Metallic Tungsten Nanostructures and Highly Nanostructured Thin Films by Deposition of Tungsten Oxide and Subsequent Reduction in a Single Hot‐Wire CVD Process

Abstract

The synthesis of metallic tungsten nanostructures and highly nanostructured thin films is presented. Crystalline tungsten oxide nanostructures are deposited on glassy carbon substrates kept at 700 AE 100 8C by oxidizing resistively heated tungsten filaments in an air flow under subatmospheric pressures. The internal morphology of the deposited tungsten oxide can be reproducibly controlled by the air pressure. After deposition, the tungsten oxides have been fully reduced to metallic tungsten by atomic hydrogen at a temperature of about 730 8C. Polycrystalline metallic tungsten nanofibers, nanocrystallites, closed crystallite films, and nanogranular films are thus obtained in a single procedure involving deposition and subsequent reduction, at relatively low temperatures.

Hetero- and homogeneous three-dimensional hierarchical tungsten oxide nanostructures by hot-wire chemical vapor deposition

“…The deposition of W 24 O 68 (O/W = 2.83), under the conditions provided in the experimental section, using an air flow at a relatively elevated subatmospheric pressure for a period of 30 min, results in crystallites [17]. By subsequently introducing H 2 , the external surfaces of the crystallites are reduced to tungsten oxide of a lower oxygen content, viz., WO 2.83 − x , as shown in earlier work of the authors [19]. The localized reduced sites act as nucleation centers for new W 24 O 68 growth when the H 2 flow is terminated and the air flow is resumed.…”

“…The external surface of the sample is highly textured, which is caused by the texture of the initial W 24 O 68 , shown previously [17]; even upon complete reduction this texture does not change [19]. W 24 O 68 nanowires are superposed on the α-W surfaces as a result of the oxidation step.…”

“…In those studies only a resistively heated W filament (as source of W atoms), an oxidizing air flow at subatmospheric pressure and a substrate heated to 700 ± 100°C, were used. The authors also reported the subsequent reduction [19] to metallic α-W of the deposited W 24 O 68 . Here, we extend the deposition process by additionally using a hydrogen (H 2 ) flow, which allows an intermediate and essential reduction step during the synthesis procedure due to the catalytic generation of atomic H by the hot filaments.…”

“…A W 24 O 68 crystallite film is deposited as described in Section 2.2. Then a H 2 flow of 100 sccm of 1.0 mbar is introduced for 30 min to completely reduce the W 24 O 68 to α-W, as shown in earlier work [19]. To subsequently re-oxidize the surface of the α-W an air flow of 25 sccm of P air = 0.039 mbar is introduced for 30 min, with the current through the filaments turned off, thus without new deposition of WO 3 − x .…”

“…Then a flow of 100 standard cubic centimeter per minute (sccm) H 2 of 1.0 mbar is introduced. The H 2 is dissociated to atomic hydrogen [19,24,22,23], which proceeds at about 1370°C [22,23]. The substrate surface temperature (T surface ) is ramped to about 750°C in a period of 15 min by heating the sample holder.…”