A method for detecting heterojunction space charge transfer based on in-situ photo-assisted XPS imaging

Abstract

The application discloses a method for detecting heterojunction space charge transfer based on in-situ photo-assisted XPS imaging, and belongs to the technical field of energy materials and material surface analysis methods. First, an FTO / TiO2 / WO3 heterojunction material is prepared as a research foundation object, the material has a ternary heterojunction structure stacked in three layers, can improve the transfer efficiency of photo-generated charges, and the TiO2 layer as an intermediate layer can accelerate the transfer of space charges, inhibit the recombination of photo-generated carriers, and effectively promote the improvement of photocatalytic efficiency. Etching is performed on the surface of the FTO / TiO2 / WO3 to obtain XPS peak strengths of different intensities. Under XPS spectrum imaging, the spectrum of different elements can be obtained, and obvious light and dark contrast is presented. Under photo-assistance, photo-generated electrons are generated on the surface of the heterojunction under light excitation, the imaging spectrum of different elements changes in intensity, the transfer process of space charges is reflected, and the transfer path of photo-generated charges can be accurately embodied.

Description

Technical Field

[0001] This invention belongs to the fields of energy materials technology and material surface analysis methods, and specifically relates to a method for fabricating a ternary heterojunction device and its application in in-situ light-assisted XPS imaging to investigate space charge transfer. Background Technology

[0002] Energy is a crucial foundation for human survival and development. To achieve green, low-carbon, and sustainable development, photocatalysis, which simulates photosynthesis in nature, is a promising new technology for converting solar energy into stable, readily accessible, and high-energy-density chemical energy. Charge separation and transfer within photogenerated charge carriers is the key "energy pump" determining the overall solar energy conversion efficiency in photocatalysis. Various advanced spectroscopic and / or microscopic tools have demonstrated their potential in improving our understanding of the complex charge separation and transfer (CST) processes in photocatalysts.

[0003] The core principle of ISI-XPS technology is the electron shielding effect. When electrons transfer between different components of a material, it causes changes in the outer electron density of atoms, which in turn causes a shift in the XPS binding energy. This technology can directly reveal the migration direction of photogenerated electrons by precisely monitoring changes in the binding energy of elements on the material surface under illumination. However, previous studies only focused on changes in surface binding energy caused by illumination, performing spectral analysis of the binding energy of material elements and making inferences based on intensity and shift changes. It is difficult to intuitively understand the migration direction of electron transfer. As described in the literature [Martin F, Lopez MC, Carrera, et al. Surface and interface analysis, vol. 36 (2004), pp. 8-16.], the surface distribution of Zn, O, Fe, and Cr spectral peak intensities in ZrO2 / 304SS (873K) material after Ar ion etching was studied using XPS parallel imaging technology. Another example is the invention patent with publication number CN106645252A, which discloses an XPS imaging analysis method for characterizing the elemental distribution on the surface of a material. By collecting photoelectron spectra of surface elements point by point within the imaging area, and then performing detailed data processing, the distribution map of the relative content of each element is accurately obtained, providing more intuitive and accurate reference data for research.

[0004] In XPS instruments, besides using spectral acquisition for chemical structure analysis of material surfaces, imaging can spatially visualize the elemental distribution on the surface. This testing mode can also distinguish different valence states of the same element, showing potential application in the study of photogenerated electron migration. XPS imaging technology, as an important tool for surface analysis, visualizes the elemental distribution and chemical state differences on material surfaces in a spatial dimension, providing crucial methodological support for studying interfacial charge transfer behavior in heterojunction catalysts. Based on the photoelectric effect, XPS imaging technology uses monochromatic X-rays to excite inner-shell electrons in the sample to generate characteristic photoelectron signals. By combining an energy analyzer with a position-sensitive detector, it ultimately generates an image of chemical state distribution with nanometer-scale detection depth and micrometer-scale spatial resolution. While this technology can distinguish different valence states of the same element and display their spatial distribution, its conventional imaging mode has significant limitations when dealing with the complex process of photocatalysis involving the dynamic generation, separation, and migration of photogenerated charges. It struggles to capture and visualize the dynamic transfer paths and spatiotemporal evolution of photogenerated charges at heterojunction interfaces in situ and in real-time under real-time photoexcitation conditions.

[0005] Therefore, traditional static spectral acquisition-based analytical methods cannot correlate the direction of light-induced electron flow with intuitive spatial distribution images, which constitutes a key bottleneck for a deeper understanding of interfacial charge transfer dynamics. Developing an in-situ, dynamic chemical imaging method that can directly correlate the transfer direction and flux of photogenerated charges with their spatial origin, migration path, and reaction endpoint under light-assisted conditions has become a crucial and urgent experimental scientific challenge in this field. Summary of the Invention

[0006] This invention addresses the technical problem that traditional XPS spectral analysis cannot intuitively present the charge migration path, and proposes a method for detecting space charge transfer in heterojunctions based on in-situ light-assisted XPS imaging.

[0007] To achieve the above objectives, the technical solution of the present invention is implemented as follows:

[0008] This invention provides a method for detecting space charge transfer in heterojunctions based on in-situ light-assisted XPS imaging, comprising the following steps:

[0009] (1) Prepare an anhydrous ethanol solution of tetrabutyl titanate as solution A. Add concentrated hydrochloric acid and deionized water to anhydrous ethanol and adjust the pH to obtain solution B. Then, add solution B dropwise to solution A under stirring. Add ethylene glycol dropwise to obtain TiO2 sol. Clean and dry the FTO glass to remove surface organic matter and particles, and perform surface treatment. Then spin-coat the TiO2 sol onto the treated FTO glass. After drying and annealing, obtain a crystalline TiO2 film.

[0010] (2) After dissolving WCl6 in anhydrous ethanol, the pH of the solution was adjusted, and then a hydrothermal reaction was carried out. After cooling, the obtained sample was washed and calcined to obtain an FTO / TiO2 / WO3 heterojunction. After surface etching treatment, the desired sample was obtained.

[0011] (3) After placing the sample obtained in step (2) into the XPS transition chamber, turn off the light source. After the vacuum level reaches the required level, send it into the test chamber for testing to obtain XPS spectrum data in the dark state.

[0012] (4) Turn on the in-situ light source and perform XPS imaging on the sample surface under illumination to obtain XPS spectrum data under illumination.

[0013] (5) Process the XPS spectrum data under dark conditions and XPS spectrum data under illumination to obtain a 2D view of the characteristic peaks of the element to be measured.

[0014] In step (1), the volume ratio of tetrabutyl titanate to anhydrous ethanol in solution A is 1:4.5-5; in solution B, the volume ratio of anhydrous ethanol, concentrated hydrochloric acid, and deionized water is 4-5:0.4-0.6:1, and the pH is adjusted to 1-2; the volume ratio of ethylene glycol to tetrabutyl titanate is 2-3:1.

[0015] The cleaning process involves sequential ultrasonic cleaning with deionized water, acetone, and isopropanol. The surface treatment utilizes an ultraviolet ozone cleaner. The principle of this ultraviolet ozone cleaner is as follows: a low-pressure mercury lamp inside the cleaner emits short-wave ultraviolet light with a wavelength of 185nm. This high-energy photon directly breaks the chemical bonds of oxygen molecules in the air, converting oxygen into ozone. Simultaneously, the 254nm wavelength ultraviolet light emitted by the mercury lamp irradiates the surface of the object to be cleaned. This light has two key functions: first, it directly acts on organic pollutants, breaking their molecular bonds and making them chemically active; second, it irradiates the ozone, decomposing it into oxygen and highly reactive oxygen atoms. The reactive oxygen atoms generated in step two have extremely strong oxidizing power. They react violently with the organic pollutant molecules activated by the ultraviolet light, completely decomposing these oil stains, residual photoresist, and other organic matter into volatile small molecules such as carbon dioxide and water. These gases then dissipate from the surface, achieving the cleaning purpose and facilitating subsequent TiO2 spin-coating.

[0016] The spin coating parameters are 2000-3000 rpm / min and the spin coating time is 20-30s; the annealing temperature is 450-500℃ and the time is 1.5-2h; the thickness of the TiO2 film is 10-100nm.

[0017] In step (2), the initial concentration of WCl6 is 3-5 mg / mL, and the pH of the solution is adjusted to 1-2 using concentrated hydrochloric acid; the temperature of the hydrothermal reaction is 100-150℃, and the time is 6-9 h; the temperature of the calcination is 400-450℃, and the time is 2-3 h; the surface etching treatment is to use laser etching to pattern the FTO / TiO2 / WO3 heterojunction surface so that the surface information of the sample at different thicknesses exhibits different binding energy intensities.

[0018] The three-layer stacked FTO / TiO2 / WO3 ternary heterojunction structure prepared by this invention can improve the transfer efficiency of photogenerated charge. The TiO2 layer, as the intermediate layer, can accelerate the transfer of space charge and suppress the recombination of photogenerated carriers, effectively promoting the improvement of photocatalytic efficiency. After surface etching, a pattern of 400-800 μm is formed on the surface of the device, which can be seen during imaging, helping to clarify the elemental distribution and determine the direction of photoelectron transfer.

[0019] The vacuum level required in step (3) is that the vacuum value of the transition chamber reaches 10. -8 Torr below, while the vacuum value of the test chamber reaches 10. -9 Below Torr, the test transmit current is 10mA and the flux is 80.

[0020] The light source in step (4) is a monochromatic Al Kα X-ray source (1486.6 eV) as the XPS excitation source, and light with a wavelength of 365 nm as the in-situ light-assisted excitation source;

[0021] In XPS imaging mode, set it to image stack mode. In excitation settings, set tuning to imaging and emission current to 15 mA.

[0022] In the Tuning Settings, the Collimation Mode is set to High Res Imaging.

[0023] Lens Mode is set to FOV3 Imaging, and Resolution is set to 80.

[0024] In the image stack settings, set the step size to 0.1 eV and the single acquisition time to 60 s.

[0025] In step (3) or step (4), the spectrum range is the range of XPS characteristic peaks of the element to be measured. The element to be measured includes Sn 3d and W 4f. The binding energy range of Sn 3d is 480-497eV, and the binding energy range of W 4f is 30-37eV.

[0026] The data processing in step (5) is performed using the Optimal Scaling Principle of CasaXPS software to obtain a total spectrum that reflects the information of the sample surface.

[0027] Specifically, the optimal scaling principle in the professional XPS data analysis software CasaXPS was used to standardize and synthesize the acquired raw spectra. The core purpose of this process is to eliminate non-chemical shift intensity fluctuations caused by minor differences in instrument status, acquisition time, or sample surface conditions by normalizing and calibrating the intensity of a series of XPS spectra obtained from different regions of the sample or under different conditions. This places multiple sets of data on the same comparable intensity benchmark. Finally, the processed data are integrated using this principle to generate a "total spectrum" with a significantly improved signal-to-noise ratio, representativeness, and accurate and consistent reflection of the average elemental composition and chemical state information of the sample surface. This provides a reliable data foundation for subsequent qualitative and semi-quantitative analysis of the sample.

[0028] The composite photocatalyst prepared by the method for detecting space charge transfer in heterojunctions based on in-situ light-assisted XPS imaging provided by this invention, and the method for preparing FTO / TiO2 / WO3 ternary heterojunctions provided by this invention, has the following characteristics: (1) Compared with single photocatalysts, constructing heterojunctions with multiple catalysts is an effective strategy to improve photocatalytic activity, and the synergistic improvement of photocatalyst activity makes it more efficient; (2) The material system has a ternary heterojunction structure that effectively promotes the separation of photogenerated electrons and holes, greatly reduces the recombination rate of photogenerated electrons and holes, and thus can achieve the effect of promoting photocatalytic reaction; This invention uses an in-situ XPS light-assisted imaging method, which can clearly reflect the sample valence state information through the binding energy of XPS, and accurately capture the sample surface binding energy intensity to reflect the specific distribution range of photoelectrons and the transfer path between heterojunctions.

[0029] This invention also provides the application of the method for detecting space charge transfer in heterojunctions based on in-situ light-assisted XPS imaging in determining the direction of photogenerated electron transfer in the fields of heterojunctions and surface catalysis.

[0030] The direction of photogenerated electron transfer is determined by comparing the changes in XPS image signal intensity of the same target element acquired under conditions of no light and light. Specifically, a decrease in image signal intensity after illumination indicates electron inflow, while an increase in image signal intensity after illumination indicates electron outflow.

[0031] The beneficial effects of this invention are:

[0032] 1. This invention discloses an imaging method for detecting space charge transfer in heterojunctions based on in-situ XPS light-assisted imaging, and successfully verifies its accuracy in imaging elemental surface information. It also enables the exploration of space charge transfer paths between heterojunctions under light-assisted in-situ imaging. This invention provides an efficient and intuitive method for detecting elemental surface information and, through imaging, explores the space charge transfer mechanism under light irradiation. This invention designs a device with a three-layer stacked FTO / TiO2 / WO3 ternary heterojunction structure. Using XPS imaging, the distribution of the elements to be detected can be easily characterized, and a series of images at different binding energies can be obtained by setting appropriate imaging programs. In the in-situ irradiation experiment, the direction of electron migration can be determined by comparing the changes in element brightness before and after irradiation. This invention combines the XPS imaging principle and the binding energy detection mechanism, based on the fact that photogenerated electrons are generated on the device surface under light irradiation excitation. Where photogenerated electrons flow in, the binding energy of the electron acceptor element decreases, and vice versa. The direction of electron flow can be detected by observing the change in color intensity before and after irradiation.

[0033] 2. Compared to traditional methods that rely solely on XPS analysis combined with energy dispersive spectroscopy under illumination, the imaging method for detecting space charge transfer in heterojunctions provided by this invention, based on in-situ XPS light-assisted imaging, transforms linear characteristic peak changes into planar 2D views, representing a significant paradigm shift. This allows researchers to intuitively and efficiently analyze the spatial separation and transfer behavior of photogenerated charges, providing an unprecedentedly powerful tool for revealing mechanisms and optimizing designs in fields such as heterojunctions and surface catalysis. It enables more intuitive and efficient analysis of light-assisted space charge transfer, clarifying changes in valence state information on material surfaces, tracing charge transfer paths, and spatially locating the origin and destination of charges. Attached Figure Description

[0034] To more clearly illustrate the technical solutions in the embodiments of the present invention or the prior art, the drawings used in the description of the embodiments or the prior art will be briefly introduced below. Obviously, the drawings described below are only some embodiments of the present invention. For those skilled in the art, other drawings can be obtained based on these drawings without creative effort.

[0035] Figure 1 This is a schematic diagram of the imaging and spectral acquisition principle of the XPS device.

[0036] Figure 2 The relationship between characteristic peaks and imaging.

[0037] Figure 3 The image shows the 2D stacking results of the corresponding binding energies obtained from the Sn 3d test and the W 4f test.

[0038] Figure 4 The spectral results of Sn 3d and W 4f obtained from the image are shown.

[0039] Figure 5 The results before and after data processing for W 4f are shown; the former is a camera image of the etched area of ​​the device within XPS; the latter is the result after fitting according to the optimal scaling principle of the W 4f spectral image data.

[0040] Figure 6 These are processed images of Sn 3d after testing in dark and lit environments.

[0041] Figure 7 The images show the processed results of testing the W4f in both dark and lit environments. Detailed Implementation

[0042] The technical solutions of the embodiments of the present invention will be clearly and completely described below with reference to the accompanying drawings. Obviously, the described embodiments are only some embodiments of the present invention, and not all embodiments. Based on the embodiments of the present invention, all other embodiments obtained by those skilled in the art without creative effort are within the scope of protection of the present invention.

[0043] The examples used an X-ray photoelectron spectroscopy (XPS) instrument from Shimadzu Corporation (model SHIMADZU AXIS SUPRA, Kratos Analytical Inc.) for testing, and CasaXPS analysis software for data analysis.

[0044] Example

[0045] This invention provides a method for detecting space charge transfer in heterojunctions based on in-situ optically assisted XPS imaging. To more intuitively illustrate this imaging method for detecting space charge transfer in heterojunctions under in-situ XPS light assistance, the fabrication of an FTO / TiO2 / WO3 ternary heterojunction is used as an example for investigation. The specific steps are as follows:

[0046] (1) Preparation of TiO2 seed layer on FTO glass using sol-gel spin coating: 2.5 mL of tetrabutyl titanate was slowly added dropwise to 12 mL of anhydrous ethanol, and magnetically stirred for 1 h to obtain a homogeneous solution A. Another 5 mL of anhydrous ethanol was taken, and 0.5 mL of concentrated hydrochloric acid and 1.0 mL of deionized water were added sequentially to adjust the pH to 1.5. The mixture was magnetically stirred for 1 h to form solution B. Under continuous stirring, solution B was slowly added dropwise to solution A, and stirring was continued for 2 h. Another 5 mL of anhydrous ethanol was then added dropwise to solution A. 40 µL of TiO2 sol was slowly added as a thickener to obtain a transparent TiO2 sol. The FTO conductive glass was then ultrasonically cleaned for 15 min each with deionized water, acetone, and isopropanol to remove surface organic matter and particles. After drying with nitrogen, the FTO glass was cleaned and dried again to remove surface organic matter and particles. It was then placed in an ultraviolet ozone cleaner for 30 min and fixed on a spin coater. 40 µL of TiO2 sol was added to completely cover the FTO glass, and the coating was spin-coated at 3000 rpm for 30 s. Subsequently, the coated FTO glass substrate was dried in an 80 °C oven for 10 min and then annealed in a tube furnace at 500 °C for 2 h (heating rate 2 °C / min) to obtain a crystalline anatase TiO2 film.

[0047] (2) WO3 was grown on FTO / TiO2 substrate by hydrothermal method: 0.2 g WCl6 was dissolved in 40 mL of anhydrous ethanol and magnetically stirred until completely dissolved. The pH of the solution was adjusted to 1.5 with concentrated hydrochloric acid (HCl, 37%) and magnetically stirred until the solution was clear and transparent. The solution was transferred to a 100 mL high-pressure reactor with a polytetrafluoroethylene liner. The FTO glass with the TiO2 seed layer was placed obliquely in the liner (conductive side down). The reactor was sealed and placed in a hydrothermal box and reacted at 120℃ for 6 h. After the reaction was completed, it was naturally cooled to room temperature. The sample was taken out and rinsed thoroughly with ethanol and deionized water alternately. Finally, it was annealed in a muffle furnace at 400℃ for 2 h (heating rate 2℃ / min) to improve the crystallinity of WO3 and obtain FTO / TiO2 / WO3 ternary heterojunction film. The heterojunction surface was patterned by laser etching so that the surface information of the sample at different thicknesses showed different binding energy intensities.

[0048] (3) Perform traditional spectral sampling tests: Turn off all light sources when placing the sample device into the transition chamber, and wait until the vacuum value of the transition chamber reaches 10. -8 Below Torr, the vacuum value of the test chamber inside reaches 10. -9 When the Torr is below 10, the device is sent to the test chamber for testing. The test emission current is 10mA and the flux is 80. The XPS characteristic peak spectrum of the corresponding element is selected for testing.

[0049] (4) Conducting light-assisted XPS imaging spectrum testing: A monochromatic Al Kα X-ray source (1486.6 eV) was used for in-situ illumination XPS testing. The light-assisted monochromatic light source was 365nm light as the light excitation source to investigate the transfer of photogenerated charges. In XPS imaging, the mode is set to Image stack mode. In Excitation Settings, Tuning is set to Imaging, and Emission Current is set to 15 mA. In Tuning Settings, Collimation Mode is set to High Res Imaging, Lens Mode is set to FOV3 Imaging, and Resolution is set to 80. In Image stack settings, the step size is set to 0.1 eV, and the binding energy range of the elemental spectra is set to 30-37 eV for W 4f and 480-498 eV for Sn 3d. The acquisition time is set to 60 s.

[0050] (5) Data processing: In the imaging tests of Sn 3d and W 4f, images were acquired every 0.1 eV. We took the integer combination of the image data and used it for post-processing and plotting. Sn 3d had a total of 171 image data in the range of 480-497 eV, and W 4f had a total of 71 image data in the range of 30-37 eV.

[0051] Figure 1 A schematic diagram of a typical XPS experimental setup is shown. Its basic workflow is as follows: An electron beam emitted from an electron gun bombards an anode target (Al target) under high voltage, generating primary X-rays. These X-rays then pass through a monochromatic crystal to filter out bremsstrahlung background and satellite lines, thus obtaining highly monochromatic characteristic X-rays Al Kα1,2. After the monochromatic X-rays irradiate the sample surface, they excite characteristic photoelectrons. These photoelectrons enter a hemispherical electron energy analyzer via a transmission lens system. Under different deflection voltages, electrons with different kinetic energies are separated and sequentially received by detectors. Finally, the data is collected by a computer system to form an energy spectrum.

[0052] like Figure 2As shown, XPS can not only acquire spectroscopic information of samples, but also achieve spatial distribution imaging of surface elements or chemical states within a specific energy window. In this imaging mode, a hemispherical energy analyzer is set with a specific pass energy and uses an energy-selective slit (often called an "aperture" or "baffle hole") at its exit for spatial filtering. This process is essentially spatial imaging through energy filtering. By precisely adjusting the analyzer's deflection voltage, characteristic photoelectrons corresponding to a specific element or chemical state with a specific binding energy can be selectively allowed to pass through the slit, while blocking the vast majority of electrons with non-characteristic energies. Thus, the signal collected by the detector spatially represents the location distribution of photoelectrons emitting that specific energy on the sample surface. Crucially, the energy analysis principle used in this imaging mode is completely consistent with that of conventional XPS spectral acquisition. Therefore, each pixel in the resulting image not only contains spatial location information but also fully carries the chemical state information of the photoelectron energy corresponding to that point.

[0053] Figure 3 Images obtained within the test range at specific binding energies are presented; Figure 4 To infer the binding energy spectra of Sn 3d and W 4f from the imaging signal, a transformation based on the "image-to-spectrum" method and carbon calibration were performed. The resulting data is as shown in the figure, which is not convenient for comparison and interpretation.

[0054] Figure 5 Image 'a' represents the CCD optical microscope image from the XPS device, which is also the main spectral range of our imaging area. The red triangles on the surface represent the etched area. After processing the image of this area using the Optimal Scaling Principle in CasaXPS software, the image... Figure 5 Figure b shows the results of processing W 4f. The bright areas represent regions with high binding energy, while the dark areas represent the opposite.

[0055] This invention processes these images using the Optimal Scaling Principle of CasaXPS software, and then... Figure 6 The results of Sn 3d tests in both dark and lit environments, after processing, are presented. Figure 7 The image shows the results of W4f tests conducted in both dark and illuminated environments, after processing. The bright areas represent regions with high binding energy, while the dark areas represent the opposite. This indicates that the WO3 signal is primarily located on the device surface. After surface etching, the binding energy signal intensity of W4f weakens, which aligns with the presence of a dark triangle in the processed image. This demonstrates that the method effectively reflects sample surface information. Figure 6As can be seen on the color caliper on the right, the value changed from a minimum of 2.8 under dark conditions to 2.6 under bright conditions, indicating that the signal of Sn 3d weakens under illumination, and photogenerated electrons flow into the surface; from Figure 7 As can be seen on the color caliper on the right, the value changed from a maximum of 0.5 under dark conditions to 0.6 under bright conditions, indicating that the signal of W4f became stronger under illumination, and photogenerated electrons flowed out of the surface. In summary, this invention can clearly point out the photogenerated electron transfer path between heterojunctions under light assistance in the method of detecting space charge transfer in heterojunctions based on in-situ light-assisted XPS imaging.

[0056] The above description is only a preferred embodiment of the present invention and is not intended to limit the present invention. Any modifications, equivalent substitutions, improvements, etc., made within the spirit and principles of the present invention should be included within the protection scope of the present invention.

Claims

1. A method for detecting space charge transfer in heterojunctions based on in-situ light-assisted XPS imaging, characterized in that, Includes the following steps: (1) Prepare an anhydrous ethanol solution of tetrabutyl titanate as solution A. Add concentrated hydrochloric acid and deionized water to anhydrous ethanol and adjust the pH to obtain solution B. Then, add solution B dropwise to solution A under stirring. Add ethylene glycol dropwise to obtain TiO2 sol. Spin-coat the sol onto FTO glass that has been cleaned, dried and surface-treated. After drying and annealing, obtain a crystalline TiO2 film. (2) After dissolving WCl6 in anhydrous ethanol, the pH of the solution was adjusted, and then a hydrothermal reaction was carried out. After cooling, the obtained sample was washed and calcined to obtain an FTO / TiO2 / WO3 heterojunction. After surface etching treatment, the desired sample was obtained. (3) After placing the sample obtained in step (2) into the XPS transition chamber, turn off the light source. After the vacuum degree reaches the required level, send it into the test chamber for testing to obtain XPS spectrum data in the dark state. (4) Turn on the in-situ light source and perform XPS imaging on the sample surface under illumination to obtain XPS spectrum data under illumination. (5) Process the XPS spectrum data under dark conditions and XPS spectrum data under illumination to obtain a 2D view of the characteristic peaks of the element to be measured.

2. The method for detecting heterojunction space charge transfer based on in-situ light-assisted XPS imaging according to claim 1, characterized in that: In step (1), the volume ratio of tetrabutyl titanate to anhydrous ethanol in solution A is 1:4.5-5; in solution B, the volume ratio of anhydrous ethanol, concentrated hydrochloric acid and deionized water is 4-5:0.4-0.6:1, and the pH is adjusted to 1-2; the volume ratio of ethylene glycol to tetrabutyl titanate is 2-3:

1.

3. The method for detecting heterojunction space charge transfer based on in-situ light-assisted XPS imaging according to claim 2, characterized in that: The cleaning in step (1) is performed by ultrasonic cleaning with deionized water, acetone and isopropanol in sequence; the surface treatment is performed by using an ultraviolet ozone cleaner; the spin coating parameters are 2000-3000 rpm / min and the spin coating time is 20-30s; the annealing temperature is 450-500℃ and the time is 1.5-2h; the thickness of the TiO2 film is 10-100nm.

4. The method for detecting heterojunction space charge transfer based on in-situ light-assisted XPS imaging according to claim 3, characterized in that: In step (2), the initial concentration of WCl6 is 3-5 mg / mL, and the pH of the solution is adjusted to 1-2 using concentrated hydrochloric acid; the temperature of the hydrothermal reaction is 100-150℃ and the time is 6-9 h; the temperature of the calcination is 400-450℃ and the time is 2-3 h.

5. The method for detecting heterojunction space charge transfer based on in-situ light-assisted XPS imaging according to claim 4, characterized in that: The vacuum level required in step (3) is that the vacuum value of the transition chamber reaches 10. -8 Torr below, while the vacuum value of the test chamber reaches 10. -9 Below Torr, the test transmit current is 10mA and the flux is 80.

6. The method for detecting heterojunction space charge transfer based on in-situ light-assisted XPS imaging according to claim 5, characterized in that: The light source in step (4) is a monochromatic Al Kα X-ray source as the XPS excitation source, and light with a wavelength of 365 nm as the in-situ light-assisted excitation source; In XPS imaging mode, set it to image pack mode, and in the excitation mode settings, set the tuning to imaging and the emission current to 15 mA. In the tuning settings, the collimation mode is set to high-resolution imaging; Lens mode is set to FOV3 imaging, and power is set to 80. In the image packet settings, the step size is set to 0.1 eV and the single acquisition time is set to 60 s.

7. The method for detecting heterojunction space charge transfer based on in-situ light-assisted XPS imaging according to claim 6, characterized in that: In step (3) or step (4), the spectrum range is the range of XPS characteristic peaks of the element to be measured. The element to be measured includes Sn 3d and W 4f. The binding energy range of Sn 3d is 480-497eV, and the binding energy range of W 4f is 30-37eV.

8. The method for detecting heterojunction space charge transfer based on in-situ light-assisted XPS imaging according to claim 7, characterized in that: The data processing in step (5) is performed using the optimal scaling principle of CasaXPS software.

9. The application of the method for detecting space charge transfer in heterojunctions based on in-situ light-assisted XPS imaging as described in any one of claims 1-8 in determining the direction of photogenerated electron transfer in the fields of heterojunctions and surface catalysis.

10. The application according to claim 9, characterized in that: The direction of photogenerated electron transfer is determined by comparing the changes in XPS image signal intensity of the same target element acquired under conditions of no light and light. Specifically, a decrease in image signal intensity after illumination indicates electron inflow, while an increase in image signal intensity after illumination indicates electron outflow.

Images