Hydrocarbon gases (especially methane CH₄ and ethylene C₂H₂) offer significant advantages as precursors for carbon nanotubes (CNTs) in chemical vapor deposition (CVD) methods, mainly in the following aspects:
1. High Carbon Content and Cracking Efficiency
- High carbon-to-hydrogen ratio: Gases like methane (CH₄) have a high carbon-to-hydrogen atomic ratio, efficiently releasing carbon atoms during pyrolysis, resulting in a high carbon yield.
- Moderate cracking temperature: In the CVD process (700-1000℃), hydrocarbon gases can be cracked on the catalyst surface, directly generating carbon atoms for CNT growth with few byproducts (mainly hydrogen).
2. Strong Process Controllability
- Easy control of gas flow rate and concentration: By adjusting the ratio of hydrocarbon gas to carrier gas (e.g., H₂, Ar), the carbon supply rate can be precisely controlled, thus affecting the diameter, number of walls, and growth rate of CNTs.
- Wide growth window: Different hydrocarbon gases (e.g., methane is stable, ethylene is highly reactive) are suitable for the growth of various CNT types (single-walled/multi-walled).
3. Catalyst Synergistic Effect
- Reduction and activation of catalysts: Hydrogen atoms produced from the cracking of hydrocarbon gases maintain the reduced state of transition metal catalysts (e.g., Fe, Co, Ni), preventing their oxidation and deactivation.
- Moderate carbon solubility: Supersaturation of carbon in the catalyst particles promotes the tip or base growth mode of CNTs.
4. Product Purity and Quality
- Few byproducts: Cracking mainly produces carbon and hydrogen gas, minimizing the production of impurities such as amorphous carbon (if the ratio is properly controlled).
- Fewer structural defects: Compared to oxygen-containing carbon sources (such as ethanol), hydrocarbon gases reduce defects caused by oxygen impurities, favoring the growth of highly crystalline CNTs.
5. Economic and Safety Aspects
- Lower cost: Methane and acetylene are petrochemical byproducts, widely available, and relatively inexpensive.
- Controllable safety: Gas flow rate and pressure are easily monitored in the CVD process, and greenhouse gas emissions are relatively low (if methane is completely cracked, the main byproduct is hydrogen).
6. Application Adaptability
- Suitable for industrial mass production: The highly fluid gaseous raw materials are easy to feed continuously, making them suitable for large-scale production equipment such as fluidized bed CVD.
- Can be used in combination with other gases: For example, adding a small amount of hydrogen can suppress amorphous carbon formation, and adding a small amount of water vapor can clean the catalyst surface ("oxygen-assisted" CVD).
