Silicon: Electrochemistry, Production, Purification and Applications

Book Preface

Silicon lies at the heart of modern technology. Silicon can be used in various fields, such as optoelectronics, sensors, batteries, optical fibers, photoelectrochemical water splitting, terahertz emitters, and numerous other applications. As an abun-dant, non-toxic, efficient, and robust material, silicon will dominate the solar energy market at least for the next few decades.
Electrochemistry deals with the chemical transformations, which are induced by an electric current, or vice versa – with the transformations, which generate an elec-tric current. These processes provide an opportunity to store or produce electricity with a minimum carbon footprint. Electrochemistry can, therefore, significantly contribute to low-carbon economy; it offers an advancement in sustainable energy solutions and environment-friendly technologies.
In the early 2000s, V. Lehman (2002) and X. G. Zhang (2004) published several books on silicon electrochemistry. Since then, various breakthrough directions in silicon electrochemistry have emerged. For instance, luminescent porous silicon nanoparticles were electrochemically produced and applied as the carriers of the drug payload in vivo. Electrochemical silicon surface modifications increased the efficiency of photovoltaic devices used for solar energy harvest or for the production of solar fuel. Silicon photoelectrodes have been successfully developed
for hydrogen and oxygen production by water splitting as well as CO2 reduction. Electrochemically produced Si surface nano-architectures showed an intrinsic quantum confinement effect. Environment-friendly and secure solutions offered silicon electrochemistry in high-temperature molten salts. Electrochemical silicon reduction from silica in high-temperature molten salts has been discovered. Elec-trochemical deposition of doped silicon as well as formation of p-–n junction have also been demonstrated.
This book aims to summarize the experimental and technological work done in recent decades on silicon electrochemistry, production, and purification, high-lighting subjects of technological significance and future perspectives. The book aims to be highly beneficial to the communities of chemists and material scientists working in academia and industrial sectors, especially in the field of sustainable energy development: photovoltaics, light harvesting efficiency, solar-to-chemical conversion, production of solar-grade silicon as well as production of batteries, photoelectrodes, or silicon-based semiconductors. The secondary market of this book includes the education and socio-economic sectors with focal points on such topicalities as the reduction in global climate change, replacement of fossil fuels by renewable energy, and strategies of low-carbon economy.

World production of silicon (Si) reached (2010–2020) about eight millions of metric tonnes in the last decade. This quantity was produced mainly by the carbothermic silica (SiO2) reduction. The process requires a large supply of energy and emits carbon oxides (COx). A fundamental challenge is the electrochemical silicon extrac-tion from silica or other solids using electricity instead of harmful chemistries. Zero carbon footprint could be attained when using electrons as absolutely clean reduction agents generated by renewable sources. Electrochemical methods can be used on a wide scale of applications: extraction, purification, surface engineering, or thin-film technologies. Thus, silicon electrochemistry has the potential to significantly contribute to low-carbon economy; this field offers an advancement in environmentally friendly and secure technologies of energy generation and storage.
Breakthrough research topics have emerged in silicon electrochemistry in recent decades. The electrochemical formation of porous silicon (P-Si) was discovered in 1956 (Uhlir 1956). Canham reported in 1990 that a visible room-temperature photoluminescence from P-Si layer formed electrochemically on Si wafer (Canham 1990). The discovery inspired wide studies of P-Si for applications in optoelectron-ics, lasers, and sensors. Luminescent porous silicon nanoparticles were applied as the carriers of the drug payload, whose infrared luminescence enabled monitoring of the particles in vivo (Park et al. 2009). Electrochemical nano-micro-structuring of silicon has been widely investigated. The surface modifications increased the efficiency of photovoltaic (PV) devices used for solar energy harvest or for produc-tion of solar fuel. Silicon photoelectrodes have been successfully used for hydrogen
and oxygen production by water splitting as well as CO2 reduction (Sun et al. 2014). Electrochemically produced Si surface nano-architectures showed intrinsic quantum confinement effect. Electrochemical reduction of silicon dioxide to silicon in a molten salt electrolyte has been reported, which formed the basis for new processes in silicon semiconductor technology and high-purity silicon production (Nohira et al. 2003). Environmentally friendly and secure solutions offered silicon electrochemistry in high temperature molten salts (Juzeli¯unas and Fray 2020). Electrochemical deposition of doped silicon as well as formation of p–n junction has been demonstrated (Zou et al. 2017, 2019; Peng et al. 2018). The approach has he potential of reducing capital cost and energy consumption for fabrication of solar cells when compared with the conventional manufacturing process.
This book features recent achievements in silicon electrochemistry, particularly, in electrochemical silicon extraction, purification, and processing in high-temperature molten salts. The introductory part of the book (Chapters 2–4) is devoted to general aspects of silicon application. A historical overview of silicon production is provided, and its importance in a low-carbon economy is considered. Chapter 4 addresses the physical and chemical properties of silicon, which are most relevant for electrochemical materials science. The subsequent material is more specific. Chapter 5 describes the major technologies used for silicon purification such as Siemens, Union Carbide, or Ethyl Corporation processes. This chapter also provides the principles of electrorefining in high-temperature molten salts, highlighting the advantages and disadvantages when compared with conventional industrial processes.
Chapter 6 addresses electrodeposition of thin layers and discusses the possibility of replacing multiple processes of Si wafer fabrication with one-step electrochemi-cal deposition. Traditional manufacturing entails an energy-intensive and environ-mentally unfriendly production of metallurgical grade silicon (MG-Si), as well as its upgrade to solar grade silicon (SoG-Si), ingot casting, and slicing. Electrodeposi-tion from molten fluoride, chloride, and oxide electrolytes on various substrates is discussed. A recently proposed strategy for electrodeposition of photoactive silicon and p–n junction is highlighted in detail. Silicon deposition from ionic liquids – the room-temperature molten salts – is also discussed in this chapter. Significant atten-tion is given to the purity level of silicon electrodeposits, which are essential for photo-electrochemical applications.
Chapter 7 discloses photoelectrochemical (PEC) properties of silicon-oxide electrodes coated with ultrathin films of silica (SiO2), hafnia (HfO2), and alumina (Al2O3). The pivotal concept of PEC methodology is to obtain information, which correlates with that of the solid-state cells so that there is no prior need to design a solar cell that characterizes Si surface photo-responsiveness. Significant attention is given to studies of Si-oxide interfacial stability by the quartz crystal nanobalance (QCN) – a sensitive mass detector, which provides information about the electrode mass change with nanogram resolution in situ and in real time.
Deoxidation of metal oxides in a molten salt electrolyte was discovered in the year 2000 (Chen et al. 2000). The process was named the FFC Cambridge process. Simplicity and rapidity of the process have attracted global interest. Over 30 metals or semimetals were extracted from solid compounds by this energy-efficient and environment-friendly route. Chapter 8 addresses the FFC principle and its application in silicon reduction from silica. The electrochemical extraction provides a green alternative to conventional carbo-thermic silicon production. Chapters 9–12
provide further details on Si–SiO2 conversion in molten salts. Voltammetry, basic reactions, and in situ studies by synchrotron X-ray diffraction are discussed, and experimental conditions used by many authors are summarized.
Technological opportunities carry out the operation at ultra-high temperatures and at liquid state of silica feedstock. Such processes are referred to as molten oxide electrolysis (MOE). Chapter 13 discusses the MOE principles of silicon extraction in aliquidstate.
This study focused majorly on electrochemical surface engineering. Chapter 14 discusses the chemical–physical methods of silicon surface structuring, such as laser engineering and various etchings: chemical, photoelectrochemical, reac-tive ion, plasma immersion ion implantation, and metal-assisted chemical. The vapor–liquid–solid method is also discussed.
Chapter 15 features a comprehensive material obtained on electrochemical Si structuring at high-temperature molten salts including formation of black silicon (B-Si). B-Si is a nano-micro-porous material, which effectively absorbs the light on a wide range of wavelengths. Electrochemical Si structuring in molten salts is attractive due to its environmental friendliness, technical simplicity, and cost-effectiveness.
The book also outlines the perspectives of electrochemical synthesis of semicon-ductors (Chapter 16), the basic principles and materials for photo-electrodes, and the preservation of solar-fuel generators (Chapter 17).
In conclusion, while silicon electrochemistry offers a range of technological oppor-tunities, most of the developments are still on the conceptual or bench-scale level. As a result, viable technological developments are still pending.