Morphological control of heterostructured nanowires synthesized by sol-flame method

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.


Background
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][2][3][4][5][6][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 TiO 2 NWs for photoelectrochemical water-splitting exhibited an incident photon-to-current conversion efficiency that is two times higher than that of bare TiO 2 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][22][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 NPchain 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 (Co 3 O 4 ) as a model system because both CuO and Co 3 O 4 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][25][26][27][28]. We study the dependence of the final morphology of the decorated Co 3 O 4 on the chemical compositions of the solvent and the cobalt salt used in the cobalt precursor solution.

Synthesis of CuO NWs
CuO NWs are first synthesized by a thermal annealing method [29][30][31][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.

Sol-flame synthesis of Co 3 O 4 decorated CuO NWs
The general procedure of the sol-flame method for the synthesis of heterostructured NWs was described previously [21][22][23] and is shown schematically in Figure 1a The burner is operated with CH 4 and H 2 as fuels, and air as the oxidizer with a fuel to oxidizer equivalence ratio (Φ) of 0.84 (the flow rates of CH 4 , H 2 , and air are 2.05, 4.64, and 36.7 SLPM (standard liter per minute), respectively). The typical temperature of the postflame region gas is 990°C that is measured by a Ktype thermocouple (1/16 in. bead diameter, Omega Engineering, Inc., Stamford, CT, USA). The typical flame annealing time is 5 s.

Effects of solvent on the morphology of Co 3 O 4 on the CuO NWs
We first investigate the effect of residual solvent in the cobalt precursor on the final morphology of Co 3 O 4 . Typically, the cobalt precursor consists of cobalt acetate (Co(CH 3 COO) 2 ·4H 2 O) dissolved in acetic acid (CH 3 COOH) 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)  residual solvent, and a monolayer coating of Co 3 O 4 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 [36]. In this case, no particles are observed at all, but instead, a conformal and dense layer of Co 3 O 4 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 Co 3 O 4 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. 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 Co 3 O 4 , 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 This faster induced gas flow carries cobalt acetate further away from the CuO NWs, forming longer NPchains. 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.

Effects of cobalt salt precursor on the morphology of Co 3 O 4 on the CuO NWs
While the morphology of Co 3 O 4 is significantly affected by the solvent, it will also depend on the properties of the cobalt salt precursors, such as their volatility. To focus on the effect of the cobalt salt precursor, the solvent is fixed to be acetic acid with the same drying condition of 0.4 h at 25°C in air, which leaves a large amount of acetic acid in the precursor coating. We study the effect of cobalt salt precursors on the   and combusts continuously and rapidly. At this stage, the volatility of the cobalt precursor affects the nucleation process. Cobalt acetate, as an organic precursor, is more volatile and evaporates together with solvent. Consequently, the nucleation of Co 3 O 4 NPs occurs in the gas phase and is a gas-to-particle conversion process (Figure 4, left panel) [37][38][39]. Therefore, the length of the NP-chains is directly affected by the induced gas flow velocity. In contrast, cobalt nitrate, as an inorganic precursor, is non-volatile and has high solubility in acetic acid. Consequently, cobalt nitrate will mostly remain in the liquid phase and decompose to form NPs in a liquid-to-particle conversion process (Figure 4, right panel) [39][40][41], leading to the formation of a shell composed of NP aggregates.

Conclusions
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 Co 3 O 4 -decorated CuO NWs. The final morphology of Co 3 O 4 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 Co 3 O 4 NP-chains. Solvents with higher combustion temperatures produce gas flows with larger velocity, leading to the formation of longer Co 3 O 4 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 Co 3 O 4 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 Co 3 O 4 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.