TiO2 / Bi2S3 heterojunction, preparation method thereof and application of TiO2 / Bi2S3 heterojunction in flexible broadband photoelectric detector

Abstract

The invention relates to a TiO2 / Bi2S3 heterojunction, a preparation method thereof and an application of the heterojunction in a flexible broadband photoelectric detector, the heterojunction is formed by compounding TiO2 with a hollow structure and Bi2S3, the overall appearance of the TiO2 with the hollow structure is spherical or cubic, and the interior of the TiO2 with the hollow structure is of a hollow structure. Bi2S3 is nanoflowers or nanoparticles, and the Bi2S3 nanoflowers are formed by dispersing and growing Bi2S3 nanorods from the center, are in a sea urchin shape and are randomly compounded around the hollow cube; bi2S3 nano-particles with small particle sizes can be attached to the outer surface of the TiO2 with the hollow structure to form a core-shell structure. The solution method adopted by the invention has the advantages of simple operation and low cost, and can be used for synthesizing the TiO2 / Bi2S3 heterojunction composite material on a large scale. In addition, a flexible self-energized broadband photoelectric detector capable of realizing response from ultraviolet light to infrared light is further prepared, and the flexible self-energized broadband photoelectric detector has application potential in the field of flexible wearable detection.

Description

Technical Field

[0001] The present invention belongs to the technical field of semiconductor detection materials and their preparation, and particularly relates to a TiO2 / Bi2S3 heterojunction, a preparation method thereof, and an application thereof in a flexible wide-band photodetector. Background Art

[0002] As a core component for converting optical signals into electrical signals, photodetectors exhibit important application values in strategic fields such as environmental monitoring, portable communication, and missile detection, and have become a research hotspot for new optoelectronic functional devices. Traditional photodetectors usually construct planar structure devices based on rigid substrates (such as silicon, glass). Although this design has advantages in terms of stability and process maturity, its inherent physical rigidity characteristics result in the inability of the device to conform to curved surfaces or withstand dynamic deformations, severely restricting its application in flexible electronic devices and making it difficult to meet the growing demands of flexible and wearable devices. Among many detector materials, TiO2 has characteristics such as non-toxicity, low preparation cost, and good carrier transport ability. In addition, compared with traditional semiconductor materials, TiO2 can be prepared by a low-temperature solution preparation process, enabling low-temperature film formation on flexible substrates and avoiding damage to flexible substrates caused by high-temperature treatment. Moreover, considering the diverse nanostructure morphologies of TiO2, it can effectively improve the deformation tolerance of the material itself, making TiO2 one of the ideal materials for constructing flexible detectors.

[0003] However, the bandgap width of TiO2 is 3.2 eV, resulting in the spectral response range of the device being strictly limited to the ultraviolet band, thereby restricting the application development of photodetectors in real life. And TiO2 also faces the problem of rapid recombination of photo-generated carriers, which leads to a reduction in the number of carriers that can participate in the optoelectronic conversion process and generate effective electrical signals, reducing the optoelectronic conversion efficiency of the detector, and further affecting key performance indicators such as its response current to ultraviolet light and detection sensitivity, ultimately limiting the detection performance of TiO2 photodetectors. In addition, there have been reports on related TiO2 flexible photodetectors, mainly by directly growing TiO2 on existing flexible substrates to realize device preparation. This is limited in the selection of substrates for growing materials, with complex preparation processes and high costs, which affects the sustainable development of TiO2 flexible optoelectronic devices. Therefore, it is of great significance to develop a TiO2 / Bi2S3 heterojunction flexible photodetector with wide-band response and more excellent detection performance. Summary of the Invention

[0004] The core objective of the present invention is to break through the performance limitations of traditional TiO2-based photodetectors and propose a solution to enhance performance based on heterojunctions. The present invention combines wide-bandgap TiO2 with narrow-bandgap Bi2S3 (1.3 - 1.7 eV) to construct a TiO2 / Bi2S3 heterojunction structure with a type-II energy band structure, expanding the device response band to the ultraviolet-visible-infrared wide-band range; in addition, the built-in electric field at the heterojunction interface is utilized to achieve efficient separation of photo-generated electron-hole pairs, thereby significantly improving the detection performance of the device. In particular, the solution method process is adopted in the present invention, which can batch synthesize TiO2 / Bi2S3 heterojunction powder at a relatively low temperature or even at room temperature; then the material is directly transferred onto a flexible substrate by the drop-casting method, and the device prepared by this method exhibits good self-powered characteristics under zero bias voltage conditions, providing technical support for the development of a new generation of low-cost, flexible, and environment-adaptive photodetectors.

[0005] The objective of the present invention is achieved through the following technical solutions:

[0006] A TiO2 / Bi2S3 heterojunction, whose structure is a TiO2 / Bi2S3 nanoflower heterojunction or a TiO2 / Bi2S3 nanoparticle heterojunction.

[0007] The overall appearance of TiO2 in the TiO2 / Bi2S3 nanoflower heterojunction is spherical or cubic, with a hollow structure inside, a shell thickness of 20 - 200 nm, and the overall shape of the Bi2S3 nanoflower is sea urchin-like, with a size of 0.5 - 2.0 μm, consisting of a large number of Bi2S3 nanorods growing divergently from the center.

[0008] A preparation method of a TiO2 / Bi2S3 nanoflower heterojunction, and the specific steps of the method are as follows:

[0009] Step 1: Add sodium hydroxide to a solution containing Cu ions, then add ascorbic acid, react at 20 - 80 °C for 10 min - 5 h, and after centrifugation, naturally dry to obtain Cu2O powder with different morphologies;

[0010] Step 2: Add the Cu2O powder to a mixed solution of ammonium hexafluorotitanate and boric acid, react in a water bath at 40 - 80 °C for 20 min - 5 h, anneal the centrifuged and dried sample at 200 - 700 °C for 1 - 5 h; then remove the internal impurities of the annealed sample in boric acid to obtain TiO2 powder with a hollow structure;

[0011] Step 3: First, add bismuth nitrate into the ethylene glycol solution and stir it at 500 - 1000 rpm for 0.5 - 1 h to prepare an ethylene glycol solution of bismuth nitrate with a concentration of 0.04 - 0.4 mol / L. Then, quickly stir the solution at 2000 - 2400 rpm, and simultaneously add a certain amount of thiourea for 0.5 - 3 h. The molar ratio of the added bismuth nitrate to thiourea is 2:3;

[0012] Step 4: Add the hollow-structured TiO2 powder into the solution obtained in Step 3 for a water bath reaction. The water bath reaction temperature is 50 - 100 °C, and the reaction stirring time is 1 - 10 h. After the reaction is completed, wash and centrifuge, and then air-dry naturally at room temperature to obtain the TiO2 / Bi2S3 nanoflower heterojunction.

[0013] The overall appearance of TiO2 in the TiO2 / Bi2S3 nanoparticle heterojunction is spherical or cubic, with a hollow structure inside. The shell thickness is 20 - 200 nm, and the overall particle size of the Bi2S3 nanoparticles is 5 - 30 nm, randomly attached to the surface of the hollow-structured TiO2 to form a core-shell structure.

[0014] A preparation method of a TiO2 / Bi2S3 nanoparticle heterojunction, and the specific steps of the method are as follows:

[0015] Step 1: Add sodium hydroxide into the solution containing Cu ions, and then add ascorbic acid. React at 20 - 80 °C for 10 min - 5 h, and after centrifugation, air-dry naturally to obtain Cu2O powders with different morphologies;

[0016] Step 2: Add the Cu2O powder into the mixed solution of ammonium hexafluorotitanate and boric acid, react in a water bath at 40 - 80 °C for 20 min - 5 h, anneal the centrifuged and dried sample at 200 - 700 °C for 1 - 5 h; then remove the internal impurities of the annealed sample in boric acid to obtain the hollow-structured TiO2 powder;

[0017] Step 3: Prepare 2 ml of a bismuth nitrate ethylene glycol solution with a concentration of 0.01 - 0.025 mol / L, add the hollow-structured TiO2 powder and ultrasonicate for 1 - 10 min to form a uniformly dispersed suspension. Then, add 0.02 - 0.05 mmol of thioacetamide or sodium sulfide into the suspension, and ultrasonicate for 10 - 30 s to form a black suspension;

[0018] Step 4: Quickly add the black suspension into 20 ml of a bismuth nitrate and thiourea ethylene glycol solution with a concentration of 0.1 - 0.5 mol / L, and stir at 20 °C for 3 - 10 h. After the reaction is completed, wash and centrifuge, and then air-dry naturally at room temperature to obtain the TiO2 / Bi2S3 nanoparticle heterojunction.

[0019] Furthermore, the hollow-structured TiO2 is a TiO2 hollow sphere or a TiO2 hollow cube.

[0020] An application of a TiO2 / Bi2S3 heterojunction in a flexible broadband photodetector. A PET-ITO substrate loaded with a TiO2 / Bi2S3 heterojunction material is used as the flexible working electrode of the detector. A solid electrolyte is uniformly coated on the material. The PET-ITO substrate is used as the flexible counter electrode, and the two substrates are adhered together with their conductive faces facing inward to obtain a flexible broadband photodetector based on the TiO2 / Bi2S3 heterojunction.

[0021] The preparation method of the TiO2 / Bi2S3 heterojunction flexible working electrode is as follows: The TiO2 / Bi2S3 heterojunction material is dispersed in deionized water and uniformly coated on the conductive face of the PET-ITO flexible substrate by the drop-casting method. The coating and drying are repeated 3 to 10 times to obtain the TiO2 / Bi2S3 heterojunction flexible working electrode.

[0022] Furthermore, the TiO2 / Bi2S3 heterojunction is one of a TiO2 hollow sphere / Bi2S3 nanoflower heterojunction, a TiO2 hollow sphere / Bi2S3 nanoparticle heterojunction, a TiO2 hollow cube / Bi2S3 nanoflower heterojunction, or a TiO2 hollow cube / Bi2S3 nanoparticle heterojunction.

[0023] Furthermore, the preparation process of the solid electrolyte is as follows: 1.5 g of PVA is added to 12 ml of deionized water, and then 5 ml of KOH solution or polysulfide electrolyte or iodine electrolyte is added. Then it is first placed in a 95°C water bath and stirred rapidly for 20 min, and then placed at 20°C and stirred slowly for 1 to 10 h.

[0024] Furthermore, the solid electrolyte is one of a KOH solid electrolyte, a polysulfide solid electrolyte, or an iodine solid electrolyte.

[0025] The advantages of the present invention compared with the prior art are as follows:

[0026] The present invention provides a method for preparing a TiO2 / Bi2S3 heterojunction. In this method, the synthesized TiO2 has a unique hollow structure with a large specific surface area, which is conducive to the scattering and reflection of incident light inside the hollow structure material, thereby achieving better light absorption ability. Bi2S3 presents a unique nanoflower structure and nanoparticle structure. Among them, the Bi2S3 nanoflower is formed by the divergent growth of Bi2S3 nanorods from the center to the periphery. The structure is regular, and the overall appearance is sea urchin-like. This morphology has a unique hierarchical structure and a large specific surface area, which can not only increase the number of times of light scattering and reflection, extend the propagation path of light inside the material, but also provide more active sites, thus significantly enhancing the ability to capture and absorb light; and due to its small particle size, Bi2S3 nanoparticles can adhere to the surface of the hollow structure TiO2 to form a core-shell structure, which is conducive to the realization of high-performance flexible broadband photodetectors.

[0027] In addition, by combining TiO2 with a wide bandgap and Bi2S3 with a narrow bandgap, the present invention constructs a TiO2 / Bi2S3 heterojunction structure with a type-II energy band structure, expands the device response band to the ultraviolet-visible-near-infrared wide band range, and utilizes the built-in electric field at the heterojunction interface to accelerate the separation of photo-generated electron-hole pairs, thereby improving the detection performance of the device. More prominently, based on the TiO2 / Bi2S3 heterojunction powder synthesized in large quantities, detectors with flexible characteristics can be mass-produced through a simple and convenient drop-casting process, broadening the selection range of flexible substrates; in addition, the prepared devices can stably perform detection work even under different degrees of bending states. By flexibly changing parameters such as the morphology, size, shell thickness of the hollow structure TiO2, and the size of Bi2S3 nanoflowers and nanoparticles, the present invention improves the detection performance of the detector, and the entire preparation process is simple and low-cost, which is conducive to realizing high-efficiency mass production. Brief Description of the Drawings

[0028] Figure 1 It is a low-magnification SEM image of the TiO2 hollow cube / Bi2S3 nanoflower heterojunction;

[0029] Figure 2 It is a high-magnification SEM image of the TiO2 hollow cube / Bi2S3 nanoflower heterojunction;

[0030] Figure 3 It is an SEM image of the TiO2 hollow cube / Bi2S3 nanoparticle heterojunction;

[0031] Figure 4 It is an EDS image of the TiO2 hollow cube / Bi2S3 nanoparticle heterojunction;

[0032] Figure 5Time-resolved current curve of the TiO2 hollow cube / Bi2S3 nanoparticle heterojunction flexible broadband photodetector under illumination of different wavelengths when in a flat state;

[0033] Figure 6 Time-resolved current curve of the TiO2 hollow cube / Bi2S3 nanoparticle heterojunction flexible broadband photodetector under infrared light (850 nm) illumination when in different bending states. Specific implementation manners

[0034] The technical solutions of the present invention will be further described below in conjunction with the accompanying drawings and embodiments, but are not limited thereto. Any modification or equivalent replacement of the technical solutions of the present invention without departing from the spirit and scope of the technical solutions of the present invention shall be covered by the protection scope of the present invention.

[0035] Example 1:

[0036] A preparation method of a TiO2 hollow cube / Bi2S3 nanoflower heterojunction, the method steps are as follows:

[0037] Step 1: Prepare 100 ml of 0.1 mol / L copper chloride solution, stir evenly, then add 10 ml of 2 mol / L sodium hydroxide solution, stir for 0.5 h, and then add 0.006 mol of ascorbic acid to the dark brown solution, and react in a water bath at 60 °C for 3 h; wash and centrifuge the reacted solution, and dry it at room temperature to obtain Cu2O nanocube powder;

[0038] Step 2: Prepare a mixed solution of 30 ml of 0.02 mol / L ammonium hexafluorotitanate and 0.06 mol / L boric acid, stir evenly, directly add the Cu2O nanocube powder obtained in Step 1 to the mixed solution, stir evenly and react at 60 °C for 2 h, centrifuge and air-dry naturally at room temperature, and then anneal at an annealing temperature of 500 °C in an air atmosphere for 2 h; add the annealed sample to 0.5 mol / L boric acid solution and react for 3 h, wash 3 times with deionized water and alcohol by centrifugation, and air-dry naturally at room temperature to obtain TiO2 hollow cubes;

[0039] Step 3: First add 4 mmol of bismuth nitrate to 40 ml of ethylene glycol solution and stir at 500 rpm for 0.5 h to prepare a 0.1 mol / L ethylene glycol solution of bismuth nitrate, then stir the solution at 2000 rpm quickly, and add 6 mmol of thiourea and stir quickly for 3 h;

[0040] Step 4: Add 0.02 g of TiO2 hollow cube powder into the solution obtained in Step 3, place it in a water bath at 60 °C and stir for 3 h. After the reaction is completed, centrifuge and air-dry naturally at room temperature to obtain the TiO2 hollow cube / Bi2S3 nanoflower heterojunction.

[0041] The low-magnification SEM image of the TiO2 hollow cube / Bi2S3 nanoflower is as Figure 1 shown. Well-dispersed hollow cubes and nanoflowers can be observed, with good size uniformity, and some broken hollow cubes can be found, indicating that the synthesized TiO2 material is hollow. In the high-magnification SEM image ( Figure 2 ), it can be found that the nanoflower is composed of a large number of Bi2S3 nanorods growing radially from the center, presenting a sea urchin-like shape as a whole, with a particle size of about 1.2 μm, while the TiO2 hollow cube has a regular shape and an average size of about 1.1 μm.

[0042] Example 2:

[0043] A preparation method of a TiO2 hollow cube / Bi2S3 nanoparticle heterojunction, the method steps are as follows:

[0044] Step 1: Prepare 2 ml of 0.01 mol / L bismuth nitrate ethylene glycol solution, add the TiO2 hollow cube powder (0.01 g) obtained in Step 2 of Example 1, and ultrasonicate for 3 min to form a uniformly dispersed suspension. Then add 0.02 mmol of thioacetamide to the suspension and ultrasonicate for 30 s to form a black suspension.

[0045] Step 2: Quickly add the black suspension to 20 ml of 0.5 mol / L bismuth nitrate and thiourea ethylene glycol solution, stir at 20 °C for 8 h, wash and centrifuge after the reaction is completed, and finally air-dry naturally at room temperature to obtain the TiO2 hollow cube / Bi2S3 nanoparticle heterojunction.

[0046] As Figure 3 shown, in the SEM image of the TiO2 hollow cube / Bi2S3 nanoparticle heterojunction, it can be observed that the hollow cubes are uniformly dispersed, and nanoparticles can be found attached to the surface of the hollow cubes; in addition, the core-shell structure formed by the composite of nanoparticles and hollow cubes has an average size of about 1.3 μm, and the overall morphology is uneven, thus forming a relatively rough surface. To explore the elemental composition of the prepared sample, energy spectrum scanning analysis was performed on this sample, as Figure 4As shown. In the figure, the elemental peaks of O, Ti, Bi, and S appear. These four peaks are from the synthesized TiO2 and Bi2S3 materials. In addition, a very obvious Si peak also appears in the figure, which is caused by the Si wafer supporting the sample test. It is worth noting that no other elemental impurity peaks appear, indicating that the prepared TiO2 hollow cube / Bi2S3 nanoparticle heterojunction is very pure.

[0047] Example 3:

[0048] A preparation method of a flexible broadband photodetector based on a TiO2 hollow cube / Bi2S3 nanoparticle heterojunction, the method steps are as follows:

[0049] Step 1: Uniformly disperse 2 mg of TiO2 hollow cube / Bi2S3 nanoparticle heterojunction material in 0.1 ml of deionized water, and uniformly coat it on the conductive surface of the PET-ITO flexible substrate by the drop-casting method. Repeat the coating and drying 3 times to obtain a TiO2 hollow cube / Bi2S3 nanoparticle heterojunction flexible working electrode;

[0050] Step 2: Add 1.5 g of PVA to 12 ml of deionized water, then add 5 ml of polysulfide electrolyte. Then, place the obtained mixed solution in a 95 °C water bath and stir rapidly for 20 min, then place it at 20 °C and stir slowly for 5 h, and then refrigerate it for 5 h to cool it into a gel-like solid electrolyte;

[0051] Step 3: Uniformly coat a layer of polysulfide solid electrolyte on the material of the PET-ITO flexible substrate loaded with the TiO2 hollow cube / Bi2S3 nanoparticle heterojunction, and then directly cover the conductive surface of another PET-ITO flexible substrate on it as a flexible counter electrode to obtain a flexible broadband photodetector based on the TiO2 hollow cube / Bi2S3 nanoparticle heterojunction.

[0052] Under the condition of no external bias voltage (0 V), use a Keithley 2400 digital source meter to study the broadband response performance of the TiO2 hollow cube / Bi2S3 nanoparticle heterojunction flexible detector. As Figure 5As shown, under ultraviolet light illumination, the photocurrent rapidly rises to 1.7 μA, and quickly drops to the initial value after turning off the ultraviolet light. Moreover, within 5 repeated on / off cycles, the device exhibits the same pattern and there is no obvious change in the magnitude of the photocurrent. Under visible light (470 nm, 530 nm, 625 nm) illumination, the photocurrent responses are 2.0 μA, 2.1 μA, and 1.9 μA respectively, and the detector still presents a similar current change pattern. The TiO2 hollow cube / Bi2S3 nanoparticle heterojunction flexible broadband photodetector can still generate a photocurrent of 0.89 μA under 850 nm infrared light, and the response speed is comparable to that in the ultraviolet-visible region. This indicates that the device can exhibit fast response and self-powered characteristics in the ultraviolet, visible light, and infrared bands.

[0053] To test the detection ability of the TiO2 hollow cube / Bi2S3 nanoparticle heterojunction flexible broadband photodetector in the bent state, the detector in different bent states and after recovering to the flat state was cyclically tested without applying an external bias voltage, as Figure 6 shown. In the initial flat state, the detector can quickly respond to 850 nm infrared light, and the photocurrent is 0.89 μA, and the current change pattern can still remain stable within 5 cycles. When the device is slightly bent so that the radius of curvature of the device is about 4.27 cm, the dark current of the detector is almost zero without infrared light illumination. When irradiated with infrared light, the photocurrent response rises from the initial value to 0.87 μA and quickly drops to the initial value after turning off the infrared light, indicating that the detector can quickly respond to infrared light energy. When the distance between the two ends of the device is further shortened, at this time the radius of curvature is about 1.35 cm, and the device generates a photocurrent of 0.83 μA after being irradiated with light. When the device is further bent so that the radius of curvature is reduced to 0.77 cm, the photocurrent is about 0.79 μA, which is 88.8% of the photocurrent in the initial flat state. This shows that the device can still achieve fast and stable detection of infrared light in different bent states. When the bent device is restored to the flat state again, it is found that the photocurrent generated by the device is 0.88 μA, which is only reduced by 1.1% compared with the photocurrent in the initial state, and the current change pattern can still quickly respond and stably detect within 5 cycle periods. This proves that the prepared TiO2 hollow cube / Bi2S3 nanoparticle heterojunction flexible broadband photodetector can quickly achieve self-powered detection of light whether it is in the bent state or the flat state.

[0054] The above content of the present invention is only the preferred embodiment of the present invention and is not used to limit the implementation of the present invention. Those of ordinary skill in the art can very conveniently make corresponding adaptations or modifications according to the main concept and spirit of the present invention. Therefore, the protection scope of the present invention should be subject to the protection scope required by the claims.

Claims

1. A TiO2 / Bi2S3 heterojunction, characterized in that, Its structure is a TiO2 / Bi2S3 nanoflower heterojunction or a TiO2 / Bi2S3 nanoparticle heterojunction.

2. The TiO2 / Bi2S3 heterojunction according to claim 1, wherein For the TiO2 / Bi2S3 nanoflower heterojunction, the overall appearance of TiO2 is spherical or cubic, with a hollow structure inside, a shell thickness of 20 - 200 nm. The overall shape of the Bi2S3 nanoflowers is sea urchin-like, with a size of 0.5 - 2.0 μm, and is composed of a large number of Bi2S3 nanorods growing divergently from the center.

3. A method for preparing the TiO2 / Bi2S3 heterojunction according to claim 2, characterized in that The specific steps of the method are as follows: Step 1: Add sodium hydroxide to a solution containing Cu ions, then add ascorbic acid, and react at 20 - 80 °C for 10 min - 5 h. After centrifugation, dry naturally to obtain Cu2O powders with different morphologies. Step 2: Add the Cu2O powders to a mixed solution of ammonium hexafluorotitanate and boric acid, react in a water bath at 40 - 80 °C for 20 min - 5 h. Anneal the centrifuged and dried sample at 200 - 700 °C for 1 - 5 h; then remove the internal impurities of the annealed sample in boric acid to obtain TiO2 powders with a hollow structure. Step 3: First, add bismuth nitrate to ethylene glycol solution and stir at 500 - 1000 rpm for 0.5 - 1 h to prepare an ethylene glycol solution of bismuth nitrate with a concentration of 0.04 - 0.4 mol / L. Then, stir the solution rapidly at 2000 - 2400 rpm, and at the same time add a certain amount of thiourea for 0.5 - 3 h, where the molar ratio of bismuth nitrate to thiourea added is 2:

3. Step 4: Add the TiO2 powders with a hollow structure to the solution in Step 3 for a water bath reaction. The water bath reaction temperature is 50 - 100 °C, and the reaction stirring time is 1 - 10 h. After the reaction is completed, wash and centrifuge, and dry naturally at room temperature to obtain the TiO2 / Bi2S3 nanoflower heterojunction.

4. The TiO2 / Bi2S3 heterojunction according to claim 1, characterized in that, For the TiO2 / Bi2S3 nanoparticle heterojunction, the overall appearance of TiO2 is spherical or cubic, with a hollow structure inside, a shell thickness of 20 - 200 nm. The overall particle size of Bi2S3 nanoparticles is 5 - 30 nm, randomly adhering to the surface of the hollow-structured TiO2 to form a core-shell structure.

5. The preparation method of the TiO2 / Bi2S3 heterojunction according to claim 4, characterized in that The specific steps of the method are as follows: Step 1: Add sodium hydroxide to a solution containing Cu ions, then add ascorbic acid, and react at 20 - 80 °C for 10 min - 5 h. After centrifugation, dry naturally to obtain Cu2O powders with different morphologies. Step 2: Add the Cu2O powders to a mixed solution of ammonium hexafluorotitanate and boric acid, react in a water bath at 40 - 80 °C for 20 min - 5 h. Anneal the centrifuged and dried sample at 200 - 700 °C for 1 - 5 h; then remove the internal impurities of the annealed sample in boric acid to obtain TiO2 powders with a hollow structure. Step 3: Prepare 2 ml of an ethylene glycol solution of bismuth nitrate with a concentration of 0.01 - 0.025 mol / L, then add the TiO2 powders with a hollow structure and ultrasonicate for 1 - 10 min to form a uniformly dispersed suspension. Then, add 0.02 - 0.05 mmol of thioacetamide or sodium sulfide to the suspension, and ultrasonicate for 10 - 30 s to form a black suspension. Step 4: Rapidly add the black suspension into 20 ml of 0.1 - 0.5 mol / L bismuth nitrate and thiourea ethylene glycol solution, stir at 20 °C for 3 - 10 h, wash and centrifuge after the reaction is completed, and air dry naturally at room temperature to obtain the TiO2 / Bi2S3 nanoparticle heterojunction.

6. The preparation method of the TiO2 / Bi2S3 heterojunction according to claim 3 or 5, characterized in that: The TiO2 with a hollow structure is a TiO2 hollow sphere or a TiO2 hollow cube.

7. Application of a TiO2 / Bi2S3 heterojunction in a flexible broadband photodetector, characterized in that: Use the PET-ITO substrate loaded with the TiO2 / Bi2S3 heterojunction material as the flexible working electrode of the detector, uniformly coat a layer of solid electrolyte on the material, use the PET-ITO substrate as the flexible counter electrode, and stick the two substrates together with the conductive sides facing inwards to obtain a TiO2 / Bi2S3 heterojunction flexible self-powered broadband photodetector. The preparation method of the TiO2 / Bi2S3 heterojunction flexible working electrode is as follows: Disperse the TiO2 / Bi2S3 heterojunction material in deionized water, and uniformly coat it on the conductive surface of the PET-ITO flexible substrate by the drop casting method, repeat the coating and drying 3 - 10 times to obtain the TiO2 / Bi2S3 heterojunction flexible working electrode.

8. The application of a TiO2 / Bi2S3 heterojunction in a flexible broadband photodetector according to claim 7, wherein: The TiO2 / Bi2S3 heterojunction is one of a TiO2 hollow sphere / Bi2S3 nanoflower heterojunction, a TiO2 hollow sphere / Bi2S3 nanoparticle heterojunction, a TiO2 hollow cube / Bi2S3 nanoflower heterojunction, or a TiO2 hollow cube / Bi2S3 nanoparticle heterojunction.

9. Use of a TiO2 / Bi2S3 heterojunction in a flexible broadband photodetector according to claim 7, characterized in that: The preparation process of the solid electrolyte is as follows: Add 1.5 g of PVA into 12 ml of deionized water, then add 5 ml of KOH solution or polysulfide electrolyte or iodine electrolyte, and then first place it in a 95 °C water bath and stir rapidly for 20 min, and then place it at room temperature and stir slowly for 1 - 10 h.

10. The application of a TiO2 / Bi2S3 heterojunction in a flexible wide-band photodetector according to claim 7, characterized in that: The solid electrolyte is one of a KOH solid electrolyte, a polysulfide solid electrolyte, or an iodine solid electrolyte.

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