Electrochemical coating of micro-structured silicon for photoelectrochemical water splitting

In this thesis the focus is on the development of materials for photoelectrochemical devices, applicable in solar-driven hydrogen generation from water splitting.

‘Worldwide, the emission of CO2 can be reduced by around 2%, if the existing hydrogen demand would be covered with hydrogen produced by renewable energies,’ says Alexander Milbrat. ‘But the big goal goes beyond this number. We want to produce and use hydrogen as an energy carrier to store excess solar energy received in the summer for the winter period.’

The Netherlands receives roughly 70% of the total yearly solar energy in the half year between April and September. ‘If we could capture and store part of this energy for the heating period, we could further drastically reduce CO2 emission,’ says Alexander. ‘The hydrogen produced with the aimed devices can be used as a fuel, or be converted into hydrocarbons and then used in transportation or heat and electricity generation.’

In this thesis, silicon was used as a base material. ‘It is nontoxic, abundantly available, and can be produced in high purity and quantity,’ Alexander says.

In two experimental chapters, a metal and a semiconductor - i.e. platinum (Pt) and tungsten oxide (WO3) - were electrodeposited on flat ITO (indium tin oxide)-coated p-type silicon. In two other experimental chapters, metals and another semiconductor - i.e. platinum (Pt) and silver (Ag), and bismuth vanadate (BiVO4) - were deposited on microwire structured silicon, with an axial p/n junction or ITO-coated p-type silicon without a junction.

Pt particle size and density proved to be well controllable, which is important for the production of an efficient and cost-effective hydrogen generating photoelectrodes. H2O2 was added to the electrolyte solution, to reduce particle size into the nanometer range and to increase their deposited density.

Flat electrode development

‘The method has a two-fold, double-functional advantage,’ Alexander says. ‘It allows to reduce the deposited mass of the precious and expensive metal and, at the same time, optimizes the photocathode for light absorption and hydrogen evolution properties.’

Also in this thesis work, a photoanode was created with a WO3 top coating. ‘WO3 is a promising, inexpensive, and environmentally benign photoanode material for the oxidation of water,’ Alexander says. ‘But it loses its activity after minutes to hours of operation within a photoelectrochemical cell. Therefore, we studied the processes taking place over time in a phosphate buffer at various pH values.’

Microwire structuring

Another main theme in this PhD work concerned microwire-structured silicon substrates. These architectures are proven to be superior to their flat equivalents in photovoltaics, due to improved light-trapping and absorption properties.

Alexander: ‘Additionally, we wanted to make use of their high internal surface area, which is an important property in (photo)electrochemistry. Also, higher loading of semiconductors becomes possible without increasing its thickness. Bismuth vanadate - which only functions as a photoanode when deposited as very thin layers - was coated on top of microwire structures. We found that geometries optimized for photovoltaics by my colleague and close collaborator Rick Elbersen, were far from optimum for photo-electrochemistry. We attributed this setback to diffusion limitations of redox species in the internal volume of the microwires. Using different geometries might solve this problem in the future. Now we better understand the actual processes.’

Novel deposition method

Alexander was very happy to design a novel method to deposit two different materials within a microwire geometry. ‘Silver (Ag) and platinum (Pt), were spatio-selectively deposited on the microwire top and bottom sections, respectively, without any masking steps. We just used the internal properties of a p/n junction and light. In our configuration, materials deposit selectively on the bottom segments when cathodic electrodeposition is performed in the dark, and on the top segments when performed under illumination. Besides, this novel fabrication method opens up opportunities to create devices that have value outside of the area of photocatalysis, I believe. For example, where micro- to nanowires are used in applications, such as electronics, sensors, and photonics.’

Two groups

Alexander performed his research within two Mesa+ Groups: PhotoCatalytic Synthesis (PCS), led by Professor Guido Mul, and Molecular Nanofabrication (MNF), led by Professor Jurriaan Huskens. ‘Part of the funding was from the VICI grant, Jurriaan Huskens was rewarded in 2011,’ Alexander says.

‘Experts of both groups helped in the progress of my work, and a lively collaboration between the two groups exists. Further, I owe the cleanroom experts a lot. The equipment is well-maintained, and they were very helpful in explaining the procedures and for pushing the boundaries of its functionality. The SEM-pictures I was able to produce, were telling more than words could have done. We were free to use the equipment after discussing the process, paying attention to keep the machines clean and save for other colleagues to perform their experiments after us.’

Future job

Just before finishing his PhD work, Alexander started working at ASML company in Veldhoven. ‘My knowledge of Si processing and its properties may be of added value,’ he says. ‘Cleanroom skills and material science knowledge are useful in this work. I am contributing in developing existing and future machines to be more efficient and long-lived. The great thing about ASML is that room exists for fundamental research. Of course, research in industry is aimed more closely at problem-solving and production than academic research does. That will make my future work even more intense and interesting.’