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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.

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Effects of solvent on the morphology of Co3O4 on CuO NWs. Schematic drawing of the sol-flame method (a), for which bare CuO NWs (b) are dip-coated with a cobalt precursor containing cobalt salt and solvent and air dried (c), followed by a rapid flame annealing process to form Co3O4-decorated CuO NW heterostructure. SEM image of Co3O4-decorated CuO NWs prepared by the sol-flame method with different air-drying conditions: 25°C for 0.4 h (d), 25°C for 22 h (e), 130°C for 1.5 h (f), and first dried at 130°C for 1.5 h, then reapplied acetic acid and dried at 25°C for 0.4 h (g). Extensive drying by increasing duration or temperature inhibits the formation of the Co3O4 NP-chain morphology.
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Figure 1: Effects of solvent on the morphology of Co3O4 on CuO NWs. Schematic drawing of the sol-flame method (a), for which bare CuO NWs (b) are dip-coated with a cobalt precursor containing cobalt salt and solvent and air dried (c), followed by a rapid flame annealing process to form Co3O4-decorated CuO NW heterostructure. SEM image of Co3O4-decorated CuO NWs prepared by the sol-flame method with different air-drying conditions: 25°C for 0.4 h (d), 25°C for 22 h (e), 130°C for 1.5 h (f), and first dried at 130°C for 1.5 h, then reapplied acetic acid and dried at 25°C for 0.4 h (g). Extensive drying by increasing duration or temperature inhibits the formation of the Co3O4 NP-chain morphology.

Mentions: Heterostructured nanowires (NWs), such as radially modulated core/shell NWs, axially modulated NWs, nanoparticle (NP)-decorated NWs, and branched NWs, are of great interest for diverse applications because they integrate dissimilar materials at the nanometer length scale on individual NWs to achieve unique and unprecedented functionalities [1-7]. Heterostructured NWs have already demonstrated their potential in applications such as photoelectrochemistry [8,9], catalysis [10], sensors [11,12], and batteries [13,14]. For instance, Ge/Si core/shell NW field-effect transistors achieve much higher performance than planar Si metal-oxide-semiconductor field-effect transistors due to one-dimensional quantum confinement effect [15]. In addition, InP NWs, for which the depletion regions are filled with InAsP quantum dots, showed an increase of carrier gain of four orders of magnitude per absorbed photon compared to a conventional diode structure as single-photo detectors [16]. Moreover, branched TiO2 NWs for photoelectrochemical water-splitting exhibited an incident photon-to-current conversion efficiency that is two times higher than that of bare TiO2 NWs resulting from increased surface area and improved charge separation and transport within the branches [17]. Controlled and scalable synthesis of heterostructured NWs is a critical prerequisite for their broad applications. Heterostructured NWs are currently synthesized by methods such as the sol–gel method [18], hydrothermal method [13], physical/chemical vapor deposition [19], and self-assembly [20]. Our group has recently developed a new sol-flame method (Figure 1a), which combines solution chemistry and rapid flame annealing to decorate NWs with other materials in the form of shells or chains of NPs to form heterostructured NWs [21-23]. Compared to other existing methods, the sol-flame method has the unique and important advantages of rapid material growth rate, low cost, versatility and scalability. Previously, we investigated the effect of flame annealing temperature on the final morphology of the heterostructured NWs and found that high temperature flame annealing leads to NP-chain formation and low temperature favors shell formation on the NWs. In this paper, we investigate the effects of solution chemical compositions on the morphology of the heterostructured NWs synthesized by the sol-flame method. We use copper (II) oxide (CuO) NWs decorated by cobalt (II, III) oxide (Co3O4) as a model system because both CuO and Co3O4 are important materials for catalysis and electrochemical applications and hence control of their composites and nanostructures during the synthesis is critical to improve their properties [24-28]. We study the dependence of the final morphology of the decorated Co3O4 on the chemical compositions of the solvent and the cobalt salt used in the cobalt precursor solution.


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 morphology of Co3O4 on CuO NWs. Schematic drawing of the sol-flame method (a), for which bare CuO NWs (b) are dip-coated with a cobalt precursor containing cobalt salt and solvent and air dried (c), followed by a rapid flame annealing process to form Co3O4-decorated CuO NW heterostructure. SEM image of Co3O4-decorated CuO NWs prepared by the sol-flame method with different air-drying conditions: 25°C for 0.4 h (d), 25°C for 22 h (e), 130°C for 1.5 h (f), and first dried at 130°C for 1.5 h, then reapplied acetic acid and dried at 25°C for 0.4 h (g). Extensive drying by increasing duration or temperature inhibits the formation of the Co3O4 NP-chain morphology.
© Copyright Policy - open-access
Related In: Results  -  Collection

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

Figure 1: Effects of solvent on the morphology of Co3O4 on CuO NWs. Schematic drawing of the sol-flame method (a), for which bare CuO NWs (b) are dip-coated with a cobalt precursor containing cobalt salt and solvent and air dried (c), followed by a rapid flame annealing process to form Co3O4-decorated CuO NW heterostructure. SEM image of Co3O4-decorated CuO NWs prepared by the sol-flame method with different air-drying conditions: 25°C for 0.4 h (d), 25°C for 22 h (e), 130°C for 1.5 h (f), and first dried at 130°C for 1.5 h, then reapplied acetic acid and dried at 25°C for 0.4 h (g). Extensive drying by increasing duration or temperature inhibits the formation of the Co3O4 NP-chain morphology.
Mentions: Heterostructured nanowires (NWs), such as radially modulated core/shell NWs, axially modulated NWs, nanoparticle (NP)-decorated NWs, and branched NWs, are of great interest for diverse applications because they integrate dissimilar materials at the nanometer length scale on individual NWs to achieve unique and unprecedented functionalities [1-7]. Heterostructured NWs have already demonstrated their potential in applications such as photoelectrochemistry [8,9], catalysis [10], sensors [11,12], and batteries [13,14]. For instance, Ge/Si core/shell NW field-effect transistors achieve much higher performance than planar Si metal-oxide-semiconductor field-effect transistors due to one-dimensional quantum confinement effect [15]. In addition, InP NWs, for which the depletion regions are filled with InAsP quantum dots, showed an increase of carrier gain of four orders of magnitude per absorbed photon compared to a conventional diode structure as single-photo detectors [16]. Moreover, branched TiO2 NWs for photoelectrochemical water-splitting exhibited an incident photon-to-current conversion efficiency that is two times higher than that of bare TiO2 NWs resulting from increased surface area and improved charge separation and transport within the branches [17]. Controlled and scalable synthesis of heterostructured NWs is a critical prerequisite for their broad applications. Heterostructured NWs are currently synthesized by methods such as the sol–gel method [18], hydrothermal method [13], physical/chemical vapor deposition [19], and self-assembly [20]. Our group has recently developed a new sol-flame method (Figure 1a), which combines solution chemistry and rapid flame annealing to decorate NWs with other materials in the form of shells or chains of NPs to form heterostructured NWs [21-23]. Compared to other existing methods, the sol-flame method has the unique and important advantages of rapid material growth rate, low cost, versatility and scalability. Previously, we investigated the effect of flame annealing temperature on the final morphology of the heterostructured NWs and found that high temperature flame annealing leads to NP-chain formation and low temperature favors shell formation on the NWs. In this paper, we investigate the effects of solution chemical compositions on the morphology of the heterostructured NWs synthesized by the sol-flame method. We use copper (II) oxide (CuO) NWs decorated by cobalt (II, III) oxide (Co3O4) as a model system because both CuO and Co3O4 are important materials for catalysis and electrochemical applications and hence control of their composites and nanostructures during the synthesis is critical to improve their properties [24-28]. We study the dependence of the final morphology of the decorated Co3O4 on the chemical compositions of the solvent and the cobalt salt used in the cobalt precursor solution.

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.


Related in: MedlinePlus