In-situ deposition of sparse vertically aligned carbon nanofibres on catalytically activated stainless steel mesh for field emission applications
Graphical abstract
Introduction
Porous metal mesh substrates are mechanically flexible, partially transparent, and have high thermal and electrical conductivities. Carbon nanofibres (CNFs), high aspect-ratio allotropes of sp2-bonded carbon, are optically, mechanically and electrically unique. One of the most important applications of such materials is in cold cathode field emission. CNFs dramatically enhance a substrate's field emission performance, though weak adhesion and poor electronic transparency at the substrate interface result in fibre de-anchoring and substantial resistive losses. Direct in-situ CNF synthesis introduces arrays of well-anchored tips [1], [2] and ultimately facilitates the development of many novel low-end field emission applications, such as flexible conducting field emitters for environmental lighting, for example.
The main strategies for composite fabrication are ex-situ coating and in-situ deposition. In ex-situ coating a variety of binding materials adhere pre-grown nanostructures to arbitrary substrates [3], [4]. CNFs are homogenously dispersed into binders, such as Teflon and Nafion, via damaging acid treatments and ultrasonication, and then cast, sprayed, or dip deposited. However, CNF immobilisation occludes much of the field emitting surface thereby reducing the emission performance. Uniform alignment is not possible. In-situ deposition negates these detrimental features. Transition metal catalysts are powder [5], slurry [6] or vacuum deposited [7]. Pre-formed nanoparticle suspensions require considerable post-processing and, in practice, during their filtration and preparation experience significant losses in the available catalytically active species due to attrition [8]. Vacuum processes obviate this, but are comparatively costly and particularly time consuming. Most studies on the in-situ growth of CNFs on metallic substrates detail the growth of highly disordered and unaligned networks and have focused, almost exclusively, on catalyst deposition or surface preparation via physical abrasion and thermal oxidation [9], [10], [8].
Kanthal, an iron-chromium-aluminium alloy, [11], [12], Inconel [13] and Nichrome/Chromel [14] have shown some promise, though are all more than one order of magnitude more expensive per unit volume than stainless steel. Moreover, the catalyst in these studies was typically solution or vapour-phase deposited iron nitrate salts rather than direct activation of the intrinsic catalyst available in the bulk substrate [15]. Masarapu et al. [16] reported unaligned CNF synthesis on stainless steel foil using strong acid dip processes. However, as with previous studies, the resulting CNFs were randomly aligned and spaghetti-like in morphology which degrades the field emission performance. The few studies that achieved vertically aligned nanostructures typically produced conformal and dense forests which offered low field emission performance. For optimal field emission, sparse arrays are desirable. Nilsson et al. [17] and Groening et al. [18] showed that the optimal array geometry of vertically aligned nanotubes and nanofibres, in order to minimise nearest neighbour electrostatic shielding, requires a pitch approximately twice the emitter's height. High density forests have nanometer-scale spacing and therefore require exceptionally short emitters. However, short CNFs have a significantly reduced field enhancement factor. Evidently, sparse vertically aligned CNFs are necessary to provide high performance field emission characteristics. In light of this it becomes apparent that there has been very limited success in producing the required geometrically defined vertically aligned CNF arrays through low-cost means on Fe- and Ni-containing alloys.
An industry compatible alternative for the direct-deposition of highly-aligned, sparse CNFs on metallic substrates is necessary. Catalysis activation, of low-cost and widely available Fe-, Ni or Co-containing metallic supports, through single-step electrochemical reactions, offers one possible solution. The technique presented here offers high levels of homogeneity, superior alignment and excellent inter-CNF spacing, which have yet to be demonstrated elsewhere. In this paper we consider the underlying catalyst activation process and investigate the field emission properties of the resulting CNF/ss-mesh composite. Our results establish a feasible, reliable, and scalable approach to the fabrication of low-end, flexible and porous field emitters.
Section snippets
Experimental details
Type 304 stainless steel (ss)-mesh (Alfa Aesar, 36% pitch, 50 μm diameter) was selected as the catalyst-containing substrate. The typical bulk composition is detailed in Table 1. Preliminary scanning electron microscopy (SEM) analysis on etchant exposed samples showed negligible surface alteration. An electrochemical approach was used to accelerate the surface preparation. 20 × 20-mm-samples were connected to a simplified potentiostat, as shown in Fig. 1(a). Unless otherwise stated, the ss-mesh
In-situ CNF synthesis
Fig. 1(a, b) shows SEM micrographs of the as-received and pre-treated ss-mesh (1 (DI):1 (etchant), 7 min). Spherical structures, herein ‘catalyst islands’, 125 ± 55 nm (± 1 S.D.) in diameter, conformally coat the electrochemically treated mesh (Fig. 1(c)). High magnification SEM showed few islands < 40 nm in diameter. Fig. 1(d) depicts the CNF diameter distribution (7 min pre-treatment) showing a dominant peak at 120 ± 50 nm (± 1 S.D.), consistent with the initial catalyst diameter given the experimental
Conclusions
In the present study a simple, inexpensive route towards the in-situ deposition of vertically aligned CNFs on stainless steel mesh via catalyst activation is reported. Bamboo-like CNFs with aspect-ratios > 150 were synthesised. Electrochemical pre-treatment increases the surface roughness which encourages pre-oxidation and thermal reduction during CNF growth by PE-CVD. Combined etching and electroplating account for the evolution of the catalytically active surface. Field emission measurements
Acknowledgements
M-T-C acknowledges the generous support of the Schiff Studentship, Cambridge University, and financial assistance from St. John's College, Cambridge and St. Edmund's College, Cambridge. M-T-C and K-H wish to thank Dr. M. Mann and Dr. D. G. Hasko for their fruitful discussions and experimental insight. C-L is grateful for the financial support of the Scientific Research Foundation, Southeast University, China (Grant No. YBJJ0926). The authors' thank the Cavendish Laboratory, Cambridge
References (36)
- et al.
Electrochem. Commun.
(2003) - et al.
Carbon
(2008) - et al.
Diamond Relat. Mater.
(2008) - et al.
Carbon
(2003) - et al.
Surf. Coat.Technol.
(2008) - et al.
Appl. Surf. Sci.
(2007) - et al.
Corros. Sci.
(1993) - et al.
Corros. Sci.
(1998) Appl. Catal., A
(2006)- et al.
Electrochim. Acta
(2007)
J. Am. Chem. Soc.
Appl. Phys. Lett.
J. Am. Chem. Soc.
J. Electrochem. Soc.
Catal. Today
Fullerenes Nanotubes Carbon Nanostruct.
Fullerenes Nanotubes Carbon Nanostruct.
Nat. Nanotechnol.
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Present address: Research Center, State Grid Electric Power Research Institute, Nanjing 210003, China.