Investigating the Optical and Structural Properties of MiniCAST soot via UV-Vis-NIR Absorption Spectroscopy and Online Raman Spectroscopy

Abstract

Soot, formed during incomplete combustion of hydrocarbons, is a challenging compound to study. It affects air quality, climate, and human health. Its structural and optical properties are not fully understood due to variations in particle size, structure, chemical composition, and degree of carbon hybridization. In this work, soot was generated using a MiniCAST 5201C soot generator, which produces various types of soot aerosols with different maturity levels. of varying maturity. Samples were collected at five operational points (OP1, OP3, OP5, OP6, and OP7), corresponding to decreasing soot maturity. UV–Vis–NIR absorption spectroscopy was employed to characterise optical properties, and both in flight and ex situ techniques were compared. Online Raman spectroscopy was also performed in a 90◦ scattering configuration with a horizontally polarised laser to suppress interfering ambient signals. The data, collected previously, was analysed in this study. From the absorption spectra, the optical band gap, an important parameter, was estimated using the Tauc approach. Tauc analysis revealed an increase in optical band gap from mature to immature soot. Systematic changes in Raman spectral features such as the increase of peak width and decrease of peak intensity ratio were observed with decreasing maturity, indicating variations in graphitic ordering and defect content. These findings show that combining absorption spectroscopy and Raman spectroscopy provides a comprehensive approach for probing the optical properties and nanostructure of soot across different maturity

Popular Abstract

The population is growing, and the economy is expanding. This connects directly to increased energy use. Although there have been efforts to switch to renewable energy sources, fossil fuels will likely still be the main energy source for the near future. As a result, emissions will continue. These emissions include carbon dioxide and soot, which is the main focus of this study. Burning any fuel, whether in a factory, automobile, or even a candle, can produce soot, which consists of tiny black particles made of carbon. Its black color is due to its properties, and its appearance can vary depending on its source.

The primary reason for studying soot goes beyond how it forms. When these particles are suspended in the atmosphere, they absorb sunlight very well, warming the air and raising temperatures. They also influence the water cycle by interacting with clouds, affect ecosystems, and harm human health by entering the body. Soot is difficult to study. It isn’t just a black particle; it is a mix of complex carbon structures and other elements. Its properties change based on the type of fuel and its composition. Researchers generate soot in controlled environments using devices like burners, engines, and soot generators. This study uses MiniCAST as a soot generator to produce soot with different properties or maturity levels, ranging from well-developed, dark, graphitic soot to less mature, yellow-brown soot rich in organic material.

This thesis examines soot using different techniques such as UV-Vis-NIR absorption spectroscopy and Online Raman spectroscopy. UV-Vis-NIR absorption measures how soot interacts with light. The spectra can be characterized with various parameters; here, the focus is on the optical band gap, which reflects electronic transitions. Mature soot has a lower band gap, indicating more ordered graphitic carbon, while immature soot has a higher band gap, signaling more disordered carbon and higher organic content. Raman spectroscopy is used to study the structural properties of soot particles. Online Raman spectroscopy allows for the analysis of soot directly in the aerosol phase as it is produced, providing real-time information. Several spectral features can be identified from Raman data. The D and G peaks in the Raman spectra indicate disorder and graphitic ordering. Mature soot has a higher D-to-G intensity ratio, while immature soot shows stronger fluorescence from organic molecules. These analyses demonstrate that mature soot absorbs more light, has a more ordered carbon structure, and contains less hydrogen-rich material. Immature soot appears lighter in color, is less graphitic, is richer in organic compounds, and is more likely to fluoresce under laser light. Connecting these optical and structural properties helps us understand soot’s effects on climate and health and informs strategies to reduce harm.

• author: Kaimoottil Sasikumar, Swetha
• supervisor: Thi Kim Cuong Le; Jefry John
• organization: Combustion Physics; Department of Physics
• course: PHYM03 20251
• year: 2025
• type: H2 - Master's Degree (Two Years)
• subject: Physics and Astronomy
• keywords: MiniCAST soot generator, soot aerosol, absorption spectroscopy, Raman spectroscopy, optical band gap
• language: English
• id: 9212206
• date added to LUP: 2025-09-16 13:44:43
• date last changed: 2025-09-16 13:44:43