Three-dimensional metal-organic framework thin film, preparation method and application thereof

Abstract

The application discloses a three-dimensional metal organic framework film and a preparation method and application thereof. The three-dimensional metal organic framework film comprises a Cu-HHTP film and a nanowire array substrate. The Cu-HHTP film is coated on the nanowires of the nanowire array substrate. The three-dimensional metal organic framework film can not only increase the specific surface area, but also accelerate the charge and mass transmission, thereby generating better performance in many applications such as catalysis, sensing and electrochemical energy storage.

Description

TECHNICAL FIELD

[0001] The present application relates to a three-dimensional metal organic framework film and a preparation method and application thereof, and belongs to the technical field of metal organic framework films. BACKGROUND

[0002] Due to the advantages of adjustable crystal structure, high surface area and highly exposed active sites, metal organic framework (MOF) materials constructed from metal ions / clusters and organic ligands have attracted more and more attention and become an ideal choice for gas storage and separation, catalysis, sensing and other applications. However, conventional metal organic framework materials are usually crystalline powders, and the powder state hinders their further application, especially in advanced electronic devices based on thin films. Therefore, scientists have tried to explore various thin film preparation methods for preparing metal organic framework thin films. For example, layer-by-layer liquid phase epitaxy (LBL-LPE) method, Langmuir-Blodgett layer-by-layer deposition method, and pre-prepared metal organic framework nanocrystal "stamp" assembly method have been developed. At present, high-quality metal organic framework thin films (two-dimensional metal organic framework oriented thin films) with high orientation and controllable nanoscale thickness can be successfully prepared on planar substrates (such as Au film / Si wafer, quartz wafer). However, there are many applications such as electrocatalysts, sensing and electrochemical energy storage that require the growth of metal organic framework thin films on three-dimensional nanostructured substrates (such as porous substrates and nanowire arrays). The prepared multi-level nanostructured metal organic framework thin films (three-dimensional metal organic framework oriented thin films) not only increase the surface area on the substrate, but also accelerate the charge and mass transport, thereby producing better performance. However, the controllable preparation of three-dimensional metal organic framework thin films is still a great challenge. On nanostructured substrates, only a few methods of direct growth of metal organic framework thin films without orientation and rough control of film thickness have been reported. SUMMARY

[0003] According to one aspect of the present application, a three-dimensional metal organic framework (MOF) thin film is provided, which can not only increase the specific surface area, but also accelerate the charge and mass transport, thereby producing better performance in many applications such as catalysis, sensing and electrochemical energy storage.

[0004] The three-dimensional metal organic framework thin film comprises a Cu-HHTP thin film and a nanowire array substrate;

[0005] The Cu-HHTP thin film is coated on the nanowires of the nanowire array substrate.

[0006] The Cu-HHTP film is coated on the nanowires of the nanowire array substrate means that the Cu-HHTP film is grown on the surface of each nanowire of the nanowire array substrate, thereby forming a three-dimensional film structure.

[0007] The nanowire array substrate of the present application is a nanowire array with hydroxyl groups on the surface.

[0008] The Cu-HHTP film means a metal-organic framework material constructed by Cu ions and HHTP ligands.

[0009] Optionally, the thickness of the Cu-HHTP film is 6-36 nm.

[0010] Optionally, the thickness of the Cu-HHTP film is selected from any one value or a range value between any two values selected from 6, 8, 10, 12, 15, 18, 20, 30, 36 nm.

[0011] Optionally, the diameter of the nanowires in the nanowire array substrate is 50-100 nm, and the length (i.e. height) is 1-2 μm.

[0012] Optionally, the diameter of the nanowires in the nanowire array substrate is any one value or a range value between any two values selected from 50, 60, 70, 80, 90, 100 nm.

[0013] Optionally, the length of the nanowires in the nanowire array substrate is any one value or a range value between any two values selected from 1, 1.2, 1.5, 1.8, 2 μm.

[0014] Optionally, the nanowires in the nanowire array substrate are single-crystal nanowires.

[0015] Optionally, the nanowire array substrate is selected from at least one of a ZnO nanowire array and a TiO2 nanowire array.

[0016] Optionally, the TiO2 nanowire array is a rutile-phase TiO2 nanowire array with -OH groups on the surface.

[0017] Optionally, the TiO2 nanowire array is a rutile-phase TiO2 nanowire array with abundant and continuous -OH groups on the surface.

[0018] According to an aspect of the present application, there is provided a preparation method of the three-dimensional metal-organic framework film of any one of the above, which is prepared in a controllable layer-by-layer self-assembly-liquid phase epitaxy method. The method not only provides an oriented growth of the metal-organic framework structure, but also precisely controls the nanoscale thickness, which is very important for further optimizing the performance of the metal-organic framework material.

[0019] The preparation method comprises the following steps:

[0020] (S1) obtaining a nanowire array substrate;

[0021] (S2) growing a Cu-HHTP thin film on the nanowire array substrate to obtain the three-dimensional metal organic framework thin film.

[0022] Optionally, the (S2) is growing a Cu-HHTP thin film on the nanowire array substrate by a layer-by-layer self-assembly-liquid phase epitaxy method to obtain the three-dimensional metal organic framework thin film.

[0023] Optionally, the (S2) comprises:

[0024] (S2-a) soaking the nanowire array substrate in a Cu source solution I;

[0025] (S2-b) soaking the nanowire array substrate in a HHTP solution II;

[0026] (S2-c) soaking the nanowire array substrate in a Cu source solution III;

[0027] (S2-d) soaking the nanowire array substrate in a HHTP solution IV;

[0028] (S2-e) repeating (S2-c) to (S2-d) n times, n is an integer between 2 and 20.

[0029] Optionally, after the soaking I, the soaking II, the soaking III or the soaking IV, further comprising a washing step:

[0030] washing the thin film with pure ethanol to remove residual reactants.

[0031] Optionally, the (S2) comprises:

[0032] (S2-1) soaking the nanowire array substrate in a Cu source solution I;

[0033] (S2-2) soaking the nanowire array substrate in anhydrous ethanol I';

[0034] (S2-3) soaking the nanowire array substrate in a HHTP solution II;

[0035] (S2-4) soaking the nanowire array substrate in anhydrous ethanol II';

[0036] (S2-5) soaking the nanowire array substrate in a Cu source solution III;

[0037] (S2-6) soaking the nanowire array substrate in anhydrous ethanol III';

[0038] (S2-7) soaking the nanowire array substrate in HHTP solution IV;

[0039] (S2-8) soaking the nanowire array substrate in anhydrous ethanol IV';

[0040] (S2-9) repeating (S2-5) to (S2-8) n times, n is an integer between 2 and 20.

[0041] Optionally, n is any one value or a range value between any two values of 2, 3, 4, 5, 6, 8, 10, 12, 15, 18, 20.

[0042] Optionally, the Cu source in the Cu source solution comprises at least one of Cu(OAc)2, copper chloride, copper nitrate.

[0043] Optionally, the concentration of Cu source in the Cu source solution is 100-1000 nM.

[0044] Optionally, the concentration of Cu source in the Cu source solution is any one value or a range value between any two values of 100, 200, 300, 400, 500, 600, 700, 800, 900, 1000 nM.

[0045] Optionally, the concentration of HHTP in the HHTP solution is 10-100 nM.

[0046] Optionally, the concentration of HHTP in the HHTP solution is any one value or a range value between any two values of 10, 20, 30, 40, 50, 60, 70, 80, 90, 100 nM.

[0047] Optionally, the solvent in the Cu source solution and the HHTP solution independently comprises at least one of ethanol, methanol.

[0048] Optionally, the temperature of the Cu source solution and the HHTP solution is independently 40-65℃.

[0049] Optionally, the temperature of the Cu source solution and the HHTP solution is any one value or a range value between any two values of 40, 45, 50, 60, 65℃.

[0050] The temperature of the Cu source solution and the HHTP solution is the temperature when the nanowire array substrate is soaked in the Cu source solution or the HHTP solution for reaction.

[0051] Optionally, the soaking I is for 10-30 min.

[0052] The soaking II is for 20-40 min.

[0053] The soaking III is for 5-15 min.

[0054] The soaking IV is for 10-30 min.

[0055] Optionally, the soaking I is for 10-20 min.

[0056] The soaking II is for 20-30 min.

[0057] The soaking III is for 5-10 min.

[0058] The soaking IV is for 10-20 min.

[0059] Optionally, the conditions of I', II', III', IV' independently comprise:

[0060] The temperature is 40-65℃, and the time is 5-20 min.

[0061] Optionally, the temperature is any one of 40, 45, 50, 60, 65℃ or a range value between any two of them, and the time is any one of 5, 10, 15, 20 min or a range value between any two of them.

[0062] The layer-by-layer self-assembly-liquid phase epitaxy method provided in the application is advantageous to improve the crystallinity of the Cu-HHTP thin film by controlling the concentration of the Cu source solution and the HHTP solution, as well as the time and temperature of soaking (the soaking temperature is the temperature of the Cu source solution or the HHTP solution during the soaking reaction), especially the temperature and time of soaking III and soaking IV, and can adjust the nucleation and growth process of the thin film, so that the thickness growth of the thin film is more relevant to the cycle / growth period of the soaking reaction, which is advantageous to the regulation of the thickness of the thin film.

[0063] Optionally, the (S1) is to grow nanowires on a substrate by a solvothermal method to obtain the nanowire array substrate.

[0064] Optionally, the (S1) comprises:

[0065] (S1-1) spin-coating a Ti source solution I on a substrate, calcining to obtain a substrate with a TiO2 nanocrystal layer;

[0066] (S1-2) immersing the substrate with the TiO2 nanocrystal layer in a Ti source solution II to obtain the nanowire array substrate.

[0067] Optionally, the Ti source in the Ti source solution I and the Ti source solution II independently comprises at least one of Ti-containing salts.

[0068] Optionally, the Ti-containing salts are selected from at least one of tetrabutyl titanate, titanium tetrachloride, titanium trichloride, titanium sulfate, titanium hydroxide.

[0069] Optionally, the solvent in the Ti source solution I is selected from at least one of ethanol, water, methanol.

[0070] Optionally, the volume ratio of the solvent and the Ti source in the Ti source solution I is 10:1-25:1.

[0071] Optionally, the volume ratio of the solvent and the Ti source in the Ti source solution I is 20:1-25:1.

[0072] Optionally, the volume ratio of the solvent and the Ti source in the Ti source solution I is any one of 10:1, 15:1, 18:1, 20:1, 23:1, 25:1 or a range value between any two of them.

[0073] Optionally, the Ti source solution II further comprises water, saturated sodium chloride aqueous solution, concentrated hydrochloric acid,

[0074] The volume ratio of the water, the saturated sodium chloride aqueous solution, the concentrated hydrochloric acid and the Ti source is 10-200:2-20:20-60:0.1-4.

[0075] Optionally, the volume ratio of the water, the saturated sodium chloride aqueous solution, the concentrated hydrochloric acid and the Ti source is 10-80:2-20:20-60:0.1-4.

[0076] Optionally, the volume ratio of the water, the saturated sodium chloride aqueous solution, the concentrated hydrochloric acid and the Ti source is 10-20:2-10:20-50:0.1-4.

[0077] Optionally, the volume ratio of the water, the saturated sodium chloride aqueous solution, the concentrated hydrochloric acid and the Ti source is 30-80:6-18:30-60:1-3.

[0078] Optionally, the substrate is selected from any one of sapphire, glass, quartz, aluminum oxide.

[0079] Optionally, the substrate is a sapphire wafer with a crystal face of (001).

[0080] Optionally, the calcination temperature is 250-500℃ and the calcination time is 20-120 minutes.

[0081] Optionally, the calcination temperature is any one of 250, 300, 400, 450, 500℃ or a range between any two of them, and the calcination time is any one of 20, 30, 40, 60, 80, 100, 120 minutes or a range between any two of them.

[0082] Optionally, the impregnation temperature is 110-150℃, and the impregnation time is 5h or more.

[0083] Optionally, the impregnation temperature is 110-150℃, and the impregnation time is 5-15h.

[0084] Optionally, the impregnation temperature is any one of 110, 120, 130, 140, 150℃ or a range between any two of them, and the impregnation time is any one of 5, 8, 10, 15h or a range between any two of them.

[0085] According to an aspect of the present application, the three-dimensional metal organic framework film according to any one of the above or prepared by the preparation method according to any one of the above is applied in catalysis, sensing or electrochemical energy storage.

[0086] Optionally, the application in sensing is to use the three-dimensional metal organic framework film as a gas sensing material.

[0087] Optionally, the use condition of the gas sensing material includes room temperature.

[0088] Optionally, the gas sensing material is an ammonia sensing material.

[0089] The application provides a three-dimensional multi-level structure conductive metal organic framework nanofilm, and a high-quality metal organic framework thin film with high orientation and nanoscale thickness controllability is very suitable for applications such as gas separation, energy storage and conversion, optical and electrical material devices and the like. At present, high-quality metal organic framework thin films can only be successfully prepared on a planar substrate, and there are disadvantages such as low exposure area and slow mass transfer. In order to overcome these problems, the application first successfully prepares a Cu-HHTP semiconductor three-dimensional multi-level structure nanofilm on a nanowire array substrate in a controllable layer-by-layer self-assembly-liquid phase epitaxy manner. The Cu-HHTP semiconductor three-dimensional thin film has the characteristics of good crystallinity, preferred orientation and nanoscale thickness accurate controllability. The three-dimensional multi-level structure nanofilm has unique advantages in applications. For example, a chemical resistance type sensor device is prepared by taking the Cu-HHTP three-dimensional thin film as a material, and excellent room temperature chemical formula ammonia gas sensing performance is shown. Compared with a two-dimensional Cu-HHTP thin film, the lowest detection limit is optimized by 1000 times, the response to ammonia gas with a concentration of 1 ppm is increased by 250%, and the response speed is increased by 130%. Notably, the Cu-HHTP three-dimensional thin film also shows the highest sensitivity, the lowest theoretical detection line (LOD) and the fastest response speed among all reported chemical resistance type ammonia gas sensing materials at room temperature.

[0090] The application not only provides a new method for accurately controlling the preparation of metal organic framework thin films on a nanostructured substrate to prepare three-dimensional thin films and heterostructures based on metal organic frameworks, but also provides a thin film preparation method with preferred pore orientation, large exposed active surface area and good thickness control at the nanoscale, which provides a new perspective for constructing heterostructures based on metal organic frameworks. It will promote the further advanced application of metal organic framework materials such as electronic devices, catalysis and energy storage devices and the like.

[0091] The three-dimensional nanostructure metal organic framework thin film provided by the application not only can increase the specific surface area, but also can accelerate the charge and mass transfer, so as to produce better performance in many applications such as catalysis, sensing and electrochemical energy storage. At present, the controllable preparation of three-dimensional metal organic framework thin films is still a great challenge, and in the reported literature, the metal organic framework thin film on the nanostructure substrate is grown directly and is not oriented, and the thin film thickness is roughly controlled. Therefore, the controllable growth of the three-dimensional metal organic framework thin film with oriented growth and accurate control of the nanofilm thickness is a key scientific problem currently faced by the application of metal organic framework materials. In addition, the three-dimensional metal organic framework thin film still has the key problems of room temperature gas sensing application to be explored and performance improvement, so it is imperative to develop a new method for preparing a multi-level structure three-dimensional metal organic framework thin film and to realize the room temperature gas sensing application.

[0092] To solve the above technical problems, the technical scheme provided by the present application is: a three-dimensional conductive metal organic framework film with a multi-level structure is constructed by layer-by-layer growth of metal organic framework films on ZnO and TiO2 nanowire array substrates respectively through an improved layer-by-layer self-assembly-liquid phase epitaxy (LBL-LPE) method.

[0093] The specific method is to grow TiO2 single crystal nanowires (NWAs) with a diameter of 50-100 nm and a length of 1-2 μm on a sapphire substrate by a hydrothermal method. Secondly, the obtained TiO2 NWAs are respectively immersed in copper acetate (Cu(OAc)2) and 2,3,6,7,10,11-hexahydroxytriphenylbenzene (HHTP) ethanol solution to grow Cu-HHTP thin films on the nanowires layer by layer. Between each immersion process, the TiO2 NWAs are carefully cleaned with ethanol to remove unreacted reagents to control the layer-by-layer growth.

[0094] The concentration of the reagent solution, the reaction temperature (i.e. the immersion temperature) and the immersion time will affect the quality of the prepared three-dimensional Cu-HHTP thin film. Compared with the LBL-LPE method reported for growing metal organic framework two-dimensional thin films, the improved preparation strategy of the present application includes: 1) using nanowire arrays instead of flat substrates; 2) by carefully optimizing the preparation conditions, the reaction temperature is increased from room temperature to 45℃ to adjust the nucleation and growth process of the thin film and improve its crystallinity. The three-dimensional metal organic framework thin film preparation method provided by the present application includes the following steps:

[0095] (1) First, clean the sapphire wafer with a (001) crystal face in deionized water, ethanol, acetone, isopropanol, acetone and ethanol by alternating ultrasonic cleaning. Then dry under N2 flow and reserve for use.

[0096] (2) TiO2 NWAs are grown on the cleaned sapphire substrate using a solvothermal method. First, a layer of TiO2 seed layer is deposited on the substrate by spin coating an ethanol solution of tetrabutyl titanate (volume ratio of pure ethanol and tetrabutyl titanate is 10:1-25:1), and then calcining in a tube furnace at 250-500℃ for 20-120 minutes. Then, the sapphire wafer with the TiO2 seed layer is immersed upside down into 5-25 mL of tetrabutyl titanate aqueous solution (50 mL of deionized water, 10 mL of saturated sodium chloride (NaCl) aqueous solution, 60 mL of concentrated hydrochloric acid and 2 mL of tetrabutyl titanate mixed at room temperature), then take 16 mL of the solution and add it to the reactor), and place it in a Teflon-lined stainless steel autoclave. Seal the autoclave and keep it at 110-150℃ for more than 5 hours.

[0097] (3) Preparation of three-dimensional Cu-HHTP-x nm thin film on TiO2 NWAs substrate

[0098] Three-dimensional Cu-HHTP-x nm thin films were grown on TiO2 NWAs nanosubstrates using layer-by-layer self-assembly-liquid phase epitaxy method at 25-65 °C. The following reagent solutions in ethanol were used to prepare the three-dimensional Cu-HHTP-x nm thin films: Cu(OAc)2-H2O (0.1 mM) and HHTP (0.01 mM). In the first cycle, the TiO2 NWAs substrates were alternately immersed in Cu(OAc)2-H2O and HHTP solutions for 10 min and 20 min, respectively. Subsequently, the substrates were alternately immersed in Cu(OAc)2-H2O and HHTP solutions for 5 and 10 min, respectively, and each cycle was repeated. In each repeated growth cycle, the thin films were washed with pure ethanol to remove residual reactants. Finally, the thin films were naturally dried at room temperature. By controlling the number of growth cycles, three-dimensional Cu-HHTP thin films with different film thicknesses can be prepared.

[0099] Preferably, the growth temperature for growing three-dimensional Cu-HHTP-x nm thin films on TiO2 NWAs nanosubstrates is limited to 45-65 °C, which is conducive to regulating the nucleation and growth process of the thin films and improving their crystallinity.

[0100] The beneficial effects that can be produced by the present application include:

[0101] (1) The three-dimensional metal organic framework thin film provided by the present application is a high-quality three-dimensional Cu-HHTP thin film grown in a preferred orientation, has highly exposed active sites and increased active surface area, and thus has efficient mass and charge transfer.

[0102] (2) The three-dimensional metal organic framework thin film provided by the present application has a larger surface area and shorter charge and mass transfer path compared with a two-dimensional Cu-HHTP thin film.

[0103] (3) The preparation method of the three-dimensional metal organic framework thin film provided by the present application successfully prepares a Cu-HHTP semiconductor three-dimensional hierarchical structure nanometer thin film on a nanowire array substrate in a controllable layer-by-layer self-assembly-liquid phase epitaxy manner for the first time. The Cu-HHTP semiconductor three-dimensional thin film has the characteristics of good crystallinity, preferred orientation, and nanoscale thickness that is precisely controllable. The three-dimensional hierarchical structure nanometer thin film has unique advantages in applications. BRIEF DESCRIPTION OF DRAWINGS

[0104] Figure 1 is a morphology diagram of Cu-HHTP structure, TiO2-NWAs and three-dimensional Cu-HHTP thin film; wherein, Figure 1 a shows the structure of Cu-HHTP; Figure 1 b shows a cross-sectional view of the three-dimensional Cu-HHTP thin film; Figure 1c shows a 20 nm thick Cu-HHTP film encapsulating a single nanowire within a three-dimensional Cu-HHTP film; Figure 1 d shows the Cu-HHTP lattice spacing of 1.82 nm within a three-dimensional Cu-HHTP film; Figure 1 e shows the angle of Cu-HHTP preferential growth within a three-dimensional Cu-HHTP film; Figure 1 f shows a uniform and ordered encapsulation of Ti02nanowires by Cu-HHTP.

[0105] Figure 2 is a TEM image of a three-dimensional Cu-HHTP film and a plot of film thickness as a function of cycle number. Figure 2a shows the initial growth of Cu-HHTP film where island nucleation is observed rather than uniform epitaxy. Figure 2 b shows that a continuous film of approximately 6 nm thickness can be obtained at the 8thcycle. Figure 2c shows the accelerated growth of Cu-HHTP film with a linear increase in thickness to 12 nm. Figure 2 d shows the increase in thickness of Cu-HHTP film to 20 nm. Figure 2 e shows the increase in thickness of Cu-HHTP film to 36 nm. Figure 2 f shows the linear increase in thickness of Cu-HHTP film with approximately 2 nm per cycle.

[0106] Figure 3 is a schematic and optical photograph of a three-dimensional metal-organic framework film device.

[0107] Figure 4 is a PXRD plot of a three-dimensional Cu-HHTP film, Ti02-NWAs / sapphire wafer.

[0108] Figure 5 is an I-V curve of a 20 nm three-dimensional Cu-HHTP film (corresponding to the grey curve, i.e. the curve that rises steeply) and a Ti02-NWAs substrate (corresponding to the black curve, i.e. the flat curve).

[0109] Figure 6 is a plot of the recovery response of a 20 nm three-dimensional Cu-HHTP film to ammonia gas.

[0110] Figure 7 is the thickness dependent response of a three-dimensional Cu-HHTP film to 10 ppm of ammonia gas.

[0111] Figure 8 is a log-log plot of the response value versus concentration for a 20 nm three-dimensional Cu-HHTP film.

[0112] Figure 9is a comparison of the response, detection limit, and response time of Cu-HHTP powder, two-dimensional thin film, and three-dimensional thin film to ammonia (1 ppm) at room temperature.

[0113] Figure 10 is a response-recovery curve of a 20 nm three-dimensional Cu-HHTP thin film to 5 ppb ammonia.

[0114] Figure 11 is a response-recovery curve of a 20 nm three-dimensional Cu-HHTP thin film to 100 ppm ammonia.

[0115] Figure 12 is a response of a 20 nm three-dimensional Cu-HHTP thin film to 100 ppm ammonia and interfering gases at room temperature.

[0116] Figure 13 is a response-recovery curve of a TiO2 nanowire array to 100 ppm ammonia. DETAILED DESCRIPTION

[0117] The present application is described in detail below with reference to Examples, but the present application is not limited to these Examples.

[0118] Unless otherwise specified, the raw materials in the Examples of the present application are purchased through commercial channels.

[0119] In the Examples, powder X-ray diffraction (PXRD) analysis is recorded using a Rigaku Smartlab X-ray diffractometer equipped with a one-dimensional array detector using Cu Kα radiation at a step size of 0.02°. Scanning electron microscopy (SEM) uses a ZEISS-300 instrument from Carl Zeiss, with a voltage of 5.0 kV. Field emission transmission electron microscopy (FETEM) uses a F200X G2 instrument from TALOS, with a condition of 200 kV and 200 pm electron beam. X-ray photoelectron spectroscopy (XPS) analysis uses an ESCALAB 250Xi XPS instrument from Thermo Fisher. I-V curve analysis uses a Keithley 4200 semiconductor characterization instrument from Keithley, with a condition of a two-probe DC method measurement.

[0120] The chemical resistance type gas sensor was measured in our self-made complex environment gas sensor testing system. The corresponding testing method is a dynamic testing method (a certain concentration of gas flows through the device surface at a constant flow rate). The specific testing method is as follows: At room temperature, the sensor is placed in an opaque, sealed quartz tube. The gas sensing test is performed by monitoring the real-time change of the DC current under different concentrations of the target gas analyte (the duration of the analytical gas flow is set to 3 minutes). Dry air is used as the carrier gas for the analytical gas, and the device current under dry air is used as the baseline current. When the gas analyte is introduced, the device current will increase or decrease. When dry air is introduced again, the current will return to the baseline current. The DC sensing circuit is supplied with a constant 5V bias voltage through a Keithley 2602B (USA) source meter. The flow rates of the carrier gas and analytical gas can be controlled by a mass flow controller (CS-200C, Beijing Qixing Optoelectronic Equipment Manufacturing Co., Ltd., China). It can also control the flow rate of a certain amount of gas entering the gas distribution tank to regulate the concentration of the analytical gas. All tests are conducted at room temperature and under light-free conditions.

[0121] Specific methods for preparing analytical gases of different concentrations: Three mass flow meters, MFC1, MFC2, and MFC3, are used to control the carrier gas (dry air for blank measurements), the mixed gas (dry air for mixing with the analytical gas), and the analytical gas (a standard gas of a certain concentration), respectively. A mass flow controller is used to control the analytical gas and the mixed gas at an appropriate ratio, and they are mixed in a gas mixing tank to produce a target analytical gas with a specific concentration. The formula for calculating the concentration is as follows:

[0122] X (ppm) = Analytical gas flow rate / (Mixed gas flow rate + Analytical gas flow rate) × Standard gas concentration (ppm)

[0123] After a 5-minute equilibration period, the analytical gas is introduced into a quartz tube containing an internal chemielectric sensor.

[0124] The coefficient of variation (CV) is calculated using the following formula: CV = RSD / Raverage × 100%. RSD and Raverage are the standard deviation (SD) and mean of the continuous cyclic response, respectively.

[0125] All reagents used were commercially available and required no further purification. Copper acetate (Cu(OAc)₂·H₂O) was purchased from Aladdin Shanghai, China; 2,3,6,7,10,11-hexahydrotriphenylene(HHTP) ligand was purchased from TCI (Shanghai), China; tetrabutyl titanate was purchased from Shanghai Titan; concentrated hydrochloric acid (36%–38% by weight) was purchased from China National Pharmaceutical Group Corporation; (001) sapphire substrate (8×10×1mm) 3) from Jinan Optoelectronic Co. Ltd, China.

[0126] As an embodiment, the three-dimensional Cu-HHTP thin film provided by the present application is a high-quality thin film grown with preferred orientation, with highly exposed active sites and increased active surface area, thus having high efficiency of mass and charge transfer.

[0127] Two-dimensional Cu-HHTP thin films were grown epitaxially on the surface of quartz plates treated with piranha solution. However, in the present patent, the number of -OH groups unevenly distributed on the surface of TiO2 nanowire is less compared to the quartz plate treated with piranha solution. The surface of the rutile phase TiO2 nanowire array is rich and continuous in -OH groups, which can promote the layer-by-layer growth of Cu-HHTP. A continuous thin film with a thickness of about 6 nm can be obtained in the 8th cycle. After that, the growth of Cu-HHTP thin film is accelerated, and the thickness increases linearly, about 2 nm per cycle. Cu-HHTP thin films with thicknesses of 6, 12, 20 and 36 nm can be obtained by 8, 12, 16 and 24 growth cycles.

[0128] Compared with two-dimensional Cu-HHTP thin films, three-dimensional Cu-HHTP thin films show larger surface area and shorter charge and mass transfer path. The I-V curve of three-dimensional Cu-HHTP-20 nm thin film and TiO2 substrate is shown in Figure 5 , in which the resistance of three-dimensional Cu-HHTP thin film is 10 times lower than that of TiO2-NWAs substrate.

[0129] Three-dimensional metal-organic framework thin films show better performance than two-dimensional metal-organic framework thin films. As a chemical resistance sensing material, Cu-HHTP shows a highly selective response to ammonia molecules. Compared with two-dimensional Cu-HHTP thin films, the minimum detection limit of room temperature chemical resistance gas sensor based on three-dimensional Cu-HHTP thin films is optimized by 1000 times, the response to 1 ppm ammonia is increased by 250%, and the response speed is increased by 130%. Notably, Cu-HHTP three-dimensional thin film also shows the highest sensitivity, the lowest LOD and the fastest response speed among all reported chemical resistance ammonia sensing materials at room temperature.

[0130] Example 1

[0131] 1. Preparation of three-dimensional metal-organic framework thin film

[0132] 1.1. Preparation of TiO2 nanowire array:

[0133] (001)The (001) oriented sapphire substrate was cleaned by ultrasonic in deionized water, pure ethanol, acetone, isopropanol, acetone, ethanol in turn, and then dried under N2flow before use. To promote the growth of Ti02 NWAs, a Ti02 seed layer was first deposited on the substrate by spin-coating a solution of tetrabutyl titanate in ethanol (volume ratio of pure ethanol and tetrabutyl titanate is 18:1), followed by calcination in a tube furnace at 500 °C for 30 min. Then, the sapphire wafer with the Ti02 seed layer was immersed in 16 mL of tetrabutyl titanate solution (50 mL of deionized water, 10 mL of saturated aqueous sodium chloride (NaCl content is 0.359 g / ml), 60 mL of concentrated hydrochloric acid and 2 mL of tetrabutyl titanate were mixed at room temperature, then 16 mL of solution was taken and added to the reactor) up and down, and placed in a Teflon-lined stainless steel autoclave. The autoclave was sealed and kept at 150 °C for 5 hours.

[0134] 1.2. Preparation of three-dimensional Cu-HHTP-x nm thin films on Ti02 NWA substrates

[0135] Three-dimensional Cu-HHTP-x nm thin films were grown on Ti02 NWA nanosubstrates using layer-by-layer self-assembly-liquid phase epitaxy method at 45 °C (i.e., the temperature of Cu(OAc)2-H20 solution and HHTP solution was controlled at 45 °C). The following reagents were used to prepare three-dimensional Cu-HHTP-x nm thin films: Cu(OAc)2-H20 (0.1 mM) and HHTP (0.01 mM) in ethanol solution. In the first cycle, the Ti02 NWA substrate was alternately immersed in Cu(OAc)2-H20 solution and HHTP solution for 10 min and 20 min, respectively. Subsequently, it was alternately immersed in Cu(OAc)2-H20 solution and HHTP solution for 5 and 10 min, respectively, and each cycle was repeated. In each repeated growth cycle, the thin film was washed with pure ethanol (condition: 45 °C, 10 min) to remove residual reactants. Finally, the thin film was naturally dried at room temperature. By controlling the number of growth cycles, three-dimensional Cu-HHTP thin films with different film thicknesses can be prepared. In the 8thcycle, a continuous thin film with a thickness of about 6 nm can be obtained. After that, the growth of Cu-HHTP thin film accelerates, and the thickness increases linearly by about 2 nm per cycle. Cu-HHTP thin films with thicknesses of 6, 12, 20 and 36 nm can be obtained by 8, 12, 16 and 24 growth cycles, respectively.

[0136] One cycle / growth cycle refers to one-time immersion in Cu(OAc)2-H20 solution, ethanol solution, and HHTP solution, ethanol solution, respectively, in turn.

[0137] The structure of Cu-HHTP is shown in Figure 1 a. Figure 1b-1d shows the cross-sectional and top-view scanning electron microscope images of the prepared TiO2-NWAs and the 3D Cu-HHTP thin film with a thickness of 20 nm.

[0138] From Figure 1 b, it can be seen that the TiO2 nanowires are intertwined in the middle, and the morphology of the TiO2-NWAs is basically unchanged after the Cu-HHTP thin film is coated on the outside. The diameter of the nanowires in the TiO2-NWAs is 100 nm, and the length is 2 μm.

[0139] From Figure 1 b and Figure 1 c, it can be seen that the Cu-HHTP thin film on the interleaved TiO2 nanowires forms a continuous core-shell 3D nanostructure.

[0140] From Figure 1 d, it can be seen that by comparison, a clear interface between TiO2 and Cu-HHTP can be observed. The lattice fringe spacing of the metal organic framework is about 1.8 nm, which is consistent with the distance between the (100) planes in the Cu-HHTP crystal structure.

[0141] Statistical analysis shows that the angle between the lattice fringes on the surface of Cu-HHTP and TiO2 ranges from 60° to 70°( Figure 1 e), and the element mapping of the high-angle annular dark-field (HAADF) energy dispersive X-ray spectroscopy (EDS) image further confirms this TiO2 / Cu-HHTP core-shell structure( Figure 1 f), indicating the oriented growth of the single-crystal domains of Cu-HHTP. It is worth noting that this oriented growth is crucial for exposing the pores and active sites in Cu-HHTP.

[0142] Figure 2 a shows that in the early stage of Cu-HHTP thin film growth, an island-type nucleation process is observed instead of uniform epitaxy. Figure 2 b shows that a continuous thin film with a thickness of about 6 nm can be obtained in the 8th cycle. Figure 2 c shows that the growth of the Cu-HHTP thin film is accelerated, and the thickness increases linearly to 12 nm. Figure 2d shows that the thickness of the Cu-HHTP thin film increases to 20 nm. Figure 2 e shows that the thickness of the Cu-HHTP thin film increases to 36 nm. Figure 2 f shows that the thickness of the Cu-HHTP thin film increases linearly, about 2 nm per cycle. Typical 6, 12, 20 and 36 nm Cu-HHTP thin films are obtained by 8, 12, 16 and 24 growth cycles.

[0143] 2、Electrical properties

[0144] Three-dimensional Cu-HHTP thin films exhibit a larger surface area and shorter charge and mass transport paths. Schematic diagrams and optical photographs of three-dimensional Cu-HHTP thin film devices are shown below. Figure 3 As shown. Figure 4 Powder X-ray diffraction (PXRD) confirmed that the three-dimensional film on TiO2-NWAs was Cu-HHTP and possessed good crystallinity. To further verify the continuity of the Cu-HHTP film on TiO2 NWAs, a pair of parallel electrodes were fabricated at both ends of the Cu-HHTP-20nm film to ensure that Cu-HHTP-20nm was the dominant conductive path. The IV curves of the three-dimensional Cu-HHTP-20nm film and the TiO2 substrate (i.e., the TiO2 nanowires prepared in Comparative Example 1) are shown in Figure 1. Figure 5 As shown, the resistance of the three-dimensional Cu-HHTP film is 10 times lower than that of TiO2-NWAs.

[0145] 3. Gas sensing performance

[0146] The response and recovery curves of three-dimensional Cu-HHTP-x nm (x = 0, 6, 12, 20, 36 nm) films to ammonia were measured. The results show that Cu-HHTP-x nm exhibits a concentration-dependent response to ammonia, with Cu-HHTP-20 nm showing the best performance. A significant increase in resistance was observed when Cu-HHTP-20 nm was exposed to ammonia. The response increased from 42% to 161% as the ammonia concentration increased from 1 ppm to 100 ppm. At 10 ppm, its coefficient of variation (CV) over five consecutive cycles was only 2.13%, indicating excellent reproducibility. Figure 6 Similar to two-dimensional Cu-HHTP films, three-dimensional Cu-HHTP films exhibit significant thickness-dependent sensitivity. Figure 7 As the thickness increases, the response value of the three-dimensional Cu-HHTP film first increases, reaches a maximum at 20 nm, and then decreases.

[0147] Response concentration of Cu-HHTP-20nm Figure 8 As shown, it exhibits good linearity in the 1-10 ppm range. The theoretical limit of detection (LOD) was calculated from the fitted linear equation to be approximately 87 ppt by setting the response to 10%. Figure 9 It can be seen that, compared with Cu-HHTP powder (i.e., the Cu-HHTP powder prepared in Comparative Example 2), the corresponding... Figure 9 The "powder" in the text) and the two-dimensional 20nm thin film (i.e., the two-dimensional Cu-HHTP thin film prepared in Comparative Example 3, corresponding to Figure 9 Compared to the "two-dimensional thin film" in the text, Cu-HHTP-20nm (corresponding to...) Figure 9The LOD of Cu-HHTP-20nm thin film was reduced by 5000 times and 1000 times, and the response value to 1 ppm ammonia was increased by 16100% and 250%, respectively. The response time of three-dimensional Cu-HHTP-20nm was reduced to 35 seconds, which may be attributed to the unique three-dimensional structure having better gas enrichment capacity. The three-dimensional Cu-HHTP-20nm thin film can detect ammonia with a concentration as low as 5 ppb, and the response curve shows good signal-to-noise ratio and repeatability Figure 10 ). Notably, Cu-HHTP-20nm exhibits one of the lowest LODs among all reported room-temperature ammonia chemical resistance sensing materials (Table 1).

[0148] The response and recovery time of Cu-HHTP-20nm to 100 ppm ammonia were 35 seconds and 15 minutes, respectively Figure 11 ). Compared with the reported Cu-HHTP powder (Tres. = 240 s) and two-dimensional thin film of 20 nm thickness (Tres. = 81.6 s), the response speed of the three-dimensional film was increased by 600% and 130%, respectively (Table 2). To our knowledge, Cu-HHTP-20nm is one of the fastest among various reported room-temperature ammonia chemical resistance sensors (Table 1). At the same time, the selectivity of Cu-HHTP-20nm to interfering gases is greater than 3 Figure 12 ), indicating its excellent selectivity.

[0149] Comparative Example 1

[0150] Preparation of TiO2 nanowires

[0151] The (001)-oriented sapphire substrate was cleaned by ultrasonic alternately in deionized water, pure ethanol, acetone, isopropanol, acetone, ethanol, and then dried under N2flow for standby. In order to promote the growth of TiO2 NWAs, a layer of TiO2 seed layer was first deposited on the substrate by spin-coating tetrabutyl titanate ethanol solution (volume ratio of pure ethanol and tetrabutyl titanate is 18:1), and then calcined in a tube furnace, keeping at 500°C for 30 minutes. Then, the sapphire sheet with TiO2 seed layer was immersed in 16 mL tetrabutyl titanate solution (50 mL deionized water, 10 mL saturated sodium chloride aqueous solution (NaCl content is 0.359 g / ml), 60 mL concentrated hydrochloric acid and 2 mL tetrabutyl titanate are mixed at room temperature, then 16 mL solution is taken and added to the reactor) up and down, and placed in a Teflon-lined stainless steel autoclave. The autoclave was sealed and kept at 150°C for 5 hours.

[0152] The sensing response of TiO2-NWAs substrate to ammonia at room temperature was negligible Figure 13 ).

[0153] Comparative Example 2

[0154] Cu-HHTP powder (Reference: M. K. Smith; K. E. Jensen; P. A. Pivak; K. A. Mirica Chemistry of Materials 2016, 28(15), 5264-5268)

[0155] Comparative Example 3

[0156] Two-dimensional Cu-HHTP film (Reference: M. S. Yao; X. J. Lv; Z. H. Fu; W. H. Li; W. H. Deng; G. D. Wu; G. Xu Angew. Chem. Int. Ed. 2017, 56(52), 16510-16514.)

[0157] Table 1. Comparison of room temperature chemical resistance type ammonia sensing materials

[0158]

[0159]

[0160] The corresponding references referred to in Table 1 are as follows:

[0161] [1] M. S. Yao; X. J. Lv; Z. H. Fu; W. H. Li; W. H. Deng; G. D. Wu; G. Xu Angew. Chem. Int. Ed. 2017, 56(52), 16510-16514.

[0162] [2] M. S. Yao; J. J. Zheng; A. Q. Wu; G. Xu; S. S. Nagarkar; G. Zhang; M. Tsujimoto; S. Sakaki; S. Horike; K. Otake; S. Kitagawa Angew. Chem. Int. Ed. 2020, 59(1), 172-176.

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[0170]

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[14] N. A. Travlou; M. Seredych; E. Rodríguez-Castellón; T. J. Bandosz J. Mater. Chem. A 2015, 3(7), 3821-3831.

[0175]

[15] L. T. Duy; T. Q. Trung; V. Q. Dang; B.-U. Hwang; S. Siddiqui; I.-Y. Son; S. K. Yoon; D. J. Chung; N.-E. Lee Adv. Funct. Mater. 2016, 26(24), 4329-4338.

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[16] R. Kumar; R. Kumar; N. Kushwaha; J. Mittal IEEE Sens. J. 2016, 16(12), 4691-4695.

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[17] Z. Sun; D. Huang; Z. Yang; X. Li; N. Hu; C. Yang; H. Wei; G. Yin; D. He; Y. Zhang IEEE Elecre. Device L 2015, 36(12), 1376-1379.

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[20] G. Jeevitha; R. Abhinayaa; D. Mangalaraj; N. Ponpandian; P. Meena; V. Mounasamy; S. Madanagurusamy Nanoscale Adv. 2019, 1 (5), 1799-1811.

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[0188] Table 2. Comparison of Cu-HHTP powder, two-dimensional film, and three-dimensional film in hydrogen sensing performance at room temperature

[0189]

[0190] The above is only a few embodiments of the present application, and does not limit the present application in any form. Although the preferred embodiments are disclosed above, they are not intended to limit the present application. Any skilled person in the art can make some changes or modifications to the above disclosed technical content without departing from the scope of the technical solution of the present application, which are equivalent to equivalent embodiments and belong to the scope of the technical solution.

Claims

1. A three-dimensional metal-organic framework thin film, characterized in that, The three-dimensional metal organic framework film comprises a Cu-HHTP film and a nanowire array substrate; The Cu-HHTP film is coated on the nanowires of the nanowire array substrate; HHTP refers to 2, 3, 6, 7, 10, 11-hexahydroxytriphenylbenzene; The nanowires in the nanowire array substrate are single-crystal nanowires; The nanowire array substrate is selected from at least one of ZnO nanowire array and TiO2 nanowire array; The TiO2 nanowire array is a rutile phase TiO2 nanowire array, and the surface has -OH groups.

2. The three-dimensional metal-organic framework thin film of claim 1, wherein, The thickness of the Cu-HHTP film is 6 nm to 36 nm.

3. The three-dimensional metal-organic framework thin film of claim 1, wherein, The diameter of the nanowires in the nanowire array substrate is 50 to 100 nm, and the height is 1 μm to 3 μm.

4. The method of claim 1-3, wherein the method is characterized by, The preparation method comprises the following steps: (S1) obtaining a nanowire array substrate; (S2) growing a Cu-HHTP film on the nanowire array substrate to obtain the three-dimensional metal organic framework film.

5. The preparation method according to claim 4, characterized in that, The (S2) is growing a Cu-HHTP film on the nanowire array substrate by a layer-by-layer self-assembly-liquid phase epitaxy method to obtain the three-dimensional metal organic framework film.

6. The preparation method according to claim 4, characterized in that, The (S2) comprises: (S2-1) soaking the nanowire array substrate in a Cu source solution for I; (S2-2) soaking the nanowire array substrate in anhydrous ethanol for I'; (S2-3) soaking the nanowire array substrate in a HHTP solution for II; (S2-4) soaking the nanowire array substrate in anhydrous ethanol for II'; (S2-5) soaking the nanowire array substrate in a Cu source solution for III; (S2-6) soaking the nanowire array substrate in anhydrous ethanol for III'; (S2-7) soaking the nanowire array substrate in a HHTP solution for IV; (S2-8) soaking the nanowire array substrate in anhydrous ethanol for IV'; (S2-9) repeating (S2-5) to (S2-8) n times, and n is an integer between 2 and 20.

7. The production method according to claim 6, wherein The Cu source in the Cu source solution includes Cu(OAc) 2、 at least one of copper chloride, copper nitrate.

8. The preparation method according to claim 6, characterized in that, The concentration of the Cu source in the Cu source solution is 100 to 1000 nM.

9. The preparation method according to claim 6, characterized in that, The concentration of HHTP in the HHTP solution is 10 to 100 nM.

10. The method of claim 6, wherein, The solvents in the Cu source solution and the HHTP solution independently comprise at least one of ethanol and methanol.

11. The method of claim 6, wherein, The temperature of the Cu source solution and the HHTP solution is independently 40 to 65 °C.

12. The method of claim 6, wherein, The soaking I is for 10 to 30 min; The soaking II is for 20 to 40 min; The soaking III is for 5 to 15 min; The soaking IV is for 10 to 30 min.

13. The preparation method according to claim 6, characterized in that, After the soaking I, the soaking II, the soaking III or the soaking IV, a washing step is further included: Washing the film with pure ethanol to remove residual reactants.

14. Use of the three-dimensional metal organic framework film according to any one of claims 1 to 3 or prepared according to the preparation method of any one of claims 4 to 13 in sensing.

15. Use according to claim 14, characterized in that, The application in sensing is to use the three-dimensional metal organic framework film as a gas sensing material.

16. Use according to claim 15, characterized in that, The use condition of the gas sensing material includes room temperature.

17. The use according to claim 15, characterized in that, The gas sensing material is an ammonia gas sensing material.

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