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Morphological control of heterostructured nanowires synthesized by sol-flame method.

Luo R, Cho IS, Feng Y, Cai L, Rao PM, Zheng X - Nanoscale Res Lett (2013)

Bottom Line: Here, we report the effects of the precursor solution on the final morphology of the heterostructured nanowire using Co3O4 decorated CuO nanowires as a model system.When a volatile cobalt salt precursor is used with sufficient residual solvent, both solvent and cobalt precursor evaporate during the flame annealing step, leading to the formation of Co3O4 nanoparticle chains by a gas-solid transition.On the other hand, when a non-volatile cobalt salt precursor is used, only the solvent evaporates and the cobalt salt is converted to nanoparticles by a liquid-solid transition, forming a conformal Co3O4 shell.

View Article: PubMed Central - HTML - PubMed

Affiliation: Department of Mechanical Engineering, Stanford University, Stanford, CA 94305, USA. xlzheng@stanford.edu.

ABSTRACT
Heterostructured nanowires, such as core/shell nanowires and nanoparticle-decorated nanowires, are versatile building blocks for a wide range of applications because they integrate dissimilar materials at the nanometer scale to achieve unique functionalities. The sol-flame method is a new, rapid, low-cost, versatile, and scalable method for the synthesis of heterostructured nanowires, in which arrays of nanowires are decorated with other materials in the form of shells or chains of nanoparticles. In a typical sol-flame synthesis, nanowires are dip-coated with a solution containing precursors of the materials to be decorated, then dried in air, and subsequently heated in the post-flame region of a flame at high temperature (over 900°C) for only a few seconds. Here, we report the effects of the precursor solution on the final morphology of the heterostructured nanowire using Co3O4 decorated CuO nanowires as a model system. When a volatile cobalt salt precursor is used with sufficient residual solvent, both solvent and cobalt precursor evaporate during the flame annealing step, leading to the formation of Co3O4 nanoparticle chains by a gas-solid transition. The length of the nanoparticle chains is mainly controlled by the temperature of combustion of the solvent. On the other hand, when a non-volatile cobalt salt precursor is used, only the solvent evaporates and the cobalt salt is converted to nanoparticles by a liquid-solid transition, forming a conformal Co3O4 shell. This study facilitates the use of the sol-flame method for synthesizing heterostructured nanowires with controlled morphologies to satisfy the needs of diverse applications.

No MeSH data available.


Effects of solvent on the degree of branching and size distribution of Co3O4 NPs. SEM images of Co3O4 NP-decorated CuO NWs synthesized using different solvents: (a) acetic acid and (b) propionic acid. (c) Histogram of distribution of Co3O4 NP size for these two solvents. Propionic acid has a higher temperature of combustion, resulting in a larger length of NP-chains and smaller size of the NPs compared to those resulting from the use of acetic acid.
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Figure 2: Effects of solvent on the degree of branching and size distribution of Co3O4 NPs. SEM images of Co3O4 NP-decorated CuO NWs synthesized using different solvents: (a) acetic acid and (b) propionic acid. (c) Histogram of distribution of Co3O4 NP size for these two solvents. Propionic acid has a higher temperature of combustion, resulting in a larger length of NP-chains and smaller size of the NPs compared to those resulting from the use of acetic acid.

Mentions: The formation of the NP-chain morphology is due to the generation of gases by the evaporation and combustion of the coated solution on the CuO NWs during flame annealing, which induces a gas flow (i.e., Stefan flow) [23]. The above results suggest that most of the gas flow comes from the evaporation and combustion of the residual solvent rather than from the cobalt salt inside the cobalt precursor solution. To investigate the effect of solvent on the morphology of Co3O4, we select another solvent, propionic acid, to compare with acetic acid. For both solvents, the dip-coated NW samples are dried for 0.4 h at 25°C to leave a large amount of solvent on the CuO NWs before flame annealing. It is assumed that a similar amount of cobalt precursor is left on CuO NWs after drying, in each case. The use of propionic acid leads to longer NP-chains (Figure 2b) and smaller average NP size (Figure 2c) than does the use of acetic acid (Figure 2a). The length of the NP-chains increases with increasing velocity (v) of the gas flow which carries the cobalt acetate precursor away from the CuO NWs as it forms NPs. The induced gas velocity is determined by the mass flux () of the evaporated solution and the density (ρ) of the solution vapor as . The mass flux () of the evaporated solution depends most strongly on the temperature of solvent combustion, and the density (ρ) of the solution vapor is inversely proportional to the temperature of solvent combustion. Since the adiabatic combustion temperature of propionic acid (2,202 K) is higher than that of acetic acid (2,074 K), propionic acid leads to higher mass flux and lower solution vapor density, hence larger induced gas velocity. This faster induced gas flow carries cobalt acetate further away from the CuO NWs, forming longer NP-chains. The higher combustion temperature also leads to reduced gas density, which in turn reduces the gas phase concentration of cobalt acetic precursors, leading to smaller average NP size (Figure 2c). Hence, the length of the NP-chain and size of the NPs are mainly controlled by the combustion temperature of the solvent, which affects the induced gas flow velocity and the NP precursor concentration.


Morphological control of heterostructured nanowires synthesized by sol-flame method.

Luo R, Cho IS, Feng Y, Cai L, Rao PM, Zheng X - Nanoscale Res Lett (2013)

Effects of solvent on the degree of branching and size distribution of Co3O4 NPs. SEM images of Co3O4 NP-decorated CuO NWs synthesized using different solvents: (a) acetic acid and (b) propionic acid. (c) Histogram of distribution of Co3O4 NP size for these two solvents. Propionic acid has a higher temperature of combustion, resulting in a larger length of NP-chains and smaller size of the NPs compared to those resulting from the use of acetic acid.
© Copyright Policy - open-access
Related In: Results  -  Collection

License
Show All Figures
getmorefigures.php?uid=PMC3750428&req=5

Figure 2: Effects of solvent on the degree of branching and size distribution of Co3O4 NPs. SEM images of Co3O4 NP-decorated CuO NWs synthesized using different solvents: (a) acetic acid and (b) propionic acid. (c) Histogram of distribution of Co3O4 NP size for these two solvents. Propionic acid has a higher temperature of combustion, resulting in a larger length of NP-chains and smaller size of the NPs compared to those resulting from the use of acetic acid.
Mentions: The formation of the NP-chain morphology is due to the generation of gases by the evaporation and combustion of the coated solution on the CuO NWs during flame annealing, which induces a gas flow (i.e., Stefan flow) [23]. The above results suggest that most of the gas flow comes from the evaporation and combustion of the residual solvent rather than from the cobalt salt inside the cobalt precursor solution. To investigate the effect of solvent on the morphology of Co3O4, we select another solvent, propionic acid, to compare with acetic acid. For both solvents, the dip-coated NW samples are dried for 0.4 h at 25°C to leave a large amount of solvent on the CuO NWs before flame annealing. It is assumed that a similar amount of cobalt precursor is left on CuO NWs after drying, in each case. The use of propionic acid leads to longer NP-chains (Figure 2b) and smaller average NP size (Figure 2c) than does the use of acetic acid (Figure 2a). The length of the NP-chains increases with increasing velocity (v) of the gas flow which carries the cobalt acetate precursor away from the CuO NWs as it forms NPs. The induced gas velocity is determined by the mass flux () of the evaporated solution and the density (ρ) of the solution vapor as . The mass flux () of the evaporated solution depends most strongly on the temperature of solvent combustion, and the density (ρ) of the solution vapor is inversely proportional to the temperature of solvent combustion. Since the adiabatic combustion temperature of propionic acid (2,202 K) is higher than that of acetic acid (2,074 K), propionic acid leads to higher mass flux and lower solution vapor density, hence larger induced gas velocity. This faster induced gas flow carries cobalt acetate further away from the CuO NWs, forming longer NP-chains. The higher combustion temperature also leads to reduced gas density, which in turn reduces the gas phase concentration of cobalt acetic precursors, leading to smaller average NP size (Figure 2c). Hence, the length of the NP-chain and size of the NPs are mainly controlled by the combustion temperature of the solvent, which affects the induced gas flow velocity and the NP precursor concentration.

Bottom Line: Here, we report the effects of the precursor solution on the final morphology of the heterostructured nanowire using Co3O4 decorated CuO nanowires as a model system.When a volatile cobalt salt precursor is used with sufficient residual solvent, both solvent and cobalt precursor evaporate during the flame annealing step, leading to the formation of Co3O4 nanoparticle chains by a gas-solid transition.On the other hand, when a non-volatile cobalt salt precursor is used, only the solvent evaporates and the cobalt salt is converted to nanoparticles by a liquid-solid transition, forming a conformal Co3O4 shell.

View Article: PubMed Central - HTML - PubMed

Affiliation: Department of Mechanical Engineering, Stanford University, Stanford, CA 94305, USA. xlzheng@stanford.edu.

ABSTRACT
Heterostructured nanowires, such as core/shell nanowires and nanoparticle-decorated nanowires, are versatile building blocks for a wide range of applications because they integrate dissimilar materials at the nanometer scale to achieve unique functionalities. The sol-flame method is a new, rapid, low-cost, versatile, and scalable method for the synthesis of heterostructured nanowires, in which arrays of nanowires are decorated with other materials in the form of shells or chains of nanoparticles. In a typical sol-flame synthesis, nanowires are dip-coated with a solution containing precursors of the materials to be decorated, then dried in air, and subsequently heated in the post-flame region of a flame at high temperature (over 900°C) for only a few seconds. Here, we report the effects of the precursor solution on the final morphology of the heterostructured nanowire using Co3O4 decorated CuO nanowires as a model system. When a volatile cobalt salt precursor is used with sufficient residual solvent, both solvent and cobalt precursor evaporate during the flame annealing step, leading to the formation of Co3O4 nanoparticle chains by a gas-solid transition. The length of the nanoparticle chains is mainly controlled by the temperature of combustion of the solvent. On the other hand, when a non-volatile cobalt salt precursor is used, only the solvent evaporates and the cobalt salt is converted to nanoparticles by a liquid-solid transition, forming a conformal Co3O4 shell. This study facilitates the use of the sol-flame method for synthesizing heterostructured nanowires with controlled morphologies to satisfy the needs of diverse applications.

No MeSH data available.