Preparation method and application of carbon layer bridged Au-C / tio2 composite photocatalyst

Abstract

The present application relates to the technical field of catalyst preparation, in particular to a preparation method and application of Au-C / TiO2 composite photocatalyst with carbon layer bridging. A ternary composite material with carbon layer (C), Au and TiO2 having a close contact interface is successfully prepared by calcination method, and the composite material exhibits excellent photocatalytic hydrogen production activity and light stability. The experimental results show that the optimized sample has a hydrogen production rate of 1473 μmol·h ‑1 ·g ‑1 , and still maintains high activity after 24 h of cycle activity test. The present application proposes to introduce C layer to extend the distance between TiO2 semiconductor and Au, which reduces the recombination of photo-generated electrons and holes, thereby ensuring more electrons to participate in the photocatalytic hydrogen production reaction. The technical scheme of the present application solves the problem of too close distance between the electron and hole aggregation center of Au / TiO2 system, which is expected to significantly improve the commercial availability of TiO2 and even promote its application in industry.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of catalyst preparation, in particular to a preparation method and application of a carbon layer bridged Au-C / TiO2 composite photocatalyst. BACKGROUND

[0002] Due to the enhancement of people's environmental awareness and the overconsumption of non-renewable energy, the use of semiconductor photocatalysts to convert solar energy into hydrogen energy has attracted widespread attention in the scientific community [1-5]. Among the existing various semiconductors, TiO2 has become an excellent photocatalyst for producing H2 due to its chemical inertness, cost-effectiveness, non-toxicity and long-term photo stability [6-8]. However, the photocatalytic efficiency of TiO2 is low, and about 90% of the photo-generated electrons and holes recombine within ~10 ns, hindering its wide application in the fields of environment and energy [9-11].

[0003] Typical methods to improve the charge separation efficiency include loading metals, constructing heterojunctions, regulating morphology and introducing defects [12-15]. In particular, coupling TiO2 with noble metal Au nanoparticles (NPs) is a traditional and practical method [16-18]. An ohmic or Schottky junction will be formed between Au and TiO2, depending on the size of their work functions. When the work function of TiO2 is larger, an ohmic contact is formed between TiO2 and Au, and vice versa

[19] . The formation of metal-semiconductor junction helps the Au cocatalyst to capture photo-generated electrons from the conduction band of TiO2. At the same time, Au nanoparticles (NPs) are considered to be favorable active sites for the hydrogen evolution reaction (HER), due to its low overpotential and low hydrogen evolution free energy

[20] . However, although the direct close contact of the Au / TiO2 system improves the separation of interface charges, the distance between photo-generated electrons and holes is still very close, so that they are easily recombined [21, 22]. Therefore, the photocatalytic efficiency of the traditional Au / TiO2 system is not ideal (about 100-400 μmol·h -1 ·g -1 ), and the photocatalytic hydrogen production efficiency of Au / TiO2 is less improved compared with pure TiO2 [23-26]. How to improve the photocatalytic activity of the Au / TiO2 system is an important challenge for the further development and wide application of TiO2-based composite materials.

[0004] To solve this problem, the common strategy is to use an intermediate conductor to extend the distance between the semiconductor and the cocatalyst, such as introducing graphene and carbon [17, 23], which can optimize the transport path of photo-generated carriers, thereby minimizing the probability of charge recombination [27, 28]. In recent years, transition metal carbide Ti3C2MXene has been considered as a new star in two-dimensional (2D) materials, which is synthesized by etching the Al layer of Ti3AlC2MAX phase [29-31]. Notably, Ti3C2 will form abundant Ti monovacancies or vacancy clusters on its surface during preparation, which has strong enough reducing property to reduce chloroauric acid (HAuCl4) to Au NPs

[32] . Taking advantage of this property, Au / Ti3C2 composite materials can be prepared by in-situ deposition without adding any reducing agent. In addition, due to the exposure of a large number of Ti metal atoms on the surface and edges of Ti3C2 nanosheets, Ti3C2 nanosheets are not stable under oxidizing atmosphere and high temperature conditions. It has been reported that Ti3C2 is oxidized to a composite of TiO2 and carbon layer (C / TiO2) after calcination in CO2 atmosphere at 700 ℃ [33, 34], and the carbon layer prepared from Ti3C2 has high electrical conductivity and large specific surface area, and the thickness is thinner than Ti3C2, so it can improve the problem of light intensity attenuation caused by Ti3C2 under light irradiation, but the photocatalytic hydrogen production activity of C-TiO2 derived from Ti3C2 is not ideal, about 400 μmol·h -1 ·g -1 However, to our knowledge, there has been no report on using the carbon layer derived from Ti3C2 as an intermediate conductor to increase the charge transfer distance between TiO2 and Au, and promoting photocatalytic hydrogen production through the synergistic effect of Au and C layer. SUMMARY

[0005] In view of the deficiencies in the prior art described above, the purpose of the present application is to provide a preparation method and application of a carbon layer bridged Au-C / TiO2 composite photocatalyst.

[0006] The problem to be solved by the present application is the problem existing in the Au-TiO2 system: the distance between the electron accumulation center (Au) and the hole accumulation center (valence band of TiO2) is too close, resulting in fast carrier recombination. The present application ingeniously introduces a carbon layer to extend the distance between TiO2 and Au, and the ohmic junction formed between the carbon layer and TiO2 can also promote the transfer of electrons from TiO2 to the carbon layer and further to Au, where the hydrogen evolution reaction occurs.

[0007] The present application uses the reducing Ti vacancies (V Ti) to drive the self-reduction of HAuCl4 to in-situ load Au NPs on the surface of Ti3C2. Subsequently, Au / Ti3C2 is calcined in a tube furnace under CO2 atmosphere and oxidized into carbon layer supported Au / TiO2 composite. During the cooling process, air is injected into the tube furnace at a certain temperature to further oxidize the powder to adjust the content of C layer in the composite. The obtained Au-C / TiO2 ternary composite is abbreviated as CTA- T wherein C, T, A and T are C layer, TiO2, Au and temperature at air injection, respectively. The comprehensive experimental characterization and theoretical calculation results prove that the C layer not only serves as a bridge for the transfer of photo-generated electrons from TiO2 to Au, but also effectively extends the distance between the semiconductor and the hydrogen production active site, reduces the probability of charge recombination, and thus ensures more electrons to participate in the photocatalytic hydrogen production reaction.

[0008] To achieve the above object, the technical scheme adopted by the present application is as follows:

[0009] A preparation method of a carbon layer bridged Au-C / TiO2 composite photocatalyst, the steps of which are as follows:

[0010] (1) Preparation of Au / Ti3C2 composite material by using the reducing Ti vacancies of two-dimensional Ti3C2 MXene

[0011] Disperse two-dimensional Ti3C2 nanosheets in deionized water and fully stir to obtain a uniform Ti3C2 suspension. Slowly add an aqueous solution of chloroauric acid to the above Ti3C2 suspension under dark conditions, and after stirring at room temperature under dark conditions for 10-180 min (preferably 30 min), centrifuge, wash, and finally freeze-dry to obtain Au / Ti3C2 composite material;

[0012] (2) Preparation of Au-C / TiO2 composite by calcination method

[0013] Place the above Au / Ti3C2 composite material in a container, and under CO2 atmosphere, increase the temperature to 300-1000 ℃ at a heating rate of 5-20 ℃ / min, then calcine at 300-1000 ℃ (preferably 700 ℃) under CO2 atmosphere for 20-180 min, and then decrease the temperature to room temperature at a rate of 5-20 ℃ / min. During the cooling process, when the temperature decreases to 25-700 ℃ (preferably 300-600 ℃, most preferably 450 ℃), stop the CO2 flow and replace it with air flow for the second oxidation until the temperature decreases to room temperature. The obtained Au-C / TiO2 ternary composite is Au-C / TiO2 composite photocatalyst.

[0014] Further, the specific operation of step (2) is as follows: the Au / Ti3C2 composite material obtained in step (1) is placed in a container, heated to 700 DEG C at a temperature increasing rate of 10 DEG C / min under a CO2 atmosphere, then calcined at 700 DEG C under a CO2 atmosphere for 30 min, and then decreased to room temperature at a rate of 10 DEG C / min; during the temperature decreasing process, when the temperature decreases to 300-600 DEG C, stop the CO2 input and change to input air to perform the second oxidation until the temperature decreases to room temperature, thereby obtaining the Au-C / TiO2 composite photocatalyst.

[0015] Further, the loading amount of Au in the Au-C / TiO2 composite photocatalyst is 0.1%-10% (preferably 0.8%); further, in step (1), the mass ratio of the two-dimensional Ti3C2 nanosheet to chloroauric acid in the chloroauric acid aqueous solution is 700: (1.28-128), preferably 700:10.24.

[0016] Further, the gas flow rate of the CO2 atmosphere is 10-70 sccm.

[0017] Further, the freeze-drying condition in step (1) is -60 DEG C, 10 Pa.

[0018] Further, in step (1), the two-dimensional Ti3C2 nanosheet is obtained by etching Ti3AlC2 to remove the Al layer through a wet chemical method with LiF-HCl as the etchant, and then ultrasonic stripping.

[0019] Further, the two-dimensional Ti3C2 nanosheet is prepared by the following method: LiF is dissolved in an HCl solution, then Ti3AlC2 is slowly added to obtain a suspension, the obtained suspension is ultrasonically mixed at room temperature, then the above solution is transferred to a 40-60 DEG C constant-temperature oil bath, and etching reaction is performed for 20-40 h (preferably, the reaction condition is 55 DEG C for 24 h), the obtained black solid powder is washed with deionized water and anhydrous ethanol alternately until the filtrate is neutral, and then dried (preferably, vacuum dried at 60 DEG C for 8 h) to obtain a black powder; finally, a certain amount of the black powder is dispersed in deionized water to form a Ti3C2 suspension (preferably, 20 mg·mL -1 ), and ultrasonic stirring is performed under an N2 atmosphere for 4-8 h (preferably, 4 h), and then the ultrathin two-dimensional Ti3C2 nanosheet is obtained after freeze-drying.

[0020] In the present application, the room temperature range is 20-25 DEG C.

[0021] The application further provides application of the Au-C / TiO2 composite photocatalyst bridged by the carbon layer in photocatalytic hydrogen production, specifically, application of the catalyst in photocatalytic water splitting for hydrogen production under ultraviolet-visible light.

[0022] The characterization method of the composite photocatalyst prepared by the method of the application is as follows:

[0023] (1) Transmission electron microscopy (TEM, Talos F200S, Thermo Scientific) and field emission scanning electron microscopy (FESEM, SU8010, Hitachi) are used to observe the microstructure of the sample. High-resolution transmission electron microscopy (HRTEM, Talos F200S, Thermo Scientific), high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM, Talos F200S, Thermo Scientific) and EDS connected to HAADF-STEM are used to study the CTA- T The composition and relative position relationship of the sample are verified to verify the successful synthesis of the Au-C / TiO2 composite material. In order to study the Brunauer-Emmet-Teller (BET) specific surface area (S BET ) and pore structure, the sample is analyzed by using a nitrogen adsorption instrument (Micromeritics ASAP 2020, USA) after degassing at 150 °C. The adsorption isotherm data are analyzed by using the Barret-Joyner-Halender (BJH) method. The photoluminescence spectrum with an excitation wavelength of 245 nm is obtained by using a fluorescence spectrophotometer (F-7000, Hitachi). The X-ray diffraction (XRD, D8Advance, Bruker) of the catalyst is measured by using a monochromatic Cu Kα generator (λ = 1.5406 Å) and a laser confocal Raman spectrometer (Thermo Fisher Scientific, DXR 2xi). The thermogravimetric curve of the sample is measured by using a thermal gravimetric analyzer (Netzsch, STA 449 C) with a heating rate of 10 K / min. The element content of Au and C in the sample is characterized by using an inductively coupled plasma emission spectrometer (ICP-OES) and a CHNS / O element analyzer (Element, Vario EL cube). The X-ray photoelectron spectroscopy (XPS) is obtained on an Escalab 250xi spectrometer (Thermo Scientific). The ultraviolet-visible diffuse reflectance spectrum (UV-vis DRS) is obtained by using an ultraviolet-visible spectrophotometer (UV2550, Shimadzu).

[0024] (2) The activity of the prepared composite photocatalyst is evaluated by photocatalytic water decomposition to produce hydrogen, and triethanolamine is used as a sacrificial agent.

[0025] The Au-C / TiO2 composite photocatalyst prepared by the preparation method is applied to photocatalytic hydrogen production.

[0026] The photocatalytic hydrogen production performance of the sample under xenon lamp (300 W) irradiation is studied by using a full-glass automatic online trace gas analysis system (Labsolar-6A, Beijing Pofei Technology Co., Ltd.). Before the hydrogen generation reaction starts, 20 mg of the photocatalyst sample is uniformly dispersed in 80 mL of a triethanolamine (TEOA) aqueous solution (10 v%). During the entire photocatalytic reaction process, 0.6 mL of the generated gas (H2) is automatically collected every hour, and then injected into a gas chromatograph (GC-2018, Shimazu, equipped with a TCD detector and a 5 Å molecular sieve column, with N2 as the carrier gas). The photocatalytic hydrogen production rate is calculated according to the average value of the hydrogen production amount in 4 hours, and the photocatalytic hydrogen production rate of the composite photocatalyst is up to 1473 μmol·h -1 ·g -1 .

[0027] Compared with the prior art, the advantages and beneficial effects of the present application are as follows:

[0028] The present application uses 2D Ti3C2 MXene with reducing Ti vacancies as a substrate, in-situ loads Au NPs, and then generates Au-C / TiO2 ternary composite photocatalyst through a CO2-air stage oxidation step, and then applies it to photocatalytic decomposition of water to produce hydrogen. The introduction of Au NPs promotes the oxidation of Ti3C2 into C / TiO2 composite material in a CO2 atmosphere, and the injection of air further oxidizes the powder to adjust the carbon content appropriately. Experimental and theoretical results show that in the Au-C / TiO2 ternary composite, the C layer can act as an electronic bridge to transport the photo-generated electrons of TiO2 to the surface of the hydrogen evolution site Au nanoparticles (NPs). The present application successfully constructs a multi-interface charge transport network, realizes a directional long-distance photoelectron transfer route from TiO2 to the C layer to Au, and H + is finally reduced to H2. Therefore, the annihilation rate of photoelectrons is greatly reduced. Due to the presence of the intermediate C layer, the hydrogen evolution rate of the Au-C / TiO2 composite material is 27 times higher than that of Au-TiO2, and also exceeds many reported C / TiO2 photocatalysts derived from Ti3C2 MXene. The present application ingeniously uses two-dimensional MXene to form a TiO2-based photocatalytic system, which is simple to operate, mild in conditions, few in steps, short in time consumption, and the obtained composite photocatalyst has significantly enhanced photocatalytic activity. Therefore, the method of the present application has broad engineering practical application prospects. Attached Figure Description

[0029] Figure 1 This is a schematic diagram of the preparation process of the Au-C / TiO2 nanocomposite material described in this invention.

[0030] Figure 2 (a) is a TEM image of Ti3C2 in Example 1; Figure 2 (b) is a TEM image of Au / Ti3C2; Figure 2 (c)-(e) are FESEM, TEM and HRTEM images of the CTA-450 sample, respectively; Figure 2 (f) shows the HAADF-STEM image of the CTA-450 sample and its EDS spectra of Ti, O, C and Au atoms.

[0031] Figure 3 (a) and (b) are CTA- in Example 1, respectively. T The N2 adsorption / desorption isotherms and corresponding pore size distribution curves of the sample.

[0032] Figure 4 (a) The XRD patterns of Ti3AlC2 and Ti3C2 in Example 1; Figure 4 (b) is the TG curve of Ti3C2 under CO2 atmosphere.

[0033] Figure 5 (a) and (b) are CTA- in Example 1, respectively. T XRD patterns and Raman spectra of CT-450 samples; Figure 5 (c) CTA under air atmosphere T Thermogravimetric curve of the sample; Figure 5 (d) is the SEM image of the CT-450; Figure 5 (e) is a side view of Ti3C2 and Au / Ti3C2 being adsorbed by CO2 molecules.

[0034] Figure 6 (a)-(d) are the high-resolution XPS spectra of Ti 2p, O 1s, C 1s and Au 4f in the CTA-450 sample in Example 1, respectively.

[0035] Figure 7 (a) and (b) are CTA- in Example 1, respectively. T DRS spectra of the sample and Tauc spectra of the CTA-450 sample.

[0036] Figure 8 (a) and (b) are CTA- in Example 1, respectively. T A comparison of the photocatalytic hydrogen production rates of the CT-450 sample and pure P25, and a cycle activity test of the CTA-450 sample.

[0037] Figure 9 (a)-(c) are CTA-, Au / C and Au-C / TiO2 interfaces, respectively, in Example 1 T Steady-state PL spectra, TPR and EIS of the samples; Figure 9 (d) and (e) are the difference in electron density (2.6 x 10 -4 e / Å 3 ) and Bader charge transfer of C / TiO2 and Au / C interfaces, respectively. Figure 9 (f) is the calculated HER free energy diagram of TiO2, Au and C.

[0038] Figure 10 (a) is the band structure of TiO2, C and Au before and after contact; Figure 10 (b) is the schematic diagram of charge transport path of Au / TiO2 (left) and Au-C / TiO2 (right) in the process of photocatalytic hydrogen production. DETAILED DESCRIPTION

[0039] The applicant will combine specific examples to explain the technical solutions of the present application in detail, so that those skilled in the art can have a further understanding of the present application, but the following examples are not explained in any way as a limitation on the scope of protection of the present application.

[0040] In the following examples: Ti3AlC2 was purchased from Henan Yi Yi Technology Co., Ltd., with a purity of 98%, 200 mesh; the room temperature refers to 25℃;

[0041] The naming rules of the composite photocatalysts prepared in the following examples are as follows:

[0042] The prepared Au-C / TiO2 ternary composite photocatalyst is named CTA- T , wherein C, T, A and T are C layer, TiO2, Au and temperature when air injection, respectively.

[0043] Example 1: An Au-C / TiO2 ternary composite photocatalyst was prepared by the following method:

[0044] (I) Preparation of two-dimensional ultrathin Ti3C2 nanosheets

[0045] Ti3AlC2 was etched by wet chemical method to remove Al layer with LiF-HCl as etchant, and then ultrasonic exfoliation was used to obtain ultrathin Ti3C2. Specifically, 5.0 g LiF was dissolved in 100 mL 9 M hydrochloric acid, and then 5.0 g Ti3AlC2 was slowly added to form a suspension. The obtained suspension was ultrasonically treated for 5 min at room temperature and stirred for 10 min to mix uniformly; then the above solution was transferred to a constant temperature oil bath at 55 ℃, and etching reaction was carried out for 24 h. The reaction product was washed with deionized water and anhydrous ethanol alternately until the filtrate was neutral (pH=7, same below), and finally vacuum dried at 60 ℃ for 8 h to obtain black powder. Finally, 5.0 g of the above black powder was dispersed in 0.25 L deionized water to form a suspension (20 mg·mL -1 ), and ultrasonic treatment was carried out under N2 atmosphere for 4 h in ice bath. After freeze-drying (freeze-drying at-60 ℃, 10 Pa), ultrathin 2D Ti3C2 nanosheets were obtained, which were hereinafter referred to as ultrathin Ti3C2.

[0046] (II) Preparation of Au-C / TiO2 composite

[0047] (1) 700 mg of the above ultrathin Ti3C2 was dispersed in 700 mL deionized water, and uniform Ti3C2 suspension was obtained by ultrasonic stirring. 10.24 mL of HAuCl4 aqueous solution (1 mg·mL -1 ) was slowly added thereto under dark conditions, and then reaction was carried out for 30 min at room temperature in the dark. After the reaction was completed, the above solution was centrifuged at a rotation speed of 14000 rpm and then washed (washed with deionized water and anhydrous ethanol alternately until the filtrate was neutral). The obtained sample after washing was freeze-dried (freeze-drying at-60 ℃, 10 Pa), and the obtained black solid powder was Au / Ti3C2 composite material.

[0048] (2) 100 mg of the above Au / Ti3C2 composite material was placed in a porcelain boat, and then the temperature was increased to 700 ℃ at a heating rate of 10 ℃ / min under CO2 atmosphere at a flow rate of 50 sccm. Calcination was carried out at 700 ℃ for 30 min under CO2 atmosphere (this step played a role of oxidizing Ti3C2 and retaining C layer), and then the temperature was decreased to room temperature at a rate of 10 ℃ / min. During the temperature decreasing process, the temperature reached a certain temperature (i.e. TAfterwards, CO2 is turned off, and air is injected to continue cooling to room temperature for secondary oxidation (in this invention, the air injection method is: first, air is squeezed in using an air bag filled with air, and then it is directly connected to the atmosphere). The purpose is to adjust the C layer content in the composite material. In this embodiment, different temperatures T are set for experiments to obtain a series of Au-C / TiO2 composite photocatalysts, namely Au-C / TiO2-T samples, denoted as CTA- T ( T = 700, 600, 450, 300 and 25 ℃), where CTA-25 is the sample obtained by calcining at 700℃ and then directly cooling to 25℃.

[0049] The preparation method of binary C / TiO2 is similar to that of Au-C / TiO2-450, except that the reaction process of adding HAuCl4·4H2O aqueous solution is not carried out. Instead, the Au / Ti3C2 sample in step (2) is replaced with ultrathin Ti3C2, and the final product is named CT-450.

[0050] Detection Example 1

[0051] The morphology and structure of the samples were characterized using TEM, FESEM, HRTEM, and HAADF-STEM. Figure 2 As shown in Figure a, Ti3C2 obtained by HCl-LiF etching is a two-dimensional ultrathin nanosheet with a smooth surface and flat edges. When Au NPs are loaded onto the Ti3C2 surface through the self-reduction of Ti vacancies using HAuCl4, the planar nanosheets are transformed into a rough surface with dispersed nanoparticles. Figure 2 b). Au NPs have a relatively uniform particle size, with a diameter of about 10 nm

[32] . SEM, TEM and STEM images of CTA-450 samples ( Figure 2 (cf) revealed that two-dimensional Ti3C2 transforms into highly translucent C nanosheets and TiO2 NPs through a calcination process, with obvious Au NPs still remaining on the nanosheet surface. In particular, HRTEM images of the CTA-450 samples show lattice spacings of 0.35, 0.25, and 0.24 nm, which correspond to anatase TiO2 (101), rutile TiO2 (111), and Au (111) lattice planes, respectively. Figure 2 e) [32-34], and HAADF-STEM images and EDS elemental mapping images of Ti, O, C and Au ( Figure 2 f) also indicates the relative positions of TiO2, Au, and C, showing that Au / Ti3C2 has been transformed into a composite material of Au, TiO2, and C layers (Au-C / TiO2) during calcination.

[0052] Detection Example 2

[0053] The Brunauer Emmett-Teller (BET) method based on N2 adsorption-desorption was used to determine a series of CTA- T Specific surface area of ​​the sample (A) BET ) and pore structure, the results show Figure 3 All samples exhibited type IV isotherms and typical H3 hysteresis loops, indicating that they all possessed slit-like mesoporous structures

[35] . The A... BET The values ​​are 51.89, 51.87, 34.49, 20.00, and 15.44 m, respectively. 2 ·g -1 ( Figure 3 a). For example Figure 3 As shown in Figure b, the pore size distribution curves of the samples were calculated using the Barret-Joyner-Halender (BJH) method. This further confirms the presence of mesopores in the samples, with average pore sizes of 5.32, 5.35, 11.62, 15.56, and 20.6 nm for CTA-25, CTA-300, CTA-450, CTA-600, and CTA-700, respectively. (The text abruptly ends here, likely due to an incomplete sentence or missing information.) BET The difference in pore size can be understood as follows: as the temperature of the second oxidation increases, the amount of C layer in the sample gradually decreases. The TiO2 particles lack the support of the C layer, leading to their accumulation and thus affecting the A content of the sample. BET Reduced and increased pore size.

[0054] Detection Example 3

[0055] The prepared samples were subjected to comprehensive analysis of phase structure and composition using XRD, TG, Raman spectroscopy, ICP-OES, and XPS. Figure 4 As shown in Figure a, the Ti3AlC2 precursor exhibits distinct XRD diffraction peaks at 39.1° and 9.5°, belonging to the (104) and (002) lattice planes, respectively (JCPDS No. 52-0875). After the etching reaction, the strongest diffraction peak at 39.1° disappeared, and the diffraction peak at 9.5° shifted to a lower angle, confirming the successful removal of the Al layer of Ti3AlC2, thus producing Ti3C2MXene with a wider interlayer spacing [29, 36]. The TG curve of Ti3C2 ( Figure 4 b) indicates that the oxidation of Ti3C2 in a CO2 atmosphere occurs at around 700 °C, which is very consistent with previously reported results

[33] . This result provides a basis for the selection of the oxidation temperature of Ti3C2 in a CO2 atmosphere in this invention.

[0056] Figure 5 a showed CTA-T XRD pattern of the sample. After oxidation treatment, the typical diffraction peaks of Ti3C2 disappeared, replaced by diffraction peaks of TiO2, indicating that Ti3C2 was completely oxidized to TiO2. For CTA- T The samples, TiO2NPs, consist of anatase and rutile phases. The diffraction peaks at 25.4°, 37.9°, 48.2°, 54.1°, and 62.9° correspond to the (101), (004), (200), and (204) crystal planes of anatase TiO2 (JCPDS NO. 73-1764), respectively. The lattice planes at 27.5°, 36.2°, 41.4°, 54.5°, 56.8°, and 69.2° correspond to the (110), (101), (111), (211), (220), and (301) crystal planes of rutile TiO2 (JCPDS NO. 65-0192), respectively [33, 37]. It is noteworthy that the peak intensity of anatase TiO2 in CTA-600, CTA-450, and CTA-300 samples exceeded that in CTA-700 and CTA-25 samples, indicating that a suitable temperature range (300-600℃) and an oxygen atmosphere are favorable for anatase formation. Conversely, high temperature (700℃) or oxygen deficiency is unfavorable for anatase formation [38,39]. Furthermore, the XRD peak of Au NPs (JCPDS NO. 04-0784) is higher in CTA- T This result is clearly visible in the sample and matches the STEM images very well. Figure 2 f) [32, 40].

[0057] Although the XRD pattern did not show any peaks corresponding to the amorphous C layer, CTA- was successfully confirmed by Raman spectroscopy. T The sample contains a C layer, as well as anatase and rutile TiO2. Specifically, at 145 cm⁻¹... -1 The peaks at 246, 438, and 606 cm⁻¹ belong to anatase TiO₂. -1 The peak at that location belongs to rutile TiO2 ( Figure 5 b). Consistent with the XRD results, the Raman intensity of anatase TiO2 in CTA-600, CTA-450, and CTA-300 samples was significantly stronger than that in CTA-700 and CTA-25 samples. (1333 and 1601 cm⁻¹) -1 The Raman peaks at these locations correspond to the D and G bands of carbon, respectively [34, 41]. This result strongly confirms that CTA- T The presence of layer C in the sample corresponds to the results of the TEM image. Figure 2d). Notably, in the CTA-700 sample, the intensity of the D and G band peaks of C decreased significantly with increasing air injection temperature, eventually disappearing completely. This indicates that changing the air injection temperature during cooling effectively controlled the C layer content in the Au-C / TiO2 composite photocatalyst. The C layer content in the composite photocatalysts, from highest to lowest, was CTA-25, CTA-300, CTA-450, CTA-600, and CTA-700. This conclusion well explains the variation law of the sample specific surface area and pore size ( Figure 3 ).

[0058] CTA- was determined using a combination of TG, CHNS / O elemental analysis, and ICP-OES. T The content of C and Au in the sample. From Figure 5 The TG curves of C show that the C content decreases at higher air injection temperatures, effectively confirming the results of Raman spectroscopy. Notably, CHNS / O elemental analysis revealed that the C layer contents of CTA-25, CTA-300, CTA-450, CTA-600, and CTA-700 samples were 7.7 wt%, 6.5 wt%, 1.8 wt%, 1.2 wt%, and 0.05 wt%, respectively. Furthermore, ICP-OES determined the CTA-... T The actual mass percentage of Au in the samples was approximately 0.8%, which is basically equal to the theoretical content. These results indicate that samples CTA-25, CTA-300, CTA-450, and CTA-600 are ternary composite photocatalysts of C, TiO2, and Au, while the C content of sample CTA-700 is negligible, and it can be considered a binary composite material of Au and TiO2.

[0059] More importantly, the introduction of Au NPs can effectively promote the oxidation of Ti3C2. For example... Figure 5 As shown in Figure a, compared with CTA-450, CT-450 without Au loading exhibits weaker XRD diffraction peaks of TiO2, and the 8.4° characteristic peak belonging to Ti3C2 does not disappear

[42] . Meanwhile, as shown in Figure a... Figure 5 As shown in b, the CT-450 is at 160 cm. -1 The Raman peak at the location is attributed to the ω1 Raman active phonon vibration mode of titanium carbide, which is consistent with the XRD results

[43] . In addition, the SEM images of CTA-450 ( Figure 2 c) Only a small number of nanoparticles of relatively small size were observed on the surface of the CT-450 nanosheets. Figure 5d), which further indicates that the oxidation degree of Ti3C2 in CT-450 is very low. To reveal the underlying reasons, density functional theory (DFT) was used to calculate and compare the adsorption energies (ΔE) of CO2 on Ti3C2 and Au / Ti3C2 surfaces. ads ).like Figure 5 As shown in e, the |ΔE| of CO2 on the Au / Ti3C2 surface ads The surface area of ​​Au NPs is significantly larger than that of Ti3C2, indicating that the adsorption of CO2 on the Au / Ti3C2 surface is thermodynamically more favorable due to the presence of Au NPs [43, 44]. It is generally believed that CO2 adsorption is the primary and key step in the oxidation of Ti3C2

[33] . The strength of CO2 adsorption determines the residence time of CO2 on Ti3C2; the stronger the adsorption, the longer the residence time, and the higher the degree of oxidation

[45] . Therefore, the introduction of Au NPs promotes the oxidation of Ti3C2 in a CO2 atmosphere.

[0060] The surface chemical composition of the samples was analyzed using XPS spectroscopy. High-resolution XPS spectra of Ti 2p, O 1s, C 1s, and Au4f in the CTA-450 sample are shown below. Figure 6 In the high-resolution XPS spectrum of Ti 2p, the two peaks at 464.3 and 458.5 eV belong to the Ti 2p bonds of the Ti-O bond in TiO2, respectively. 1 / 2 and Ti 2p 3 / 2 Typical peak ( Figure 6 a)

[17] . For example, Figure 6 As shown in b, three oxygen-containing bonds were observed in the O1s high-resolution XPS spectrum, corresponding to the Ti-O bond in TiO2, the OH bond in the adsorbed water, and the CO bond in C-TiO2, respectively [33, 46]. The C 1s spectrum could be fitted to three peaks with binding energies of 284.8, 286.3, and 288.8 eV ( Figure 6 c), these are attributed to CC / C=C, CO, and CO-Ti bonds, respectively [6, 47]. This indicates that some C atoms achieve C doping by substituting O atoms to form CO-Ti bonds. Additionally, the deconvolution peaks at 87.2 and 83.5 eV correspond to Au 4f 7 / 2 and Au4f 5 / 2 The binding energy difference between the two is 3.7 eV, indicating that the chemical state of Au NPs is mainly Au. 0 ( Figure 6 d). The two acromions located at 88.8 and 85.3 eV belong to Au. 3+ This may be because the sample was synthesized under high temperature conditions, which easily causes Au NPs to be slightly oxidized.

[0061] Detection Example 4

[0062] The light absorption properties of each sample were investigated by UV-vis DRS. As shown in Fig. Figure 7 a, the CTA- T composite photocatalysts exhibited excellent light absorption in the range of 400-800 nm, and the absorption intensity decreased proportionally with the increase of air injection temperature due to the reduction of C layer. In addition, the color change of the CTA- T composite photocatalysts also reflected the difference in absorption intensity (see Fig. Figure 7 a). The absorption edge at 400 nm in the CTA- T samples was attributed to the intrinsic absorption of TiO2, and the significant enhancement of absorbance at 600 nm might be related to the localized surface plasmon resonance absorption of Au NPs [22, 48]. In addition, by converting the UV-vis DRS curves into Tauc plots through Kubelka-Munk function, the band gap of CTA-450 was calculated to be 2.98 eV (Fig. Figure 7 b). The band gap of CTA-450 was smaller than that of pure TiO2 (3.0-3.2 eV), which was due to the formation of Ti-O-C bonds between TiO2 and C (as shown in XPS results) [33, 34].

[0063] Test Example 5

[0064] The photocatalytic hydrogen evolution activity of the CTA- T samples was evaluated by using triethanolamine (TEOA) as a sacrificial agent, and a full-glass automatic online trace gas analysis system (Labsolar-6A, Beijing Phoibos Technology Co., Ltd.) under xenon lamp (300 W) irradiation. Before the hydrogen generation reaction started, 20 mg of photocatalyst sample was uniformly dispersed in 80 mL of triethanolamine (TEOA) aqueous solution (10 v%). During the entire photocatalytic reaction process, 0.6 mL of generated gas (H2) was automatically collected every hour, followed by injection into a gas chromatograph (GC-2018, Shimazu, equipped with a TCD detector and a 5 Å molecular sieve column, with N2 as the carrier gas), and the photocatalytic hydrogen evolution rate was calculated according to the average value of 4-hour hydrogen production.

[0065] The comparison of average hydrogen evolution rates of CTA- T samples, CT-450 and pure P25 titanium dioxide after 4 hours of irradiation is shown in Fig. Figure 8 a. Obviously, with the increase of T , the hydrogen evolution activity of the CTA- T samples showed a trend of first increasing and then decreasing. The photocatalytic hydrogen evolution rate of the CTA-25 sample without secondary air oxidation treatment was only 65 μmol·h -1 ·g -1, which is due to the excessive C layer that is not conducive to the light absorption of TiO2, and blocks most of the active sites on the surface of TiO2[49, 50]. The photocatalytic hydrogen production rate of the ternary CTA-450 sample reaches 1473 μmol·h -1 ·g -1 , which is 57 and 17 times that of the binary CT-450 and pure P25, respectively. Pure P25 has higher photocatalytic activity than binary CT-450, which is due to its higher content of anatase and rutile TiO2. In contrast, when the air injection temperature is as high as 700 ℃, the CTA-700 sample has almost no C layer and can be regarded as an Au-TiO2 binary composite, and its photocatalytic rate is only 55 μmol·h -1 ·g -1 , which is 27 times lower than the best CTA-450 ternary composite. By adjusting the C content by changing the air injection temperature, it can be considered that the appropriate C content and Au NPs can effectively synergistically enhance the photoactivity of TiO2.

[0066] For semiconductor photocatalysts, light stability and photoactivity are equally important in future practical applications. The cyclic activity of the CTA-450 sample was tested to evaluate its light stability. As shown in Figure 8 b, the CTA-450 sample still maintains high activity after six consecutive photocatalytic hydrogen production reaction cycles (4 h for each cycle), indicating that the CTA-450 sample has good cyclic stability.

[0067] The above results fully demonstrate that the synergistic effect of the C layer and Au NPs significantly enhances the photocatalytic activity of TiO2. It is well known that the performance of a photocatalyst mainly depends on the generation, separation, and transfer efficiency of photoexcited carriers. Therefore, we carried out a series of photo / electrochemical experiments and DFT calculations to elucidate the reasons for the significant improvement in the photocatalytic activity of CTA-450.

[0068] First, the effects of the separation and migration efficiency of photoexcited electron-hole pairs on the photocatalytic performance were analyzed by PL, TPR, and EIS. In general, a decrease in PL intensity means that photoelectrons and holes have a longer lifetime and higher separation efficiency[34, 51]. As shown in Figure 9 a, the CTA- T sample has a clear PL emission peak at 400 nm after excitation at 245 nm, which is consistent with the absorption edge of the DRS ( Figure 7 a). Both the CTA-25 and CTA-700 samples exhibit strong PL intensity, which is due to excessive and insufficient C layers, respectively. In contrast, the fluorescence of the CTA-450 sample is significantly quenched, indicating that the opportunity for photoexcited charges to combine on the surface of TiO2 is greatly reduced due to the coexistence of Au and C.

[0069] Subsequently, TPR and EIS again verified that Au NPs and C layer synergistically improved the charge separation and transfer efficiency of TiO2. Figure 9 The TPR in b shows that the CTA-450 sample produces the highest photocurrent density among all samples, indicating that the CTA-450 sample can generate the most photo-generated electrons under light, thereby promoting the H2 evolution reaction. At the same time, the smaller the semicircle diameter of the EIS Nyquist plot, the lower the charge transfer resistance in the photocatalyst [52, 53]. As shown in c, the arc diameter of CTA-450 is the smallest among all samples, and its resistance value is the lowest, which is more conducive to charge transfer. The results of PL, TPR and EIS are in good agreement with the trend of H2 generation activity of the samples (a). In summary, in the CTA-450 sample, the appropriate amount of Au NPs and C layer synergistically promote the migration of photo-generated charges. The results show that the CTA-450 sample has the best photocatalytic hydrogen generation activity. Figure 9 Figure 8 a). In summary, in the CTA-450 sample, the appropriate amount of Au NPs and C layer synergistically promote the migration of photo-generated charges. The results show that the CTA-450 sample has the best photocatalytic hydrogen generation activity.

[0070] In order to accurately clarify the charge transfer mechanism in the Au-C / TiO2 ternary system, theoretical calculations based on density functional theory (DFT) were carried out. Figure 9 d and Figure 9 e are the differential charge density and Bader charge analysis results along the Z-axis direction of the C / TiO2 and Au / C interfaces, respectively, where the yellow area represents electron accumulation and the blue area represents electron depletion. They intuitively depict the changes in charge density during the formation of the C / TiO2 and Au / C interfaces. There is a strong chemical interaction between TiO2, Au and the C layer, leading to a redistribution of interface charges. Obviously, in the C / TiO2 interface, electron depletion occurs on the C layer and electron accumulation occurs on TiO2. Therefore, electrons tend to spontaneously transfer from the C layer to TiO2. Such a direction of electron transfer indicates that the work function of TiO2 is greater than that of the C layer, and when a new Fermi level balance is established between C / TiO2, an ohmic junction will be formed at the C / TiO2 interface

[19] . For the Au / C interface, electrons transfer from the C layer to the Au layer. Further Bader charge analysis was carried out to quantify the amount of transferred charge, and the results show that the amount of charge transferred from the C layer to TiO2 and Au is 0.74 e and 0.04 e, respectively.

[0071] Considering that the Gibbs free energy (ΔG H* ) of the intermediate adsorbing H* on the photocatalyst is a key factor in determining HER activity, the Gibbs free energy of TiO2, Au and C during the H2 evolution process was calculated, including the initial state H + + e - , the intermediate adsorbing H* and the final product 1 / 2H2 (g). ΔG H* ​The positive value indicates H2 adsorption kinetics, and the negative value indicates H2 release kinetics. The optimal value |ΔG H* | is zero or close to zero, which is most conducive to the reaction

[54] . The theoretical calculation results are shown in Fig. 2. Figure 9 The ΔG H* of TiO2, Au and C are 0.48, 1.84 and -0.10 eV, respectively. Therefore, among the three components of the Au / C-TiO2 composite material, Au is proved to be the most suitable active site for reducing H + to H2.

[0072] According to the experimental and theoretical calculation results, a reasonable mechanism of the photocatalytic hydrogen production of the ternary CTA-450 photocatalyst is proposed, as shown in Fig. 3. Figure 10 Before compounding, the Fermi level of the C layer is higher than that of TiO2. After compounding, electrons migrate from the C layer to TiO2 until their Fermi levels align to a new equilibrium state. This forces the energy band of TiO2 to bend downward, forming an ohmic junction at the C / TiO2 interface (a). Figure 10 After light irradiation, TiO2 absorbs light energy to produce electrons and holes, and the photo-generated electrons accumulated at the conduction band are transferred to the C layer and further to the surface of Au NPs. Finally, H + adsorbed on the surface of Au is reduced to H2 by the accumulated electrons. In contrast, in the traditional Au / TiO2 system, due to the close distance between the accumulation centers of electrons (Au) and holes (TiO2), the carriers are prone to recombination (b). Figure 10 In the Au-C / TiO2 system involving the C layer, the C layer not only serves as an intermediate bridge for the transfer of electrons from TiO2 to Au, but also extends the distance between the semiconductor and the hydrogen production active site, greatly ensuring the spatial separation of photo-generated electrons and holes. This well explains the phenomenon that the photocatalytic activity of the CTA-450 sample (i.e., the ternary Au-C / TiO2) in the present application is significantly better than that of the CTA-700 sample (i.e., the binary Au-TiO2).

[0073] In summary, the present application uses 2D Ti3C2 MXene and reductive Ti vacancies as a substrate, in-situ loads Au NPs, and then performs a CO2-air staged oxidation process to generate an Au-C / TiO2 (CTA) ternary composite photocatalyst, which is then applied to photocatalytic water splitting for hydrogen production. The introduction of Au NPs promotes the oxidation of Ti3C2 into a C / TiO2 composite material under a CO2 atmosphere, and the further oxidation of the powder by injecting air can appropriately adjust the carbon content. In the composite material, the activity of the CTA-450 sample with Au and C layers as dual-reduction type cocatalysts is significantly improved (1473 μmol·h -1 ·g -1), which are 27, 57 and 17 times of the binary Au-TiO2 (CTA-700), C-TiO2 (CT-450) and pure TiO2 (P25), respectively. Experimental and theoretical results show that in the Au-C / TiO2 ternary composite, an ohmic junction is formed to facilitate the transfer of photo-generated electrons from TiO2 to the C layer, and the C layer further transfers electrons to Au NPs. H + is finally reduced to H2. The introduction of the C layer effectively extends the distance between the semiconductor (TiO2) and the hydrogen production active site (Au) and significantly reduces the probability of charge recombination. The modification of introducing an intermediate conductor ensures the catalytic activity and durability of the photocatalyst, which may help to bridge the gap between future photocatalytic technology and practical applications.

[0074] The specific method of the above theoretical calculation is as follows: the density functional theory (DFT) calculation is performed by the Vienna ab initio simulation package (VASP) method with a projection augmented wave and the Perdew-Burke-Ernzerhof formula of generalized gradient approximation. The cutoff energy is set to 500 eV, and the structure is relaxed until the energy and force convergence criteria are 1 × 10 - 5 eV and 0.02 eV Å -1 , respectively. In addition, the vacuum layer is set to 15 Å, and the DFT-D3 method of Grimme is used to correct the van der Waals interaction.

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Claims

1. A method for preparing a carbon layer-bridged Au-C / TiO2 composite photocatalyst, characterized in that, Includes the following steps: (1) Au / Ti3C2 composite material was prepared by utilizing the reducing Ti vacancies of two-dimensional Ti3C2 MXene. Two-dimensional Ti3C2 nanosheets were dispersed in deionized water and stirred thoroughly to obtain a uniform Ti3C2 suspension. Under dark conditions, chloroauric acid aqueous solution was slowly added to the above Ti3C2 suspension and stirred at room temperature and in the dark for 10-180 min. After centrifugation and washing, Au / Ti3C2 composite material was obtained by freeze drying. The two-dimensional Ti3C2 nanosheets were obtained by etching Ti3AlC2 with LiF-HCl as an etchant to remove the Al layer through a wet chemical method, followed by ultrasonic exfoliation. (2) Au-C / TiO2 composite was prepared by calcination. The Au / Ti3C2 composite material was placed in a container and heated to 300-1000℃ at a rate of 5-20℃ / min under a CO2 atmosphere. Then, it was calcined at 300-1000℃ under a CO2 atmosphere for 20-180 min. After that, it was cooled to room temperature at a rate of 5-20℃ / min. During the cooling process, when the temperature dropped to 300-600℃, the CO2 was stopped and air was introduced to carry out a second oxidation until the temperature dropped to room temperature, thus obtaining the Au-C / TiO2 ternary composite, i.e., the Au-C / TiO2 composite photocatalyst.

2. The preparation method according to claim 1, characterized in that, The specific operation of step (2) is as follows: the Au / Ti3C2 composite material obtained in step (1) is placed in a container and heated to 700°C at a heating rate of 10°C / min under a CO2 atmosphere. Then, it is calcined for 30 min under a CO2 atmosphere at 700°C. Then, it is cooled to room temperature at a rate of 10°C / min. During the cooling process, when the temperature drops to 300-600°C, the CO2 is stopped and air is introduced into it for a second oxidation until the temperature drops to room temperature, thus obtaining the Au-C / TiO2 composite photocatalyst.

3. The preparation method according to claim 1, characterized in that, In step (2), the rate at which CO2 is introduced is 10-70 sccm.

4. The preparation method according to claim 1, characterized in that, The Au loading in the Au-C / TiO2 composite photocatalyst is 0.1%-10%.

5. The application of the carbon layer-bridged Au-C / TiO2 composite photocatalyst obtained by the preparation method according to any one of claims 1-4 in photocatalytic hydrogen production.

6. The application according to claim 5, characterized in that, The application is as follows: the catalyst is used for photocatalytic water splitting to produce hydrogen under ultraviolet-visible light.

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