Using gold nanoparticles in hydrogen economy

An international team of researchers from Peter the Great St. Petersburg Polytechnic University (SPbPU), Leibniz University Hannover (LUH), and the Ioffe Institute in Russia found a way to improve nanocomposite material, which opens up new opportunities for use in hydrogen economy and other industries. The project outcome is illustrated in the academic article "The mechanism of charge carrier generation at the TiO2-n-Si heterojunction activated by gold nanoparticles" published in the scientific journal Semiconductor Science and Technology.

The study discusses composite material, a semiconductor based on titanium dioxide. Its applications are widely studied by researchers all over the world. However, the processes, which take place inside this material, are very complex. In order to use the semiconductor more effectively, it is necessary to ensure that the energy enclosed between its layers can be released and transmitted.

In the framework of the experiments conducted at SPbPU, LUH, and the Ioffe Institute, researchers proposed a qualitative model to explain the complex processes.

The research group used a composite material consisting of a silicon wafer (standard silicon wafer used in electronic devices), gold nanoparticles, and a thin layer of titanium dioxide. In order to transfer the energy inside the material, the researchers insulated nanoparticles from the silicon. If the nanoparticles are not insulated from the silicon wafer, the energy cannot be transmitted - neither onto the silicon nor onto the titanium dioxide. This leads to loss of energy.

"The obtained material was a silicon wafer with pillar-like structures grown on its surface. It was used as a substrate for the sample. Gold nanoparticles were situated on top of these pillars and the whole structure was coated with titanium oxide. Thus, nanoparticles were only exposed to titanium dioxide and insulated from the silicon at the same time. As the number of boundaries between the layers decreased, we tried to describe the processes in the material. In addition, we assumed that this structure would increase the efficiency of using the energy of light illuminating the surface of our material", says Dr. Maxim Mishin, professor of Physics, Chemistry, and Technology at the Microsystems Equipment Department of SPbPU.

In St. Petersburg, an international research group established a model of the new structure. Subsequently, the main part of the structure was realised in Hannover: a silicon wafer with pillars and gold nanoparticles situated on top.

The experiment was conducted as follows. First, the wafer was oxidized, i.e. it was covered with an oxid layer, and gold nanoparticles were put on top.

"After that, we faced the next task: creating pillars and etching the substrate so that it remained under the particles and not in between them. Considering that we were dealing with dimensions at the nanoscale, the diameter of gold nanoparticles being approximately 10 nanometres and the height of the pillars being 80 nanometres, this was no easy feat. The progress of modern nanoelectronics allowed us to use so-called "dry" etching methods such as reactive ion etching", adds Dr. Marc Christopher Wurz from the Institute of Microproductiontechnology at Leibniz University Hannover.

Developing the technology has been a protracted process: at the early stages of the experiment, while using the ion etching, all gold nanoparticles were simply demolished from the oxidized wafer. It took researchers one week to identify suitable parameters for etching the plasma, so that the gold nanoparticles remained on the surface. The entire experiment was conducted over a period of 10 days.

The research project is ongoing. According to the researchers, the nanocomposite material can be used in optical devices operating in the visible light spectrum. In addition, it can be used as a catalyst to produce hydrogen from water, or to purify water by stimulating the decomposition of complex molecules. This material may also be useful as an element of a sensor, which detects gas leaks or an increased concentration of harmful substances in the air.

The project was supported by the DAAD programme "Strategic Partnership of Peter the Great St. Petersburg Polytechnic University and Leibniz University Hannover".