Morphological control of heterostructured nanowires synthesized by sol-flame method
© Luo et al.; licensee Springer. 2013
Received: 28 June 2013
Accepted: 2 August 2013
Published: 8 August 2013
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.
Synthesis of CuO NWs
CuO NWs are first synthesized by a thermal annealing method [29–32], where copper wires (wire diameter 0.0045 in.; McMaster, Atlanta, GA, USA) with a length of 1 cm are annealed at 550°C for 12 h in air in a tube furnace (Lindberg/Blue M, Waltham, MA, USA) to grow CuO NWs perpendicularly to the copper wire surface.
Preparation of cobalt precursor solutions
The cobalt precursor solutions with a typical concentration of 0.04 M are prepared by mixing cobalt acetate tetrahydrate (Co(CH3COO)2·4H2O, 99%, Sigma-Aldrich Chemicals, St. Louis, MO, USA) or cobalt nitrate hexahydrate (Co(NO3)2·6H2O, 99%, Sigma-Aldrich Chemicals) with acetic acid (CH3COOH, 99.7%, EMD Chemicals, Merck KGaA, Darmstadt, Germany) or propionic acid (C2H6COOH, 99%, Mallinckrodt Chemicals, St. Louis, MO, USA). After mixing the cobalt salt with the solvent, the cobalt precursor solutions are sonicated for 10 min to completely dissolve the cobalt salt and then aged overnight at room temperature before use.
Sol-flame synthesis of Co3O4 decorated CuO NWs
The burner is operated with CH4 and H2 as fuels, and air as the oxidizer with a fuel to oxidizer equivalence ratio (Φ) of 0.84 (the flow rates of CH4, H2, and air are 2.05, 4.64, and 36.7 SLPM (standard liter per minute), respectively). The typical temperature of the post-flame region gas is 990°C that is measured by a K-type thermocouple (1/16 in. bead diameter, Omega Engineering, Inc., Stamford, CT, USA). The typical flame annealing time is 5 s.
The morphology, crystal structure, and elemental composition of the prepared heterostructured NWs are characterized by scanning electron microscopy (SEM, FEI XL30 Sirion, 5 kV, Hillsboro, OR, USA), transmission electron microscopy (TEM, Philips CM20 FEG, 200 kV, Amsterdam, The Netherlands), and TEM-energy dispersive X-ray spectroscopy (EDS), respectively.
Results and discussion
Effects of solvent on the morphology of Co3O4 on the CuO NWs
We first investigate the effect of residual solvent in the cobalt precursor on the final morphology of Co3O4. Typically, the cobalt precursor consists of cobalt acetate (Co(CH3COO)2·4H2O) dissolved in acetic acid (CH3COOH) solvent. We study the effect of residual acetic acid on the CuO NWs by varying the drying conditions immediately after the dip-coating step. We test three different drying conditions in air: (1) 0.4 h at 25°C, (2) 22 h at 25°C, and (3) 1.5 h at 130°C, corresponding to increasing amounts of solvent evaporation during the drying process and, therefore, decreasing amounts of residual solvent, and then we anneal all the samples in the same flame condition. First, when the dip-coated NW sample is dried at 25°C for 0.4 h (highest amount of residual solvent), a Co3O4 NP-chain morphology is formed on the CuO NWs after flame annealing (Figure 1d). Second, the longer drying duration of 22 h at 25°C leads to a smaller amount of residual solvent, and a monolayer coating of Co3O4 NPs is formed after flame annealing (Figure 1e). Third, the amount of residual solvent is minimized by drying at 130°C, which is higher than the boiling temperature of acetic acid (118°C) but is lower than the decomposition temperature of cobalt acetate (230°C) to avoid precursor decomposition . In this case, no particles are observed at all, but instead, a conformal and dense layer of Co3O4 is coated onto CuO NWs (Figure 1f). In order to confirm the importance of the residual solvent, we reapply the solvent acetic acid by drop casting to the dip-coated NW that has been dried for 1.5 h at 130°C, and then air dry the NW again at 25°C for 0.4 h, and the NP-chain morphology is formed after flame annealing. These results clearly indicate that the amount of the residual solvent in the precursor coating layer (Figure 1c) before flame annealing has a strong impact on the final morphology of Co3O4 on the CuO NWs. A larger amount of residual solvent leads to the formation of the NP-chain morphology, and a smaller amount of residual solvent leads to the formation of shells, or equivalently a thin film coating.
Effects of cobalt salt precursor on the morphology of Co3O4 on the CuO NWs
To summarize, we have investigated the fundamental aspects of morphology control of heterostructured NWs synthesized by the sol-flame method for the model system of Co3O4-decorated CuO NWs. The final morphology of Co3O4 on the CuO NWs is greatly influenced by the properties of both the solvent and the cobalt salt used in the cobalt precursor solution. First, the evaporation and combustion of the solvent induces a gas flow away from the NWs that is responsible for the formation of Co3O4 NP-chains. Solvents with higher combustion temperatures produce gas flows with larger velocity, leading to the formation of longer Co3O4 NP-chains with smaller NP size. Second, the volatility of the cobalt precursor determines the precursor decomposition and nucleation process. A volatile cobalt precursor evaporates during flame annealing and converts to NPs in a gas-solid transition, forming Co3O4 NP-chains. Non-volatile cobalt precursor mainly remains in the liquid and converts to NPs in a liquid–solid transition, favoring the formation of a Co3O4 shell. Finally, we believe that this new understanding will facilitate the use of the sol-flame method for the synthesis of heterostructured NWs with tailored morphologies to satisfy the needs of diverse applications such as catalysis, sensors, solar cells, Li-ion batteries, and photosynthesis.
This research was funded by the ONR/PECASE program and Army Research Office under the grant W911NF-10-1-0106.
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