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Highly efficient electronic sensitization of non-oxidized graphene flakes on controlled pore-loaded WO3 nanofibers for selective detection of H2S molecules.

Choi SJ, Choi C, Kim SJ, Cho HJ, Hakim M, Jeon S, Kim ID - Sci Rep (2015)

Bottom Line: A tentacle-like structure with randomly distributed pores on the surface of electrospun WO3 NFs were achieved, which exhibited improved surface area as well as porosity.Porous WO3 NFs with enhanced surface area exhibited high gas response (Rair/Rgas = 43.1 at 5 ppm) towards small and light H2S molecules.In contrast, porous WO3 NFs with maximized pore diameter showed a high response (Rair/Rgas = 2.8 at 5 ppm) towards large and heavy acetone molecules.

View Article: PubMed Central - PubMed

Affiliation: Department of Materials Science and Engineering, Korea Advanced Institute of Science and Technology, 291 Daehak-ro, Yuseong-gu, Daejeon 305-701, Republic of Korea.

ABSTRACT
Tailoring of semiconducting metal oxide nanostructures, which possess controlled pore size and concentration, is of great value to accurately detect various volatile organic compounds in exhaled breath, which act as potential biomarkers for many health conditions. In this work, we have developed a very simple and robust route for controlling both the size and distribution of spherical pores in electrospun WO3 nanofibers (NFs) via a sacrificial templating route using polystyrene colloids with different diameters (200 nm and 500 nm). A tentacle-like structure with randomly distributed pores on the surface of electrospun WO3 NFs were achieved, which exhibited improved surface area as well as porosity. Porous WO3 NFs with enhanced surface area exhibited high gas response (Rair/Rgas = 43.1 at 5 ppm) towards small and light H2S molecules. In contrast, porous WO3 NFs with maximized pore diameter showed a high response (Rair/Rgas = 2.8 at 5 ppm) towards large and heavy acetone molecules. Further enhanced sensing performance (Rair/Rgas = 65.6 at 5 ppm H2S) was achieved by functionalizing porous WO3 NFs with 0.1 wt% non-oxidized graphene (NOGR) flakes by forming a Schottky barrier (ΔΦ = 0.11) at the junction between the WO3 NFs (Φ = 4.56 eV) and NOGR flakes (Φ = 4.67 eV), which showed high potential for the diagnosis of halitosis.

No MeSH data available.


Related in: MedlinePlus

(a) Base resistance characteristics of dense WO3 NFs, PS (500)-WO3 NFs, PS (200&500)-WO3 NFs, and PS (500)-WO3 NFs funtionalized with 0.1 wt% NOGR at 350°C, (b) BET surface area and average pore diameter of the dense WO3 NFs, PS (500)-WO3 NFs, and PS (200&500)-WO3 NFs, (c) pore distribution analysis using N2 vapor in the range of 2–16 nm, and (d) pore distribution analysis using Hg in the range of 3–5000 nm.
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f6: (a) Base resistance characteristics of dense WO3 NFs, PS (500)-WO3 NFs, PS (200&500)-WO3 NFs, and PS (500)-WO3 NFs funtionalized with 0.1 wt% NOGR at 350°C, (b) BET surface area and average pore diameter of the dense WO3 NFs, PS (500)-WO3 NFs, and PS (200&500)-WO3 NFs, (c) pore distribution analysis using N2 vapor in the range of 2–16 nm, and (d) pore distribution analysis using Hg in the range of 3–5000 nm.

Mentions: To understand the sensing mechanism depending on the pore size and pore distribution, the base resistance, Brunauer-Emmett-Teller (BET) surface area, and average pore diameters were measured (Figure 6). The base resistances of all the sensors were confirmed, which are closely related with the packing density of the sensing materials42. Similar base resistances were observed with dense WO3 NFs (15.57 MΩ), PS (500)-WO3 NFs (10.22 MΩ), and PS (200&500)-WO3 NFs (16.31 MΩ) at an operating temperature of 350°C, which implies that the three sensors have similar packing densities (Figure 6a). The BET surface area and average pore diameter were measured by a surface area analyzer using N2 vapor. Although dramatic changes were not observed in the BET surface area and average pore diameter, the PS (200&500)-WO3 NFs exhibited the highest BET surface area (19.22 m2/g) while possessing the lowest average pore diameter (7.98 nm) among the three samples (Figure 6b). In the case of PS (500)-WO3 NFs, the highest average pore diameter was observed (8.96 nm) with a moderate BET surface area (18.60 m2/g). The dense WO3 NFs showed the lowest BET surface area (17 m2/g) with a moderate average pore diameter of 8.25 nm. It can be explained that the BET surface area was increased by the pore formation on the surface of WO3 NFs, in which a higher increase in BET surface area was achieved with the mixture of two different PS colloids (i.e., 200 nm and 500 nm in diameter) than the use of single PS colloids (i.e., 500 nm in diameter). The pore distribution analysis in a mesoscale range of 2–15 nm revealed that both PS (500)-WO3 NFs and PS (200&500)-WO3 NFs exhibited higher pore volume compared to the dense WO3 NFs (Figure 6c). The higher average pore diameter of the dense WO3 NFs than the PS (200&500)-WO3 NFs was mainly assumed to be the increased density of pores at the surface of the WO3 NFs and the formation of the dense WO3 layer around pores. However, the pore sizes created by the removal of PS colloids must be larger than 30 nm. Therefore, we investigated the macroscale (>50 nm) pore distribution in the range of 50–5000 nm using a porosimeter using Hg (Figure 6d). The result revealed that the higher pore density was observed with the PS (500)-WO3 NFs and PS (200&500)-WO3 NFs compared to the dense WO3 NFs. Similar pore diameters of 678 nm and 677 nm were measured with PS (500)-WO3 NFs and PS (200&500)-WO3 NFs, respectively, which were noticeably larger than that (433 nm) of dense PS-WO3 NFs. Macroscale pores (433 nm), formed in dense PS-WO PS-WO3 NFs, are originated from voids between the WO3 NFs. In the case of PS (200&500)-WO3 NFs, linked pores, which might be formed by interconnection between pores formed by PS colloids having different diameters of 200 nm and 500 nm, can contribute to the counting of larger pore sizes. This feature can lead to comparable average pore sizes (677 nm) of PS (200&500)-WO3 NFs as compared to that (678 nm) of PS (500)-WO3 NFs by compensating the small pore sizes formed by discrete PS colloids having diameter of 200 nm. However, pore density of PS (200&500)-WO3 NFs is higher than that of the PS (500)-WO3 NFs, which is ascribed to the increase in pore density at the surface of the WO3 NFs by forming linked pores formed by PS colloids having different diameters of 200 nm and 500 nm. The gas sensing characteristics, therefore, can be interpreted to mean that the larger the BET surface, i.e., PS (200&500)-WO3 NFs, the higher the H2S sensing performance (Figure 5a), which is attributed to the fact that the small and light molecule of H2S can penetrate deep into the sensing layers, thereby enhancing the surface reaction between H2S and chemisorbed oxygen species (O2−, O−, and O2−). In contrast, large and heavy molecules such as acetone cannot penetrate deep into the sensing layer, which resulted in less of a surface reaction, leading to the lowest acetone response (Figure 5b) to the PS (200&500)-WO3 NFs having relatively small pore diameter (average 7.98 nm). For this reason, the highest acetone sensing performance was achieved with the PS (500)-WO3 NFs with an increased pore diameter on the mesoscale (2–50 nm) as well as the macroscale (>50 nm) dimensions, in which the response is controlled by the acetone diffusion into the sensing layers and corresponding surface reaction. This observation is consistent with a previous report using SnO2 nanoparticle for sensing different analytes4344. We expect further improvement of gas response via the optimization of pore size and distribution by using PS colloids with various sizes and concentrations.


Highly efficient electronic sensitization of non-oxidized graphene flakes on controlled pore-loaded WO3 nanofibers for selective detection of H2S molecules.

Choi SJ, Choi C, Kim SJ, Cho HJ, Hakim M, Jeon S, Kim ID - Sci Rep (2015)

(a) Base resistance characteristics of dense WO3 NFs, PS (500)-WO3 NFs, PS (200&500)-WO3 NFs, and PS (500)-WO3 NFs funtionalized with 0.1 wt% NOGR at 350°C, (b) BET surface area and average pore diameter of the dense WO3 NFs, PS (500)-WO3 NFs, and PS (200&500)-WO3 NFs, (c) pore distribution analysis using N2 vapor in the range of 2–16 nm, and (d) pore distribution analysis using Hg in the range of 3–5000 nm.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC4308697&req=5

f6: (a) Base resistance characteristics of dense WO3 NFs, PS (500)-WO3 NFs, PS (200&500)-WO3 NFs, and PS (500)-WO3 NFs funtionalized with 0.1 wt% NOGR at 350°C, (b) BET surface area and average pore diameter of the dense WO3 NFs, PS (500)-WO3 NFs, and PS (200&500)-WO3 NFs, (c) pore distribution analysis using N2 vapor in the range of 2–16 nm, and (d) pore distribution analysis using Hg in the range of 3–5000 nm.
Mentions: To understand the sensing mechanism depending on the pore size and pore distribution, the base resistance, Brunauer-Emmett-Teller (BET) surface area, and average pore diameters were measured (Figure 6). The base resistances of all the sensors were confirmed, which are closely related with the packing density of the sensing materials42. Similar base resistances were observed with dense WO3 NFs (15.57 MΩ), PS (500)-WO3 NFs (10.22 MΩ), and PS (200&500)-WO3 NFs (16.31 MΩ) at an operating temperature of 350°C, which implies that the three sensors have similar packing densities (Figure 6a). The BET surface area and average pore diameter were measured by a surface area analyzer using N2 vapor. Although dramatic changes were not observed in the BET surface area and average pore diameter, the PS (200&500)-WO3 NFs exhibited the highest BET surface area (19.22 m2/g) while possessing the lowest average pore diameter (7.98 nm) among the three samples (Figure 6b). In the case of PS (500)-WO3 NFs, the highest average pore diameter was observed (8.96 nm) with a moderate BET surface area (18.60 m2/g). The dense WO3 NFs showed the lowest BET surface area (17 m2/g) with a moderate average pore diameter of 8.25 nm. It can be explained that the BET surface area was increased by the pore formation on the surface of WO3 NFs, in which a higher increase in BET surface area was achieved with the mixture of two different PS colloids (i.e., 200 nm and 500 nm in diameter) than the use of single PS colloids (i.e., 500 nm in diameter). The pore distribution analysis in a mesoscale range of 2–15 nm revealed that both PS (500)-WO3 NFs and PS (200&500)-WO3 NFs exhibited higher pore volume compared to the dense WO3 NFs (Figure 6c). The higher average pore diameter of the dense WO3 NFs than the PS (200&500)-WO3 NFs was mainly assumed to be the increased density of pores at the surface of the WO3 NFs and the formation of the dense WO3 layer around pores. However, the pore sizes created by the removal of PS colloids must be larger than 30 nm. Therefore, we investigated the macroscale (>50 nm) pore distribution in the range of 50–5000 nm using a porosimeter using Hg (Figure 6d). The result revealed that the higher pore density was observed with the PS (500)-WO3 NFs and PS (200&500)-WO3 NFs compared to the dense WO3 NFs. Similar pore diameters of 678 nm and 677 nm were measured with PS (500)-WO3 NFs and PS (200&500)-WO3 NFs, respectively, which were noticeably larger than that (433 nm) of dense PS-WO3 NFs. Macroscale pores (433 nm), formed in dense PS-WO PS-WO3 NFs, are originated from voids between the WO3 NFs. In the case of PS (200&500)-WO3 NFs, linked pores, which might be formed by interconnection between pores formed by PS colloids having different diameters of 200 nm and 500 nm, can contribute to the counting of larger pore sizes. This feature can lead to comparable average pore sizes (677 nm) of PS (200&500)-WO3 NFs as compared to that (678 nm) of PS (500)-WO3 NFs by compensating the small pore sizes formed by discrete PS colloids having diameter of 200 nm. However, pore density of PS (200&500)-WO3 NFs is higher than that of the PS (500)-WO3 NFs, which is ascribed to the increase in pore density at the surface of the WO3 NFs by forming linked pores formed by PS colloids having different diameters of 200 nm and 500 nm. The gas sensing characteristics, therefore, can be interpreted to mean that the larger the BET surface, i.e., PS (200&500)-WO3 NFs, the higher the H2S sensing performance (Figure 5a), which is attributed to the fact that the small and light molecule of H2S can penetrate deep into the sensing layers, thereby enhancing the surface reaction between H2S and chemisorbed oxygen species (O2−, O−, and O2−). In contrast, large and heavy molecules such as acetone cannot penetrate deep into the sensing layer, which resulted in less of a surface reaction, leading to the lowest acetone response (Figure 5b) to the PS (200&500)-WO3 NFs having relatively small pore diameter (average 7.98 nm). For this reason, the highest acetone sensing performance was achieved with the PS (500)-WO3 NFs with an increased pore diameter on the mesoscale (2–50 nm) as well as the macroscale (>50 nm) dimensions, in which the response is controlled by the acetone diffusion into the sensing layers and corresponding surface reaction. This observation is consistent with a previous report using SnO2 nanoparticle for sensing different analytes4344. We expect further improvement of gas response via the optimization of pore size and distribution by using PS colloids with various sizes and concentrations.

Bottom Line: A tentacle-like structure with randomly distributed pores on the surface of electrospun WO3 NFs were achieved, which exhibited improved surface area as well as porosity.Porous WO3 NFs with enhanced surface area exhibited high gas response (Rair/Rgas = 43.1 at 5 ppm) towards small and light H2S molecules.In contrast, porous WO3 NFs with maximized pore diameter showed a high response (Rair/Rgas = 2.8 at 5 ppm) towards large and heavy acetone molecules.

View Article: PubMed Central - PubMed

Affiliation: Department of Materials Science and Engineering, Korea Advanced Institute of Science and Technology, 291 Daehak-ro, Yuseong-gu, Daejeon 305-701, Republic of Korea.

ABSTRACT
Tailoring of semiconducting metal oxide nanostructures, which possess controlled pore size and concentration, is of great value to accurately detect various volatile organic compounds in exhaled breath, which act as potential biomarkers for many health conditions. In this work, we have developed a very simple and robust route for controlling both the size and distribution of spherical pores in electrospun WO3 nanofibers (NFs) via a sacrificial templating route using polystyrene colloids with different diameters (200 nm and 500 nm). A tentacle-like structure with randomly distributed pores on the surface of electrospun WO3 NFs were achieved, which exhibited improved surface area as well as porosity. Porous WO3 NFs with enhanced surface area exhibited high gas response (Rair/Rgas = 43.1 at 5 ppm) towards small and light H2S molecules. In contrast, porous WO3 NFs with maximized pore diameter showed a high response (Rair/Rgas = 2.8 at 5 ppm) towards large and heavy acetone molecules. Further enhanced sensing performance (Rair/Rgas = 65.6 at 5 ppm H2S) was achieved by functionalizing porous WO3 NFs with 0.1 wt% non-oxidized graphene (NOGR) flakes by forming a Schottky barrier (ΔΦ = 0.11) at the junction between the WO3 NFs (Φ = 4.56 eV) and NOGR flakes (Φ = 4.67 eV), which showed high potential for the diagnosis of halitosis.

No MeSH data available.


Related in: MedlinePlus