DOI:
10.1039/C6RA19436B
(Paper)
RSC Adv., 2016,
6, 91860-91869
RGO enveloped vertically aligned Co3O4 nanowires on carbon fabric: a highly efficient prototype for flexible field emitter arrays†
Received
1st August 2016
, Accepted 30th August 2016
First published on 12th September 2016
Abstract
A surfactant-free new approach is formulated for synthesizing Co3O4 nanowires on a flexible carbon fabric substrate by a simple chemical route. The devised protocol yields uniform and highly dense nanowire distributions that are able to sustain their structural and morphological integrities even under severe bending and twisting. To understand the growth process of these Co3O4 nanowires on the chemically activated carbon fabric, the morphological evolutions are investigated in a step-by-step manner using FESEM and HRTEM analysis. As-synthesized Co3O4 nanowires, endowed with sharp edge features of their morphology, are found to exhibit excellent p-type field emitting traits with a very low turn on field of 0.86 V μm−1 with a significant current density of ∼0.5 mA cm−2 at a relatively lower applied electric field of ∼1.4 V μm−1. To improve the current density and field enhancement factor even further, RGO sheets are anchored over the Co3O4 nanowires so that the conducting RGO sheets could potentially enrich the field emission performance due to its high electron affinity. Thus the designed hybrid nanostructures are found to display a turn-on field of 0.8 V μm−1 and current density twice (1 mA cm−2) that of bare Co3O4 nanowires. Experimental observations indicate that these hybrid RGO wrapped Co3O4 nanostructures are highly promising as cold cathodes in flexible field emission display applications.
Introduction
Electron sources are one of the pivotal components in multitudes of modern instruments including (but not limited to) high energy accelerators, electron microscopes, X-ray sources, cathode-ray tube monitors and microwave amplifiers. There exist several methods for extracting electrons from a suitable source material, including by thermionic emission (heating to high temperatures), photoelectric emission (by electromagnetic wave of appropriate frequency) or by field emission (applying a large electrostatic field). Among these, field emission based systems have traditionally had high distinction due to their fast turn-on, low working temperatures, ultrahigh brightness, large area and miniaturized device size. Lately flexible field emission emitters have become a hotspot of applied research owing to their unique light-weight, conformable and flexible natures which endow them with significant edges that can potentially be exploited in upcoming novel fields such as flexible field emission (FE) displays,1 e-papers,2 X-ray tubes,3 etc.
The advent of nanotechnology in recent times has completely rejuvenated the science of designing field emitters from the ground up which now boasts discreet strategies such as: (i) incorporation of nanostructures with sharp tips on the emitting surface for massively enhancing local electric field,4,5 (ii) increasing the density of emission sites by surface decoration and incorporating micron order roughness,6 (iii) tailoring the band gap of utilized nanomaterials via doping7a,7b and last but not the least (iv) enveloping nanometer scale sharp emitter tips by suitable 2D materials such as graphene.8 These high density protrusions further localize and enhance the electric field, thereby permitting electrons to tunnel through the tips at very low electric fields and to produce stable emission in a large area. Till date, both organic (carbon nanotubes, graphene oxide (GO), graphene, carbon nanosheets) and inorganic nanostructures9–13 have been successfully implemented to design functional field emitting devices. Organic semiconductors have many innate advantages over their inorganic counterparts such as good flexibility, low-temperature synthesis protocol and inherent compatibility with plastic substrates. Thus they are increasingly finding themselves at the forefront of applied research in light-emitting diodes, radio frequency identification tags and sensors etc.14,15 However, issues such as low mobility and stability still constricts large scale commercial adaption of these organic devices. In contrast, 1D inorganic semiconductors have controllable size as well as tunable electrical/optoelectronic properties; thus they have been proffered as one of the most functional materials in nanoscience and technology. At the same time, 1D inorganic semiconductors can conveniently be transferred to various kinds of substrates so that they can be utilized as building blocks for flexible electronics in a wide-ranging inter disciplinary area.16 But lack of adequate long-term or high-temperature FE stabilities and unsatisfactory mechanical properties have partially hindered the development of these materials for practical applications.
Among the family of inorganic semiconductors (TiO2, SiC, CdS, ZnO, WO3, In2O3, Ga2O3, CuO),17–23 Co3O4, as a wide-band-gap, p-type semiconductor, has been extensively studied for possible utilization in the disciplines of electrochemistry, magnetism, catalysis as well as in the field of lithium ion battery.24,25 In addition, the vertically aligned 1D nanostructures of Co3O4 have also generated significant enthusiasm in field emission oriented disciplines due to their sharp tips, low work function (4.5 eV), high aspect ratio, vertical orientation, tunable nanoporosity, large surface area and direct path for electron. However high turn on voltage and low current density of them still mount a serious obstacle in their commercial deployment. In this regard carbon 2D (two dimensional) nanomaterials can play an important role in both economic and performance front as they can endow higher surface area, electrical conductivity and electron affinity to Co3O4 at nanoscale in the form of hybrid nanomaterials. The graphene hybrid nanostructures have been extensively explored since the past decade for its highly-efficient field emission performances.26 But in particular, the protrusions of reduced graphene oxide (RGO) on vertically aligned nanoporous Co3O4 nanowires hybrid structure for FE studies has not been demonstrated till date.
In the present work, we report fabrication of vertically aligned, poly-crystalline 1D (nanowires) Co3O4 with nanoporosity; directly grown on flexible carbon fabric (CF) by simple wet chemical route. In this approach, the morphology, dimensions and structural parameters of as synthesized Co3O4 nanowires can conveniently be controlled by modulating synthesis parameters. We incorporated both inorganic and organic semiconductors in a single framework by wrapping these nanoporous Co3O4 nanowires with reduced graphene oxide (RGO) on CF and investigated their FE performances. We also probed the distribution of local electric field at the exact emission sites by finite element method simulation.
Experimental details
Methods
Materials. Cobalt nitride (Co(NO3)2), ammonium fluoride (NH4F) were purchased from Sigma-Aldrich and urea (CO(NH2)2), hydrazine hydrate (NH2NH2), ammonia solution, potassium permanganate (KMnO4), con. H2SO4 were of analytical grade (AR) and used as received without further purification. Graphite flakes (SP-1 graphite, ∼150 μm size) was purchased from Bay Carbon Corporation. Deionized water was used throughout the experiments.
Synthesis of vertically aligned nanoporous Co3O4 nanowires on carbon fabric. Prior to the deposition on carbon fabric, it was subsequently cleaned by sonication in acetone and DI water and dried properly. Then the carbon fabric was chemically activated by KMnO4. In the preceding steps, 0.05 M Co(NO3)2·6H2O, 0.1 M NH4F and 0.25 M urea solution was prepared separately. Then NH4F and urea solutions were added drop wise respectively to Co(NO3)2·6H2O solution and stirred for 15 min to get homogeneous solution. The pink colored solution was transferred into a 50 mL stainless steel autoclave and the CF mounted on a glass slide was inserted into the solution, keeping the activated surface down. Then the autoclave was kept at 120 °C in oven for 1–6 (C1, C2, C3, C4, and C6) h. After the reaction the autoclave was cooled down naturally to the room temperature. The as obtained CF was found to be covered evenly with pink colored film. The light pink shaded CFs were washed several times by DI water and ethanol successively to remove the impurities. Furthermore, the deposited substrates were annealed in a box furnace at 400 °C for 3 h at heating rate 5 °C min−1. Finally, the pink color of CFs turned into black which confirms the formation of cobalt oxide. The uniform dark and intense black colour of the carbon fabric appear in the middle portion (shown at ESI Fig. S1†) confirms the formation cobalt oxide on CF.
Synthesis of graphene oxide (GO). GO was prepared from graphite powder by modified Hummer's method (see the ESI†).
Fabrication of RGO–Co3O4 nanowire hybrid 3D arrays. Fabrication of RGO–Co3O4 hybrid nanostructure were carried out by employing a simple ex situ method. At first, 5 mg of as-prepared GO powder in 100 mL distilled water was sonicated thoroughly until the powder become well dispersed and then stirred for 1 h to form homogeneous dispersion. Thereafter, the as-prepared carbon fabric with Co3O4 nanostructures was mounted on a glass slide and dipped into the above dispersed solution keeping the growth surface (i.e., where the Co3O4 nanostructures was grown) downward. After that 5 μL hydrazine hydrate and 35 μL NH3·H2O was added drop wise with continuous stirring at 90 °C and stirred until the solution colour turned black. Finally the carbon fabric was rinsed properly by DI water to remove the impurities. The obtained RGO–Co3O4 hybrid nanostructures were finally dried in air oven at 80 °C for 12 h. The schematic presentation of synthesis process for highly oriented architecture of RGO–Co3O4 hybrid nanostructures are shown in the Fig. 1.
 |
| Fig. 1 The schematic presentation of synthesis process for highly oriented architecture of RGO–Co3O4 hybrid nanostructures. | |
Materials characterizations. The crystallinity and phase purity of the as prepared nanostructures were verified by X-ray diffraction (XRD) using Cu Kα radiation (λ = 1.5406 Å) (XRD, D8 Advanced, Bruker). Raman spectra was obtained with the excitation of a 532 nm laser source (WITECH). The morphologies and lattice structures of as synthesized nanostructures were inspected by field emission scanning electron microscope (FESEM, S-4800, Hitachi) and high resolution transmission electron microscope (HRTEM, JEOL, JEM 2100).
Field emission measurements. The field emission measurement setup was in a high vacuum (2 × 10−6 mbar) chamber. At the time of measurement, the CF with sample and a stainless steel electrode with tip diameter (2R) of ∼1.5 mm treated as cathode and anode respectively keeping 1.5 mm separation between them. Cathode was connected with 1.3 MΩ resistor.
Results and discussion
Structural and morphological analysis
The crystallinity and phase purity of samples were analysed through X-ray diffraction technique. Fig. 2, shows the XRD pattern of as-synthesized Co3O4 nanowires (NW) on CF. The broad diffraction peak at a 2θ value of 26° is ascribed to a typical diffraction peak of amorphous carbon fabric. Not only that the intensity of Co3O4 diffraction peaks increased accordingly with the increasing hydrothermal reaction time which also indicating the higher degree of crystallinity of Co3O4 nanostructures. The strong and sharp diffraction peaks at 2θ values of 31.3, 36.9, 44.9, 59.5, and 65.3° are associated to the lattice planes of (220), (311), (400), (511) and (440) respectively (JCPDS card no: 42-1467). No other diffraction peaks were observed in the XRD pattern which confirms the phase purity and higher degree of crystallinity. As (002) plane of carbon cloth is overlapping with the (002) plane of graphene so further Raman analysis is carried out for better confirmation of RGO over Co3O4 nanowires. At first Raman spectra of Co3O4 NW on CF (C6) were measured at room temperature (Fig. 3a). There are five peaks at 197, 483, 522, 621, and 693 cm−1 in the pattern, which correspond to F12g, Eg, F22g, F32g, A1g modes of Co3O4 respectively.27 The D and G band of carbon fabric have been completely disappeared which confirms the uniform, homogeneous deposition of nanostructures all over CF. Then the Raman spectrum of RGO–Co3O4 NW hybrid structure was measured (Fig. 3b) which shows two other new vibration mode of D-band at 1354 cm−1 and G band at 1600 cm−1 in addition to different Co3O4 modes. The G vibration mode at 1600 cm−1 is the assessing of graphite ordering of carbon materials, which includes the in-plane bond stretching motion of pairs of C sp2 bonding, while the D peak is only active in the presence of defects and is a measure of the sp3 bonding in graphene. This confirmed the presence of RGO in this RGO–Co3O4 NW hybrid nanostructure. The intensity ratio of D to G band (ID/IG) has been proposed to be an indication of disorder in the RGO sheets and a low ratio indicates a greater disorder arising from structural defects. The RGO–Co3O4 NW hybrid nanostructure has an intensity ratio (ID/IG) near to 1.2, representing the formation of small sized sp2 domains in the RGO. This observation further confirms the formation of RGO–Co3O4 NW hybrid nanostructures. Other than these afore mentioned peak, one peak at 2676 cm−1 was observed. The 2D peak is the second order of the D peak is due to the defect activated combination of phonons scattering process which is an indicative of thin layer of graphene sheets.28,29 The shape of the 2D peak was very symmetric thus from this it could be inferred that the thickness of the graphene layer is very thin approximately 1–2 layers.
 |
| Fig. 2 (a–c) Optical images of the carbon fabric with Co3O4 NWs, under slight bending and twisting condition, respectively. (d) X-ray diffraction patterns of carbon fabric and Co3O4 nanowire at different oxidation time 3 h and 6 h (C3, C6) respectively on carbon fabric. The broad hump peaking at 2θ values of 26° is due to the graphitic nature of carbon fabric. | |
 |
| Fig. 3 The Raman spectra of (a) annealed Co3O4 NW, absence of any C peak indicate the uniform deposition of Co3O4 NW on CF (lower) and hybrid RGO–Co3O4 NW on CF where D, G, 2D peaks indicate better graphitization over the Co3O4 NW. (b) Only Co3O4 NW. | |
The field emission scanning electron microscope (FESEM) and transmission electron microscope (TEM) were utilized to characterize the morphologies of as-synthesized Co3O4 NW and hybrid nanostructures on CF under different magnifications. To elucidate the growth mechanism of Co3O4 NW, the synthesis process was terminated at different growth stages and the morphology of the resultant products were imaged (see ESI Fig. S2†) stepwise. The typical FESEM micrographs for two stages of hydrothermal time, 3 h (C3) & 6 h (C6) of the as synthesized nanostructures are displayed in Fig. 4 which shows that the CF is uniformly covered with Co3O4 NWs. Fig. 4a shows the micrographs for 3 h reaction time where nanowires were grown over CF but were not so uniform in their overall geometry. The magnified FESEM image (Fig. 4b) of C3 depicts the length of individual nanowires are nearly 2 μm with tip diameter ∼15–20 nm. Further extension of the reaction time till 6 h; uniform, well aligned, well distributed nanostructure has been achieved all over the carbon fabric with relatively larger length and sharp tip with a significant tendency to form nanoneedle like shape. Fig. 4c and d shows distinct and uniform formation of NWs thoroughly over the CF with approximately 3–3.5 μm in length and nearly 10–12 nm of tip diameter. Their magnified image reveals that the NWs are highly porous, which is due to the release of CO2 and H2O during the calcination process. However, longer reaction time gives rise to a uniform, well aligned, well distributed nanostructure with higher aspect ratio than that of C3. So the morphology have been greatly influenced and can be moderated by changing the time of reaction. Nanostructure of individual strand of CF resembles a woolly caterpillar which have a furry body and legs beneath. Similarly, nanostructure of individual strand of CF has no growth underneath due to the absence of seed layer at back side.
 |
| Fig. 4 FESEM images showing morphological evolution of Co3O4 NWs on carbon fabric for hydrothermal reaction time (a) and (b) 3 h and (c) and (d) 6 h. Right panel images are presenting the higher magnification images of left panel. | |
In addition, elemental mapping by energy dispersive X-ray spectroscopy (EDS) was used to analyse the distribution of the Co and O elements in a particular strand of CF as shown in Fig. 5. The images of Co and O elements mapping of C6 sample are shown in Fig. 5a and b. The atomic% ratio of Co and O was found to be almost 3
:
4 as expected in the Co3O4 molecule. Fig. 5e and f shows the TEM/HRTEM images of isolated vertically aligned Co3O4 NW annealed at 400 °C for 3 h. It can be observed that small crystals are self-assembled one upon another and formed the NWs. Along with every eight to ten individual NWs are clubbed together and formed conical needle like nanostructures which promotes nano-porosity alongside of the entire nanostructures. Fig. 5f demonstrates the HRTEM image of Co3O4 NWs having high quality lattice fringes without any distortion, which clearly demonstrates clear lattice fringes of Co3O4 NWs after annealing process. The estimated interplanar spacing of adjacent lattice fringes is about ∼0.283 nm, which corresponds to the (220) plane of spinel Co3O4 phase. Further the poly-crystallinity nature of the as-prepared Co3O4 NWs has been confirmed from SAED pattern shown in (see ESI Fig. S3†). In the next panel of Fig. 5g shows a few layers of RGO covered the apexes of the nanowires and formed sharp protrusions. The folds and wrinkles existing in the RGO sheets are clearly noticeable from this FESEM images. Fig. 5h shows TEM images of Co3O4 NWs–RGO sheets hybrid structures, which indicate uniform coverage of RGO on the entire nanowire. For clear visualization of RGO wrapping over the Co3O4 nanowires, an atomic model is presented in Fig. 5i.
 |
| Fig. 5 (a–c) EDS elemental mapping of Co, O elements Co3O4 NWs on CF, (d) morphology of Co3O4 NWs on CF imaged by FESEM (e and f) TEM and HRTEM image showing an individual nanowire which evidencing the self-assembly of multiple nanowires into single nanotips. (g) RGO wrapped over a large area of the Co3O4 NWs arrays, (h) TEM image of wrapped RGO on an individual NW of Co3O4 with several wrinkling and nano-protrusions at the top of tip (i) ball-stick representation of RGO on Co3O4 at atomistic level. | |
The plausible formation mechanism. Formation process of the Co3O4 NWs was further investigated at different reaction stages. We propose a four step growth mechanism of Co3O4 NWs on seeded carbon fabric. In the absence of seeds, no shape control is possible, instead of various large particle with different shapes usually forms. Therefore, we applied KMnO4 activation to the CF to control the nucleation and growth of the secondary structure to the final shape.30 The chemical reactions involved in the preparation process can be expressed as follows:
H2NCOHNH2 + H2O → 2NH3 + CO2 |
At first, the additive NH4F in the solution release F− ions which further gets coordinated with bivalent Co2+ ions and form a complex CoFx(x−2)− in the as prepared homogeneous solution. As the temperature of the reactant solution ramped to 70 °C in the oven, urea hydrolyzation takes place and a number of CO32− and OH anions starts forming gradually. In a second step: with the increasing temperature of oven during hydrothermal (120 °C), the concentration of anions (CO32−, OH−) in the as-prepared reactant solution increased which further led to the formation of a nucleus of cobalt-hydroxide-carbonate. The formation of CoFx(x−2)− prevents the formation of brucite-like layered structure of Co(OH)2. Furthermore, in the third succession, the growing cobalt-hydroxide-carbonate nuclei were beginning to impinge on other neighboring crystals and assemble along the specific orientation preferentially. As Co3O4 has a spinel structure containing Co3+ in an octahedral coordination and Co2+ in a tetrahedral coordination. Oxygen anions form a distorted face-centered cubic sub-lattice, in which Co2+ cations occupy one-eighth of the tetrahedral interstices and Co3+ cations occupy half of the octahedral interstices. The {001} and {111} planes contain only Co2+ cations, while the {110} plane is composed mainly of Co3+ cations.31 Certainly, the {100}, {110}, {111}, and {112} crystal planes present different atomic arrangement thus evolves different nanostructures according to their surface energies. The surface energy follows the general sequence: γ2.31{111} > γ1.46{112} > γ1.31{110} > γ0.92{100}, and the coordination number (CN) follows the reverse order. Hence the minimum surface energy requirements adopt the preferential growth along {100} then {110} after stabilizing {112} and {111} planes respectively.32 In brief the nanowires of Co3O4 grows mainly along [110] direction and preferentially exposes {100} planes. As the concentration of the nutrient solution quenched with the progression of reaction, the flux of cobalt-hydroxide-carbonate reduces thus forms wires like nanostructures with sharper tips and broader bases. Systematic time to time illustrations of growth mechanism is depicted in Fig. 6. In the final stage, the prolonged annealing time in air decomposes cobalt-hydroxide-carbonate gradually into CO2 and Co3O4. Henceforth, transformation of black Co3O4 1D nanostructured arrays on substrates takes place which in turns left porous. Every single nanowire having a tendency of inclining together to bundle up at the top. Thus by suitably monitoring the reaction conditions such as kinetic parameters (heating rate) and also thermodynamic parameter (temperature), it is possible to tune the shape, alignment, and assembly of the Co3O4 nanostructures from the cobalt hydroxide-carbonate precursor.
 |
| Fig. 6 Schematic of successive growth of Co3O4 NWs on CF with the different hydrothermal reaction time. | |
Field emission (FE) is a quantum tunnelling phenomenon where electrons are emitted from a solid surface due to the effect of a strong electrostatic field. In our field emission experiments, the cathode was made up of RGO sheets supported on bundled Co3O4 NWs on CF and the anode was a probe positioned at a distance (d) of 1.5 mm above the surface which is schematically represented in Fig. 7a. The field emission characteristics of Co3O4 NWs (C3, C6) and RGO–Co3O4 hybrid nanostructures is shown in Fig. 7b. As the applied voltage is gradually increased, the emission current is observed to increase very rapidly, indicating that the emission is indeed as per the Fowler–Nordheim (F–N) theory. The values of the turn-on and threshold fields, (defined as the fields required to draw emission current densities of ∼1 and ∼10 μA cm−2), are found to be ∼0.8 (RGO–Co3O4), ∼0.87 (C6) and ∼1.02 (C3) V μm−1 respectively (Table 1). When RGO sheets supported on Co3O4 NWs, there was a significant enhancement of the electron field emission, producing a significant current density of ∼1 mA cm−2 at a relatively lower applied electric field of ∼1.4 V μm−1. Thus the field emission of RGO–Co3O4 hybrid nanostructures was improved efficiently compared to the pure Co3O4 NWs. The FE current–voltage characteristics are further analyzed by the Fowler–Nordheim (F–N) theory:33,34
Table 1 Morphology dependent field emission properties of as synthesized products
Sample |
Observed product morphology |
Turn on (V μm−1) at 10 μA cm−2 |
Threshold (V μm−1) at 0.1 mA cm−2 |
β |
C3 |
Short nanowires |
1.02 |
1.36 |
7548 |
C6 |
Long nanowires nanoknife |
0.86 |
1.19 |
10 470 |
RGO-C6 |
RGO wrapped over long nanowires |
0.8 |
1.07 |
15 340 |
Or
|
 | (1) |
where
A and
B are constants with values of 1.54 × 10
−6 eV AV
−2 and 6.83 × 10
3 V μm
−1 eV
−3/2, respectively,
E =
βE0 is the local electrical field,
E0 is the mean field between the cathode and anode,
β is the field enhancement factor and
φ is the work function of the emitting materials, which is 4.5 eV for Co
3O
4 (
ref. 35a and 35b) and 4.6 eV for RGO
36 as the electron emitted finally from the top surface of the RGO–Co
3O
4 NWs hybrid nanostructures. The field enhancement factor (
β) is an important parameter of the emitter which quantify the ability of the emitter to amplify the applied electric field at the emitting site. In the presence of an applied electric field (
Eapp), the emission properties were obtained using

where
Eloc corresponds to the local field and is estimated from the slope (
m) of the electrostatic potential at an inter electrode distance (
d) of 1.5 mm. The field enhancement factor (
β) has been calculated from the F–N plot of
Fig. 7c about 7548, 10
![[thin space (1/6-em)]](https://www.rsc.org/images/entities/char_2009.gif)
470, 15
![[thin space (1/6-em)]](https://www.rsc.org/images/entities/char_2009.gif)
340 for C3, C6 and RGO–Co
3O
4 respectively. For comparison of the previous reports was furnished in the
Table 2. The FE measurements were also carried out for more three set of samples of individual compositions to verify the reproducibility which showed that the turn on values are well consistent with the samples C3, C6 and RGO–Co
3O
4 (as shown in ESI, Fig. S4 and Table T1
†). These Co
3O
4 nanostructures on the carbon cloth substrate satisfy the most requirements as a flexible cold cathode, with high aspect ratio, excellent electrical conductivity, thermal and chemical stability. In comparison to C6, the field emission performance of C3 is somewhat limited owing to its large tip diameter and small length. The geometrical enhancement factor
β, which indicates the field emission capacity of the nanostructures, depend on several factors such as the shape or geometry of emission sites, screening effect (arising from the extent of separation between the adjacent NWs), and most importantly the work function of the emitting materials. Although Co
3O
4 NWs (C6) possess sharp needle like tips and high-aspect ratio, but the emission field could be screened due to the interaction of adjacent NWs. However, after attachment of RGO sheet on Co
3O
4 NWs, the emission current density might increase remarkably. Furthermore, when two different classes of nanostructures are combined, the hybrids would also serve as a two-stage field emitter. The field enhancement factor of the cascade emitter can be presented as:
βheterostructures = βCo3O4βRGO |
where
βCo3O4 and
βRGO are field enhancement factor of the Co
3O
4 NWs and RGO, respectively. In addition to the oxygen content of RGO edge, their uniform wrinkle resulting from the well-aligned Co
3O
4 NWs and its rough top surface of Co
3O
4 NWs tips also lead to nanoscale protrusions. Each protrusion serves as an individual field emitter. Therefore, the value of
βRGO indicates distributed wrinkles and protrusions in the RGO, and so the
β value of heterostructures is usually larger than the Co
3O
4 NWs.
Table 2 Turn-on field values of various metal oxide/sulphide–RGO nanocomposites reported in the literature
Composite |
Turn-on field (current density) |
Field enhancement factor (β) |
Ref. |
SnS2/RGO |
2.65 V μm−1 (1 μA cm−2) |
3700 |
44 |
WS2/RGO |
2.0 V μm−1 (1 μA cm−2) |
2978 |
45 |
RGO–Ag NWs |
2.4 V μm−1 (1 μA cm−2) |
1985 |
46 |
GO/TiO2 |
2.6 V μm−1 (10 μA cm−2) |
2300 |
47 |
RGO/TiO2 |
1.0 (10 μA cm−2) |
6000 |
17 |
Ni nanotips/GO |
0.5 V μm−1 (1 μA cm−2) |
— |
8 |
G/SiNWs |
2.59 (10 μA cm−2) |
4044 |
48 |
CuO/NRs-NGS Cu(OH)2/2NRs-NGS |
0.724, 0.956 V μm−1 (10 μA cm−2) |
7609, 3304 |
49 |
RGO : HμSi |
1.3 V μm−1 (10 μA cm−2) |
4819 |
43 |
RGO + P3HT |
2.2 V μm−1 (10 μA cm−2) |
395 |
50 |
Co3O4/HEG |
1.12 V μm−1 (10 μA cm−2) |
6223 |
51 |
CNF–RGO |
2.4 (1 μA cm−2) |
16 432 |
42 |
RGO–Co3O4 |
0.8 (10 μA cm−2) |
15 340 |
Present work |
Moreover, the observed enhancement in FE of RGO–Co3O4 can be due to the following reasons: first, RGO sheets over these highly dense Co3O4 nanowires formed dense localize protrusions in the nanoscale order; along with the gradual formation of high aspect ratio wrinkles which thereby increased the number of emitter sites; thus high current density was observed at a lower field.42 Secondly, the work function of the Co3O4 (4.5 eV) and RGO (4.6 eV) is nearly same. The field emission mechanism is greatly influenced by the electron affinity (η) and nanostructure of the materials.37–39 Thus apart from nanostructures, the increment of current density in RGO wrapped Co3O4 NWs can be described by considering the electron affinity of these two compounds. As the electron affinity of the RGO (∼4.2 eV)40 is much higher than that of Co3O4 (∼3.55 eV),41 the electron transfer is favorable from the Co3O4 NWs into the conducting RGO sheet which then can easily surmount the surface potential barrier of graphene in response to the applied external electric field. Furthermore, the emission stability of RGO–Co3O4 on CF was examined at a fixed voltage and an average emission current density 0.83 mA cm−2 was achieved for more than 2 h as shown in Fig. 7d, with a minimal emission degradation and ∼2% of fluctuation.
To further investigate the origin of this efficient field emission, we computationally investigated the local electric field profile of Co3O4 and RGO–Co3O4 hybrid nanostructures by finite displacement method using ANSYS Maxwell simulation package. As per the nanoscale geometry is concerned, a local field not only exists at the top but also at the side edges near the tip and the effective enhancement factor is the contribution of both. The simulated results provide accurate insight regarding the effect of nanoscale geometry on local field and screening effect after and before the RGO envelop which is practically difficult to estimate otherwise.52 Simulation parameters were chosen accordingly with the experimental counterpart to maintain a direct analogy: i.e. length ∼3.5 μm, tip diameter ∼10–12 nm, applied potential = 2 kV, separation between anode and the grounded cathode = 1.5 mm etc. were chosen. The simulated profiles of electric field vector's magnitude for aligned Co3O4 and RGO–Co3O4 hybrids are shown in Fig. 7e and f. The magnitude of the electric field on the 2D plane was mapped by a rainbow color coordinate where blue is the indicative of minimum and red is maximum. The figure clearly depicts that the gradient of the local electric field is significantly higher on the nanoscale protrusions produced by RGO and at its wrinkles edge sites than on bare Co3O4 NWs. The possible band diagrams of RGO enveloped vertically aligned Co3O4 nanowires are schematically represented in Fig. 8 for the illustration of field emission mechanism.
 |
| Fig. 7 (a) Schematic representation of electron emission from cathode (RGO–Co3O4 NWs) to the anode, (b) field emission current density as a function of electric field for RGO–Co3O4 NWs, Co3O4–C6, Co3O4–C3 (c) corresponding FN plots at vacuum separation distances of 2 mm. (d) The field emission current stability curve, shows the stability of current density of RGO–Co3O4 NWs over a long time. (e) Sectional view of the electric distributions from the three dimensional Co3O4 and (f) graphene–Co3O4 (G–Co3O4) composite nanostructures. | |
 |
| Fig. 8 Schematic band diagram of RGO and Co3O4 for probable mechanism of field emission properties. | |
In retrospective, the admirable mechanical and electron emission stability of the RGO–Co3O4 hybrid nanostructures could be attributed to following (i) high stability of the base Co3O4 NWs (ii) large interfacing area between the carbon fabric substrate and heterostructures, (iii) nanoscale dense localize protrusions and sharp ridges resulting from RGO encapsulation.
Conclusion
An effective approach for morphology control pertaining to large scale growth of Co3O4 nanowires over flexible carbon fabric is demonstrated. The sharp edges of Co3O4 nanowires with higher aspect ratio were obtained by modulating hydrothermal reaction time. Their uniform nanostructural registry and high aspect ratio exhibited excellent field emission performance. In order to further boost field emission performances, in the next step, RGO sheets were transferred on the as synthesized Co3O4 nanowires. The RGO sheets were subsequently observed to wrinkle and envelop the nanotip apexes. The effects of RGO incorporation to form hybrid nanostructures is two-fold: primarily it can augment the innate field emission capacity of the underlying nanostructures owing to its own high electron affinity and secondarily it can populate local field emitting sites by supplying nanoscale protrusions in the form of wrinkles and ridges. Consequently, RGO–Co3O4 nanowire hybrid nanostructures, grown on a flexible carbon fabric substrate are found to display lower turn-on field of 0.8 V μm−1 and much higher current density of 1.02 mA cm−2. Finite element method simulation of the emitter geometries both with and sans RGO envelop to probe local electric field also corroborated the experimental findings. The current tactics of utilizing flexible carbon fabric substrate and observed high stability of the deployed hybrid nanostructures on it might play a critical role in achieving lightweight as well as flexible field emission displays with higher tunnelling probability and remarkable stability. Specially, observed low turn on field and formation of 3D heterojunction of 2D RGO and 1D Co3O4 might be instrumental in integration of the nanoscale building blocks into multifunctional devices from next generation flat panel display applications to photonics, energy conversion, and storage.
Acknowledgements
One of the authors (PH) wishes to thank the University Grants Commission (UGC), the Government of India, for awarding her a junior research fellowship during the execution of the work. We thank the UGC also for the ‘University with Potential for Excellence (UPE-II)’ scheme. Authors wish to thank Dr Nirmalya Shankar Das for his help in drawing the artworks used in this manuscript.
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Footnotes |
† Electronic supplementary information (ESI) available. See DOI: 10.1039/c6ra19436b |
‡ These authors contributed equally to this work. |
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