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45244-G10
Multilayer Assembly of Nanoparticles Using Carbon Nanotube as Backbone Phase

Kathy Lu, Virginia Polytechnic Institute and State University

During this project reporting period (09/01/2007-08/31/2008), we have developed a method to tailor multi-walled carbon nanotube (MWCNT) length from tens of microns to a couple of hundreds of nanometers. Also, we were able to successfully activate the chemically inert CNT surfaces in order for them to have certain charge characteristics. TiO₂ nanoparticles were dispersed in an aqueous suspension. By zeta-potential analysis, we were able to obtain opposite surface charges for CNTs and TiO₂ nanoparticles under the same pH and enable electrostatic assembly.

In our research, we started with MWCNTs (Helix Material Solutions, Inc. Richardson TX) made by chemical vapor deposition (CVD) and 3:1(v/v) concentrated H₂SO₄:HNO₃ mixture for MWCNT shortening. MWCNTs were suspended in 3:1 acid mixture and ultrasonicated in a water bath at 35-40^oC for different time such as 1h, 5h, and 24h. The resultant suspension was diluted with 250 ml of DI water. The shortened MWCNTs were obtained after centrifugation and thorough washing with distilled water. The hybrid mechanical-chemical acid treatment can not only shorten CNTs but also make the inert CNT surfaces to bear charges.

To obtain stable aqueous dispersions of CNTs, sodium dodecyl sulfate (SDS) was used to prepare aqueous dispersions of CNTs. pH values were adjusted by adding either sodium hydroxide (NaOH) or hydrochloric acid (HCl).

TiO₂ nanoparticles (MTI Corporation, Richmond, CA) of mean particle size 5 nm and purity 99.99% was used. TiO₂ nanoparticles were first dispersed in DI water. Ultrasound was used to improve TiO₂ nanoparticle dispersion and to separate possible agglomerates. Then the suspension was mixed overnight by ball milling. The suspension was allowed to stand for at least 1 day after that. Only the upper layer TiO₂ nanoparticle suspension was used in this study.

In order for TiO₂ nanoparticles to coat on MWCNTs by electrostatic attraction, the two species need to have opposite surface charges under the same pH. By zeta-potential analysis (Nano ZS Zetasizer, Malvern Instruments, Southborough, MA), we found that only the original MWCNT dispersion shows the isoelectric point within the 1.5-10 pH range at about pH 3. At pH<3, the zeta potential of original MWCNTs dispersions without SDS is positive. At pH>3, the zeta potential is negative. When MWCNTs were shortened by mechanical-chemical treatment using acid mixture, there is obvious difference in surface charge characteristics. Within pH range of 2-10, the shortened MWCNTs without SDS are negative and have high absolute zeta potential. The maximum zeta potential is as low as -50 mV. At pH 4, TiO₂ nanoparticles have positive zeta-potential. Also, the zeta-potential difference is large enough for electrostatic attraction. As a result, well dispersed TiO₂ suspension at pH 4.0 was added into MWCNT suspension at the same pH. CNTs, TiO₂ nanoparticles and the assembled CNT-TiO₂ were characterized by scanning electron microscopy (LEO 1550, Carl Zeiss MicroImaging, Inc, Thornwood, NY) and transmission electron microscopy.

Our future work will involve TiO₂ nanoparticle size and dispersion improvement. Also, the electrostatic coating process needs to be enhanced for multi-layer TiO₂ nanoparticle assembly. After that, we will freeze dry the assembled CNT-TiO₂ and carry out nanostructure characterization as well as property testing.

Impact of the research

On my career: The support of this research project has played significant role in expanding the research capabilities of my program and enhancing my career growth. Fundamentally, it improved our understanding of CNT length modification and electrostatic assembly process, an important area in nanomaterial research. We successfully demonstrated that the length of MWCNTs can be tailored and the surface charges of CNTs and TiO₂ nanoparticles can be matched for electrostatic assembly. Also, we were able to publish our papers and present our research findings in international conference.

On the scholar participated in the project: The project provided an excellent opportunity for the post-doc researcher to advance her career. During this project, she was to able to conduct challenging experiments and solve problems independently. She also got the opportunities to analyze and summarize the research findings in the leading-edge nano-research areas. Also, she presented the research results during international conference.

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