Enhancing microwave absorption of TiO 2 nanocrystals via hydrogenation
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Department of Chemistry, University of Missouri – Kansas City, Kansas City, Missouri 64110, USA (Received 25 May 2014; accepted 12 August 2014)
TiO2 has attracted tremendous research interest for photocatalytic water splitting, solar hydrogen generation, environmental pollution removal, dye-sensitized solar cells, lithium-ion batteries, supercapacitors, and field emission. Microwave absorption materials (MAMs) play important roles in many military (e.g., the stealth coating on the B-2 bomber) and civil (e.g., telecommunications, noise reduction, information security, signal, and data protection) applications. However, TiO2 is not a good MAM due to its poor absorption in the microwave region. Here, we report that via hydrogenation excellent and tunable microwave absorption is achieved with hydrogenated TiO2 nanocrystals. After hydrogenation, 4.3x and 103x improvements have been obtained in storing and dissipating the electric energy of the microwave electromagnetic field. Their permittivity values are higher than those of the current carbonaceous MAMs. Instead of relying on the dipole rotation or ferromagnetic resonance mechanisms for traditional MAMs, the hydrogenated TiO2 nanocrystals work as good MAMs based on a newly proposed collective-movement-of-interfacial-dipole (CMID) mechanism. Although there is still no direct physical evidence of the interface effects of the CMID mechanism, the CMID as a hypothesis at this point successfully explained the origin of the enhanced microwave absorption of the hydrogenated TiO2 nanoparticles. This study thus may open new applications for TiO2 nanocrystals and also stimulate new approaches for new MAM development.
I. INTRODUCTION
Address all correspondence to this author. e-mail: [email protected] This author was an editor of this journal during the review and decision stage. For the JMR policy on review and publication of manuscripts authored by editors, please refer to http://www.mrs. org/jmr-editor-manuscripts/. DOI: 10.1557/jmr.2014.227
movement-of-interfacial-dipole (CMID) mechanism. This study paves a way for new applications of TiO 2 and other oxide materials beyond their traditional fields and opens a new avenue for developing new generations of microwave absorption materials (MAMs). Gigahertz MAMs play important roles in many military (e.g., the stealth coating on the B-2 bomber) and civil (e.g., telecommunications, noise reduction, information security, signal, and data protection) applications.22–28 Increasing the absorption capability of shielding materials in the microwave region (218 GHz) is key to minimize radar reflection from aircraft surfaces to avoid radar detection, and to reduce data and information leakage in various electronic devices.22–28 The absorption of the microwave electromagnetic field can be attributed to the losses of the electrical and magnetic fields of the microwave radiation within the materials. 24,25 The electrical loss is primarily caused by dielectric relaxation effects associated with the alignment of polar groups (permanent and indu
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