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COVER ARTICLEZhang et al. Controlled growth of nickel nanocrystal arrays and their fi eld electron emission performance enhancement via removing adsorbed gas molecules

CE015007_cover_PRINT.indd 1CE015007_cover_PRINT.indd 1 1/18/2013 7:58:02 AM1/18/2013 7:58:02 AM

Cite this: CrystEngComm, 2013, 15,1296

Controlled growth of nickel nanocrystal arrays and theirfield electron emission performance enhancement viaremoving adsorbed gas molecules

Received 8th September 2012,Accepted 10th October 2012

DOI: 10.1039/c2ce26456k

www.rsc.org/crystengcomm

Jian Wang, Liangming Wei, Liying Zhang, Jing Zhang, Hao Wei, Chuanhai Jiangand Yafei Zhang*

We present the fabrication of Ni nanocone arrays on Ni foil substrate as well as gas exposure field electron

emission experiments using them as cold electron cathodes. The self-assembly of nanoscale building blocks

into organized conical superstructures is achieved by the assistance of oriented attachment. These Ni

nanocones grow only along the [111] direction and to be controlled mainly rely on two factors: the surface

energies and the lattice matching extent of the attached surfaces. On the basis of the equilibrium between

nucleation rate and crystallization rate, Ni nanocones with different aspect ratio (0.2–2.2) are prepared by

adjusting the concentration of hydrazine hydrate and growth temperature. Field electron emission

measurements indicate that the as-grown Ni nanocone array is an excellent field emitter exhibiting low

turn-on field, high current density, and large field enhancement factor due to sharper tips and better

contact with the Ni substrate. Meanwhile, we find that adsorbed gas molecules greatly hindered the field

electron emission performance of the Ni nanocone array, and oxygen and nitrogen gases show different

suppressive behaviors, which strongly correlate with the electronegativity of the individual species.

Repeated applying voltage or ‘vacuum J–E annealing’ could significantly improve field emission properties

and stability, which is attributed to desorption of the adsorbed gas molecules through Joule heating.

Introduction

Intensive research has been carried out over the last severaldecades on electron emitters in view of numerous applicationssuch as flat panel displays, parallel electron-beam lithographyequipment, X-ray sources, electron microscopes, cathode-ray tubemonitors, high energy accelerators, and vacuum microwaveamplifiers.1–7 Electrons can be produced from a metal orsemiconductor surface into the vacuum either via thermionicemission by heating to high temperatures or via field emission(i.e., cold emission) by applying sufficiently large electrostaticfields.8,9 The electron emitters utilize thermionic emission togenerate high-current electron beams but are problematic becausethey are unstable, bulky, have high energy consumption and timedelay, and cause heating of the surrounding device housing.10

Therefore, the cold cathode emitter attracts more attention due toits fast turn-on process, low working temperatures, ultrahighenergy efficiency and miniaturized device size, and provides ahigh emission current density at low electric fields.11,12 In fieldemission (i.e., Fowler–Nordheim tunneling), electrons tunnelthrough a surface potential-energy barrier in the presence of a

strong external electrostatic field.13 This potential barrier, which isidentical to the work function of the cold cathode material at zeroelectric field, becomes low and narrow in an external electric field,enabling the Fowler tunneling of electrons.9,12,13 For thecommercial applications of field emission, it is desirable toincrease current density and reduce the strength of the turn-onelectric fields. Generally, one of the most effective ways that canmeet these requirements is optimizing the geometric factor (suchas needlelike shape with sharp tip) by increasing the alignment orreducing the tip size of the nanostructures.14,15

Recently, nanomaterials with nanoscale sharp tips onplanar substrates exhibit excellent electron emission charac-teristics due to local field enhancement at the tip.Considerable efforts have been made with respect to thegrowth of various nanostructures in a part of the fieldemission community, particularly for carbon nanotubes,graphene, graphene oxide (GO), semiconducting nanotipsand metallic nanotips.16–20 Among all these nanostructures,metallic nanocones exhibiting superior electrical conductivityand excellent field emission performance are expected to beuseful in the fabrication of electron emitters for flat paneldisplays and scanning probe microscopy. In the last twentyyears, only a few examples of the fabrication of metallicnanocones were reported, and the synthesis methods weremainly electrodeposition method. For example, Nagaura et al.

Key Laboratory for Thin Film and Microfabrication of the Ministry of Education,

Research Institute of Micro/Nano Science and Technology, School of Materials

Science and Engineering, Shanghai Jiao Tong University, Shanghai 200240, China.

E-mail: yfzhang@sjtu.edu.cn; Fax: +86 21 3420 5665; Tel: +86 21 3420 5665

CrystEngComm

PAPER

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synthesized hexagonally ordered nickel (Ni) nanocone arraywith an interval of 100 nm using electrodeposition andanodization method.21 Hang et al. produced large-scale Ninanocone array via directional electrodeposition method, andthis Ni nanocone array showed good field emission propertieswith low turn-on field of 5–6.7 V mm21 and large emissioncurrents of 155–206 mA.22 A new and promising route that weexplore in this article is arrays of Ni nanocones on Ni foilsubstrates via a simple hydrothermal reaction. These as-grownNi nanocones with nanoscale sharp tips on planar Nisubstrates can effectively increase emission current densityand reduce the strength of the turn-on electric fields,especially for flat panel displays and electron microscopes.

For the commercial applications of field emission, the coldcathodes sometimes require operation in poor vacuumconditions or low pressure ambient air environments.23

However, the field emission performance is strongly relatedto the residual gas or liberated by electron bombardmentinside the sealed field emission devices, which can affect theemission stability of the cold cathode. For example, Yeonget al. found that the oxygen gas exposure suppresses fieldemission, while hydrogen gas reduced the emission turn-onvoltage and increased emission current.24 However, contra-dictory results on the effect of oxygen on the field emissionhave been reported in literature.25 In order to achieve emissionstability of electron emitters and illustrate specific emissionmechanism, understanding the effects of gas adsorption anddesorption on the field emission properties is an importantissue both technologically and fundamentally.26 In the presentwork, we systematically investigated the effects of gas exposureon the field emission properties of Ni nanocone array and apossible electron emission mechanism was discussed.

Experimental section

Preparation of the Ni nanocone array

All chemicals used in this work were of analytical reagentgrade and were directly used without further purification. Inorder to prepare Ni nanocones on Ni foil substrates, before-hand commercial Ni foils were ultrasonically washed withacetone, ethanol and distilled water for 20 min successively.Then Ni foils were dipped into a 20 wt% solution of sulfuricacid for 60 min at 80 uC to remove the thin oxidation layer. In atypical procedure, 0.952 g of NiCl2?6H2O (Sinopharm ChemicalReagent Co., Ltd, China) was dissolved in 50 mL distilled waterunder continuous magnetic stirring at room temperature. Halfan hour ultrasound treatment was carried out to ensure thatnickel ions were dispersed uniformly in the water solution anda grass-green color was observed in this solution. And then 50mL water solution of N2H4?H2O (10 mL, 85 wt%, SinopharmChemical Reagent Co., Ltd, China) were added into the abovesolution. This reaction mixture was stirred constantly to obtaina homogeneous navy blue solution and subsequently trans-ferred into a Teflon-lined stainless steel autoclave containingpre-treated Ni foil. The Teflon reaction kettle was closed tightlyto perform hydrothermal processes at 100 uC for 15 h. After the

reaction was completed, the obtained grey–black products onNi foil substrate were rinsed three times with acetone, ethanoland distilled water, and then the products were dried in avacuum oven at 60 uC for 10 h. Fig. 1 (A) shows the flowchart ofNi nanocone array preparation.

Characterization of Ni nanocones

The crystalline phase identification of the as-preparedproducts were performed by X-ray powder diffraction (XRD)using a 18 kW advanced X-ray diffractometer (D8 ADVANCE,Bruker AXS, Germany) in a two theta range from 30 to 100uwith Cu Ka radiation (l = 0.154056 nm) rotating anode pointsource operating at 40 kV and 40 mA. The surface morphologyand elemental composition of Ni nanocones were investigatedby using a field emission scanning electron microscopy (SEM,Zeiss Ultra 55, Germany) combined with energy dispersivespectroscopy (EDS) at accelerating voltage of 5 kV and 20 kV,respectively. The microstructure of as-synthesized product wasfurther studied by a transmission electron microscopy (TEM,JEM-2100, JEOL, Japan). Samples for TEM were obtained viapeeling nanocones from Ni foil and then diluted with ethanol.High-resolution transmission electron microscopy (HRTEM)images and selected area electron diffraction (SAED) patternswere recorded using a JEM-2010 transmission electron micro-scope operating at 200 kV.

Preparation and measurement of the field emission devices

For the fabrication of the field emission devices (see Fig. 1 (B)),the as-prepared Ni nanocone array on Ni substrate used for thecathode were attached onto an indium tin oxide (ITO)aluminosilicate glass using silver paste and then dried at 80uC for 3 h. Another piece of ITO glass was used as the anode tocollect electrons from Ni nanocones. The gap between theanode and the cathode separated via mica was kept at 300 mm.A voltage from 0 to 3000 V was applied under a two-parallel-plate configuration and the emission current was measured atthe same time using a homemade measure unit connected inseries with a 10 MV resistor. The emission current density wascalculated from the obtained emission current and theemission area of the Ni nanocone array. All of the measure-ments were carried out in a closed testing chamber with avacuum of 5.0 6 1024 Pa at room temperature.

In order to investigate the effect of residual gases on thefield emission properties of Ni nanocone array, the testconditions were kept constant except the gas condition, sothat other factors can be avoided. For this consideration, ahomemade field emission device testing system was set up, asshown in Fig. 1 (C). During the gas exposure experiments,separate high purity gases (air, N2 and O2) were introducedinto the vacuum chamber by means of a manual valve andmass flow controller (MFC). After each gas exposure experi-ment, the test chamber was pumped down to the initialpressure of 5.0 6 1024 Pa, and field emission measurementswere continued to carry out in terms of field emission currentversus applied voltage (I–V) and current versus time (I–t)characteristics. Fresh samples were used for each gas exposureexperiment in order to avoid other factors’ interference.

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Results and discussion

Structure and morphology of Ni nanocones

Pure Ni nanocones on Ni foil substrate were synthesized by thehydrothermal reaction of an aqueous solution of nickelchloride hexahydrate with hydrazine hydrate in a Teflon-linedstainless steel autoclave. At a fixed mole fraction of divalentnickel and water, the reaction time (15 h), reaction tempera-ture (100 uC), and concentration of hydrazine hydrate (10%)were experimental variables. Fig. 2 (A) shows the typical XRDpatterns of the as-prepared Ni nanocone array. From theliterature (Joint Committee on Powder Diffraction Standards(JCPDS) card No. 04-0850 (Ni), space group, fm3m), all thediffraction peaks in this pattern (Fig. 2 (A)) were found tomatch with pure Ni phase having face-centered cubic (fcc)

structure. These peaks at the scattering angles (2h) correspondto crystal planes of (111), (200), (220) and (311) of crystallineNi. No other characteristic peaks, such as nickel oxide orhydroxide, can be detected, indicating that the pure Ni phasewas obtained under the current synthesis conditions. Thetexture coefficient of each crystal face can be calculated by thefollowing formula:

TChkl~I(hkl)=I0(hkl)P

I(hkl)=I0(hkl)|100% (1)

where I(hkl) and I0(hkl) indicate the diffraction intensities of theNi deposits and the standard Ni power, respectively. Theresults are TC111 = 40%, while TC200 = 23%, TC220 = 18% andTC311 = 19%, which indicates that the preferred orientation of

Fig. 1 (A) Flowchart of samples preparation shows the specific method of synthesis of Ni nanocone array on Ni foil substrate. The nanocones were prepared byhydrothermal reaction of nickel salt with hydrazine hydrate in Teflon-lined stainless steel autoclave. (B) Schematic diagram shows configuration of the field emissiondevices, which mainly contained cathode of Ni nanocone array to emit electrons by applying a strong external electrostatic field, and anode to collect electrons fromNi nanocones. (C) The homemade field emission device testing system.

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Ni nanocones on substrate is (111). Energy DispersiveSpectroscopy (EDS) analysis (see Fig. 2 (B)) on severalnanocones only revealed the peaks of Ni (no formingamorphous NiO layer in air), which further confirms that theas-synthesized Ni nanocones are very stable in ambientatmosphere.

Fig. 2 (C) presents the low-magnification scanning electronmicroscopy (SEM) image of the Ni nanocones with aremarkably uniform shape and size. The Ni nanocones havebase diameters ranging from 50 to 450 nm and heightsranging from 50 to 200 nm. More importantly, the nanoconeyield is rather high and the purity is outstanding as is evidentfrom the Ni foil substrate. Fig. 2 (D–F) clearly demonstrate themorphology of a single nanocone from different angles ofview. Obviously, the top view image indicated that thenanocone has four side planes forming a rectangular pyramidtip. From the side view image it can be seen that the tipdiameter of the nanocone is about 10 nm, and the apex angleof nanocone is about 40u. In order to better understand thestructure of the conical crystals in more detail, further

transmission electron microscopy (TEM) observation at highermagnification (Fig. 2 (G)) shows that the Ni nanocone has avery smooth surface and good symmetry. The typical high-resolution TEM (HRTEM) image recorded from the edge of theNi nanocone is shown in Fig. 2 (H), as indicated by the symbolof 1 (the dashed red circle part in Fig. 2 (G)), which shows thecrystal lattice structure of the nanocone. These clear latticefringes indicate that the entire nanocone is a single crystalstructure, and no visible line or planar defects, implying thehigh crystallinity of Ni nanocone. The crystals are imaged tohave nearly parallel lines, which are nickel atomic planesseparated by about 2.1 Å in Fig. 2 (H), corresponding to the{111} planes of a face-centered cubic Ni crystal. This meansthat the growth orientation of the Ni nanocone is along the[111] direction. The corresponding selected area electrondiffraction (SAED) pattern of the Ni nanocone is shown inFig. 2 (I). These pattern spots also demonstrate the singlecrystal of this nanocone, and pattern spots can readily beindexed as (111), (200), (220), (311) of the face-centered cubicNi.

Fig. 2 Structure and morphology characterization of Ni nanocones on Ni foil substrate synthesized via a hydrothermal approach (100 uC, 15 h). (A) XRD pattern of the sampleconfirming formation of face-centered cubic Ni crystal. (B) EDS analysis showing that the as-prepared products are composed of only Ni element. (C) Low-magnification SEMimage of Ni nanocone array demonstrating the high yield and good uniformity. (D–F) High magnification SEM image of a single Ni nanocone from different angle of view: (D)top view, (E) 45u to the substrate, (F) side view. (G) TEM image of a single Ni nanocone showing the conical structure and good symmetry structure. (H) HRTEM image recordedfrom the area labeled 1 (the dashed red circle part in figure (G)). (I) SAED pattern recorded from a single Ni nanocone, which shows the single crystalline structure.

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Growth mechanism of Ni nanocone array

In order to illustrate the growth process of the Ni nanoconearray in more detail, time-dependent experiments were carriedout at 100 uC with changing growth times from 0, 2, 5, to 10 hwhile keeping all the other reaction conditions constant (seeFig. 3). As the hydrothermal reaction proceeded, the initialnickel atoms experienced a morphological evolution andformed into the nanocones. The representative SEM imagesof the nanocone array prepared at certain reaction timeintervals are shown in Fig. 3 (A1–A4). Before the hydrothermalreaction, it can be clearly seen that there is only a roughsurface on Ni foil substrate due to acid washing at 80 uC for 60min (see Fig. 3 (A1)). When the reaction time is prolonged to 2h, there are nanoscale island-like structures (Fig. 3 (A2)) withan average size of about 100 nm. After the reaction time isfurther prolonged to 5 h, we are surprised to find that somesmall nanocones have formed on the surface of Ni substrate atthis time. From Fig. 3 (A3) it can be clearly seen that the basediameter and height of nanocones are about 80 nm and 100nm, respectively. These small nanocones’ appearance isprobably the result of self-assembly and oriented attachmentof nickel atoms based on the nanoislands. Meanwhile, we

found that the sizes of as-formed nanoislands becomerelatively smaller. When the reaction time is prolonged to 10h, as shown in Fig. 3 (A4), we clearly observed that the averageheight of the nanocones was increased and perfect conicalstructures were gradually formed. If the growth time is furtherprolonged, the size and morphology of the nanocones remainthe same even at longer growth time such as 15 h. However,the prolonged growth time favors the crystallization of Niphase, as seen in Fig. 2. Moreover, no obvious crack orexfoliation from the substrate was observed even undercontinuous ultrasonic treatment for more than two hours,indicating that the as-prepared Ni nanocone array is ratherstable on the Ni foil substrate.

From the above experimental results with the morphologicalevolution from Ni nanoislands to Ni nanocones, we proposethe possible growth mechanism of the Ni nanocone array(Fig. 3 (B1–B4) and (C1–C4)). In preparation of the reactionsolution, the hydrazine hydrate was added to the watersolution containing nickel chloride, the solution turned turbidand navy blue, indicating the formation of a stable complex([Ni(N2H4)2]Cl2) between Ni2+ and N2H4 at room temperature(eqn (2)). As the ultrasonic time extended, the [Ni(N2H4)2]Cl2

Fig. 3 Proposed model of Ni nanocone array formation on Ni substrate and supporting data. SEM images of Ni nanocone array prepared at 100 uC, showing themorphological evolution from Ni nanoislands to Ni nanocones for different growth times: (A1) 0 h, (B1) 2 h, (C1) 5 h, (D1) 10 h. (B1–B4) and (C1–C4) Schematicillustration of the possible growth mechanism of Ni nanocone array.

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complex begins to decompose into [Ni(NH3)6]Cl2 accompaniedby the color change from navy blue to light violet (eqn (3)), asshown in Fig. 3 (B1) and (C1).27 The formation of nickelcomplex precursor [Ni(N2H4)2]Cl2 and [Ni(NH3)6]Cl2 sharplydecreased the free nickel ions concentration in the solution,which resulted in a relatively slow rate of generation of Ninuclei. The present synthesis approach might be a typicalOstwald ripening model.28,29 When the hydrothermal reactiontemperature is raised to a certain point, the [Ni(NH3)6]Cl2

slowly dissolved and reduced by surplus hydrazine hydrate toform Ni nuclei and nanoparticles (eqn (4)). The obtained Ninanoparticles adsorbed onto Ni foil substrate to form Ninanoislands (Fig. 3 (B2) and (C2)), which could serve as seedsfor the further growth of nanocones at the expense of the othersmall nanoparticles through Ostwald ripening. The self-assembly and oriented attachment play an important role inthe formation of Ni nanocones. Self-assembly in the hydro-thermal system is guided by both entropic and enthalpicinteractions, and the system can spontaneously form orderedphases to decrease its overall free energy.30 The self-assemblyof nanoscale building blocks into organized conical super-structures was achieved by the assistance of oriented attach-ment. Attachment is observed primarily in one direction todecrease the overall energy.31 At the same time, we know thatthe primary driving force for conical morphological evolutionis the surface energy reduction due to minimization of the areaof high surface energy faces. The Ni nuclei self-assemble intonanocones by the oriented attachment along the [111] whichmainly rely on two factors, the same or similar surfaceenergies and the lattice matching extent of the attachedsurfaces, as shown in Fig. 3 (B3) and (C3). When the reactiontime is further prolonged, the underdeveloped nanocones arefurther assembled to form perfect Ni nanocones on substratealong [111] direction (Fig. 3 (B4) and (C4)).

NiCl2 + 2N2H4 A [Ni(N2H4)2]Cl2 (2)

2[Ni(N2H4)2]Cl2 + 5N2H4 A 2[Ni(NH3)6]Cl2Q + 3N2q (3)

2[Ni(NH3)6]Cl2 + N2H4 A 2N2Q + N2q + 12NH3q + 4HCl (4)

Effect of reaction parameters

The equilibrium between nucleation rate and crystallizationrate is very important for the formation of nanoscale Niconical structure. Reaction velocity can be adjusted bychanging the concentration of hydrazine hydrate and growthtemperature, which can regulate the kinetics of nucleation andcrystallization and further efficiently control the morphologyand structure of Ni nanocones. In our control experiments, themorphologies of Ni conical structures obtained from thesolutions with different concentrations of hydrazine hydrateare significantly different. Representative SEM images of thesamples prepared with concentration of hydrazine hydrate of1%, 5%, 10% and 20% while maintaining other parametersunchanged are presented in Fig. 4 (A1–A4), respectively. Whenthe N2H4?H2O concentration is 1%, the nanoscale island-likestructures with an average height of about 40 nm and aspectratio of about 0.3 are formed (Fig. 4 (A1)). Increasing the

N2H4?H2O concentration to 5%, the structure is featured bynanocones with symmetrical structures, which are approxi-mately 200 nm in height and 200 nm in base diameter (Fig. 4(A2) and (C)). After further increasing the N2H4?H2O concen-tration to 10%, the perfect cone-like structures is observed, asshown in Fig. 4 (A3). When the N2H4?H2O concentration is toohigh such as 20%, we would observe that the thick nanoplatesappear and the nanocones gradually disappear (Fig. 4 (A4)). Itcan be concluded that too high a concentration of N2H4?H2Oaccelerates the formation of Ni nuclei, which then lead to thesimultaneous assembly and attachment in several differentdirections to form nanoplates.

To further investigate the effect of growth temperature onthe nanocones morphology, Ni nanocones formed at differenttemperatures were examined by SEM. When the growthtemperature was raised from 90 uC to 120 uC and finally to180 uC while keeping the other reaction conditionsunchanged, an obvious morphology evolution from cone-liketo pagoda-like structures was observed (Fig. 4 (B1–B4)). At alow growth temperature of 60 uC, it can be clearly seen thatthere are only spherical particles with a diameter of about 200nm adsorbed onto Ni foil substrate (Fig. 4 (B1)). By increasingthe growth temperature to 90 uC, elegant Ni conical structuresare observed (Fig. 4 (B2)). These nanocones have an averageheight of about 300 nm and aspect ratio of about 2.0 (Fig. 4(D)). Because the optimum growth temperature is favorable forthe required energy, thermodynamic condition controls theassembly and attachment rate in one higher-energy surface todecrease the overall energy as fast as possible. As the growthtemperature proceeds to 120 uC, there are pagoda-likestructures on the lower part of nanocones, which means somesmall branches have formed on some parts of the conicalsurface (Fig. 4 (B3)). When the growth temperature is furtherincreased to 180 uC, there are perfect pagoda-like structures,and the trunk and branches of the pagoda-like structurebecome thicker (Fig. 4 (B4)). On the basis of the results aboveand our understanding, it is possible to obtain differentnanocones morphologies by controlling the growth condi-tions, such as the concentration of the hydrazine hydrate andgrowth temperature.

Field emission performance of Ni nanocone array

We believe the Ni nanocone array on Ni foil substrate is theideal field emitter due to their sharp tips, controlled growthand good conductivity. Up until now, the studies on the fieldemission performance of Ni nanocone array are quite limitedcompared to other nanomaterials such as carbon nanotubes,graphene, graphene oxide (GO), Co3O4 nanowalls, ZnOtetrapod structures.12,18–20,32,33 The field emission propertiesof these as-synthesized Ni nanocone array were measuredusing a homemade field emission measurement system atpressure of about 5.0 6 1024 Pa with a tip-to-anode distance ofabout 300 mm. The field emission was initiated by cycling thevoltage applied between the anode and the cathode up to 3000V for ten times, and the field emission current was recorded asa function of the applied voltage. In general, Fowler–Nordheim(F–N) theory is used to describe field emission characteristicsof metal materials, which are expressed as:

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J~I

S~

AE02

W

� �

exp {BW3=2

E0

� �

(5)

where J is the field emission current density, I is the totalemission current in the unit of microampere, S is the effectivearea (centimeter square) of electron emission, A = (emissionarea) 6 1.54 6 1026 cm2 A(m V21)2 eV, B = 6.83 6 109 (V m21)eV23/2, and W is the work function of metal material, which is5.15 eV for nickel. E0 is the local electric field (V mm21) on theemitting tips, which can be expressed by the following equation:

E0~bE

d

� �

(6)

where E is the applied voltage between the anode and thecathode, d is the inter-electrode distance (about 300 mm in this

work), and b is the field enhancement factor that depends onthe morphology of the materials, crystal structures, tip radiusand the areal density of the emitters.34 The field enhancementfactor is introduced to quantify the degree of enhancement ofany tip over a flat surface, which represents the true value of theelectric field at the tip compared to its average macroscopicvalue.22 Eqn (5) and (6) can be further derived as:

lnJ

E2

� �

~ lnAb2

d2W

!

{BdW3=2

b

� �

|1

E

� �

(7)

From eqn (7), it can be derived from a linear relationshipbetween ln(J/E2) versus (1/E) with slope given by the followingequation:

Fig. 4 Shape controls of Ni nanocones on Ni foil substrate. (A1–A4) SEM images of Ni nanocones grown in environments with different hydrazine hydrateconcentrations. (B1–B4) SEM images of Ni nanocones grown in environments with different temperature. (C) Nanocone aspect ratio and height versus hydrazinehydrate concentration. (D) Nanocone aspect ratio and height versus growth temperature.

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k = (BdW3/2)/b (8)

Therefore, the field enhancement factor b can be easilydeduced from the slope k of the F–N plot resulting by thelinear fitting of the experimental data.

The typical curves of the emission current density as afunction of the applied field (J–E plots) from the as-grown Ninanocone array are shown in Fig. 5 (A1 and A2) (blue starsymbols). The turn-on field (Eto, which is defined as the valueof E when the current density reaches 10 mA cm22) of Ninanocone array is 4.1 V mm21 (Fig. 5 (A1)), which is relativelylower than previous reports.22 At an applied electric field of 5 Vmm21, the obvious field emission current has been observedand is 26 mA (Fig. 5 (A1)). When applied electric field increasesto 10 V mm21, the field emission current of Ni nanocone arrayis up to 1730 mA (Fig. 5 (A2)). The linear relationship betweenln(J/E2) and (1/E) indicates that the field electron emissionfrom Ni nanocone array follow the F–N behavior, and theenhancement factor of b # 3720 can be calculated from theslop of ln(J/E2) and (1/E), or the F–N plot (Fig. 5 (A3), blue starsymbols). As a comparison, the enhancement factor value b

(3720) of Ni nanocone array on Ni substrate is larger than thatof CdS nanowires grown on Si substrate (b = 555),35 Ni

nanocones on Cu substrate (b = 2000),22 and ZnS nanowiresarray Cu substrate (b = 3400),36 which is likely due to thesharper tips than that of above-mentioned nanomaterials. Allof the other Ni nanocone arrays prepared by ourselves alsodisplay pretty good field emission properties, which have thelow turn-on field, the high field emission current density andthe large enhancement factor.

Field emission performance is strongly related to theabsorbed residual gas which can desorbed through Jouleheating, electron emission or ion impact, and further affectingthe emission stability of the cold cathode. In order to achievefield emission stability of Ni nanocone array and illustratespecific emission mechanism, understanding the effects ofresidual gas adsorption and desorption on the field emissionproperties is a key point. The residual gas (air) is composed ofvarious gases such as nitrogen, oxygen, water vapor, and so on.To distinguish a different gas effect from air, we introduced asingle above-mentioned gas species (nitrogen or oxygen) at atime into a vacuum system. For gas exposure experiments, ourhomemade testing system was evacuated first to a basepressure of 5.0 6 1024 Pa. The high purity gases wereintroduced into the closed testing chamber until the pressureincreased to 2.0 6 1024 Pa for 24 h. After each gas exposure,

Fig. 5 (A1, A2) Field emission current density from the air-exposed Ni nanocone array on Ni foil substrate as a function of applied electric field at pressure of about 5.06 1024 Pa. Thismeasurement was repeated ten times. The green dotted line is the emission current density at 10 mA cm22, which is used to define the turn-on electric field. The similar I–V curves fornitrogen and oxygen are shown in (B1, B2) and (C1, C2), respectively. According the corresponding Fowler–Nordherim plots (A3, B3, C3), the field enhancement factor can be evaluated.

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the testing chamber was again pumped down to a basepressure of 5.0 6 1024 Pa.

Fig. 5 (B1 and B2, C1 and C2, blue star symbols) present theJ–E plots after two different gas exposures on the Ni nanoconearray, indicating that: 1) the turn-on field of nitrogen gas (4.0 Vmm21) is smaller than that of oxygen gas (4.5 V mm21); 2) theemission current density of nitrogen gas (1.94 mA cm22) islarger than that of oxygen gas (1.54 mA cm22) at 10 V mm21.This behavior is closely related to the electronegativity, i.e., theelectronegativity of oxygen atom (3.44) is stronger than that ofthe nitrogen atom (3.04). It is expected that the adsorbed gasatoms with strong electronegativity present at the sharp tips ofnanocones depress the electron emission. On the other hand,the adsorbed oxygen molecules exothermically dissociate onthe tip edge of Ni nanocones, which is followed by oxidativeetching.37 The degradation of the sharp tips leads to the poorfield emission performance. Therefore, although air comprisesvarious molecules, oxygen gas dominates the field emissioncharacteristics of the Ni nanocone array more severely thanother gases.

During the field emission experiment, we found a veryinteresting phenomenon that the field emission currentdensity kept increasing from quite a low value to a maximumvalue when we repeatedly applied voltage from 0 to 3000 Vmany times under the same conditions. The similar phenom-enon also was observed during various gas exposure experi-ments. Fig. 5 shows the J–E plots of the Ni nanocone arraycorresponding to the repeated applied voltage. Using thenitrogen gas exposure as an example, initially, the emission

current density was only 1.94 mA cm22 when the applied fieldreached a value of 10 V mm21 (Fig. 5 (B2), blue star symbols),the turn-on field was as high as 4.0 V mm21 (Fig. 5 (B1)), andthe enhancement factor was only 3720 (Fig. 5 (B3)). However,through repeated applying electric field five times withoutchanging any other parameters, the current density rose to2.86 mA cm22 at the same electric field (Fig. 5 (B2), redtriangular symbols). And the more we repeated the appliedvoltage, the higher the measured current density and the lowerthe turn-on field. After ten times repeated applied voltages, theemission current density reached a larger value of about 3.32mA cm22 at the same electric field value (Fig. 5 (B2), blacksquare symbols), the enhancement factor also reached a largevalue of approximately 5910 (Fig. 5 (B3)), and the turn-on fielddecreased to 3.0 V mm21 (Fig. 5 (B1)). The similar improvedfield emission performance were obtained using the sametreatment, namely, repeated applying voltage many times, asshown in Fig. 5 (A1–A3 and C1–C3). This means field emissionproperties can be significantly improved just via repeatedapplying voltage without changing any other parameters.

Field emission mechanism of Ni nanocone array

On the basis of experimental results mentioned above, thepossible field emission mechanism of the air-exposed Ninanocones is tentatively proposed in the present system, asshown in Fig. 6 (A1–A3). When the Ni nanocone array isexposed to air, various gas molecules such as oxygen andnitrogen adsorb not only on the conical surfaces but also at thesharp tips of the nanocones, which can deeply influence the

Fig. 6 (A1–A3) Schematic illustration for the field emission mechanism of the air-exposed Ni nanocones. (B1) Field emission current density from the air-exposed Ninanocone array as a function of field emission time via applied electric field of 3000 V at pressure of about 5.0 6 1024 Pa. (B2) Field emission J–E curves from the Ninanocones before and after an hour ‘vacuum J–E annealing’. (B3) The corresponding F–N plots of the Ni nanocones before and after an hour ‘vacuum J–E annealing’.

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field emission properties (Fig. 6 (A1)). Inside the high vacuumtesting chamber, with only the help of vacuum pump, it is notsufficient to remove all the gas molecules adsorbed on thenanocones surface. Electrons that flow along nanocones couldbe scattered by natural defects and sharp tips to generate Jouleheating. When a high electric field is applied to the nanoconearray, the large emission currents will allow electrons to flowalong nanocones and give rise to Joule heating at defects andapex of nanocones. These heated nanocones could desorb theadsorbed gas molecules and thus lower the concentration ofgas adsorbed layer (Fig. 6 (A2)). With increasing voltages, heatenergy would be spread from the heated nanocones to theneighboring nanocones, causing the gas molecules adsorbedon these nanocones to desorb and hence improving the fieldemission properties. For example, after ten times repeatedapplying voltages, the emission current density of the oxygen-gas-exposed Ni nanocones increased about 1.6 times at thesame electric field value of 10 V mm21 (Fig. 5 (B3)), theenhancement factor also increased approximately 1.5 times(Fig. 5 (C3)), and the turn-on field decreased 0.8 times (Fig. 5(C1)). In summary, the field emission performance of Ninanocones is dominated via both the gas adsorption anddesorption.

From the J–E curves of the gas-exposed Ni nanocone array(Fig. 5), it is obvious that the emission current densitiesfluctuate very rapidly for the initial J–E measurements. Thisfluctuation can be reduced via applied constant electric field of3000 V at pressure of about 5.0 6 1024 Pa for 1 h (called‘vacuum J–E annealing’), as shown in Fig. 6 (B1). This ‘vacuumJ–E annealing’ was carried out until the emission currentdensities fluctuated disappeared. During vacuum annealing,the emission current densities increased gradually and theirfluctuation was reduced. Fig. 6 (B2) shows the field emission J–E curves from the Ni nanocone array before and after an hour‘vacuum J–E annealing’, which illustrate that ‘vacuum J–Eannealing’ results in the emission current densities increasingand the field emission after ‘vacuum J–E annealing’ remainedvery stable over many times for repeated measurement. At anapplied electric field of 10 V mm21, the field emission currentdensity of Ni nanocone array was about 1.6 mA cm22 initiallyand increased to 3.6 mA cm22 after an hour ‘vacuum J–Eannealing’. This increase of the initial current density may bedue to outgassing on the surface of Ni nanocone array. Aftercyclic electric field experiment ten times, the emission currentdensity remained at approximately 3.6 mA cm22, and nodegradation of the current density or shift of the fieldemission curve was observed. For the turn-on field and theenhancement factor, a similar phenomenon was observed. Forinstance, the field enhancement factor increased from 3720before ‘vacuum J–E annealing’ to 6910 after ‘vacuum J–Eannealing’ and remained about the same during later fieldemission experiments, as shown in Fig. 6 (B3).

Conclusions

In summary, we have achieved a large-scale synthesis of Ninanocone array on Ni foil substrate by a facile wet chemicalmethod. The as-grown Ni nanocones have base diameters

ranging from 50 to 450 nm and heights ranging from 50 to 200nm, and the tip diameter of the nanocone is about 10 nm andthe apex angle of nanocone is about 40u. The Ni nuclei self-assemble into cone-like structures by the oriented attachmentalong the [111] direction which mainly rely on two factors, thesame or similar surface energies and the lattice matchingextent of the attached surfaces. Reaction velocity can beadjusted by changing the concentration of hydrazine andgrowth temperature, which can regulate the kinetics ofnucleation and crystallization and further control the mor-phology and structure of nanocones. Field emission measure-ments show that the as-prepared Ni nanocone array is asuperior field emitter exhibiting low turn-on voltage, highcurrent density and large field-enhancement factor due tosharp tips. In addition, we find that adsorbed gas moleculesgreatly hindered the field emission performance of thenanocones. Repeated applying voltage or ‘vacuum J–E anneal-ing’ could drive the adsorbed gas molecules away andsignificantly improve field emission properties and stability.

Acknowledgements

We are thankful for the financial support from National High-Tech R & D Program of China (863, No. 2011AA050504),Natural Science Foundation of Shanghai (No. 10ZR1416300),the Foundation for SMC Excellent Young Teacher, ‘PCSIRT’and the Analytical and Testing Center in Shanghai Jiao TongUniversity.

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