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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 assembled TiO₂ nanoparticles onto MWCNTs via electrostatic attraction, developed a method to oxidize MWCNTs and produce acid functional groups on the surfaces of MWCNTs, and assembled TiO₂ sol onto MWCNTs via esterfication.

The experimental process related to TiO₂ nanoparticle assembly onto MWCNTs has been detailed in last year’s report. Based on last year’s work, we have developed a mechanical and chemical hybrid method to tailor MWCNT length. Under the same pH, opposite zeta-potentials have been obtained for CNT and TiO₂ nanoparticle suspensions. Based on electrostatic attraction principle, TiO₂ nanoparticles have been homogeneously attached onto CNT surfaces to produce CNT/TiO₂

nano-clusters.

During this reporting period, we started with MWCNTs (Helix Material Solutions, Inc. Richardson TX) made by chemical vapor deposition for surface functionalization. First the MWCNTs were added into a three-neck flask containing a 3:1 (v/v) mixture of sulfuric acid and nitric acid. The mixture was sonicated at room temperature for 2 hrs using a bath sonicator (Model 08895-04, Cole-Parmer Instrument Co., Vernon Hills, IL) and refluxed in an oil bath at different temperatures for 8 hrs. The CNTs were then filtered out and washed with DI water until the pH of the filtrate became neutral. The as-purified MWCNTs were dried at 60°C for 20 hrs in an oven.

For TiO₂ sol preparation, DI water and ethanol were mixed with a 1.5:1 volume ratio and stirred for 30 min. After that, the mixture was adjusted with ice acetic acid to pH 2.0 and further stirred for 60 min. Tetrabutyl titanate (TBTA, Ti(OC₄H₉)₄, CAS, ACROS Organics, Morris Plains, NJ) and ethanol with a volume ratio of 1:10 was added dropwise into the above mixture and vigorously stirred for 60 min. After further stirring for 3 hrs, transparent titanium hydroxide sol solution was obtained and allowed to age at room temperature for 2 days.

The main surface functional groups of oxidized MWCNTs, namely, carboxylic, lactonic, and phenolic groups, were quantified with a direct acid-base titration method using 0.1 N NaOH. MWCNT suspension (10 mg/ml) was added to TiO₂ sol suspension at 1:2 volume ratio. The volume concentration of TiO₂ sol used in this study was 16.7, 25, and 50 vol%, respectively. After sonication for 20 min, the suspensions were kept for 3 days at room temperature to allow complete reaction between the TiO₂ sol and MWCNTs. After that, the CNT/TiO₂ sol assemblies were filtered out and washed with DI water. The collection was dried at 80°C for 16 hrs.

The morphologies of CNTs and CNT/TiO₂ sol assemblies were examined by TEM (JEOL 2010F, 200 kV TEM, JEOL Ltd., Tokyo, Japan). EDS (Noran System 6, Thermo NanoTrace, Waltham, MA) was employed to measure the relative amount of TiO₂ sol attached onto MWCNTs. The oxidation of MWCNTs was characterized by FT-IR (Nicolet 8700, Thermo Scientific, Waltham, MA).

From this work, we have observed that the functional group content increases with increasing oxidation temperature. Also, TiO₂ sol has been successfully assembled onto MWCNT surfaces. Surface modification of MWCNTs with acid oxidation greatly improves the reactivity of MWCNTs with TiO₂ sol. The thickness of the TiO₂ sol layers increases with TiO₂ sol concentration. The assembly mechanism is shown to be primarily esterification between the –COOH groups of the oxidized MWCNT surfaces and the –OH groups of TiO₂ sol. This research provides a more controlled approach of assembling nano-species onto MWCNT surfaces. Also, the resultant assembly is more stable due to chemical bonding.

This research provides a more controlled approach of assembling nano-species onto MWCNT surfaces. This nanoparticle-CNT assembly approach can be used for performance improvement of solar cells, catalyst, sensors, and nanoelectronic devices.

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 surface modification and TiO₂ sol assembly process, an important area in nanomaterial research. We successfully demonstrated that the surface of MWCNTs can be functioanlized and CNTs and TiO₂ sols can be assembled via esterification. Also, we were able to publish our papers and present our research findings in international conferences.

On the scholars participated in the project: The project provided an excellent opportunity for the post-doc researchers to advance their careers. During this project, they were able to conduct challenging experiments and solve problems independently. They also got the opportunities to analyze the research findings in the leading-edge nano-research areas. Also, Dr. Dong presented her research results during an international conference.

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