analysis of NiS on CdS nanomaterials and different NiS / CdS heterojunction materials; wherein: (a) represents the XPS spectra and chemical state analysis of the Cd 3d 5 / 2 and Cd 3d 3 / 2 peaks of pure CdS, the NiS / CdS heterojunction material formed when the reaction time in step three was 12h (NCS-12h) and the NiS / CdS heterojunction material formed when the reaction time in step three was 24h (NCS-24h); (b) represents the XPS spectra and chemical state analysis of the S 2p orbitals of pure CdS, NCS-12h and NCS-24h. DETAILED DESCRIPTION

[0028] In order to make the purposes, technical solutions and advantages of the embodiments of the present application clearer, the technical solutions in the embodiments of the present application will be described clearly and completely below with reference to the drawings in the embodiments of the present application. Obviously, the described embodiments are only some of the embodiments of the present application, rather than all the embodiments of the present application. Based on the embodiments in the present application, all other embodiments obtained by those of ordinary skill in the art without creative work fall within the scope of protection of the present application.

[0029] Unless otherwise specifically indicated, the terms used herein are understood to have the meanings as commonly used in the art. Thus, unless otherwise defined, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this application belongs. If there is a conflict between the present specification and the patent specification, the present specification prevails.

[0030] Preparation method: The present application provides a preparation method of a NiS / CdS heterojunction material, comprising: dispersing hexagonal phase CdS exposing (001) crystal surface (in terms of the amount of substance of CdS), a soluble nickel source (in terms of the amount of substance of Ni) and a soluble sulfur source (in terms of the amount of substance of S) in ethylene glycol at a molar ratio of CdS:Ni:S = 1:(0.5-1.5):(0.5-1.5), and reacting at a constant temperature of 180±10℃ to obtain a NiS / CdS heterojunction material.

[0031] The preparation method realizes in-situ phase-morphology synergistic control of NiS on the (001) crystal surface of hexagonal phase CdS in a single mild system only by time control: for the first time, gradient phase transition of NiS from amorphous-NiS→α-NiS (hexagonal phase)→β-NiS (rhombohedral phase) is realized, accompanied by multi-dimensional morphology evolution (quantum dots→amorphous state→crystal face exposure type→block type→long rod type). More specifically, continuous crystal phase conversion of "NiS quantum dots→amorphous NiS→α-NiS phase→α-NiS+β-NiS coexistence→β-NiS phase" is realized.

[0032] The preparation method can precisely control the crystal phase transition of NiS and realize atomic-level interface coupling of NiS and CdS. The crystal phase transition of NiS can induce interface stress reconstruction and electronic state redistribution, thereby breaking through the performance limit of static heterojunction. For example, the lattice contraction (~4%) in the process of α-NiS→β-NiS phase transition can release the interface stress and promote the formation of coherent interface; at the same time, the high conductivity of β-NiS and the exposed (110) high-activity crystal face can synergistically optimize the charge transport and surface reaction path.

[0033] The preparation method can be used for constructing a semiconductor heterojunction nano device with high carrier separation and transport; stress-interface dynamic balance: through the lattice distortion→ stress release→ co-lattice matching interface reconstruction mechanism, the Schottky barrier is reduced and the built-in electric field is induced; performance-stability synergistic improvement: the high crystallinity of beta-NiS (rhombohedral phase) and the stable interface structure can significantly inhibit the photo-corrosion of CdS, and its high conductivity accelerates the surface reaction kinetics.

[0034] The transformation of alpha-NiS (high-temperature metastable phase) to beta-NiS (stable phase) has a unique nature. Alpha-NiS is usually synthesized at high temperature and is difficult to exist stably, while the method realizes the generation and transformation of alpha-NiS at a lower temperature (180±10℃) through kinetic control, providing a new model for the study of metal sulfide phase transformation.

[0035] The prior art usually needs to synthesize different phase structures by changing the reaction conditions (such as temperature, precursor), while the method realizes the controllable synthesis of multiple types of heterojunctions by adjusting the reaction time, greatly simplifying the process flow.

[0036] The hexagonal phase CdS with exposed (001) crystal faces is selected as the substrate in the present application, because it is found that NiS only accumulates on the end face of the hexagonal phase CdS nanowire with exposed (001) crystal faces, and there is no NiS formed around the hexagonal phase CdS nanowire. The (001) face of the hexagonal phase CdS usually exhibits stronger photocatalytic activity due to its high surface energy, but this crystal face is easily lost in conventional synthesis. The method directly uses this crystal face as a heterojunction substrate, providing a structural basis for improving catalytic performance.

[0037] The present application can obtain NiS / CdS heterojunction materials with different NiS morphologies by controlling the reaction time, for example: In some embodiments, the reaction time is 18-24 hours, and a first NiS / CdS heterojunction material is obtained. Within this reaction time, NiS forms a NiS crystal group with alpha-NiS and beta-NiS coexisting on the exposed (001) crystal face of the hexagonal phase CdS, and the alpha-NiS and beta-NiS bicrystal phase coexisting NiS / CdS heterojunction material is first prepared, laying a foundation for the research of alpha-NiS and beta-NiS bicrystal phase coexisting NiS / CdS heterojunction material in the field. Figure 4As shown, 18h-NiS / CdS and 24h-NiS / CdS have both α-NiS and β-NiS characteristic peaks, indicating that in the first NiS / CdS heterojunction material obtained by the reaction time of 18-24 hours, NiS exists in the form of α-NiS and β-NiS, which overcomes the problem of coexistence of α-NiS and β-NiS in the prior art. Within this time period, α-NiS (hexagonal phase) is converted to β-NiS (rhombohedral phase), and the β-NiS grain grows along the end face of the (001) crystal plane of the hexagonal phase CdS nanowire to form a block structure with a length-diameter ratio of (1-3): 1.

[0038] In some embodiments, the reaction time is greater than 24 hours, and a second NiS / CdS heterojunction material is obtained. Within this reaction time, the crystal face exposed type β-NiS (rhombohedral phase) is restructured under hydrothermal conditions to form a long rod type β-NiS / CdS heterojunction, the (001) crystal plane of CdS is connected with the (300) crystal plane of NiS, a coherent interface is formed by heterojunction, and the crystallinity of the two phases is significantly improved. By forming a β-NiS / CdS coherent heterojunction, atomic level matching between the (100) crystal plane of β-NiS and the (001) crystal plane of CdS is observed by HRTEM (lattice mismatch degree ≤1.0%). That is, in the second NiS / CdS heterojunction material, NiS is in the form of long rod type β-NiS.

[0039] In some embodiments, the reaction time is not more than 3 hours, and NiS quantum dots are formed on the (001) crystal plane exposed by the hexagonal phase CdS to obtain a third NiS / CdS heterojunction material. Within this reaction time, NiS grows on the end face of the (001) crystal plane exposed by the hexagonal phase CdS nanowire from nothing to something, and NiS is in the form of quantum dots at 3 hours. That is, in the third NiS / CdS heterojunction material, NiS is in the form of quantum dots.

[0040] In some embodiments, the reaction time is 3-12 hours, and amorphous NiS is formed on the (001) crystal plane exposed by the hexagonal phase CdS to obtain a fourth NiS / CdS heterojunction material. Within this reaction time, NiS grows on the end face of the (001) crystal plane exposed by the hexagonal phase CdS nanowire, and is still amorphous NiS at 12 hours. That is, the NiS in the fourth NiS / CdS heterojunction material is in the form of amorphous.

[0041] In some embodiments, the crystal orientation relationship of the heterojunction in which the hexagonal phase CdS nanowire (001) end face grows along a specific crystal direction is: α-NiS (100) / / CdS (100), α-NiS (001) / / CdS (001), β-NiS (300) / / CdS (001).

[0042] The glycol is used as the solvent, because the glycol can dissolve the soluble nickel source and the soluble sulfur source, and the boiling point is higher than 180±10 DEG C, which meets the requirement of the reaction temperature; the coordination effect controls the nucleation rate of the NiS, which is a key factor for realizing the time-dependent phase change.

[0043] In some embodiments, the hexagonal CdS is a hexagonal CdS nanowire; and / or, The end surface of the hexagonal CdS nanowire exposes the (001) crystal surface; and / or, The aspect ratio of the hexagonal CdS nanowire is (50-100):1.

[0044] When the hexagonal CdS nanowire with the end surface exposing the (001) crystal surface is used as the catalyst base, the NiS extends along the axis of the CdS nanowire to form a nanowire type NiS / CdS heterojunction material. The commercially available product can be used as the hexagonal CdS.

[0045] In some embodiments, the preparation method further comprises a preparation step of the hexagonal CdS, and the preparation step of the hexagonal CdS comprises: The soluble diethyl dithiocarbamate and the soluble cadmium salt are co-dissolved in water in a molar ratio of diethyl dithiocarbamate: Cd = 1: (1-1.2) to generate cadmium diethyl dithiocarbamate precipitate; The cadmium diethyl dithiocarbamate is dispersed in ethylenediamine, and is reacted at a constant temperature of 180±10 DEG C for more than 10 hours to obtain the hexagonal CdS nanowire.

[0046] The preparation step of the hexagonal CdS can prepare the hexagonal CdS nanowire with the end surface exposing the (001) crystal surface, so as to realize the extension of the NiS along the axis of the CdS nanowire to form the nanowire type NiS / CdS heterojunction material.

[0047] Preferably, the constant temperature reaction time is 12 hours.

[0048] In some embodiments, the soluble nickel source is nickel nitrate hexahydrate; and / or, the soluble sulfur source is thiourea. The nickel nitrate hexahydrate and the thiourea can be dissolved in the glycol, and no solid product other than the NiS is generated, which reduces the risk that the solid impurities affect the crystallization of the NiS.

[0049] The catalyst: The NiS / CdS heterojunction material prepared by the preparation method of the NiS / CdS heterojunction material provided by the application.

[0050] Application of the catalyst: The application provides application of a NiS / CdS heterojunction material in the fields of photocatalysis and semiconductors, and preferably, the NiS / CdS heterojunction material is used as a catalyst for photocatalytic hydrogen production.

[0051] The technical solutions of the application are described in detail below through specific examples. Unless otherwise specified, the chemicals used in the examples are obtained from commercial channels. Example

[0052] The preparation method of the NiS / CdS heterojunction material provided in this example comprises the following steps: Step one: 22.53 g of sodium diethyldithiocarbamate is dissolved in 500 mL of deionized water at room temperature, and stirred for 1 h until completely dissolved; 22.84 g of CdCl2·2.5H2O is added, and the stirring is continued for 5 h to generate a white precipitate; the white precipitate is separated by centrifugation, washed with deionized water for three times, and dried at 180℃ for 12 h to obtain cadmium diethyldithiocarbamate.

[0053] Step two: 1.57 g of cadmium diethyldithiocarbamate is dispersed in 56 mL of ethylenediamine, and stirred for 1 h, and then transferred to a reaction kettle for reaction at 180℃ for 12 h; the product is collected by centrifugation, washed with deionized water and ethanol alternately for three times, and vacuum dried at 180℃ for 12 h to obtain hexagonal phase CdS nanowires with (001) crystal plane exposed on the end surface. The CdS nanowires have a diameter of 30-50 nm, a length of 2-5 μm, and an aspect ratio (50-100):1.

[0054] Step three: multiple parallel experimental groups are prepared: 1.0 g of the CdS nanowires prepared in step two, 0.5 g of nickel nitrate hexahydrate, and 2.0 g of thiourea are dispersed in 50 mL of ethylene glycol, and reacted at 180℃ for 3 h, 6 h, 9 h, 12 h, 15 h, 18 h, 21 h, and 24 h, respectively; the products are collected by centrifugation, washed and dried to obtain NiS / CdS heterojunction materials.

[0055] Characterization results: 1. High-resolution electron microscope (HRTEM) The heterojunction crystal planes of the NiS / CdS heterojunction materials obtained in each group are observed by a high-resolution electron microscope (HRTEM), as shown in FIG. 1: the NiS grows only along the axial direction of the CdS nanowires, that is, the (001) crystal planes exposed on the end surfaces of the NiS and the CdS nanowires form a heterojunction. Figures 1-3

[0056] Figure 1 ​The TEM images of CdS nanowires and NiS / CdS heterojunction materials can clearly show that when the reaction time in step three is 3 h, the NiS on the NiS / CdS heterojunction material is in the form of quantum dots. The HRTEM observation shows that the end face of CdS is (001) crystal face and the local bonding distortion of amorphous phase of NiS, and the crystal lattice distortion rate is ≥7%.

[0057] With the continuation of the hydrothermal reaction, the NiS quantum dots continue to grow on the end face of the (001) crystal face of the CdS nanowire, and build an amorphous state. When the reaction time in step three reaches 6 h, the NiS forms an amorphous or metastable phase.

[0058] With the continuation of the hydrothermal reaction, the amorphous NiS amorphous particles gradually grow into α-NiS (hexagonal phase) grains, and the α-NiS grows along the axis of the CdS nanowire. When the reaction time in step three reaches 12 h, the α-NiS forms a crystal face exposure type α-NiS / CdS heterojunction with a length-diameter ratio (2-4:1) on the end face of the CdS nanowire. In the heterojunction, the (001) crystal face of the α-NiS is combined with the (001) crystal face of the CdS, and the (101) crystal face of the α-NiS is exposed.

[0059] Subsequently, the α-NiS (hexagonal phase) begins to transform into β-NiS (rhombohedral phase), and the β-NiS phase grains grow into a block structure along the axis of the CdS nanowire. When the reaction time in step three reaches 18 h, the block structure has a length-diameter ratio (1-3):1.

[0060] The crystal face exposure type β-NiS (rhombohedral phase) restructures under hydrothermal conditions. When the reaction time in step three reaches 24 h, a long rod type β-NiS / CdS heterojunction is formed. The (001) crystal face of the hexagonal phase CdS nanowire end face is connected with the (300) crystal face of the β-NiS, and a coherent interface is formed by heterojunction, and the crystallinity of the two phases is significantly improved. The HRTEM observation shows that the (100) crystal face of the β-NiS is atomically matched with the (001) crystal face of the CdS (lattice mismatch degree ≤1.0%).

[0061] Figure 2 The HRTEM image of the crystal face of the nanoheterojunction formed when the reaction time in step three is 12 h. In this image, the crystal lattices of the CdS nanomaterial and the NiS nanocrystal can be accurately observed. The NiS grains gradually grow into α-NiS, and a crystal face exposure type α-NiS / CdS heterojunction is obtained. At this time, the (001) crystal face of the NiS grain at the bottom of the NiS grain is connected with the (001) crystal face of the CdS, and the (001) crystal face and the (101) crystal face of the NiS grain are exposed.

[0062] Figure 3Figure 1 is a high-resolution electron microscopy (HRTEM) image of a nanoheterojunction surface formed when the reaction time of step three is 24h. When the solvothermal time is up to 24h, the structure of the exposed β-NiS is restructured under hydrothermal conditions to form a long rod type β-NiS / CdS heterojunction, the (001) crystal surface of CdS is connected with the (300) crystal surface of NiS, the heterojunction forms a coherent interface and the crystallinity of the two phases is significantly improved.

[0063] In summary, the present application realizes the dynamic optimization of the crystal phase, morphology and interface characteristics of the NiS / CdS heterojunction by precisely controlling the solvothermal reaction time. Specifically, by precisely controlling the solvothermal reaction time (3-24h), the gradient phase transition of NiS from amorphous-NiS→α-NiS (hexagonal phase)→β-NiS (rhombohedral phase) is realized for the first time, accompanied by multi-dimensional morphology evolution (quantum dots→amorphous→exposed crystal surface→block→long rod type), breaking through the limitations of single crystal phase and static composite interface in traditional processes.

[0064] In the aspect of interface engineering, the lattice contraction (about 4%) in the α-NiS→β-NiS phase transition process is used to induce interface stress release, and atomic level coherent matching is realized through epitaxial growth: for example, the β-NiS grows into a block structure along the CdS end face (interface lattice strain≤1.5%) at 18h, and the long rod type β-NiS / CdS is coherently connected through (300) and (001) crystal surfaces (lattice mismatch degree≤1.0%) at 24h, forming a high-efficiency built-in electric field, significantly reducing the Schottky barrier and inhibiting carrier recombination.

[0065] 2. XRD spectrum Figure 4 Figure 2 is an XRD spectrum of the nanoheterojunction formed by CdS nanomaterials and NiS nanocrystals at different stages of solvothermal synthesis. The XRD spectrum also well verifies the conclusion that the corresponding crystal phase controllable NiS composite heterojunction photocatalyst is constructed on the surface of CdS with the change of the solvothermal reaction time.

[0066] 3. XPS spectrum The present application reveals the evolution of the interface electronic structure of the NiS / CdS composite material under different treatment times through the XPS spectrum. Figure 5 Figure 3 shows the XPS spectrum of CdS nanomaterials and NiS nanocrystals at different treatment times and its chemical state analysis. Figure 5 (a) is a fine scanning spectrum of Cd 3d orbit, and the Cd 3d 5 / 2 and Cd 3d 3 / 2The peaks are located at 404.0 eV and 410.8 eV, respectively, while the binding energies of samples treated for 12 hours (NCS-12h) and 24 hours (NCS-24h) shift to 405.21 eV, 411.95 eV and 405.56 eV, 412.30 eV, respectively. The gradual increase in binding energy indicates that the electron density of Cd further decreases with the extension of NiS loading time, which is attributed to the formation of a heterojunction at the CdS-NiS interface and the electronic interaction of the Cd-S-Ni structure. The electronegativity difference (Cd: 1.69, Ni: 1.91) leads to electron transfer from Cd to Ni, thereby enhancing the separation efficiency of photogenerated carriers. Figure 5 (b) shows the peak fitting results of the S 2p orbital in pure CdS. 2- 2p 3 / 2 and 2p 1 / 2 The peaks are located at 161.23 eV and 162.44 eV, respectively, while the S 2p peaks of the NCS-12h and NCS-24h samples are... 3 / 2 The peaks shifted to 161.50 eV and 162.68 eV, respectively, and the increase in binding energy further confirmed the strong interaction between CdS and NiS. Furthermore, the peaks appearing in the 168.7–170.0 eV range can be attributed to sulfate (SO42-). 2- The 2p orbital of ) indicates that part of S 2- The catalyst is oxidized during the recombination process. The existence of this oxidized state may originate from interfacial reactions or the participation of surface-adsorbed oxygen species. Differences in binding energy among samples treated at different times reflect the dynamic adjustment of the interfacial electronic structure, providing a crucial basis for optimizing heterojunction photocatalysts. This technology, through dynamic phase transformation and interfacial stress reconstruction mechanisms, provides a new paradigm for the synergistic design of crystal planes / phases in CdS-based photocatalysts, possessing significant scientific value and application prospects for developing highly efficient and stable photocatalytic systems.

[0067] The above description is merely a specific embodiment of the present invention, enabling those skilled in the art to understand or implement the invention. Various modifications to these embodiments will be readily apparent to those skilled in the art, and the general principles defined herein may be implemented in other embodiments without departing from the spirit or scope of the invention. Therefore, the present invention is not to be limited to the embodiments shown herein, but is to be accorded the widest scope consistent with the principles and novel features claimed herein.

Claims

1. A method for preparing a NiS / CdS heterojunction material, characterized in that, include: Hexagonal CdS with exposed (001) crystal planes, soluble nickel source, and soluble sulfur source were dispersed in ethylene glycol at a molar ratio of CdS:Ni:S=1:(0.5~1.5):(0.5~1.5) and reacted at a constant temperature of 180±10℃ to obtain NiS / CdS heterojunction material.

2. The method for preparing NiS / CdS heterojunction material according to claim 1, characterized in that: The reaction time is 18-24 hours, and the first NiS / CdS heterojunction material is obtained.

3. The method for preparing NiS / CdS heterojunction material according to claim 1, characterized in that: The reaction time was greater than 24 hours, and a second NiS / CdS heterojunction material was obtained.

4. The method for preparing NiS / CdS heterojunction material according to claim 1, characterized in that: The reaction time is no more than 3 hours, and the third NiS / CdS heterojunction material is obtained.

5. The method for preparing NiS / CdS heterojunction material according to claim 1, characterized in that: The reaction time is 3 to 12 hours to obtain the fourth NiS / CdS heterojunction material.

6. The method for preparing NiS / CdS heterojunction material according to claim 1, characterized in that: The hexagonal CdS is hexagonal CdS nanowires; and / or The end faces of the hexagonal CdS nanowires expose (001) crystal planes; and / or, The aspect ratio of the hexagonal CdS nanowires is (50~100):

1.

7. The method for preparing NiS / CdS heterojunction material according to claim 6, characterized in that: The preparation method further includes a step for preparing hexagonal CdS, wherein the preparation step for hexagonal CdS includes: With a molar ratio of diethyldithiocarbamate:Cd=1:(1~1.2), soluble diethyldithiocarbamate and soluble cadmium salt are co-dissolved in water to form cadmium diethyldithiocarbamate precipitate. Cadmium diethyldithiocarbamate was dispersed in ethylenediamine and reacted at a constant temperature of 180±10℃ for more than 10 hours to obtain hexagonal CdS nanowires.

8. The method for preparing NiS / CdS heterojunction material according to claim 1, characterized in that: The soluble nickel source is nickel nitrate hexahydrate; and / or, The soluble sulfur source is thiourea.

9. A NiS / CdS heterojunction material, characterized in that... It is prepared by the method for preparing NiS / CdS heterojunction material according to any one of claims 1 to 8.

10. The application of the NiS / CdS heterojunction material according to claim 9 in the fields of photocatalysis and semiconductors.

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