Metal Oxide Nanofiber Ultra-Sensitive Gas Sensor Prototype and Nanofabrication Process Unveiled by Korea Institute of Science and Technology Researchers

FIG. 3 illustrates images of a  prototype test sensor device using a TiO₂ metal oxide nanofiber fabricated by performing electrospinning, thermocompression, and thermal treatment on a sensor electrode formed on an alumina substrate at the Korea Institute of Science and Technology.

An ultra-sensitive metal oxide gas sensor and a fabrication method, in which a fine structure of a metal oxide has a nanorod or nanograin structure with excellent gas diffusivity and remarkably increased specific surface area earned U.S. Patent 7,640,789 for the Korea Institute of Science and Technology (KIST) (Seoul, KR)

The sensor is made using thermocompression (hot pressing) to attain a fast response time and high sensitivity sensing, and adhesion between a porous metal oxide fiber and a sensor substrate is enhanced, say inventors Il-Doo Kim,  Jae-Min Hong, Dong-Young Kim,  Seong-Mu Jo, Avner Rothschild and Harry L. Tuller  

The ultra-sensitive metal oxide gas sensor  makeup includes a sensor electrode and a porous metal oxide thin layer formed on the sensor electrode, having a network structure of nanofibers consisting of a single crystalline nano-rod. In this case, the porous metal oxide thin layer has a macro-pore between nanofibers and a meso-pore between nano-rods.

 Another version of an ultra-sensitive metal oxide gas sensor includes a sensor electrode; and a porous metal oxide thin layer formed on the sensor electrode, having a network structure of nanofibers in which single crystalline nano-grains are conglomerated and twisted. In this case, the porous metal oxide thin layer has a macro-pore between nanofibers and a meso-pore between nano-grains.

In yet another version an ultra-sensitive metal oxide gas sensor includes a sensor electrode; and a porous metal oxide thin layer formed on the sensor electrode, having a network structure of nano-rods consisting of nano-grains. In this case, the porous metal oxide thin layer has a meso-pore between nano-rods and a meso-pore between nano-grains.
 
An ultra-sensitive metal oxide gas sensor may also be comprised of a sensor electrode; and a porous metal oxide thin layer formed on the sensor electrode, having a network structure of nanofibers consisting of at least one of nano-grain and nano-rod. The metal oxide thin layer includes ZnO, SnO₂, VO₂, TiO₂, In₂O₃, CaCu₃Ti₄O₁₂, NiO, MoO₃, SrTiO₃, Fe₂O₃, TiO₂ doped with at least one of Nb, Fe, Co, and V, SrTiO₃ doped with Fe, or ZnO doped with at least one of In and Ga.

The method for fabricating an ultra-sensitive metal oxide gas sensor, includes the steps of spinning a mixture solution including a metal oxide precursor and a polymer onto a sensor electrode to form a metal oxide precursor-polymer composite fiber; thermally compressing or thermally pressurizing the composite fiber; and thermally treating the thermally compressed or thermally pressurized composite fiber to remove the polymer from the composite fiber.

The metal oxide precursor includes a precursor constituting ZnO, SnO2, VO2, TiO2, In₂O3, CaCu₃Ti4O1₂, NiO, MoO₃, SrTiO₃, Fe₂O₃, a precursor constituting TiO₂ doped with at least one of Nb, Fe, Co, and V, a precursor constituting SrTiO₃ doped with Fe, or a precursor constituting ZnO doped with at least one of In and Ga. 

FIG. 1 is a schematic view illustrating an electrospinning device used in forming nanosensors

FIG. 2 illustrates electrospinning, thermocompression, and thermal treatment for fabrication of a sensor

FIG. 4 illustrates an array sensor using a nanofiber mats fabricated by electrospinning, thermocompression, and thermal treatment for different kinds of metal oxides on a sensor electrode having an array sensor structure, provided with a high temperature heater, wherein examples of the metal oxide nanofiber sensor include ZnO, SnO₂, TiO₂, CaCu₃Ti₄O₁₂.

FIG. 5 illustrates (a) a SEM image of a TiO₂/PVAc composite fiber formed by electrospinning from a mixture solution fabricated by using a TiO₂ precursor and PVAc, on an interdigital capacitor (IDC) consisting of Pt electrodes on an Al₂O₃₂ nanofiber obtained by thermal treatment of a TiO₂/PVAc composite fiber at 450.degree. C. for 30 minutes. 

FIG. 6 illustrates (a) a SEM image of a TiO₂/PVAc composite fiber obtained by thermocompression under the pressure of 1.5 Kgf/cm² (213.4 psi) at 120.degree. C for 10 minutes in accordance with the first embodiment of the present invention. Thermal compression is introduced to drive the polymer (PVAc) above its glass transition temperature resulting in markedly better adhesion due to the partial or entire melting of PVAc. Subsequent heat treatment resulted in nanofibers consisting of nanorods as shown in FIG. 6(b). The adhesion is markedly enhanced. The inset of FIG. 6(b) exhibits the microstructure of nanorods. 

FIG. 7 illustrates (a) a TEM image of a TiO₂ nanorod generated by thermal treatment after melting of polymer through thermocompression, (b) a TiO₂ nanorod having a single crystalline structure in the range of width 10 nm to 20 nm and length of 50 nm to 100 nm, (c) lattice image of a single crystalline TiO₂ nanorod and (d) Fourier transform (FFT) electron diffraction pattern showing a single crystalline TiO₂ nanorod having an anatase structure. substrate in accordance with a preferred embodiment (b) a SEM image of a TiO2,