Introduction
Oxide nanofibers consisting of nanoparticles are excellent nanostructures for designing high performance gas sensors. To date, randomly tangled oxide nanofiber networks prepared by ...far-field electrospinning process (FFES) have been widely explored as the sensing materials. When the distance between spinneret needle and substrate is very short, the straight configuration of nanofibers can be directly written on the substrate without whipping. This is referred as near-field electrospinning (NFES), which can be applied to well-defined patterning of one-dimensional nanofibers for various new applications such as sensors, optoelectronic circuits, and functional nanoelectronics. The patterning of polymeric nanofibers has been demonstrated over a decade ago. In contrast, the fabrication of one-dimensional (1D) metal oxide nanopatterns remains challenging because it becomes difficult to control the chaotic whipping motion of the jet due to the acceleration of metal ions under the electric field. In the present study, grids, diamonds, and hexagrams composed of In
2
O
3
, Co
3
O
4
, and NiO nanofibers were prepared by NFES and their potential applications as gas sensors were investigated. The sensor using In
2
O
3
nanofiber pattern exhibited an unprecedentedly high response (resistance ratio = 239) to 5 ppm trimethylamine (TMA) with negligible cross-responses to ubiquitous ethanol and other interference gases.
Method
Transparent solution containing 7 g of methanol (99 %), polyvinylpyrrolidone (PVP, 17.4 wt%, Mw = 1,300,000) and metal nitrates (indium nitrate hydrate, 1.4 wt%, 99.999%; cobalt nitrate hexahydrate, 1.7 wt%, 99.99%; nickel nitrate hexahydrate, 1.7 wt%, 99.99%) were stirred for 24 h at room temperature. The prepared clear solution was transferred to a glass syringe (1000 μL) with a 21-gauge needle, and electrospun onto a 4-inch Si/SiO
2
wafer containing 76 sensor substrates (size: 1 × 1 cm
2
) in which Pt interdigitated electrodes (electrode width: 50 μm, electrode spacing: 5 μm) above a metallic collector. The flow rate is fixed as 0.005 μL min
−1
at an applied voltage of 50 V and the distance between the tip and the collector is 1 mm. During electrospinning, the loader and collector moved toward x- and/or y-axis at a controlled moving velocity of 800 mm s
−1
, leading to the fabrication of precursor fiber patterns, for example, grids, diamonds, and hexagrams. The precursor patterns were annealed at 600 °C for 5 h in an air atmosphere, and converted into the corresponding patterns of In
2
O
3
, Co
3
O
4
, and NiO pattern sensors, respectively. The sensors were placed in a quartz tube furnace and heated at sensing temperature (In
2
O
3
: 350 °C; Co
3
O
4
: 225 °C; NiO: 300 °C). The dc-probe resistance of the sensor was measured using an electrometer interfaced with a computer.
Results and Conclusions
The well-defined patterns of continuous and cylindrical oxide nanofibers could be prepared by using highly volatile solvent (methanol) and by controlling the viscosity and electrical conductivity of spray solution, the distance between spinneret needle and substrate, and the moving velocity of spinneret needle. All the parameters had an optimum window, indicating that the sophisticated control or tuning of the parameters is essential to prepare well-organized and continuous precursor and/or metal oxide fibers via NFES. It is worthwhile to note that the longstanding problem of preparing elaborate nano-architectures consisting of 1D metal oxides has been solved for the first time by investigating the interrelations of the solution/working parameters in NFES systematically. Interestingly, the sensor using In
2
O
3
nanofiber pattern on Pt electrodes exhibited unprecedented high selectivity and responses to TMA compared with other metal oxide nanostructures reported in the literature, whereas random networks of In
2
O
3
nanofibers and dense thin In
2
O
3
film with negligible Pt electrode exposure showed low response and selectivity to TMA. The intriguing trimethylamine sensing characteristics are explained in relation to the promotion of gas sensing reaction by catalytic Pt electrode, which is supported again by the high TMA selectivity of the sensors using the patterns of Co
3
O
4
and NiO nanofibers. In addition, the highly increased sensitivity to analyte gases compared with thin film sensors can be attributed by the increased gas accessibility due to the sensor structures, which is composed of mono-layered In
2
O
3
nanofibers with 200 nm diameter. The NFES-assisted patterning of metal oxide nanofibers can provide a new platform to design high performance gas sensors for a range of emerging applications.
Introduction
Oxide nanofibers consisting of nanoparticles are excellent nanostructures for designing high performance gas sensors. To date, randomly tangled oxide nanofiber networks prepared by ...far-field electrospinning process (FFES) have been widely explored as the sensing materials. When the distance between spinneret needle and substrate is very short, the straight configuration of nanofibers can be directly written on the substrate without whipping. This is referred as near-field electrospinning (NFES), which can be applied to well-defined patterning of one-dimensional nanofibers for various new applications such as sensors, optoelectronic circuits, and functional nanoelectronics. The patterning of polymeric nanofibers has been demonstrated over a decade ago. In contrast, the fabrication of one-dimensional (1D) metal oxide nanopatterns remains challenging because it becomes difficult to control the chaotic whipping motion of the jet due to the acceleration of metal ions under the electric field. In the present study, grids, diamonds, and hexagrams composed of In
2
O
3
, Co
3
O
4
, and NiO nanofibers were prepared by NFES and their potential applications as gas sensors were investigated. The sensor using In
2
O
3
nanofiber pattern exhibited an unprecedentedly high response (resistance ratio = 239) to 5 ppm trimethylamine (TMA) with negligible cross-responses to ubiquitous ethanol and other interference gases.
Method
Transparent solution containing 7 g of methanol (99 %), polyvinylpyrrolidone (PVP, 17.4 wt%, M
w
= 1,300,000) and metal nitrates (indium nitrate hydrate, 1.4 wt%, 99.999%; cobalt nitrate hexahydrate, 1.7 wt%, 99.99%; nickel nitrate hexahydrate, 1.7 wt%, 99.99%) were stirred for 24 h at room temperature. The prepared clear solution was transferred to a glass syringe (1000 μL) with a 21-gauge needle, and electrospun onto a 4-inch Si/SiO
2
wafer containing 76 sensor substrates (size: 1 × 1 cm
2
) in which Pt interdigitated electrodes (electrode width: 50 μm, electrode spacing: 5 μm) above a metallic collector. The flow rate is fixed as 0.005 μL min
−1
at an applied voltage of 50 V and the distance between the tip and the collector is 1 mm. During electrospinning, the loader and collector moved toward x- and/or y-axis at a controlled moving velocity of 800 mm s
−1
, leading to the fabrication of precursor fiber patterns, for example, grids, diamonds, and hexagrams. The precursor patterns were annealed at 600 °C for 5 h in an air atmosphere, and converted into the corresponding patterns of In
2
O
3
, Co
3
O
4
, and NiO pattern sensors, respectively. The sensors were placed in a quartz tube furnace and heated at sensing temperature (In
2
O
3
: 350 °C; Co
3
O
4
: 225 °C; NiO: 300 °C). The dc-probe resistance of the sensor was measured using an electrometer interfaced with a computer.
Results and Conclusions
The well-defined patterns of continuous and cylindrical oxide nanofibers could be prepared by using highly volatile solvent (methanol) and by controlling the viscosity and electrical conductivity of spray solution, the distance between spinneret needle and substrate, and the moving velocity of spinneret needle. All the parameters had an optimum window, indicating that the sophisticated control or tuning of the parameters is essential to prepare well-organized and continuous precursor and/or metal oxide fibers via NFES. It is worthwhile to note that the longstanding problem of preparing elaborate nano-architectures consisting of 1D metal oxides has been solved for the first time by investigating the interrelations of the solution/working parameters in NFES systematically. Interestingly, the sensor using In
2
O
3
nanofiber pattern on Pt electrodes exhibited unprecedented high selectivity and responses to TMA compared with other metal oxide nanostructures reported in the literature, whereas random networks of In
2
O
3
nanofibers and dense thin In
2
O
3
film with negligible Pt electrode exposure showed low response and selectivity to TMA. The intriguing trimethylamine sensing characteristics are explained in relation to the promotion of gas sensing reaction by catalytic Pt electrode, which is supported again by the high TMA selectivity of the sensors using the patterns of Co
3
O
4
and NiO nanofibers. In addition, the highly increased sensitivity to analyte gases compared with thin film sensors can be attributed by the increased gas accessibility due to the sensor structures, which is composed of mono-layered In
2
O
3
nanofibers with 200 nm diameter. The NFES-assisted patterning of metal oxide nanofibers can provide a new platform to design high performance gas sensors for a range of emerging applications.
