Directed Self Assembly Of Copper-Based Hierarchical Nanostructures

Jan 8, 2015 - ... doping of graphene-carbon nanotube (CNT) hybrids in a plasma medium and their field emission properties. Aarti Tewari , Pankaj Sriva...
0 downloads 0 Views 9MB Size
Download PDF
Article pubs.acs.org/JPCC

Directed Self Assembly Of Copper-Based Hierarchical Nanostructures on Nitrogen-Doped Graphene and Their Field Emission Studies Pranati Nayak and S. Ramaprabhu* Alternative Energy and Nanotechnology Laboratory (AENL), Nano Functional Materials Technology Centre (NFMTC), Department of Physics, Indian Institute of Technology Madras, Chennai 600036, India S Supporting Information *

ABSTRACT: We report a large scale root to assemble hierarchical copper-based nanostructures on nitrogen-doped graphene sheets by a pH followed by a temperature-directed self-assembly process and their electron field emission studies. Starting with a controlled pH directed self-assembly root, we assembled Cu(OH)2 NRs on NGS and further reassembled it to 1D Cu NPAs and CuO NRs by thermal annealing in H2 and Ar atmosphere, respectively. The field emission characteristics are precisely studied, which depicts a significantly lower threshold and turn-on field for 1D Cu NPAs-NGS-based emitter compared to CuO NRs-NGS and Cu(OH)2 NRs-NGS. Also it exhibited about 3 and 27 times higher emission current density than its oxide and hydroxide counterpart under a moderate field of 1 V/μm. The enhanced field emission behavior of 1D Cu NPAs-NGS is attributed to the low work function, the easy electron tunneling from the one-dimensional arrangement of Cu NPs, which increases the emission sites and hence the FE current density. On the basis of easy large-scale synthesis techniques and better FE performances, these hierarchical nanostructures offer prospects for understanding the effect of linear arrangement of nanoparticles on FE, which can be envisioned for design, fabrication, and optimization of cold cathode devices.

■

INTRODUCTION Graphene, the material of the 21th century has garnered tremendous attention due to its unique physical properties such as room temperature quantum Hall effect, exceptional charge carrier mobility, good electrical and thermal conductivity, and ultra large specific surface area.1−6 Doping heteroatoms like nitrogen in carbon lattice of graphene can modulate the band structure and dramatically alter its electrical properties.7,8 In addition to this, the defects and disordered surface morphology induced by doping could potentially produce localized active sites for anchoring and strong binding of foreign materials, leading to hybrid materials for various field of applications.9−12 Being n-type doped, its excellent electrical conductivity and low work function lead nitrogen doped graphene sheets (NGS) thought to be more promising for electron field emission.13,14 Electron field emission is a quantum mechanical tunneling phenomena, where under high field of the order of 109 V/m, the potential barrier at the material−vacuum interface narrows down, which enhances the tunneling probability and therefore the emission current density. Bind up with the morphology and geometry of the emitter, the field emission characteristics are highly subjective to materials with more emission sites, low work function, and good stability.15,16 Being a flat twodimensional (2D) structure, the emission sites in graphenebased emitters are mainly the defects, protrusions, and edges.17,18 In this contest, the FE characteristics can preferably be enhanced by creating more protrusions19−22 or assembling © XXXX American Chemical Society

various low work function metal/metal oxide heterostructures of different dimensions over flat 2D G/NG.23−26 To date, with rising interest in construction of novel hetero structures over 2D graphene or NGS for various fields of applications, less effort has been paid to copper-based nanostructures over NGS. Motivated from its low work function, ease of fabrication, and tunable electronic properties, copper-based nanostructures of various shape and size have been demonstrated to exhibit outstanding field electron emission performance with low turn-on/threshold electric fields, high emission density, and good stability.27,28 In view of potential applications of hierarchical copper-based nanostructures, there have been a few approaches on copper-based one-dimensional (1D)/zero-dimensional (0D) novel structures on graphene surfaces. In most of the studies, the approach has been multistep involving lack of uniformity and shape control. Recently, Ruoff et al. prepared hybrid film of CuNW-RGO by spray-coating presynthesized CuNWs solution on RGO film and revealed improved electrical conductivity, oxidation resistance, substrate adhesion, and stability, which were used as transparent electrodes in Prussian blue (PB)-based electrochromic devices.29 Sun et al. developed CuO−Cu2O−Cu nanorod decorated RGO composites by hydrothermal technique and applied in photocurrent generation in the visible Received: December 15, 2014

A

DOI: 10.1021/jp512476b J. Phys. Chem. C XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry C spectra region.30 Chen et al. prepared graphene-copper nanoparticle composite by chemical reduction using potassium borohydride as a reducing agent and applied for electrochemical sensing of carbohydrate.31 Despite enhanced performance in various fields, the synthetic roots, however, limit the shape control, requires complicated experimental setups, involves multisteps, and are not easy to be scaled up for practical implementation. Therefore, it is important to have a catalyst free, large scale, easy synthesis strategy consisting of controlled size distribution, shape evolution, and a desired composition. Directed self-assembly refers to the process by which discrete components organize themselves to a defined novel structure triggered by controlling the external synthesis parameters like temperature, pressure, pH, solvent polarity, reaction atmosphere, etc.32 The unique physical properties of the assembled structure varies distinctly from those of individual components and vastly depends on the size, shape, and composition of the building blocks. Nanoparticle assemblies in 1D array or 1D nanostructure over a conducting template give specific properties arising from the synergistic effect of the assembled nanostructures. In this trend, nitrogen-doped graphene, having large specific surface area, good electrical conductivity, and surface defects can ensure good bonding, strong interfacial interactions, and electrical contact with the assembled nanostructures. Herein, we report a pH-directed self-assembly root for the large scale synthesis of uniformly decorated very thin Cu(OH)2NRs over 2D NGS sheets and further extending the method by reassembling it to CuO NRs and 0D CuNPs arranged in a 1D arraylike structure by low temperature thermal annealing in Ar and H2 atmosphere. The hierarchical nanostructure of very thin Cu(OH)2NRs on NGS and its shape and composition evolution to 1D CuO and 0D Cu NPs arranged in an arraylike structure has been studied by different characterization techniques. Furthermore, we extensively investigated the electron field emission properties of the hybrid composites and their dependence on shape and composition of the emitter.

the above prepared dispersion under vigorous stirring. A calculated amount of ammonium persulfate (APS, molar ratio of aniline to APS 1.5) was dissolved in 1 M H2SO4, cooled in ice bath, and rapidly poured to the GO aniline dispersion under stirring. Subsequently, the polymerization was continued by stirring the dispersion in ice bath for 12 h. A green color precipitate was obtained, indicating formation of emeraldine polymerized form of PANI. The suspension was filtered, washed thoroughly with deionized (DI) water and ethanol. Finally the product was dried at 45 °C in vacuum oven for 24 h to obtain PGO. In order to obtain N doping in graphene, the above synthesized product was annealed in an Ar atmosphere at different temperatures. Many important features perceived from the characterization of the products supports N doping which are discussed briefly in the Supporting Information. Synthesis of Cu(OH)2 NRs-NGS, CuO NRs-NGS and 1D Cu NPAs-NGS Hierarchical Nanostructures. In a typical synthesis procedure, a homogeneous suspension of NGS was made by ultrasonicating a 100 mg of NGS in 100 mL DI water for 30 min. The solution was made highly basic, maintained at pH 12 by adding NaOH solution. Further, a solution containing 100 mg of copper chloride in DI water was added to the NGS solution under vigorous stirring. The reaction was allowed to continue for 30 min. The suspension was filtered, washed several times with DI water, and dried at 60 °C in vacuum. The Cu(OH)2 nanowires start to assemble under the same conditions when the pH of the medium approaches high basicity (pH 12). The influence of pH on self-assembly was further studied, maintaining the same synthetic conditions at different pH medium (6, 8, 10, and 12). Further, we reassembled it to 0D CuNPs arranged in a 1D arraylike structure and CuO NRs over NGS by thermal annealing at 200 °C in H2 and Ar atmosphere, respectively. 2.3. Material Characterization Techniques. The powder X-ray diffraction (XRD) analysis was performed by PANalytica X’pert pro X-ray diffractometer with Cu Kα as the X-ray source. Raman spectra were recorded by a WITec alpha 300 confocal Raman spectrometer equipped with Nd:YAG laser (532 nm) as the excitation source. The sample heating was avoided by using low laser intensity. FTIR spectra were recorded on a PerkinElmer spectrometer using KBr powder pressed pellets. The morphology of the synthesized products were characterized by field emission scanning electron microscopy (FESEM, Quanta 3D), high-resolution transmission electron microscopy (HRTEM, Tecnai G2 20 S-TWIN). To investigate the chemical state, the X-ray photoelectron spectra were recorded with SPECS X-ray photoelectron spectrometer using Mg Kα as the X-ray source and PHOIBOS 100MCD energy analyzer at an ultra high vacuum (10−10 mbar). The data were analyzed using CASA XPS. UPS measurements were done with Omicron ESCA Probe spectrometer with an HIS 13 UV source (21.8 eV He (I) source) and EA 125 energy analyzer. 2.4. Fabrication of Flexible Field Emitter. The electron field emission (FE) properties were studied using an indigenously fabricated set up comprising a high vacuum chamber with base pressure of 1 × 10−6 mbar. The anode is a cylindrical gold-coated copper rod of 1 cm diameter. The cathode is made up of stainless steel of diameter 5 cm which acts as a substrate holder for field emission studies. The field emitter was fabricated on flexible conducting carbon cloth by spin coating by taking a fine dispersion of 5 mg of 1D Cu NPAs-NGS in 0.5 mL (0.5%) nafion solution. Then the

2. EXPERIMENTAL METHODS 2.1. Materials. Flake Graphite powder (99.99% SP-1, average particle size 45 μm) was purchased from Bay Carbon, Inc. Zinc acetate (C4H10O6Zn·2H2O, 99.5%), tin(II) chloride dihydrate (SnCl2·2H2O), ruthenium(III) chloride hydrate (RuCl3, ×H2O) were procured from Aldrich. Potassium permanganate (KMnO4), sodium nitrates (NaNO3), concentrated sulfuric acid (H2SO4, 99%), and concentrated nitric acid (HNO3, 98%) (Rankem chemicals, India) were used as received. Hydrogen peroxide (H2O2, 30 wt %/V) was purchased from Fisher Scientific. Ultrapure water (18.2 MΩ cm) from Millipore system was used throughout all the experiments. 2.2. Synthesis Of Materials. Synthesis of NGS. N-Doped graphene was synthesized by thermal annealing of polyanilinegraphite oxide (PGO) composite in Ar atmosphere. GO was synthesized using natural flake graphite powder by Hummer’s method.33 PGO composite was prepared by established chemical oxidation polymerization process with little modification.34 In a typical root, aqueous GO solution was made by adding 100 mg of GO in 100 mL of DI water. The mixture was ultrasonicated until fully dispersed before transferring into an ice bath. Then a solution containing 10 μL of aniline monomer in 20 mL of 1 M H2SO4 precooled in the ice bath was added to B

DOI: 10.1021/jp512476b J. Phys. Chem. C XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry C

Figure 1. Sketch model for the nucleation and self-assembly of Cu(OH)2 NRs-NGS, CuO NRs-NGS, and 1D Cu NPAs-NGS hierarchical nanostructures compared with FESEM images.

Figure 2. (A) TEM images of Cu(OH)2 NRs-NGS prepared at (a) pH 6, (b) pH 8, (c) pH 10, and (d) pH 12. (B) Powder XRD patterns of (a) 1D Cu NPAs-NGS, (b) CuO NRs-NGS, and (c) Cu(OH) 2 NRs-NGS.

of Cu(OH)2 NRs is revealed from the morphology analysis. When compared, the synthesized samples at different pH medium show the formation mechanism of a 1D nanostructure. The large NG sheets act as excellent supporter and stabilizers for growth of Cu(OH)2NRs and restricts its unavoidable selfaggregation. Further, the reassembling of Cu(OH)2 to 1D Cu NPAs and CuO were carried out by annealing the synthesized product in H2 and Ar atmosphere at 200 °C. The best plausible explanation to the self-assembly process is given by carrying out the reaction at different pH medium (pH 6, 8, 10, and 12) and analyzing the sample by TEM. As displayed in Figure 2A(a), in pH 6 medium, the samples prepared are NG sheets without any attached assembly. When pH was increased to 8, very tiny nanoparticles of diameter 5−10 nm are formed over the NG surface, which is depicted from Figure 2A(b). It is interesting that very thin nanorods of a 5−10 nm diameter rather than nanoparticles are formed with further increase of the pH up to 10. Including this shape evolution from particles to 1D rod, also some tiny particles are embedded to the rods in this pH

prepared dispersion was spin-coated on a carbon cloth at 500 rpm in the first stage for 20 s followed by 2000 rpm in the second stage for 30 s. In order to evaporate the solvent, the film was dried at 100 °C for 12 h in a vacuum oven. The other field emitters were fabricated using equal weight % following the same fabrication technique. The fabricated emitters were fixed to the cathode plate by electrical cotact using silver paste. In order to maintain the anode at a desired distance from the cathode, the anode−cathode separation was controlled by the micrometer screw gauge with a least count of 10 μm vertically fixed on the anode. The field emission current was measured by sweeping the voltage using Keithley 237 SMU with nanoampere current sensitivity.

3. RESULTS AND DISCUSSION 3.1. Self-Assembly of Hierarchical Copper Nanostructures on N-Doped Graphene. The synthesis strategy for different novel copper-based hierarchical nanostructures over NGS is depicted as Figure 1. The detailed structural assembly C

DOI: 10.1021/jp512476b J. Phys. Chem. C XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry C

Figure 3. (a and b) Low- and high-magnification FESEM and (c and d) TEM images of self -assembled Cu(OH)2 NRs-NGS hybrids, respectively. (e and f) FESEM and (g and h) TEM images for reassembled CuO NRs-NGS at different magnifications.

Figure 4. (a and b) Low- and high-magnification FESEM and (c and d) TEM images of reassembled 1D Cu NPAs-NGS hybrids showing NPs arranged in 1D arraylike assembly. High-magnification TEM images of (e) Cu(OH)2 NRs-NGS, (f) CuO NRs-NG, (g) Cu NPs arranged in 1D at different resolutions, and (f) a single Cu NP showing good crystallinity.

medium [shown in Figure 2 A(c)]. By further increasing the pH to 12, pure tiny nanorods of Cu(OH)2uniformly decorated on NGS sheets are formed finally, which is depicted from Figure 2A(d). It is worth noting that the nanorods assembled on NG sheets under high pH conditions follows from the hydrolysis of copper salt and self-assembly to particles and then 1D rods with an increase in pH. Further, we reassembled the nanostructures to CuO NRs and 1D Cu NPAs over NGS. By annealing at 200 °C in Ar atmosphere, it led to the formation of CuO NRs without any change in morphology and remains homogeneously distributed over NGS. In contrast, when annealed in H2 atmosphere, the nanorods break into Cu NPs arranged in a 1D chainlike structure over NGS sheets. This can be due to the high reactivity of hydroxyl groups in Cu(OH)2

with H2 molecules releasing water vapor. The following reaction mechanism can be proposed. Ar

Cu(OH)2 ⎯⎯⎯⎯⎯→ CuO + H 2O 200 ° C

H2

Cu(OH)2 + H 2 ⎯⎯⎯⎯⎯→ Cu + 2H 2O 200 ° C

3.2. Characterization of Hierarchical Nanostructures. In order to investigate the crystallinity of the synthesized hierarchical nanostructures, the powder X-ray diffraction (XRD) pattern were recorded in the range from 5° to 90° with a step size of 0.016°. For Cu(OH)2 NRs-NGS, the XRD data in Figure 2B(c) shows major peaks between 15° to 70° D

DOI: 10.1021/jp512476b J. Phys. Chem. C XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry C

chemical state and purity of the synthesized products, we performed X-ray photoelectron spectroscopy (XPS) analysis in a binding energy ranging from 0 to 1000 eV. As shown in Figure 5A, the extensive survey spectrum of the composites

along with the C(002) peak of NGS which can be indexed to an end-centered orthorhombic (Cmc21(36), a = 0.294 nm, b = 0.105 nm, and c = 0.525 nm) structure of Cu(OH)2 (JCPDS no. 80-0656).35 However, it is interesting that with annealing at 200 °C in Ar atmosphere all the peaks disappear; instead the two strongest peaks at 35.7 and 38.9° appeared along with some small peaks and C(002) which are indexed to endcentered monoclinic (C2/c(15), a = 0.468 nm, b = 0.342 nm, and c = 0.512 nm) CuO structure (JCPDS no. 89-5895) in the patterns shown in Figure 2B(b).36 But when annealed in an H2 atmosphere, four highly crystalline diffraction peaks at 43, 50, 74, and 87° appears [Figure 2B(a)], which can be ascribed to well-crystallized metallic Cu with a face-centered cubic (fcc, 37 Fm3m ̅ (225), a = 0.361 nm) structure (JCPDS no. 85-1326). Other than this, a small peak appears at 36.5, which can be due to the presence of the trace amount of CuO2 phase.38 Being highly reactive to oxygen, it is expected that it can be due to surface oxidation of copper NPs in ambient conditions. The XRD pattern gives a strong evidence of self-assembly of Cu(OH)2NRs over NGS and its reassembly to CuO NRs and 1D Cu NPAs in thermal annealing at different atmosphere. To study the morphology and structural analysis of the nanocomposites, electron microscopy was employed. The representative scanning electron microscopy image shown in Figure 3 (panels a and b) reveals the one-dimensional rodlike morphology of Cu(OH)2 over flat 2D NGS and demonstrates homogeneous distribution of nanorods on NG sheets without agglomerations. The typical lateral size of NRs are measured to be several hundred nanometers where as the diameter varies from 5 to 10 nm. Further, the TEM images (Figure3, panels c and d) validate the intimate contact between densely assembled Cu(OH)2NRs over NGS. After thermal annealing in Ar atmosphere, the crystal structure changes from Cu(OH)2 to CuO, which is discussed earlier in XRD analysis. However, the one-dimensional morphology over NGS surface persists, which is evident from FESEM images shown in Figure3 (panels e and f). Both the well-dispersed CuO NRs and NGS are clearly observable in the higher magnification images of Figure 3 (panels g and h). Figure 4 depicts the FESEM and TEM images revealing the change in morphology of the composite with annealing in H2 atmosphere. It clearly predicts the decomposition of Cu(OH)2 NRs and its reassembly to spherical Cu nanoparticles aligned in 1D. It is expected that the Cu atoms from individual Cu(OH)2NR decomposes in thermal annealing in the presence of H2 and nucleates to form copper nanoparticles arranged in 1D fashion. The high-resolution TEM images shown in Figure 4e illustrates the high crystallinity of Cu(OH)2 NRs, which remains consistent with the XRD results. The lattice fringes are separated by a 0.263, 0.251, and 0.364 nm distance corresponding to the (002), (111), and (021) planes of orthorhombic Cu(OH)2. The high-resolution TEM image (Figure 4f) of a single CuO NR reveals a lattice fringe separation of 0.252 nm, 0.230 nm corresponding to (002), and (200) lattice planes of monoclinic CuO. The HRTEM images for 1D Cu NPAs are presented in Figure 4 (panels g and h) at different magnifications which depict a lattice fringe separation of 0.2 nm corresponding to the Cu (111) plane. The additional plausible explanation for the transformation of Cu(OH)2NRs to 1D Cu NPAs is the loss of the hydroxide ion by reacting with the H2 molecule to form water vapor through the annealing process. In contrast, it forms CuO NRs at the same temperature in Ar atmosphere. To investigate the

Figure 5. (A) XPS survey spectrum and (B) high-resolution XPS spectra in Cu 2p region of (a) 1D Cu NPAs-NGS, (b) CuO NRsNGS, and (c) Cu(OH)2 NRs-NGS.

depicts the presence of elemental C, O, and Cu. Compared to the as-synthesized Cu(OH)2 [Figure 5A(a)], the oxygen content predictably decreases through annealing in Ar [Figure 5A(b)] and H2 atmosphere [Figure 5A(c)]. The presence of no other elemental peak validates the high purity of the composite. Figure 5B depicts the high-resolution spectrum of Cu 2p region in the synthesized products. The deconvoluted Cu 2p spectra for Cu(OH)2 NRs-NGS [Figure 5B(a)] exhibits a doublet at the binding energy of 936.6 and 956.5 eV, corresponding to the Cu 2p3/2 and Cu 2p1/2 spin orbit coupling levels. The observation of the broad shakeup peak in higher-binding energy direction well agrees with the Cu(OH)2 standard spectra.39 The peak position of Cu 2p spectrum shifts toward lower binding energy (BE) values of 933.68 and 963.68 eV with an energy separation of 20 eV after annealing in Ar atmosphere [Figure 5B(b)]. This is a signature of conversion of Cu(OH)2 to CuO.40 Furthermore, the spectra for H2-treated samples [Figure 5B(c)] shows again a shift to 932.2 and 952.2 eV BE values, which is assigned to Cu(0) in the metallic state.41 Also, the shakeup peak profile diminishes, which agrees with the standard data for metallic copper. Again, an asymmetry of peak E

DOI: 10.1021/jp512476b J. Phys. Chem. C XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry C

Figure 6. (A) Field emission current density as a function of applied field for (a) 1D Cu NPAs-NGS, (b) CuO NRs-NGS, (c) Cu(OH)2 NRs-NGS. (B) Fowler-Northeim plot showing linear dependence in low field regions. (C) UPS valence bands spectra of (a) 1D Cu NPAs-NGS, (b) CuO NRsNGS, and (c) Cu(OH)2 NRs-NGS. (D) Sketch model showing mechanism of electron emission from different emitters.

where J is the emission current density, E is the applied field, ϕ is the work function of the field emitter, and β is the field enhancement factor. A and B are constants with values 1.56 × 10−6 AeV V−2 and 6.83 × 103 V/μm eV−3/2, respectively. Replotting the J−E plot data as ln(J/E2) versus (1/E) concludes that the emitters follow the F−N tunneling mechanism which is depicted in Figure 6B. The F−N plots were linearly fit in order to calculate the geometric field enhancement factor (β) based on the following relation,

profile is observed for 1D Cu NPAs-NGS, which depicts the presence of the Cu(2+) phase in the sample. This can be due to surface oxidation of metallic copper during sample preparation for XPS. 3.3. Electron Field Emission. The electron field emission properties for the synthesized hybrid structures were investigated using an indigenously fabricated set up at room temperature and a base pressure of 10−6 mbar (refer to Experimental Methods for details). Figure 6A shows the plot of the emission current density (J) as a function of the applied field (E) measured at an anode−cathode separation of 500 μm. It is observed that the turn-on field (ETO at J = 10 μA/cm2) and the threshold field (Eth at J = 0.2 mA/cm2) of 1D Cu NPAsNGS (ETO = 0.648 and Eth = 0.77 V/μm) are significantly lower than that of CuO NRs-NGS (ETO = 0.724 and ETh = 0.96 V/ μm) and Cu(OH)2NRs-NGS (ETO = 0.956 and ETh = 1.11 V/ μm), respectively. The typical field emission behavior are further analyzed by the Fowler-Northeim (F−N) tunneling mechanism based on F−N equation.42 ⎛ Aβ 2E2 ⎞ ⎛ −Bϕ3/2 ⎞ ⎟ J=⎜ ⎟exp⎜ ⎝ ϕ ⎠ ⎝ βE ⎠

(1)

⎛ Aβ 2 ⎞ ⎛ Bϕ3/2 ⎞ ⎛ J ⎞ ⎟ ln⎜ 2 ⎟ = ln⎜ ⎟−⎜ ⎝E ⎠ ⎝ ϕ ⎠ ⎝ βE ⎠

(2)

β=

Bϕ3/2d slope

where d is anode cathode separation and ϕ is the work function of the hybrid composite. The work function of the synthesized samples was measured using UV photoelectron spectroscopy (UPS), as shown in Figure 6C. The work function varies depending on the decorated nanostructures. Compared to CuO NRs-NGS (4.65 eV) and Cu(OH)2NRs-NGS (4.73 eV), the ϕ value of 1D Cu NPAs-NGS decreases considerably (4.15 eV). With a constant anode cathode separation (d value), calculated ϕ values and slope of F−N plots, the β value for 1D Cu NPAsNGS were calculated to be 7680, which is larger than those of CuO NRs-NGS (7609) and Cu(OH)2 NRs-NGS (3304), respectively. Being closely dependent on the geometry and the work function of the emitter, the better field enhancement in 1D Cu NPAs-NGS-based emitter is believed to be from the electric field localization (Eloc= βEappl) at individual CuNPs

or

F

DOI: 10.1021/jp512476b J. Phys. Chem. C XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry C

NPAs emitter exceeds the FE properties of Cu(OH)2 NRsNGS, CuO NRs-NGS-based emitters, with ETO = 0.648 V/μm and ETh = 0.77 V/μm and a large field-enhancement factor of about 7680. These results demonstrate that the 1D Cu NPAsNGS emitter has better emission activity compared to Cu(OH)2 NRs-NGS, CuO NRs-NGS based on its increased effective emission sites, decreased work function, and 1D arraylike architecture. This approach offers prospects for understanding the effect of linear arrangement of nanoparticles on FE, which can be envisioned for design, fabrication, and optimization of cold cathode devices.

arranged in arraylike 1D structure, which could act like tiny nanotips for electron emission. In contrast, the CuO NRs-NGS shows a lower β value which can be due to local field enhancement at the ends of the rod. As the rods are aligned horizontally on NG sheets, it is assumed that the outer surface of the CuO NRs remains less sensitive for electron emission, which is consistent with some recent studies on 1D CNT-based field emitters.43,44 The mechanism of emission is illustrated schematically in Figure 6D. In addition, the current densities at a field of 1 V/μm for 1D Cu NPAs-NGS is 1001.76 μA/cm2, which is about 3 times higher for CuO NRs-NGS (275.86 μA/ cm2) and about 27 times higher for Cu(OH)2 NRs-NGS (36.96 μA/cm2). The higher current densities of 1D Cu NPA-based field emitters is likely to be based on several factors. One is the metallic nature of 1D Cu NPAs, which has a lower work function compared to its metal oxide and metal hydroxide counterparts. The other probable reason is the low resistance in electron conduction at individual Cu NPs and NGS interface aroused from the intimate contact in the nucleation process with NGS. This predominantly concentrates the electric field at individual NPs, which acts as a major emission site. In contrast, the electric field on a 1D flat rod remains constant all over the rod surface, which fails in concentrating the local field. Another possible mechanism can be the effect of 1D arraylike arrangement of NPs, in which each isolated NPs act like separate emission tips and this could effectively result in more emission sites compared to aligned rods.45 Judging from the considerably enhanced current density and large field enhancement factor, which intuitively suggests that in spite of being 1D, both CuO NRs and Cu(OH)2 NRs on NGS surface remains less sensitive for FE compared to Cu NPs arranged in a 1D arraylike structure. Being of practical importance, the stability study of the emitters was carried out at an applied field of 1 V/μm, which is shown in Figure7. It is apparent that no significant degradation of emission current density over a time period of ∼4.3 h occurs. This illustrates good stability of the fabricated emitters.

■

ASSOCIATED CONTENT

S Supporting Information *

The synthesis and characterization details, including XRD patterns, FESEM and HRTEM images, and FTIR, Raman, and XPS spectra, of nitrogen-doped graphene sheets. This material is available free of charge via the Internet at http://pubs.acs.org.

■

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel: +91-44-22574862. Fax: +91-4422570509. Notes

The authors declare no competing financial interest.

■

ACKNOWLEDGMENTS The author thanks Indian Institute of Technology Madras (IITM), Chennai, India, for financial support.

■

REFERENCES

(1) Novoselov, K. S.; Jiang, Z.; Zhang, Y.; Morozov, S. V.; Stormer, H. L.; Zeitler, U.; Maan, J. C.; Boebinger, G. S.; Kim, P.; Geim, A. K. Room-Temperature Quantum Hall Effect In Graphene. Science 2007, 315, 1379. (2) Geim, A. K. Graphene: Status and Prospects. Science 2009, 324, 1530−1534. (3) Balandin, A. A.; Ghosh, S.; Bao, W.; Calizo, I.; Teweldebrhan, D.; Miao, F.; Lau, C. N. Superior Thermal Conductivity Of Single-Layer Graphene. Nano Lett. 2008, 8, 902−907. (4) Gómez-Navarro, C.; Weitz, R. T.; Bittner, A. M.; Scolari, M.; Mews, A.; Burghard, M.; Kern, K. Electronic Transport Properties Of Individual Chemically Reduced Graphene Oxide Sheets. Nano Lett. 2007, 7, 3499−3503. (5) Castro Neto, A. H.; Guinea, F.; Peres, N. M. R.; Novoselov, K. S.; Geim, A. K. The Electronic Properties Of Graphene. Rev. Mod. Phys. 2009, 81, 109−162. (6) Eda, G.; Fanchini, G.; Chhowalla, M. Large-Area Ultrathin Films Of Reduced Graphene Oxide as a Transparent And Flexible Electronic Material. Nat. Nanotechnol. 2008, 3, 270−274. (7) Wei, D.; Liu, Y.; Wang, Y.; Zhang, H.; Huang, L.; Yu, G. Synthesis Of N-Doped Graphene By Chemical Vapor Deposition And Its Electrical Properties. Nano Lett. 2009, 9, 1752−1758. (8) Joucken, F.; Tison, Y.; Lagoute, J.; Dumont, J.; Cabosart, D.; Zheng, B.; Repain, V.; Chacon, C.; Girard, Y.; Botello-Ḿ endez.; et al. Localized State and Charge Transfer in Nitrogen-Doped Graphene. Phys. Rev. B 2012, 85, 161408. (9) Gracia-Espino, E.; Jia, X.; Wagberg, T. Improved Oxygen Reduction Performance Of Pt−Ni Nanoparticles By Adhesion On Nitrogen-Doped Graphene. J. Phys. Chem. C 2014, 118, 2804−2811. (10) Vinayan, B. P.; Nagar, R.; Rajalakshmi, N.; Ramaprabhu, S. Novel Platinum-Cobalt Alloy Nanoparticles Dispersed On NitrogenDoped Graphene as a Cathode Electrocatalyst For Pemfc Applications. Adv. Funct. Mater. 2012, 22, 3519−3526.

4. CONCLUSION In summary, a large scale synthesis strategy for anchoring hierarchical copper based nanostructures over NGS was shown by pH-directed self-assembly and further low-temperature thermal annealing in Ar and H2 atmosphere. The 1D Cu

Figure 7. Emission current density with respect to time for (a) 1D Cu NPAs-NGS, (b) CuO NRs-NGS, and (c) Cu(OH)2 NRs-NGS, depicting stability of the emitters. G

DOI: 10.1021/jp512476b J. Phys. Chem. C XXXX, XXX, XXX−XXX

Article

The Journal of Physical Chemistry C (11) Wu, Z. S.; Yang, S.; Sun, Y.; Parvez, K.; Feng, X.; Müllen, K. 3D Nitrogen-Doped Graphene Aero Gel-Supported Fe3o4 Nanoparticles as Efficient Electrocatalysts For The Oxygen Reduction Reaction. J. Am. Chem. Soc. 2012, 134, 9082−9085. (12) Ma, Z.; Huang, X.; Dou, S.; Wu, J.; Wang, S. One-Pot Synthesis Of Fe2O3 Nanoparticles On Nitrogen-Doped Graphene as Advanced Supercapacitor Electrode Materials. J. Phys. Chem. C 2014, 118, 17231−17239. (13) Soin, N.; Roy, S. S.; Roy, S.; Hazra, K. S.; Misra, D. S.; Lim, T. H.; Hetherington, C. J.; McLaughlin, J. A. Enhanced And Stable Field Emission From In Situ Nitrogen-Doped Few-Layered Graphene Nanoflakes. J. Phys. Chem. C 2011, 115, 5366−5372. (14) Yu, S. S.; Zheng, W. T. Effect Of N/B Doping On The Electronic And Field Emission Properties For Carbon Nanotubes, Carbon Nanocones, and Graphene Nanoribbons. Nanoscale 2010, 2, 1069−1082. (15) Xu, J.; Hou, G.; Mori, T.; Li, H.; Wang, Y.; Chang, Y.; Luo, Y.; Yu, B.; Ma, Y.; Zhai, T. Excellent Field-Emission Performances Of Neodymium Hexaboride (Ndb6) Nanoneedles With Ultra-Low Work Functions. Adv. Funct. Mater. 2013, 23, 5038−5048. (16) Baby, T. T.; Ramaprabhu, S. Cold Field Emission From Hydrogen Exfoliated Graphene Composites. Appl. Phys. Lett. 2011, 98, 183111. (17) Guo, Y.; Guo, W. Electronic And Field Emission Properties Of Wrinkled Graphene. J. Phys. Chem. C 2013, 117, 692−696. (18) Yamaguchi, H.; Murakami, K.; Eda, G.; Fujita, T.; Guan, P.; Wang, W.; Gong, C.; Boisse, J.; Miller, S.; Acik.; et al. Field Emission From Atomically Thin Edges Of Reduced Graphene Oxide. ACS Nano 2011, 5, 4945−4952. (19) Ye, D.; Moussa, S.; Ferguson, J. D.; Baski, A. A.; El-Shall, M. S. Highly Efficient Electron Field Emission From Graphene Oxide Sheets Supported By Nickel Nano Tip Arrays. Nano Lett. 2012, 12, 1265− 1268. (20) Nayak, P.; Santhosh, P. N.; Ramaprabhu, S. Enhanced Electron Field Emission Of One-Dimensional Highly Protruded Graphene Wrapped Carbon Nanotube Composites. J. Phys. Chem. C 2014, 118, 5172−5179. (21) Zhang, S.; Zhang, Y.; Huang, S.; Liu, H.; Wang, P.; Tian, H. First-Principles Study Of Field Emission Properties Of Graphene-Zno Nanocomposite. J. Phys. Chem. C 2010, 114, 19284−19288. (22) Kaushik, V.; Shukla, A. K.; Vankar, V. D. Improved Electron Field Emission From Metal Grafted Graphene Composites. Carbon 2013, 62, 337−345. (23) Baby, T. T.; Ramaprabhu, S. Effect Of Metal Nanoparticles Decoration On Electron Field Emission Property Of Graphene Sheets. Nanoscale 2011, 3, 4170−4173. (24) Lee, D. H.; Lee, J. A.; Lee, W. J.; Choi, D. S.; Lee, W. J.; Kim, S. O. Facile Fabrication And Field Emission Of Metal-Particle-Decorated Vertical N-Doped Carbon Nanotube/Graphene Hybrid Films. J. Phys. Chem. C 2010, 114, 21184−21189. (25) Chen, L.; He, H.; Lei, D.; Menggen, Q. Q. G.; Hu, L.; Yang, D. Field Emission Performance Enhancement of Au Nanoparticles Doped Graphene Emitters. Appl. Phys. Lett. 2013, 103, 233105. (26) Hwang, J. O.; Lee, D. H.; Kim, J. Y.; Han, T. H.; Kim, B. H.; Park, M.; No, K.; Kim, S. O. Vertical Zno Nanowires/Graphene Hybrids for Transparent and Flexible Field Emission. J. Mater. Chem. 2011, 21, 3432−3437. (27) Hsieh, C. T.; Chen, J. M.; Lin, H. H.; Shih, H. C. Field Emission from Various CuO Nanostructures. Appl. Phys. Lett. 2003, 83, 3383− 3385. (28) Das, S.; Saha, S.; Sen, D.; Ghorai, U. K.; Banerjee, D.; Chattopadhyay, K. K. Highly Oriented Cupric Oxide Nanoknife Arrays On Flexible Carbon Fabric as High Performing Cold Cathode Emitter. J. Mater. Chem. C 2014, 2, 1321−1330. (29) Kholmanov, I. N.; Domingues, S. H.; Chou, H.; Wang, X.; Tan, C.; Kim, J. Y.; Li, H.; Piner, R.; Zarbin, A. J. G.; Ruoff, R. S. Reduced Graphene Oxide/Copper Nanowire Hybrid Films As High-Performance Transparent Electrodes. ACS Nano 2013, 7, 1811−1816.

(30) Tian, J.; Li, H.; Xing, Z.; Wang, L.; Luo, Y.; Asiri, A. M.; Abdulrahman, O. A.; Sun, X. One-Pot Green Hydrothermal Synthesis Of Cuo-Cu2o-Cu Nanorod-Decorated Reduced Graphene Oxide Composites and Their Application In Photocurrent Generation. Catal. Sci. Technol. 2012, 2, 2227−2230. (31) Chen, Q.; Zhang, L.; Chen, G. Facile Preparation Of GrapheneCopper Nanoparticle Composite By In Situ Chemical Reduction For Electrochemical Sensing Of Carbohydrates. Anal. Chem. 2012, 84, 171−178. (32) Grzelczak, M.; Vermant, J.; Furst, E. M.; Liz-Marzán, L. M. Directed Self-Assembly Of Nanoparticles. ACS Nano 2010, 4, 3591− 3605. (33) Hummers, W. S., Jr.; Offeman, R. E. Preparation of Graphitic Oxide. J. Am. Chem. Soc. 1958, 80, 1339−1339. (34) Xu, J.; Wang, K.; Zu, S.-Z.; Han, B.-H.; Wei, Z. Hierarchical Nanocomposites Of Polyaniline Nanowire Arrays On Graphene Oxide Sheets with Synergistic Effect for Energy Storage. ACS Nano 2010, 4, 5019−5026. (35) Park, S. H.; Kim, H. J. Unidirectional Aligned Copper Hydroxide Crystalline Nanorods From Two-Dimensional Copper Hydroxy Nitrate. J. Am. Chem. Soc. 2004, 126, 14368−14369. (36) Cao, A.; Monnell, J. D.; Matranga, C.; Wu, J.; Cao, L.; Gao, D. Hierarchical Nanostructured Copper Oxide And Its Application in Arsenic Removal. J. Phys. Chem. C 2007, 111, 18624−18628. (37) Yin, M.; Wu, C. K.; Lou, Y.; Burda, C.; Koberstein, J. T.; Zhu, Y.; O’Brien, S. Copper Oxide Nanocrystals. J. Am. Chem. Soc. 2005, 127, 9506−9511. (38) Tran, P. D.; Batabyal, S. K.; Pramana, S. S.; Barber, J.; Wong, L. H.; Loo, S. C. J. A Cuprous Oxide−Reduced Graphene Oxide (Cu2o− Rgo) Composite Photo Catalyst For Hydrogen Generation: Employing Rgo as an Electron Acceptor To Enhance The Photo Catalytic Activity And Stability Of Cu2o. Nanoscale 2012, 4, 3875−3878. (39) Wagner, C. D.; Naumkin, A. V.; Kraut-Vass, A.; Allison, J. W.; Powell, C. J.; Rumble, J. R. Jr. NIST Standard Reference Database 20, version 3.4 (http://srdata.nist.gov/xps/), 2003. (40) Ko, J. W.; Kim, S. W.; Hong, J.; Ryu, J.; Kang, K.; Park, C. B. Synthesis Of Graphene-Wrapped Cuo Hybrid Materials By CO2 Mineralization. Green Chem. 2012, 14, 2391−2394. (41) Ghosh, M.; Padma, N.; Tewari, R.; Debnath, A. K. Structural, Optical, And Electrical Properties Of In Situ Synthesized Zno-Cupc Nanocomposites. J. Phys. Chem. C 2014, 118, 691−699. (42) Fowler, R. H.; Nordheim, L. Electron Emission In Intense Electric Field. Proc. R. Soc. A 1928, 119, 173−181. (43) Konishi, Y.; Hokushin, S.; Tanaka, H.; Pan, L.; Akita, S.; Nakayama, Y. Comparison Of Field Emissions From Side Wall And Tip Of An Individual Carbon Nanotube. Jpn. J. Appl. Phys. 2005, 44, 1648−1651. (44) Chai, G.; Chow, L. Electron Emission From The Side Wall Of An Individual Multiwall Carbon Nanotube. Carbon 2007, 45, 281− 284. (45) Shang, N.; Papakonstantinou, P.; Wang, P.; Zakharov, A.; Palnitkar, U.; Lin, I. N.; Chu, M.; Stamboulis, A. Self-Assembled Growth, Microstructure, And Field-Emission High-Performance Of Ultrathin Diamond Nanorods. ACS Nano 2009, 3, 1032−1038.

H

DOI: 10.1021/jp512476b J. Phys. Chem. C XXXX, XXX, XXX−XXX