Flame Vapor Deposition Research Articles

Flame synthesis of WO3 nanotubes and nanowires for efficient photoelectrochemical water-splitting

Tungsten trioxide (WO3) is a technologically important material for photoelectrochemical (PEC) water-splitting for the solar production of hydrogen fuel from water. For PEC water-splitting, high aspect ratio WO3 nanostructures such as nanowires (NWs) and nanotubes (NTs) are superior to planar WO3 films because they orthogonalize the directions of light absorption (along the long axis) and charge transport (across the short radius), leading to both efficient light absorption and charge carrier collection. However, PEC water-splitting requires the growth of WO3 on delicate transparent conducting oxide (TCO) substrates that cannot tolerate high temperature processing. To date, the large-scale, rapid, economical synthesis of high aspect ratio WO3 nanostructures on these delicate TCO substrates remains a major challenge. Previously, we synthesized WO3 NW arrays by a rapid, atmospheric and scalable flame vapor deposition (FVD) method, in which a flame oxidizes and evaporates tungsten metal to produce tungsten oxide vapors that condense onto a colder substrate in the form of NWs. Nevertheless, at substrate temperatures low enough to ensure the health of the TCO, the growth of WO3 NW arrays was non-uniform and sparse due to limitations of the experimental design. Herein, we significantly improve the FVD design to grow uniform and densely packed WO3 nanostructures on TCO substrates, thereby enabling the application of these WO3 nanostructures to PEC water-splitting. The morphology of the nanostructures varied from densely packed multi-shell NTs and single-shell NTs to NWs as we increased the substrate temperature in the range 530–700°C. Importantly, the WO3 NTs synthesized by FVD had higher areal number density and longer length than state-of-the-art WO3 NW photoanodes grown by chemical vapor deposition and hydrothermal methods, resulting in stronger light absorption and superior PEC water-splitting performance. Thus, in addition to being scalable, rapid and economical, the FVD method also synthesizes materials of high quality.

Flame synthesis of 1-D complex metal oxide nanomaterials

One dimensional (1-D) complex metal oxide nanomaterials, such as ternary oxides, doped oxides, and hierarchical structures containing several oxides, not only benefit from large aspect ratios, but also offer exciting opportunities to design materials with desired properties by tuning their chemical compositions and tailoring their sizes and morphologies at the nanometer scale. Flame synthesis is an attractive method to grow 1-D complex metal oxide nanostructures because of its high temperature, scalability, low-cost and rapid growth rate. Here, we present three new combined flame synthesis methods: (1) simultaneous vapor–vapor growth, (2) simultaneous solid–vapor growth, and (3) sequential solid–vapor growth, to grow 1-D complex metal oxide nanostructures with well-defined compositions and morphologies. These three methods combine the previously reported flame vapor deposition and solid diffusion growth methods that were separately used to grow 1-D simple binary metal oxide nanostructures, and significantly advance the capabilities of existing flame synthesis methods for the growth of 1-D nanomaterials. The first method, simultaneous vapor–vapor growth, combines the flame vapor deposition growth of two different metal oxides by oxidizing and evaporating two different metal sources. With this we have successfully grown W-doped MoO3 nanoplates and nanoflowers. In the second method, simultaneous solid–vapor growth, one precursor is again provided by oxidizing and evaporating metal oxide from a metal, while the other precursor diffuses out from a different growth substrate. With this we have successfully grown ternary Cu3Mo2O9 nanowires. The third method, sequential solid–vapor growth, essentially uses the 1-D nanostructures firstly grown by solid diffusion as the substrates for subsequent flame vapor deposition. With this we have successfully grown hierarchical CuO/MoO3 core/shell nanowires and MoO3-branched CuO nanowires. We believe that these three new combined flame synthesis methods will provide a general platform for the synthesis of 1-D complex metal oxide nanostructures with tailored properties.

Unique Magnetic Properties of Single Crystal γ-Fe2O3 Nanowires Synthesized by Flame Vapor Deposition

Single crystal γ-Fe(2)O(3) nanowires with 40-60 nm diameters were grown for the first time by single-step atmospheric flame vapor deposition (FVD) with axial growth rates up to 5 μm/minute. Because of their superior crystallinity, these FVD γ-Fe(2)O(3) nanowires are single magnetic domains with room temperature coercivities of 200 Oe and saturation magnetizations of 68 emu/g.