Synthesis of Three-Dimensional Tungsten Oxide Cactus-Shaped Nanostructures Supported on Carbon Nanotubes Template

Abstract

We report that three-dimensional cactus-shaped tungsten oxide WO x (x ¼ 2-3) nanostructures have been synthesized over a large area using a simple one-step metal-organic chemical vapor deposition (MOCVD) technique at unusually lower temperatures than previously reported, making use of vertically aligned carbon nanotubes (CNTs) as the base template in a core-shell configuration. Each WO 3 crystallite forms independently perpendicular to the CNT spine resulting in the cactus-like structures. The presence of these structures has been shown to lead to improved electron emission properties, where the turn-on field was lowered from pristine CNTs at 3.47-2.31 V lm À1 with an increase in the field enhancement factor from $1225 to $3985. The growth mechanism of the nanostructures is discussed.Among all the transition metal oxides, tungsten oxides WO x (x ¼ 2-3) have been extensively studied due to their wide range of unique material properties, which includes electrochromic, optochromic and gasochromic properties. 1-9 These unique properties make WO x highly flexible for applications such as electrochromic windows, 1,2 solar cells, 3 gas sensors, 4,5,9 and photocatalyst materials. 6 One way to control and enhance its unique materials properties is to modify its morphology and physical structure. Different structures and morphologies have been reported. For example, different WO x reported morphologies include nanoparticles, 10,11 nanoplates, 12 whiskers, 13 nanotubes, 14 and nanorods, 6,15 etc. excluding conventional thin and thick films.There are many advantages in obtaining these unique nanosized structures. For example, there is a significant increase in the surface area of the nanostructured WO x as compared with thin films, leading to an increase in the efficiency of WO x based sensors. In the recent few years, nanostructured WO x , especially WO x nanowires, has also found potential applications in field emission applications due to their unique high-aspect ratio structures, 16-18 with good robustness and longevity. 19,20 One of the most attractive features lies with the morphology of WO x , where it has the capability to form tree-like structures when physical growth methods, such as conventional thermal deposition, 11,21,22 are used. This was reported by Zhu et al. as early as 1999, 21 where unique structures could be obtained through heating a W foil partly covered by a SiO 2 plate in an Ar atmosphere at 1600 C. There were also reports of three-dimensional regular (3D) network of WO x nanowires obtained through thermal evaporation of W powders in the presence of oxygen at 1400-1500 C. 22 In this letter, we report a simple approach to obtain 3D WO x nanostructures at a much lower temperature process of 600 C (as compared to >1000 C for standard furnace process). The uniqueness lies in using highly conductive multiwalled carbon nanotubes (CNTs) as the base template for the WO x nanowires to first encapsulate around forming a core-shell structure. Subsequently, unique 3D cactus-shaped nanostructures can be obtained ...

Tungsten Oxide Film and Nanorods Grown by Aerosol-Assisted Chemical Vapor Deposition Using κ2-β-Diketonate and β-Ketoesterate Tungsten (VI) Oxo-Alkoxide Precursors

“…The W 4f peaks at BEs of 35.5 and 37.5 eV are well matched to oxidized tungsten (WO x ). 44,46,47 This result is in agreement with the observation that tungsten oxide approaches WO 3 as deposition temperature increases.…”

“…The XPS spectra reveal W 4f 5/2 and 4f 7/2 peaks at low BEs of 31.5 and 33.5 eV, indicative of tungsten carbide. 41,45,46 The peak intensities decrease as deposition temperature increases, in agreement with the increase in free carbon. The W 4f peaks at BEs of 35.5 and 37.5 eV are well matched to oxidized tungsten (WO x ).…”

“…This crystalline growth behavior following the initial growth of an amorphous region is consistent with other reports that observed growth of crystalline WO x , and, in particular, W 18 O 49 whisker growth on a tungsten suboxide layer via metal oxidation. 46,64 Formation of high work function tungsten oxide buffer layers is important to forming low resistance contacts in organic electronics. The energy band state was determined from UPS measurements and the work function is plotted in Figure 7 for samples grown at 300, 400, and 500 • C from precursors 1-5.…”

Partially fluorinated oxo-alkoxide tungsten(vi) complexes as precursors for deposition of WOx nanomaterials

“…2,3,6,7 Tungsten oxide nanostructures can be prepared by CVD from the volatile carbonyl complex W(CO) 6 , but a carbon nanotube template or a co-reactant was required. 26 AACVD of WO 3 nanorods from W(OPh) 6 24,27 and W(CO) 6 28 has also been reported. In addition, tungsten oxo complexes with ancillary aminoalkoxide ligands have been used to prepare WO x needles by hydrolysis but water is not compatible with some applications.…”

Synthesis and evaluation of κ2-β-diketonate and β-ketoesterate tungsten(vi) oxo-alkoxide complexes as precursors for chemical vapor deposition of WOx thin films

“…high BE peaks at 34.8 and 37.3 eV, which may match to W 6+ . [60][61][62] Meanwhile, the W peaks of sample grown at 500 °C were deconvoluted to minor peaks at BEs of 32.4 and 33.7 eV, which suggest W 4+ , [63][64][65] and major peaks at BEs of 35.6 or 37.7 eV, which correspond to W 6+ , 61,66,67 thus suggesting increasing tungsten oxidation state as deposition temperature increases. The XPS results were used to estimate overall film composition (Fig.…”