Growing clean carbon nanotubes
Scientists at the University of Warwick, UK, have grown clean carbon nanotubes directly onto a disc surface. The creation of these ultramicroelectrodes, which are 25-100µm in diameter, could be used for extremely sensitive sensors.
Researchers used catalysed chemical vapour deposition (cCVD) to grow single-walled nanotubes on insulating silicon oxide (SiO2) surfaces. The surfaces were covered with a catalyst material, such as iron, and placed in a quartz tube running through a furnace. The substrate was then heated to between 775 and 900ºC, while hydrogen was passed over it. At its peak growth temperature, a carbon source such as methane was introduced to the tube for five minutes, and then the system was left to cool. The resulting nanotubes were grown in an even and controlled manner, allowing them to overlap and create a single metallic microcircuit across the surface of the SiO2.
‘By playing around with the growth conditions [temperature and the hydrogen:methane ratio], we can control the density of the tubes growing on the surface – using SiO2 promotes random growth with no preferred orientation,’ explains Professor Julia MacPherson of the Electrochemistry and Interfaces Group at Warwick. ‘If we get the conditions correct, the density is high enough for us to work with metallic-like films.’
Unlike other CVD techniques, the method allows nanotubes to be grown directly onto a surface, using the catalyst, in a desired pattern. ‘Also, if you get the conditions right, you can grow tubes straight away without the need to post-process the samples,’ adds MacPherson. ‘Other [CVD] techniques often require a clean-up stage to get rid of the high metal content and amorphous carbon.’
Researchers have so far created ultramicroelectrodes from nanotubes that have lasted through several experiments running over repeated days. Their low surface area on the disc means they are able to screen out background noise and cope with low signal-to-noise ratios. Combined with their quick response time – which MacPherson says is in the microseconds – the nanotube electrodes could be thousands of times more sensitive than other ultramicroelectrodes.
The team sees potential for combining the electrodes with online analysis flow systems for low concentration detection work, such as heavy metal analysis. They are also experimenting with quartz surfaces, whose smooth parallel path orientation would allow the nanotubes to grow in an aligned manner, creating faster response times.