Method for improving back contact of tantalum nitride photo-anode

Abstract

The invention belongs to the technical field of photoelectrochemical water decomposition photoelectrode material preparation, and particularly relates to a method for improving tantalum nitride photoanode back contact. According to the method, an electrochemical polishing technology is used for treating the niobium foil substrate, pollutants and a complex original oxidation film on the surface of the niobium foil substrate are removed, meanwhile, the surface flatness of the niobium foil substrate is improved, the tantalum nitride crystallization process is promoted, and the quality of the tantalum nitride photo-anode film prepared on the surface of the niobium foil substrate is improved; then, the niobium foil substrate subjected to electrochemical polishing is subjected to high-temperature nitridation, an NbNx crystal phase matched with crystal lattices of the tantalum nitride photo-anode thin film is generated on the front face of the niobium foil substrate, crystal nucleus sites are provided for growth of the tantalum nitride photo-anode thin film, and therefore back contact between the tantalum nitride photo-anode thin film and the niobium foil substrate is improved, and the crystallinity of the tantalum nitride thin film is improved; and the water decomposition efficiency of the tantalum nitride photo-anode is improved. Compared with the prior art, the method is simple in processing mode and low in cost, and can be expanded and applied to other metal substrates.

Description

Technical Field

[0001] The present invention belongs to the technical field of the preparation of photoanode materials for photoelectrochemical water splitting, and particularly relates to a method for improving the back contact of tantalum nitride photoanodes. Background Art

[0002] Artificial photosynthesis can convert solar energy into renewable clean energy, which is an effective way to reduce the consumption of fossil fuels and protect the environment. Photoelectrochemical water splitting (PEC), as a form of artificial photosynthesis, is considered a highly promising means to achieve solar-to-hydrogen (STH) conversion. Among the photoanode materials used for PEC water splitting, Ta 3 N 5 stands out with its unique advantages. Ta 3 N 5 is an n-type semiconductor with a band gap of 2.1 eV, so it can absorb most of the solar spectrum (the solar spectrum with an absorbable wavelength less than 600 nm). In addition, the energy band positions of its conduction band and valence band span the oxidation-reduction potential of water, which means that it is energetically suitable for overall water splitting. Theoretically, under standard sunlight irradiation, Ta 3 N 5 can generate a maximum photocurrent density of 12.9 mA·cm -2 , corresponding to a theoretical maximum STH conversion efficiency of 15.9%.

[0003] Improving the back contact between tantalum nitride and the metal substrate to enhance the carrier transport efficiency at the rear interface is crucial for improving the water splitting efficiency of tantalum nitride photoanodes. Currently, when preparing tantalum nitride photoanodes, metals such as niobium (Nb) and tantalum (Ta) that have both electrical conductivity and high-temperature resistance are usually selected as the substrates. Among them, niobium foil, as the back electrode of the tantalum nitride photoanode, plays a crucial role in the electron collection process. It not only provides nucleation microdots for the growth of the tantalum nitride thin film but also directly affects the growth quality of the tantalum nitride thin film. However, due to the uneven surface and complex composition of the metal substrate, its crystal lattice is mismatched with that of Ta 3 N 5 , which will have an adverse effect on the quality of the Ta 3 N 5 thin film. Therefore, a suitable method needs to be found to improve the back contact between tantalum nitride and the metal substrate. Summary of the Invention

[0004] In view of this, the present invention proposes a method for improving the back contact of tantalum nitride photoanodes to solve the problems of lattice mismatch and poor contact between the niobium foil substrate and tantalum nitride, improve the crystallinity of the tantalum nitride thin film, and provide a good foundation for the preparation of tantalum nitride photoanodes with high water splitting performance.

[0005] To solve the above problems, the present invention adopts the following technical solutions:

[0006] A method for improving the back contact of a tantalum nitride photoanode, comprising the following steps:

[0007] Step 1, cleaning the metal substrate and drying the metal substrate;

[0008] Step 2, putting the metal substrate obtained in Step 1 into a polishing solution, and polishing the niobium foil by electrochemical polishing to reduce the surface roughness of the metal substrate and remove the native oxide film;

[0009] Step 3, cleaning and drying the metal substrate obtained in Step 2;

[0010] Step 4, preparing a NbN x thin film on the front surface of the metal substrate obtained in Step 3 by high-temperature nitridation;

[0011] Step 5, preparing a gradient Mg-doped Ta x precursor thin film on the NbN 2 O 5 thin film by dual-source electron beam deposition;

[0012] Step 6, nitriding the Mg-doped Ta 2 O 5 precursor thin film by high-temperature nitridation to obtain a Mg:Ta 3 N 5 / NbN x thin film;

[0013] Step 7, welding a conductive wire to the back of the metal substrate of the product obtained in Step 6 using indium metal, and then encapsulating and covering it with a solidifying glue to form an ohmic contact, thereby preparing a Mg:Ta3N5 / NbN x photoanode.

[0014] Furthermore, the purity of the metal substrate used in Step 1 is greater than or equal to 99.98%, and the thickness is greater than or equal to 0.07 mm.

[0015] Furthermore, the implementation method of Step 1 is: ultrasonically cleaning the metal substrate with soapy water and deionized water for 15 minutes in sequence, and drying it with a nitrogen gun.

[0016] Furthermore, the polishing solution used in Step 2 is prepared by mixing hydrofluoric acid and sulfuric acid in a volume ratio of 1:9 to 1:14.

[0017] Furthermore, the voltage for the electrochemical polishing treatment in Step 2 is 5-15 V, and the polishing time is 5-12 minutes.

[0018] Further, in Step 3, the metal substrate obtained in Step 2 is ultrasonically cleaned successively with soapy water, deionized water, acetone, and isopropyl alcohol, each for 15 minutes.

[0019] Further, in Step 4, high-temperature nitridation is used to prepare NbN on the front side of the metal substrate obtained in Step 3. x The process for the thin film is as follows:

[0020] The metal substrate obtained in Step 3 is placed in a quartz boat and sealed in the quartz tube of a high-temperature tube furnace, and high-temperature nitridation is carried out in an ammonia atmosphere with a purity of 99.999%. The nitriding gas flow rate is 100 - 300 sccm, the temperature is 1073 - 1323 K, and the time is 1 - 12 hours.

[0021] Further, in Step 5, dual-source electron beam deposition is used to prepare a gradient Mg-doped Ta x O 2 O 5 precursor thin film on the NbN thin film. The process is as follows:

[0022] The metal substrate obtained in Step 4 is placed on the sample stage of an electron beam deposition system. Subsequently, Ta 2 O 5 and MgO oxide precursors are placed in crucibles in a vacuum electron beam deposition chamber. The vacuum chamber is evacuated to a vacuum degree of 5×10 -6 ~1×10 -5 , and then 1 - 10 sccm of oxygen is introduced;

[0023] During the deposition process, two quartz crystal oscillators are used to control the deposition rates and film thicknesses of the two sources respectively. Specifically, when the deposition rate of Ta 2 O 5 is fixed at and the deposition thickness is 600 nm, for every 100 nm of Ta 2 O 5 deposited, the deposition rate of the MgO source decreases until to achieve gradient doping of Mg 2+ in the Ta 2 O 5 thin film; the initial deposition rate of MgO is

[0024] Further, in Step 6, high-temperature nitridation is used to nitride the Mg-doped Ta 2 O 5 precursor thin film to obtain a Mg:Ta 3 N 5 / NbN x thin film. The process is as follows:

[0025] Dope Ta with Mg 2 O 5 Put the precursor film into a quartz boat and seal it in the quartz tube of a high-temperature tube furnace. Conduct high-temperature nitridation in an ammonia atmosphere with a purity of 99.999%, and finally obtain Mg:Ta 3 N 5 / NbN x film; The high-temperature nitridation conditions are: gas flow rate is 100 - 300 sccm, temperature is 1073 - 1323 K, and time is 1 - 12 hours.

[0026] A method for improving the back contact of a tantalum nitride photoanode provided by the present invention. This method uses electrochemical polishing technology to treat the niobium foil substrate. While removing surface contaminants and complex native oxide films on the niobium foil substrate, it improves the surface flatness of the niobium foil substrate, promotes the crystallization process of tantalum nitride, and enhances the quality of the tantalum nitride photoanode film prepared on the surface of the niobium foil substrate; Then, conduct high-temperature nitridation on the electrochemically polished niobium foil substrate to generate NbN with lattice matching to the tantalum nitride photoanode film on the front side of the niobium foil substrate x crystalline phase, providing nucleation sites for the growth of the tantalum nitride photoanode film, and further improving the back contact between the tantalum nitride photoanode film and the niobium foil substrate; Through the synergistic effect of electrochemical polishing treatment and high-temperature nitridation treatment, a quasi-Ohmic contact with lattice matching is formed between the tantalum nitride photoanode film and the niobium foil, thereby increasing the crystallinity of the tantalum nitride film and improving the water splitting efficiency of the tantalum nitride photoanode.

[0027] Compared with the prior art, the treatment method of the present invention is simple, low in cost, and can be extended and applied to other metal substrates. Brief Description of the Drawings

[0028] Figure 1 is the process flow chart of the present invention;

[0029] Figure 2 is the surface AFM pattern of the Mg:Ta3N5 / NbNx film in Example 1;

[0030] Figure 3 is the surface AFM pattern of the Mg:Ta3N5 film in Example 2;

[0031] Figure 4 is the surface SEM pattern of the Mg:Ta3N5 / NbNx film in Example 1;

[0032] Figure 5 is the surface SEM pattern of the Mg:Ta3N5 film in Example 2;

[0033] Figure 6 is the XRD pattern of the NbNx film in Example 1;

[0034] Figure 7 XRD patterns of Mg:Ta3N5 / NbNx thin film in Example 1 and XRD pattern of Mg:Ta3N5 thin film in Example 2

[0035] Figure 8 Photocurrent-voltage curve of the Mg:Ta3N5 / NbNx photoanode in Example 1 Detailed implementation manners

[0036] To better understand the purpose, structure and function of the present invention, the present invention will be further described in detail below with reference to the accompanying drawings. It should be noted that the given examples cannot be used as a limitation to the present invention. Non-essential improvements and adjustments made by other technicians to the present invention still fall within the protection scope of the present invention

[0037] A method for improving the back contact of tantalum nitride photoanode provided in this embodiment is to use electrochemical polishing method to improve the surface flatness of the metal substrate and remove the complex components on its surface; then high-temperature nitridation is carried out to introduce NbN x conductive layer. By using the synergistic effect of the contact optimization and lattice matching between NbN x and Ta 3 N 5 thin film, by depositing a gradient Mg-doped Ta 3 N 5 thin film on the NbN x thin film with lattice matching with Ta 3 N 5 thin film, the crystallinity of Mg-doped tantalum nitride is improved. The technological process is as Figure 1 shown, including ultrasonic cleaning of the metal substrate, electrochemical polishing of the metal substrate, ultrasonic cleaning of the polished metal substrate, preparation of NbN x thin film on the front of the metal substrate by using high-temperature nitridation method, deposition of magnesium gradient-doped tantalum oxide precursor thin film on the NbN x thin film by using a dual-source vacuum electron beam deposition system, preparation of Mg:Ta 3 N 5 / NbN x thin film and preparation of Mg:Ta 3 N 5 / NbN x photoanode in seven steps. The effect of the above method for improving the back contact of tantalum nitride photoanode will be verified by examples below

[0038] Example 1

[0039] Step 1: Ultrasonically clean the niobium foil substrate with soapy water and deionized water for 15 minutes in sequence, and thoroughly dry it with a high-purity nitrogen gun

[0040] Step 2: Vertically place the niobium foil substrate obtained in Step 1 into the polishing solution, which is prepared from hydrofluoric acid and sulfuric acid in a volume ratio of 1:9. Use the electrochemical polishing method to polish the niobium foil substrate. During the polishing process, use a constant voltage potentiostat as the energy supply device, with the niobium foil substrate as the anode and the aluminum foil as the cathode. The polishing voltage is 14 V, and the polishing time is 10 minutes to remove the native oxide film on the surface of the niobium substrate and reduce its surface roughness.

[0041] Step 3: Ultrasonically clean the niobium foil substrate obtained in Step 2 in sequence with soapy water, deionized water, acetone, and isopropanol for 15 minutes to remove other impurities such as the residual polishing solution on the surface of the niobium foil again.

[0042] Step 4: Place the niobium foil substrate obtained in Step 3 into a quartz boat and seal it in the quartz tube of a high-temperature tube furnace. Place it in an ammonia atmosphere with a purity of 99.999%, and carry out high-temperature nitridation at a gas flow rate of 280 sccm and a temperature of 1298 K for 6 hours to obtain NbN x thin film.

[0043] Step 5: Place the niobium foil substrate obtained in Step 4 on the sample stage of a vacuum electron beam deposition system. Respectively place MgO and Ta 2 O 5 as the deposition sources of the oxide precursors and put them into the crucibles in the chamber. After pumping to a vacuum of 5×10 -6 Torr, introduce 5 sccm of oxygen. During the deposition process, use two quartz crystal oscillators to control the deposition rates and film thicknesses of the two sources respectively. Fix the deposition rate of Ta 2 O 5 at and the deposition thickness is 600 nm. The initial deposition rate of MgO is Ta 2 O 5 For every 100 nm of deposition thickness, the deposition rate of the MgO source decreases until to achieve gradient doping of Mg 2+ in Ta 2 O 5 thin film.

[0044] Step 6: Place the prepared Mg:Ta 2 O 5 / NbN x thin film into a quartz boat and seal it in the quartz tube of a high-temperature tube furnace. Place it in an ammonia atmosphere with a purity of 99.999%, and carry out high-temperature nitridation at a gas flow rate of 280 sccm and a temperature of 1298 K for 6 hours to finally obtain Mg:Ta 3 N 5 / NbN x thin film.

[0045] Step 7: Use indium metal to weld the conductive wire to the back of the niobium foil substrate loaded with the Mg-doped tantalum nitride photoanode film, and use solidifying glue to encapsulate and cover it to form a quasi-ohmic contact, thereby preparing Mg:Ta 3 N 5 / NbN x photoanode.

[0046] Example 2

[0047] The steps are the same as those in Example 1, except that: the steps of electrochemical polishing and high-temperature nitridation of the niobium foil substrate are not carried out, and Mg:Ta is directly prepared on the niobium foil substrate by the above method 3 N 5 / NbN x photoanode.

[0048] Figure 2 is the surface AFM map of the Mg:Ta3N5 / NbNx film in Example 1, Figure 3 is the surface AFM map of the Mg:Ta3N5 film in Example 2. By comparing Figure 2 and Figure 3 it can be seen that there are obvious grooves on the surface of the Mg:Ta 3 N 5 film, while the surface of the Mg:Ta 3 N 5 / NbN x film is very flat. It shows that the electrochemical polishing and high-temperature nitridation treatments do reduce the surface roughness of the niobium foil substrate, optimize the quasi-ohmic contact between Mg:Ta 3 N 5 and the tantalum foil substrate, and the prepared Mg:Ta 3 N 5 photoanode film is flatter, which is beneficial to improving the crystallinity of the Mg:Ta 3 N 5 photoanode film.

[0049] Figure 4 is the surface SEM map of the Mg:Ta3N5 / NbNx film in Example 1, Figure 5 is the surface SEM map of the Mg:Ta3N5 film in Example 2; Referring to Figure 4 and Figure 5 it can be seen that the films in both Example 1 and Example 2 have formed surfaces with some cracks. The cracks on the surface of the Mg:Ta 3 N 5 / NbN x film are significantly fewer than those on the surface of the Mg:Ta 3 N 5 film, which corresponds to the results obtained by AFM.

[0050] Figure 6 For the NbN in Example 1 x XRD pattern of the thin film. It can be seen from Figure 6 that the surface of the niobium foil substrate after electrochemical polishing and high-temperature nitridation treatment has become a mixed phase of NbN and Nb 2 N, and its diffraction pattern corresponds to the standard XRD cards of NbN and Nb 2 N (JCPDS no. 40-1274-Nb 2 N) and (JCPDS no. 20-0801-NbN).

[0051] Figure 7 For the Mg:Ta 3 N 5 / NbN x XRD pattern of the thin film and the XRD pattern of the Mg:Ta 3 N 5 thin film in Example 2. It can be seen that the diffraction pattern of the Mg:Ta 3 N 5 thin film prepared by the dual-source electron beam deposition system corresponds to Ta 3 N 3 N with a rutile-type (TiO 5 )(JCPDS no. 89-5200) orthorhombic crystal structure, which indicates that the substitution of Mg for Ta does not cause a change in the crystal structure of the Ta 3 N 5 thin film and no other impurity phases are introduced. Most notably, the diffraction intensity of the Mg:Ta 3 N 5 / NbN x thin film in Example 1 is increased compared with that of the Mg:Ta 3 N 5 thin film in Example 2, and the intensity of the peak representing metallic niobium at θ = 38° in the XRD diffraction of the Mg:Ta 3 N 5 / NbN x thin film is significantly reduced, which is more conducive to the crystallization of tantalum nitride. This result shows that the presence of the NbN Figure 2 layer is beneficial to improving the crystallinity of the Ta x N 3 N 5 thin film.

[0052] Figure 8 For the photocurrent-voltage curve of the Mg:Ta 3 N 5 / NbN x photoanode in Example 1. Before performing photoelectrochemical water splitting, in Mg:Ta3 N 5 / NbN x Deposition of NiCoFe - B on the photoanode surface i Oxygen - evolving cocatalyst. As can be seen from Figure 6 that under 1.23V vs. RHE, Mg:Ta 3 N 5 / NbN x photoanode has a saturated photocurrent density as high as 8 mA / cm 2 , and only a simple treatment method is used to optimize the back contact, which significantly improves the photocurrent of tantalum nitride.

[0053] In summary, the present invention uses electrochemical polishing and high - temperature nitridation technologies to optimize the back contact of Mg:Ta 3 N 5 and niobium foil substrate, and prepares a lattice - matched niobium nitride / tantalum nitride heterojunction. Electrochemical polishing removes the complex native oxides on the surface of the niobium foil and reduces the surface roughness. High - temperature nitridation of the niobium foil substrate prepares a NbN x layer, and then a Mg:Ta 3 N 5 thin film is deposited on this layer. The lattice match between the two is very high, so the crystallinity of the Mg:Ta 3 N 5 thin film is improved. This method uses polishing technology and lattice matching to improve the crystallinity of the Mg:Ta 3 N 5 thin film, and finally realizes a Mg:Ta 3 N 5 / NbN x photoanode with high PEC performance.

[0054] It can be understood that the present invention is described through some embodiments. Those skilled in the art know that without departing from the spirit and scope of the present invention, various changes or equivalent replacements can be made to these features and embodiments. In addition, under the teaching of the present invention, these features and embodiments can be modified to adapt to specific situations and materials without departing from the spirit and scope of the present invention. Therefore, the present invention is not limited by the specific embodiments disclosed herein, and all embodiments falling within the scope of the claims of this application belong to the scope protected by the present invention.

Claims

1. A method for improving the back contact of a tantalum nitride photoanode, characterized in that: The following steps are involved: Step 1, cleaning the metal substrate and drying the metal substrate; Step 2, placing the metal substrate obtained in step 1 into a polishing solution, and using an electrochemical polishing method to polish the niobium foil to reduce the surface roughness of the metal substrate and remove the native oxide film; Step 3, cleaning and drying the metal substrate obtained in step 2; Step 4: Prepare NbN on the front side of the metal substrate obtained in step 3 by high temperature nitridation method x film; Step 5: Using dual-source electron beam deposition method to deposit NbN x A gradient Mg-doped Ta2O5 precursor film is prepared on the film; Step 6: Nitridize the Mg-doped Ta2O5 precursor film using a high temperature nitridation method to obtain Mg:Ta3N5 / NbN x film; Step 7: Use metal indium to weld the conductive wire to the back of the metal substrate of the product obtained in step 6, and then use a solidifying glue to encapsulate and cover it to form an ohmic contact to prepare Mg:Ta3N5 / NbN x Photoanode.

2. A method for improving the back contact of a tantalum nitride photoanode according to claim 2, characterized in that: The metal substrate used in step 1 has a purity greater than or equal to 99.98% and a thickness greater than or equal to 0.07 mm.

3. A method for improving the back contact of a tantalum nitride photoanode according to claim 2, characterized in that: The polishing liquid used in step 2 is prepared by mixing hydrofluoric acid and sulfuric acid in a volume ratio of 1:9 to 1:14; the voltage of the electrochemical polishing treatment is 5 to 15 V, and the polishing time is 5 to 12 minutes.

4. A method for improving the back contact of a tantalum nitride photoanode according to claim 2, characterized in that: In step 3, the metal substrate obtained in step 2 is ultrasonically cleaned using soap water, deionized water, acetone and isopropanol in sequence, and the cleaning time is 15 minutes.

5. A method for improving the back contact of a tantalum nitride photoanode according to claim 2, characterized in that: In step 4, NbN is prepared on the front side of the metal substrate obtained in step 3 by high temperature nitridation. x The film process is: The metal substrate obtained in step 3 is placed in a quartz boat and sealed in a quartz tube of a high-temperature tube furnace, and high-temperature nitridation is performed in an ammonia atmosphere with a purity of 99.999%, with a nitriding gas flow rate of 100-300sccm, a temperature of 1073-1323K, and a time of 1-12 hours.

6. A method for improving the back contact of a tantalum nitride photoanode according to claim 2, characterized in that: In step 5, a dual-source electron beam deposition method is used to deposit NbN x The process of preparing gradient Mg-doped Ta2O5 precursor film on the film is: The metal substrate obtained in step 4 was placed on the sample stage of the electron beam deposition system. Then, Ta2O5 and MgO oxide precursors were placed in crucibles in the vacuum electron beam deposition chamber, and the chamber was evacuated to 5×10 -6 ~1×10 -5 After reaching the vacuum level, 1-10 sccm of oxygen is introduced; During the deposition process, two quartz crystal oscillators are used to control the deposition rate and film thickness of the two sources respectively. Specifically, when the Ta2O5 deposition rate is fixed at When the deposition thickness is 600nm, the deposition rate of MgO source decreases for every 100nm of Ta2O5 deposition. Until To achieve Mg 2+ Gradient doping in Ta2O5 thin films; The initial deposition rate of MgO is 7. A method for improving the back contact of a tantalum nitride photoanode according to claim 2, characterized in that: In step 6, the Mg-doped Ta2O5 precursor film is nitrided by a high temperature nitridation method to obtain Mg:Ta3N5 / NbN x The film process is: The Mg-doped Ta2O5 precursor film was placed in a quartz boat and sealed in a quartz tube of a high-temperature tube furnace. It was subjected to high-temperature nitridation in an ammonia atmosphere with a purity of 99.999%, and finally Mg:Ta3N5 / NbN was obtained. x Thin film; high temperature nitridation conditions are: gas flow rate of 100-300sccm, temperature of 1073-1323K, time of 1-12 hours.

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