Nghiên cứu chế tạo vật liệu nano TiO2 ứng dụng trong lĩnh vực quang điện hóa

HUE UNIVERSITY UNIVERSITY OF SCIENCES NGHIA VAN NGUYEN RESEARCH ON THE FABRICATION OF FIBER - LIKE TiO2 NANOCOMPOSITES FOR APPLICATION IN THE PHOTOELECTROCHEMICAL FIELD Major: Solid State Physics Code: 9440104 SUMMARY OF THESIS PH. PHYSICS HUE - 2021 The thesis has been completed at Faculty of Electricity, Electronics and Material Technology, University of Sciences, Hue University. Thuy Minh Doan 2. Son Manh Nguyen Reviewer 1:.

The thesis will be defended in front of Hue University’s Doctoral thesis committee at. The thesis will be accessible at: 1. National library of Vietnam. Library of University of Sciences, Hue University.

REASONS TO CHOOSE THE TOPIC The increase in population and the development of life in the world have led to an increase in energy demand globally. Currently, fossil fuels (coal, oil, gas) are still the main source of human energy (about 90%). In addition to the fact that fossil fuels will be depleted soon, fossil fuel consumption increases CO2 emissions, which has been exacerbating environmental pollution and gas change. According to a report by the Global Energy and Climate Outlook (GECO) in 2016, global energy demand and greenhouse gas emissions are expected to increase 37% and 30% respectively over the period to 2030.

The solution to this problem is to find and exploit clean, sustainable, and environmentally friendly energy sources. This is an urgent issue that attracts the attention of researchers around the world. Over the past decades, many renewable energy sources have been developed to replace fossil fuels such as wind, geothermal, hydroelectric and solar energy. These are relatively clean and sustainable energy sources.

However, each of those energy sources has several limitations that make development challenging. For example, electricity generated by wind turbines cannot be stored, hydroelectricity is limited in construction due to high costs and possible adverse environmental impacts. In this situation, hydrogen gas emerges as the ideal energy storage and transport means for the following reasons: First, it is the most abundant element in nature and exists both in water and in biological gas; Second, hydrogen is very light (0.8988 g/L) so it has a greater energy density (~ 140 kJ/g) than any other fuel, such as gasoline (~ 40 kJ/g); Third, it is environmentally friendly because its use does not generate pollutants or any negative impact on the environment. Like electricity, hydrogen can be produced from a variety of sources such as natural gas, coal, water, and other renewable energy sources.

Currently, the basic methods of hydrogen generation are the Converting hydrocarbons (fossil fuels, biomass) by heat (Reforming); the Electrolysis water method; The Biological method. However, these methods have a major limitation in consuming a lot of 2 energy from fossil fuel sources. In recent years, the photochemical method to generate hydrogen has been interested in research and development. Many scientists believe that the hydrogen energy is generated by photochemical water splitting is one of the most promising because it is based on the conversion of sunlight, an available energy source, and endless.

The photochemical system that produces hydrogen from solar water splitting can be divided into three main categories: (1) photoelectric – electrolytic (PV-E) cells, (2) photocatalytic ( PC), and (3) photoelectrochemical cell (PEC). PEC is a technology integrating light absorption on semiconductor materials and electrochemical process in photoelectric cells, oxidation and dehydration reactions take place on electrodes to separate water into hydrogen and oxygen. PEC technology is considered to be the middle ground between PV-E technology and PC technology, it has a relatively high efficiency of converting solar energy into hydrogen at a reasonable cost. Fujishima and Honda were the first to report the water separation PEC using a titanium dioxide (TiO2) electrode in 1972 in the Nature [26].

This technique allows the directly generation of hydrogen gas from solar energy. In the simplest case, the photoelectrochemical principle of water separation is based on the conversion of light energy into electric current in a cell consisting of two electrodes embedded in an electrolyte solution. This electric current is used to electrolyze water. One of the two electrodes is a semiconductor that can absorb light, called an working electrode (WE) or a photoelectrode, the other electrode is a metal, called a counting electrode (CE).

In which, the photoelectrode plays a very important role in the operation of PEC. The mechanism of this process is that, when the electrode WE are irradiated with an photon greater than the bandgap of TiO2, electron/hole pairs are formed in the conduction band (CB) and valence band (VB). Then electrons (e-) move toward the CE. The electrons involved in the reduction reaction produce H2 gas, while the holes (h+) oxidize to produce O2.

Theoretically, there are three ways of using electrode material in PEC, namely: WE is a semiconductor of type n and 3 CE is a metal; WE is n-type semiconductor and CE is a p-type semiconductor or WE is p-type semiconductor and CE is metal. In principle, the behavior of these three electrode layouts is the same. In PEC technology, the semiconductor material used as an photoelectrode must satisfy the following two basic conditions: First, the bandgap (Eg) must be greater than 1.23 eV, which is the energy needed to dissociate water molecules. Furthermore, for the reaction to occur without applying an external voltage, the conductor band bottom must be higher than the water oxidation level and the valence band peak must be higher than the hydrogen generation potential, that is, the two oxidation potentials.

- the reduction must be within the energy gap [37]; Second, since the electrodes are immersed in the electrolyte solution, they must be chemically stable to avoid electrochemical corrosion. It is also worth noting that, as mentioned above, for the water separation reaction to take place, Eg must be greater than 1.23 eV, but if Eg is too high (> 3 eV), the energy conversion efficiency is low. The reason is that it can only absorb ultraviolet (UV) light but cannot absorb visible light, while visible light contributes nearly half of the solar energy reaching the earth and light. Ultraviolet only accounts for about 4%.

After the report by Fujishima and Honda, a series of semiconductors used as photoelectrodes have been discovered and studied but finding a semiconductor that satisfies the above conditions remains a challenge. For example, semiconductor materials such as ZnO, SnO2 [48] have high chemical stability, but Eg are quite large so they cannot absorb most visible light, so the efficiency is quite low. The materials with low Eg such as Si, GaAs and InP [11] are susceptible to corrosion when in contact with electrolyte. Titanium, despite having a large Eg, is still one of the most attractive materials for PEC technology.

It has the conduction band bottom and valence band top position including both the oxidation-reduction potentials of H2O, thus satisfying the required for water splitting process. However, the optical conversion efficiency of this material system is still quite low 4 for practical application. Therefore, it was forgotten for a long time. Currently, in order to increase the sunlight absorption of the photoelectrodes, reasearch groups often combine TiO2 material and good optical absorbing material in the visible region.

In the most recent reports, the researchers have to combine TiO2 materials with low energy gap sizes such as CdS [18], CdSe [39], g-C3N4 [55]. The results showed that the efficiency of water splitting increased significantly under the radiation of sunlight. In these material systems, CdS is most interested in research because the small bandgap and the position of the conduction band bottom and the top of the valence band are consistent with the combination with TiO2. The publications of Jingshan et al.

[39]; Liu et al. [56]; Zhang et al. showed a significantly increased optical conversion efficiency of the CdS/TiO2 structure. Recently, metal nanoparticles are being developed to increase water separation efficiency.

The plasmonic effect from the nano- structured metals allows the broadening wavelength range of light absorption as well as the transport of "hot" electrons from the metal to the semiconductor. Fang Xu et al. [93]; Gong et al [78]; Wang et al [98] showed that when coated with the Au nanoparticles on the TiO2 surface, it was possible to shift the absorbance area of the photoelectrode towards the long wavelength. This is therefore increasing the optical current density.

In addition, the fabrication of electrodes with the nanocomposite structure of various layers has been published by some research groups showing the effectiveness of this structure in photoelectrochemistry such as N.Wu et al. [50]; Li et al. The optical conversion efficiency of the PEC is determined primarily by the photoelectrode material. An ideal photoelectrode for high efficiency and stability must satisfy the two following requirements: high light absorption and fast electron transportability.

These factors, not only depend on the crystal structure, but also depend on the structural morphology of the material. One-dimensional nanostructures such as nanowires, nanotubes, and nanorods are the most studied 5 because these structural types can improve carrier transportability. They reduce optical loss thanks to their constant path and anti-reflection resistance [35]. There are two main methods to fabricate one-dimensional nanostructured TiO2 used in the photoelectrochemical field: the electrochemical method and the hydrothermal method.

Many research results, as published by Grimes et al. showed that the nanotubes TiO2 with vertical order on the Ti substrate were fabricated by electrochemical method. In general, this method has the advantage of a simple fabrication tool that can control the diameter size. However, due to the anode of Ti, the number of photons absorbed into the material cannot be optimized.

Second, groups of Grimes et al [23]; Qin et al [66]; Leu et al. [85] successfully fabricated TiO2 with vertical order nanorod structure on transparent conductive FTO. Optical current density is relatively high, but the size of the nanorods is relatively large, so their surface area is not high. In addition to the above two methods, the electrospinning method has shown simplicity and efficiency in fabricating one- dimensional nanostructured materials with different materials.

This method is capable of controlling the porosity of the electrode film easily by controlling the nanofiber size. The electrospinning technique was invented in the US in 1902, but it was not until the late 20th century that scientists noticed this technique when making one-dimensional nanomaterials. By this method, Mali et al [59]; Li et al [52];. has successfully fabricated TiO2 photoelectrode with nanofiber structure.

Although there have been many reports on PEC technology, the study of the structure of the material on the photoelectrode to increase the conversion efficiency is still a research direction that has received great attention. On the other hand, the published works just stop at the level of investigating the materials, measuring the optical current and optical conversion efficiency. There are very few studies that directly absorb hydrogen gas generated in the reaction of the PEC. For the above reasons, we choose the thesis topic as: "Research on the fabrication of fiber-like TiO2 6 nanocomposites for application in the photoelectrochemical field".

THESIS TARGET The goal of the thesis is to fabricate nanomaterial nanostructured TiO2 composite nanomaterials, fabricated by the electrospinning method, used as a photoelectrode in the PEC model. The electrode has a high chemical, uniformity, good surface properties, good adhesion on ITO (Tin-doped indium oxide) electrode substrates, and gives high optical water separation performance in the visible light region. SUBJECTS AND SCOPE OF THE STUDY 3. Research subjects - TiO2 nanocomposites.

- Three-electrode electrochemical system. - The system captures hydrogen gas generated in the water separation reaction. Research scope - Investigating the fabrication conditions of a one- dimensional TiO2 nanostructured photoelectric electrode on ITO substrates by electrospinning method (controlling parameters affecting the size, thickness, and diameter of nanofibers). - Fabrication of CdS/Au/TiO2 composite nanomaterials on TiO2 fiber materials to ensure that the photoelectrode absorbs a large part of the sunlight spectrum.

- Measurement of the optical current characteristics of the photoelectrodes in the three-electrode system. - Design and obtain the amount of hydrogen generated by the reaction.