CN105845195B - A kind of transition metal oxide/graphene composite film and preparation method thereof - Google Patents

Abstract

The invention relates to a transition metal oxide/graphene composite film and a preparation method thereof. After dissolving and diluting transition metal alkoxide with alcohol, it is spin-coated on the surface of graphene to dope it, and after heat treatment, transition metal oxide/graphene is obtained. A graphene composite film, the transition metal oxide/graphene composite film includes a graphene layer and a transition metal oxide layer deposited on the graphene layer. The invention has low cost and simple and convenient operation. After doping treatment, the surface resistance of graphene decreased significantly and remained stable for a long time. After the doping treatment, the square resistance of graphene can be reduced by 40-50%, and it shows very excellent stability and light transmittance. The invention has great significance for expanding the application of the graphene transparent conductive film. The graphene film prepared by the invention is expected to be widely used in the fields of solar cells, touch screens, electric heating films and the like.

Description

A kind of transition metal oxide/graphene composite film and preparation method thereof

technical field

The invention relates to a preparation method of a transition metal oxide/graphene composite thin film, belonging to the technical field of thin film materials.

Background technique

Transparent conductive films with excellent conductivity and light transmission are widely used in industrial production. So far, the material used for transparent conductive films has been ITO, but ITO has many disadvantages, such as ITO is not stable in acid-base environment, low transmittance in the near-infrared region, lack of elasticity and lack of In resources. leading to its rising price. Graphene has extremely high strength, chemical stability, good flexibility and electrical conductivity, making graphene an excellent material to replace ITO to prepare transparent conductive films.

However, there are still a large number of defects in the graphene prepared by CVD, which leads to the unsatisfactory conductivity of graphene, that is, the high square resistance (100-500Ω/□) and the transmittance of 85%, which is ITO More than ten times that of the transparent conductive film, plus the zero band gap due to the nature of graphene itself, it cannot be used as a semiconductor. In addition, the relatively low work function of graphene (4.2-4.6eV) makes it less competitive as an electrode. These are all obstacles to the widespread application of graphene in the field of electronic devices. For example, if graphene is to be used as a cathode material of a solar cell, the work function of the electrode is preferably around 5.0eV, so graphene needs to adjust the conductivity and work function to meet the needs of the solar cell electrode.

Increasing the carrier concentration of graphene by doping is one of the most effective ways to increase its electrical conductivity. The doping of graphene is mainly divided into chemical (substitution) doping and surface physical doping. Different from chemical doping, surface physical doping will not destroy the six-membered ring structure of graphene, but only achieves carrier injection into graphene by virtue of the difference in work function between the dopant and graphene. Therefore, surface physical doping can significantly increase the number of carriers without reducing the carrier mobility, showing great advantages. The reported graphene surface doping substances mainly include organic molecules (TFSA, DDQ, F ₄ -TCNQ, etc.), inorganic salts/acids (HNO ₃ , AuCl ₃ , SOCl ₂ , etc.), metal particles, and oxidizing gases. etc. (O ₂ , NO ₂ ). However, the doping effects of inorganic acids, organic molecules, and gas dopants are extremely unstable. For example, the resistance of graphene electrodes doped with _HNO3 increases by 90% after being placed in air for 480 hours. However, the cost of metal particles and AuCl ₃ is too high, and it is difficult to apply them on a large scale.

Contents of the invention

Aiming at the shortcomings of the existing graphene doping methods, such as high cost and unstable resistance after doping, the present invention aims to provide a transition metal oxide/graphene composite thin film and a simple, convenient, and low-cost graphene-doped The method can significantly improve the conductivity of graphene and ensure that the resistance remains stable for a long time, thereby solving the bottleneck problem of graphene application.

In order to solve the above problems, the present invention provides a transition metal oxide/graphene composite thin film, which dissolves and dilutes the transition metal alkoxide with alcohol and spin-coats it on the surface of graphene to dope it. After heat treatment, a transition metal oxide film is obtained. Object/graphene composite film, the transition metal oxide/graphene composite film includes a graphene layer and a transition metal oxide layer deposited on the graphene layer.

In the invention, the transition metal alkoxide is dissolved and diluted with alcohol, spin-coated on the graphene surface, and finally heat-treated to obtain a transition metal oxide/graphene composite film. The present invention completes the doping of transition metal oxides on graphene during the film preparation process, which belongs to surface physical doping. When the surface of graphene is spin-coated with transition metal oxide, since the work function of the oxide is higher than that of graphene, the energy band bending will occur at the interface between the two, so that electrons are injected from graphene into the metal oxide, that is, graphene undergoes p type doping. Unlike substitutional doping, the doping of graphene with transition metal oxides will not destroy the six-membered ring structure of graphene, so the number of graphene carriers can be significantly increased without reducing the carrier mobility. In turn, the square resistance of graphene is significantly reduced, and it shows great advantages in improving the conductivity of graphene. The doping of transition metal oxides can significantly improve the work function of graphene and reduce the square resistance of graphene while ensuring a certain light transmittance. The work function of doped graphene can be increased by more than 0.3eV at most, and the square resistance can be reduced by up to 50 %above.

Preferably, the composition of the transition metal oxide layer is at least one of metal oxides of Mo, V, W and Ni, preferably metal V oxide.

Preferably, the thickness of the transition metal oxide layer is 1 nm to 20 nm, preferably 10 nm.

The present invention also provides a preparation method of transition metal oxide/graphene composite film, comprising:

(1) transition metal alkoxide solution is spin-coated onto the graphene surface;

(2) Place the spin-coated graphene obtained in (1) in an air atmosphere at 25°C to 800°C, preferably 100 to 500°C, and calcinate for 0.5 to 24 hours, preferably 1 to 5 hours, to obtain a transition metal oxide / graphene composite film.

Preferably, the transition metal alkoxide is isopropoxide, preferably at least one of molybdenum isopropoxide, tungsten isopropoxide, vanadium isopropoxide and nickel isopropoxide.

Preferably, the solvent of the transition metal alkoxide solution is at least one of ethanol, n-propanol, isopropanol, glycerol, n-butanol and ethylene glycol.

Preferably, the concentration of the alkoxide solution is greater than 0.001 mol/L and below 0.1 mol/L, preferably between 0.001 mol/L and 0.016 mol/L.

Preferably, the spin coating speed of the spin coating is between 0 and 5000 rpm, preferably between 1000 and 5000 rpm.

Preferably, the calcination atmosphere is air atmosphere.

The invention adopts metal alkoxide as raw material, forms film by spin coating method, heats treatment in air, has low cost and is easy to operate. After doping treatment, the surface resistance of graphene decreased significantly and remained stable for a long time. After the doping treatment, the square resistance of graphene can be reduced by 40-50%, and it shows very excellent stability and light transmittance. The invention has great significance for expanding the application of the graphene transparent conductive film. The graphene film prepared by the invention is expected to be widely used in the fields of solar cells, touch screens, electric heating films and the like.

Description of drawings

Fig. 1 is the XRD diffraction spectrum of transition metal oxide/graphene composite film in embodiment 1;

Fig. 2 is the SEM photo of transition metal oxide/graphene composite film in embodiment 1;

Fig. 3 is the Raman spectrum of transition metal oxide/graphene composite film in embodiment 1;

Fig. 4 is the change diagram of square resistance with time of transition metal oxide/graphene composite thin film in embodiment 1;

Fig. 5 is the light transmittance-wavelength curve of transition metal oxide/graphene composite film at different calcination temperatures in embodiment 1;

Fig. 6 is the change trend diagram of the square resistance of the transition metal oxide/graphene composite film obtained by heat treatment under different atmospheres;

Fig. 7 is the percentage decrease of the square resistance of the composite film obtained by doping with different substances in Comparative Example 1 as a function of the heat treatment temperature variation curve (the negative sign of the ordinate indicates that the square resistance increases);

Fig. 8 is a graph showing the percentage decrease of square resistance of the composite film obtained in Comparative Example 2 as a function of the concentration of the vanadium isopropoxide solution.

detailed description

The present invention will be further described below in conjunction with the accompanying drawings and embodiments. It should be understood that the following embodiments are only used to illustrate the present invention, not to limit the present invention.

The invention uses the alcohol solution of transition metal alkoxide as raw material, deposits film on the surface of graphene, heats in the air, and obtains transition metal oxide/graphene composite film. In the present invention, transition metal doping is completed in the film preparation process, wherein the composition of the transition metal oxide layer is at least one of metal Mo, V, W and Ni oxides, preferably metal V oxide. The metal V oxide has the best effect in reducing graphene resistance, can reduce graphene square resistance by more than 50%, and can increase graphene work function by more than 0.3eV.

The following examples illustrate the preparation method of the transition metal oxide/graphene composite film provided by the present invention.

Spin-coat the transition metal alkoxide solution onto the graphene surface. The deposition method of the alkoxide on the graphene surface of the present invention is a spin coating method. Because the method is easy to operate and consumes less energy.

The aforementioned transition metal alkoxide may be but not limited to isopropoxide, preferably at least one of molybdenum isopropoxide, tungsten isopropoxide, vanadium isopropoxide and nickel isopropoxide. The present invention selects transition metal isopropoxide because of its low cost and easy availability.

The above-mentioned solvent for dissolving isopropoxide may generally be alcohol, wherein alcohol includes but not limited to at least one of ethanol, propanol, isopropanol, glycerol, n-butanol, preferably isopropanol. Because isopropanol is cheap and has good wettability with graphene, the film-forming effect is better.

The concentration of the transition metal alkoxide solution is greater than 0.001 mol/L and less than 0.1 mol/L, preferably between 0.001 mol/L and 0.016 mol/L. The higher the concentration, the thicker the film will affect the transmittance; the lower the concentration, the doping effect will not be obvious.

The spin coating speed of the above spin coating is between 0 to 5000 rpm, preferably between 1000 to 5000 rpm. If the spin coating speed is too high or too low, the film thickness is not ideal.

The transition metal oxide/graphene composite thin film is obtained by heat treatment after the graphene surface is spin-coated to form a film.

The temperature of the heat treatment is between 25°C and 800°C, preferably between 100°C and 500°C. Due to the heat treatment process, the oxide film will continue to undergo chemical or physical changes. If the temperature is too low, the reaction has not yet progressed, and the doping effect is not significantly improved, or even decreased. If the temperature is too high, the graphene film may be destroyed, resulting in poor conductivity of doped graphene. In conclusion, even with heat treatment at 25-100°C, the electrical resistance of doped graphene films can be significantly reduced compared with undoped graphene, and thus the cost of this method is lower.

The atmosphere of the heat treatment mentioned above may be air. Because the oxygen in the air will increase the work function of the metal oxide, inject more carriers into the graphene, and consolidate the doping effect.

The above-mentioned heat treatment time is between 0.5 hour and 24 hours, preferably between 1 hour and 5 hours. If the calcination time is too short, the doping degree of graphene will be affected, and if the calcination time is too long, energy will be wasted.

The thickness of the transition metal oxide layer in the transition metal oxide/graphene composite film prepared by the present invention is about 10 nm (see FIG. 2 ). The light transmittance of the composite film is about 85-92% (550nm) (see Figure 5), and the square resistance is about 100-300Ω/sq (see Figure 4).

Compared with the non-doped graphene film, the doped graphene film prepared by the present invention has the work function of the graphene increased by about 0.01-2.0eV, and the square resistance is reduced by 40%-50%. After being placed in the air for 700 hours, the square resistance only increases by 20% to 60%.

Examples are given below to describe the present invention in detail. It should also be understood that the following examples are only used to further illustrate the present invention, and should not be construed as limiting the protection scope of the present invention. Some non-essential improvements and adjustments made by those skilled in the art according to the above contents of the present invention all belong to the present invention scope of protection. The specific process parameters and the like in the following examples are only examples of suitable ranges, that is, those skilled in the art can make a selection within a suitable range through the description herein, and are not limited to the specific values exemplified below.

Example 1

Dilute 0.01ml of vanadium isopropoxide (purity 99%, Sigma-Aldrich) with 1ml of isopropanol, drop it on the surface of the undoped graphene transferred to the quartz substrate, spin-coat at 5000rpm for 1 minute. The above-mentioned graphene sheets were placed in a tube furnace, and the temperature was raised to 300° C. at a rate of 20° C./minute, and kept at a temperature of 30 minutes to obtain a VO _x doped graphene transparent conductive film. Accompanying drawing 1 is its XRD diffraction pattern. It can be seen that at this temperature, VO _x is in a certain crystalline state, and its diffraction peaks at 20.26° and 41.26° correspond to crystalline V ₂ O ₅ (JCPDF#41-1426), while the small peak at 24.45° The diffraction peaks correspond to crystalline V ₃ O ₇ (JCPDF#27-0940). The SEM photo of the surface of the film is shown in Figure 2a. It can be seen that the VO _x grains are in the shape of short rods with a length of less than 100 nm. The cross-sectional SEM photo (Fig. 2b) shows that the thickness of the VOx layer _is about 10nm. The Raman spectra of graphene films before and after doping are shown in Figure 3. It can be seen that compared with undoped graphene, the graphene G peak after doping has a large red shift, which indicates that graphene has p-type doping. The graphene square resistance change and stability test results before and after doping are shown in Figure 4. The square resistance of the graphene film before doping is 400/sq, and the square resistance of the graphene film after doping is 176/sq. to 56%. It can also be seen from the figure that the stability of the sample is excellent, and the square resistance only increases by 59% after being placed in the air for 700 hours. Accompanying drawing 5 is the light transmittance curve of undoped and doped graphene, it can be seen that doped graphene has excellent light transmittance in the near-infrared and visible light regions, and the light transmittance at 550nm is about 86%.

Example 2

Dilute 0.01ml of tungsten isopropoxide (purity 99%, Sigma-Aldrich) with 1ml of isopropanol, drop onto the graphene surface transferred to the quartz substrate in advance, and spin-coat at 5000rpm for 1 minute to form a surface covering thin layer WO _x graphene film. The above film was raised to 300° C. at a rate of 20° C./min, and kept at a temperature of 30 minutes to obtain a WO _x doped graphene transparent conductive film. It is determined that the surface resistance of the doped graphene film is 250/sq, and compared with the undoped graphene film (400/sq), the resistance drops by 37.5%. Its light transmittance at 550 nm is 92%.

Comparative example 1

Dilute 0.01ml of vanadium isopropoxide and tungsten isopropoxide with 1ml of isopropanol respectively, drop onto the graphene surface transferred to the quartz substrate in advance, spin-coat at 5000rpm for 1 minute, and form a thin layer of VO _x and WO on the surface respectively. _x graphene film. The above film was placed in a tube furnace, and in an air atmosphere, the temperature was raised to 150°C, 300°C, and 400°C at a rate of 20°C/min, and kept for 30 minutes to obtain a graphene-doped transparent conductive film. The percentage decrease in square resistance of the graphene film after doping is shown in Figure 7. It can be seen that the doping effect of vanadium isopropoxide is better than that of tungsten isopropoxide.

Comparative example 2

Measure 0.0035ml, 0.005ml and 0.015ml of vanadium isopropoxide and dilute it with 1ml of isopropanol, drop it onto the graphene surface transferred to the quartz substrate in advance, and spin-coat at 5000rpm for 1 minute to form a thin layer of VO _x on the surface. Graphene film. The above-mentioned graphene film was placed in a tube furnace, heated to a certain temperature in the air at a rate of 20°C/min and then kept for 30 minutes to obtain a transparent conductive film doped with graphene. The decrease percentage of square resistance of graphene film after doping is shown in Figure 8. It can be seen that after calcination in air, the higher the concentration of vanadium isopropoxide, the better the doping effect.

Comparative example 3

Take 0.01ml of isopropoxide solution (vanadium and tungsten) and dilute with 1ml of isopropanol, drop onto the graphene surface transferred to the quartz substrate in advance, spin-coat at 5000rpm for 1 minute, and form a thin layer of VO _x on the surface and graphene films of WO _x . The above-mentioned graphene film was placed in a tube furnace, and after the temperature was raised to 300° C. at a rate of 20° C./min in air, it was kept for 30 minutes, 1 hour and 3 hours. Doped graphene with different heat treatment temperatures is obtained. The test results show that the electrical resistance and light transmittance of the doped films are basically the same, indicating that the heat treatment time has little effect on the doping effect.

Test method: Use the four-probe method to test the surface resistance of the doped graphene film, use the Raman spectrometer to test the graphene doping degree, and use the ultraviolet-visible-near-infrared spectrophotometer to test the light transmittance of the film.

Fig. 6 is a graph showing the change trend of the square resistance of doped graphene obtained by heat treatment under different atmospheres. It can be seen from Figure 6 that the annealing effect of the doped graphene film in air is more significant than that in argon, and the conductivity of the doped graphene transparent conductive film is more obvious.

Claims (11)

1. a kind of transition metal oxide/graphene composite film, it is characterised in that by transition metal alkoxide alcohol dissolved dilution After be spin-coated on graphenic surface it be doped, be heat-treated by being calcined in air atmosphere, obtain transition metal oxide/ Graphene composite film, the transition metal oxide/graphene composite film includes graphene layer and is deposited on graphene layer On transition metal oxide layer, the transition metal oxide layer composition for metal Mo, V, W and Ni oxide in extremely Few one kind.

2. transition metal oxide/graphene composite film according to claim 1, it is characterised in that the transition gold Category oxide layer thicknesses are the nm of 1 nm~20.

3. a kind of preparation method of transition metal oxide/graphene composite film as claimed in claim 1 or 2, its feature exists In, including：

（1）Transition metal alkoxide salting liquid is spun to graphenic surface；

（2）Will（1）Graphene at 25 DEG C~800 DEG C, is calcined 0.5~24 hour, obtained in air atmosphere after gained spin coating Cross metal oxide/graphene composite film.

4. preparation method according to claim 3, it is characterised in that will（1）Graphene is in air atmosphere after gained spin coating In at 100~500 DEG C, calcine 1~5 hour, obtain transition metal oxide/graphene composite film.

5. preparation method according to claim 3, it is characterised in that the transition metal alkoxide is isopropoxide.

6. preparation method according to claim 5, it is characterised in that the isopropoxide be isopropanol molybdenum, isopropanol tungsten, At least one of isopropanol vanadium, isopropanol nickel.

7. the preparation method according to any one of claim 3-6, it is characterised in that the transition metal alkoxide salting liquid Solvent is at least one of ethanol, normal propyl alcohol, isopropanol, glycerine, n-butanol, ethylene glycol.

8. preparation method according to claim 3, it is characterised in that the concentration of the alkoxide solution is more than 0.001 Mol/L and in 0.1 below mol/L.

9. preparation method according to claim 8, it is characterised in that the concentration of the alkoxide solution is 0.001 mol/L To between 0.016 mol/L.

10. preparation method according to claim 3, it is characterised in that the spin speed of the spin coating is 0 to 5000 rpm Between.

11. preparation method according to claim 10, it is characterised in that the spin speed of the spin coating is 1000 to 5000 Between rpm.