Enrichment of the field emission properties of NiCo 2 O 4 nanostructures by UV/ozone treatment †

Herein, UV/O 3 treatment was imposed on the hydrothermally synthesized spinel NiCo 2 O 4 urchin like structure to study its enhancement eﬀect on the field electron emission properties. The FE studies revealed that UV/O 3 treated NiCo 2 O 4 possesses a 4.38 V m m (cid:2) 1 turn on field at 1 m A and an emission current density of 386 m A cm (cid:2) 2 , whereas pristine NiCo 2 O 4 retains 5.69 V m m (cid:2) 1 turn on field at 1 m A and 168 m A cm (cid:2) 2 emission current density. The enhancement in the FE characteristics of UV/O 3 treated NiCo 2 O 4 is attributed to the change in the electronic structure of the emitter due to UV/O 3 treatment. Experimental data were supported through electronic structure simulations using Density Functional Theory technique. We have achieved the modification of the electronic properties of spinel NiCo 2 O 4 due to the oxygen vacancy created by UV/O 3 treatment. There is an enhancement of electronic states near the Fermi level due to oxygen vacancy. The computed work function reduces when oxygen vacancy is introduced on the surface of pristine NiCo 2 O 4 which justifies the enhancement in the field electron emission after UV/O 3 treatment. Due to O vacancy, there is charge gain by the 3d orbital of Ni and Co whereas there is a charge loss by the 2p orbital of the O atom near the vacancy. The results suggest UV/O 3 treatment can be practiced in the field electron emission investigation to retain a good current density for practical applications.


Introduction
The extraction of electrons from the surface of a material through a solid vacuum potential barrier by quantum mechanical tunnelling induced by an external electric field is known as field electron emission (FE). Field electron emission has attracted considerable attention because of its numerous applications in technologies such as electron sources in free electron lasers, 1 field emission scanning electron microscopy (FESEM), 2 microwave generation, X-ray tubes, 3 RF amplification, and field emission displays. 4,5 The extraction of an appreciable field emission current at a low applied electric field is technologically important for field emission devices. Hence the development of field emitters with a low turn on electric field, capable of delivering stable large emission currents is of significance. The field emission performance is strongly dependent on the work-function, electrical conductivity, mechanical strength, aspect ratio and chemical alertness of the emitter. 6 Modification of the cathode materials leads to the enhancement of the local electric field on the emitter surface, resulting in a reduced turn on field. 7,8 Transition metal oxides are fascinating functional materials having the s-shell of the positive metallic ions and are always fully filled with electrons; however, their d-shells may not be completely filled. 9 This characteristic leads to unique properties which involve reactive electronic transition, good electrical characteristics and so on. A number of single-phase binary metal oxide systems such as NiMoO 4 16 etc. have been explored for the FE properties. Furthermore, ternary metal oxides have drawn particular attention due to their better electronic conductivity. Recently, ternary nickel cobalt oxide (NiCo 2 O 4 ) has been studied widely because of its high electronic conductivity as it contributes charges from both nickel and cobalt ions. 17 Spinel nickel cobaltite (NiCo 2 O 4 ) is an intriguing functional metal oxide as an important member of the Co-based binary transition metal oxides family. 18 Spinel NiCo 2 O 4 retains superior electronic conductivity compared to nickel oxide or cobalt oxide, owing to the blended valence of the same positive ions in the structure. 19,20 Low cost and non-toxicity make this material a promising candidate. This feature is highly beneficial for the development of NiCo 2 O 4 as a field emitter. However, it suffers from poor electrical conductivity. 17 The electrical conductivity is a main factor for the field emission improvement and is strongly dependent on the work function. Recently researchers have been trying various methods to improve the conductivity in transition metal oxides by surface modification, doping and defects or vacancy creation. Several strategies have been used to introduce oxygen vacancies in metal oxides to overcome the drawback, as it helps in improving the charge carrier concentration and reduces resistance. The modification using UV/O 3 treatment is much effective for defect creation as it does not involve any chemical additives. While effective delocalization of electrons in UV/O 3 treated NiCo 2 O 4 can be observed due to the loss of oxygen atom from previously occupied 2p orbitals of Co 3+ and Ni 2+ . The oxygen vacancy can suitably tune the electronic structure. The oxygen vacancy promotes the reactivity of active sites and also, they are known as donor defects that results in increased current density. The half-metallic behaviour of NiCo 2 O 4 decreases by introducing oxygen vacancy which enhances the conductivity. 41 For the improvement of FE properties, various treatments are carried out by researchers such as plasma treatment, 21 laser irradiation, 22 and coating of low work function material. 23 UV/O 3 treatment suggests the possible formation of oxygen vacancies. Whereas short time exposure (20-30 s) occupies the oxygen vacancy defects on the surface and creates interstitial species. 53,54 Most commonly in spinel's oxygen vacancy and cation inversion (change in the valence state of cations) are the main factors that lead to change in their properties. 55

Synthesis
For the synthesis of spinel NiCo 2 O 4 , 30 mM Ni(NO 3 ) 2 Á6H 2 O and 60 mM Co(NO 3 ) 2 Á6H 2 O were initially dissolved in 30 ml of distilled water followed by the gradual addition of 100 mM urea (dissolved in 10 ml of distilled water), the solution was stirred magnetically for 45 minutes at room temperature until a homogeneous light pink coloured solution was obtained. The mixture was then transferred to a 50 ml Teflon lined stainless steel autoclave. The hydrothermal reaction was carried out at 200 1C for 12 h. The obtained precipitate was filtered and washed with distilled water and ethanol. Finally, the product was vacuum dried at 60 1C for 12 h and annealed at 350 1C for 3 h. The material was ready for further characterization.

Characterization
The crystallographic information and phase purity of the product was investigated using an X-ray diffractometer (XRD, Rigaku Ultima IV having an NI-filter for Cu-Ka radiation, l = 0.1541 nm). The detailed morphological analysis was obtained by field emission scanning electron microscopy (FESEM, JEOL JSM-7100F, JEOL Ltd, Singapore) and elemental analysis was carried out using an energy dispersive spectroscope (EDS, JEOL with a 129 eV resolution silicon drift detector with 15 kV accelerating voltage). To study the vibrational modes, Raman spectra were collected in a range of 100-1200 cm À1 on a confocal laser micro-Raman spectrometer (Reinshaw via Raman microscopy, a He-Ne laser with a power of 1 mW cm À2 and an excitation wavelength of 532 nm). The compositional analysis was performed using X-ray photoelectron spectroscopy under ultrahigh vacuum conditions (XPS, Thermo K Alpha+ spectrometer, Al Ka X-rays with 1486.6 eV energy). Further detailed morphological analysis was carried out using transmission electron microscopy (TEM, TALOS F200S G2, 200 kV, FEG, CMOS camera 4k Â 4k). The specific surface area and pore size distribution of the materials were obtained using BET (Brunnauer-Emmett-Teller) and BJH (Barrett-Joyner-Halenda) analysers (BELSORP max, Japan).

UV/O 3 treatment
The material was exposed for 3 minutes to UV/O 3 irradiation using a UV system along with four custom built UVC ozone producing lamps (ACS-40W4-UVO, ACS UV technologies, Bangalore). The mechanism involves three basic steps: ozone generation, ozonolysis and cleaning or surface modification. For ozone generation, a lowpressure discharge mercury lamp that generates two wavelengths viz., 184.9 and 253.7 nm is used for the photosensitized oxidation process where molecular oxygen is dissociated due to exposure to UV light to produce ozone. At 184.9 nm the UV line decomposes the oxygen molecule to produce ozone and can be given by the following equation, Furthermore, the ozonolysis process involves the decomposition of ozone to produce a high energy oxygen radicle O* upon exposure to UV light of 253.7 nm wavelength, shown by the following equation, The highly reactive atomic oxygen reacts with the material to clean the contaminants from the surface of the material. Also, it increases the oxygen-rich stoichiometric properties and surface functionalities and creates defects in the material upon prolonged exposure.

Field emission
The field electron emission investigation, viz. current density ( J) versus applied electric field (E) and emission current (I) versus time (t) were measured in a planar 'diode' configuration at a base pressure of 1 Â 10 À8 mbar. A typical 'diode' configuration consists of a phosphor coated semi-transparent screen (a circular disc having a diameter of 4 cm) as an anode. The cathode (pristine and UV/O 3 treated NiCo 2 O 4 ) was pasted on a stainless steel holder with the help of carbon tape which is connected to a linear motion drive. Before recording the FE measurements, pre-conditioning of the cathode was carried out by maintaining it at B2 kV for 30 minutes, so as to remove loosely bound particles or contaminants by residual gas ion bombardment. FE measurements were performed at a constant separation of B1000 mm, between the anode and cathode. The emission current was measured on a Keithely electrometer (6514) with a sweeping dc voltage applied to the cathode with a step of 40 V (0-40 kV, Spellman, U.S.). Using a computer-controlled data acquisition system, the field emission current stability was recorded at the pre-set (5 mA) current value. The reproducibility of the FE results was checked for two samples synthesized under identical conditions.

Computational details
We have executed the theoretical calculations using the Density Functional Theory (DFT) code VASP [24][25][26][27] combined with PAW-GGA as the exchange-correlation functional 28 for the simulations. The plane wave cutoff energy is 500 eV and the Brillion zone is sampled employing a Monkhorst-Pack 29 mesh of 8 Â 8 Â 1 and 8 Â 8 Â 8 k-points for surface and bulk structure, respectively. We have considered the convergence criterion for Hellmann-Feynman forces as 0.01 eV Å À1 and total energy convergence as 10 À6 eV.

Results and discussion
Well defined diffraction peaks confirm the phase formation and crystallinity of the as-synthesized material by XRD analysis.  (220) plane towards lower angle also suggests the formation of oxygen vacancy defects. 30,31 The surface morphology of the samples has been analysed using field emission scanning electron microscopy (FESEM). Fig. 2(a and b) shows an urchin like morphology consisting of numerous nanorods epitaxially growing from the centre. No significant change in the morphology was observed in NiCo 2 O 4 and UV/O 3 treated NiCo 2 O 4 . The flower like morphology of NiCo 2 O 4 -UVT is displayed in Fig. 2(c and d). The chemical composition is confirmed by energy dispersive X-ray spectroscopy (EDAX) analysis.     Fig. S1(b), ESI †) shows well-defined rings, which further confirms the polycrystalline intrinsic quality. Fig. S3 (ESI †) shows the N 2 adsorption-desorption isotherms of NiCo 2 O 4 and UV/O 3 treated NiCo 2 O 4 . Hysteresis loops of the type IV isotherm (Fig. S3, ESI †) reflect the formation of a mesoporous structure. 32 Also, the BET surface area of UV/O 3 treated NiCo 2 O 4 is calculated to be 96.06 m 2 g À1 . The increased surface area of NiCo 2 O 4 -UVT is due to defect sites, which reflects the probable oxygen defects. 33 The reduction in the surface area resulting from the lattice oxygen vacancy implies the inverse dependency of the lattice oxygen content. [34][35][36] The pore diameter distribution calculated by the Barrett-Joyner-Halenda (BJH) method using the desorption branch of the nitrogen isotherm is shown in Fig. S3(b) (ESI †), a major pore size distribution ranges from 3 to 6 nm. Further detailed compositional analysis including defects due to oxygen vacancies were depicted by the powerful Raman spectroscopic technique. The Raman spectra of both the samples were collected within the spectral range of 150-800 cm À1 are displayed in Fig. 3(a). Raman active vibrational bands correspond to A 1g + E g + 3F 2g phonon excitation modes, reassuring the polarization selection rules for an ideal spinel structure. 37 Peaks appearing at 191, 515 and 617 cm À1 are attributed to the F 2g mode whereas the peaks at 652 and 456 cm À1 correspond to the A 1g and E g modes of vibration. Spectra from both the samples show Co-O and Ni-O vibrations. Signals belonging to the -OH group were not observed hence confirming the decomposition of hydroxide species after thermal treatment. The peak assigned to 652 cm À1 of the A 1g mode depicts the stretching of the oxygen atoms at the octahedral sites associated with Co 3+ ions in the tetrahedral sites, however, a notable reduction in the same peak may indicate the formation of defects. 38 In the detailed observation of Raman bands (Fig. S4(a), ESI †), a sudden peak at 595 cm À1 indicates the formation of oxygen vacancies. The peaks at 456 and 515 cm À1 correspond to the combined vibration of octahedral and tetrahedral oxygen present in the lattice. 39 The blue shift at 453 cm À1 in the NiCo 2 O 4 -UVT spectrum confirms the oxygen vacancy defect creation and is displayed in Fig. S4(a) (ESI †). 40 XPS analysis was performed to study the defects and oxygen vacancies as well as for the structural and compositional analysis. Fig. S4(b) (ESI †) shows the recorded survey spectra. The comparative high resolution XPS spectra of Co2p, Ni2p and O1s of NiCo 2 O 4 and NiCo 2 O 4 -UVT is illustrated in Fig. 3(b-d). a well-defined Gaussian fitting method was implemented for the deconvolution of the core level elemental spectra. The best fitted spectra for Ni 2p with two spin orbit doublet and two shakeup satellites (denoted by Sat.) are displayed in Fig. 3(b). The peaks around the binding energy of 854 and 871 eV are assigned to the Ni 2+ state and the peaks at 855 and 872 eV are attributed to the Ni 3+ state, respectively. The atomic ratios of each element were obtained by the comparison of the covered area obtained by the fitting of curves, as summarized in Table S1 (ESI †). The fitted peak area in high resolution Ni 2p spectra shows a significant reduction in intensity by a slight degree, implying the reduction of Ni 3+ to Ni 2+ . Also, the higher Ni 3+ /Ni 2+ atomic ratio for UV/O 3 treated NiCo 2 O 4 indicates the formation of defects due to oxygen vacancies at the surface. 41,42 The O defects that are formed may be attributed to the B-type of oxygen vacancy as the charge balance is counted by partial substitution Ni 3+ for Ni 2+ at the octahedral B-sites which indicates the decreased interaction of the Ni-O vacancy with octahedral Ni and increased interaction between tetrahedral Ni and O. 38,[55][56][57] Relatively not much change in the atomic ratio of the Co 2p spectra is observed. The peaks at 780 and 796 eV belong to Co 2+ whereas the peaks at 779 and 794 eV are ascribed to the Co 3+ state. 43 Besides, the core level spectra of O 1s with three major peaks provide the confirmation of oxygen vacancy defect formation after UV/O 3 treatment. The typical metal-oxygen (O 2À ) bonding is represented by the peak at around 529 eV. The peak at a slightly higher binding energy of 533 eV can be observed due to the coordination of the hydroxyl (-OH) species of adsorbed water molecules on the surface and defect caused due to the low oxygen coordination content. 58 The obvious peak at 531 eV is mainly attributed to defects sites and oxygen vacancy sites. 44,45 The relative enhancement in the O x peak clarifies the oxygen deficient region in the lattice. Also, the high O x ratio in NiCo 2 O 4 -UVT indicates the increased oxygen vacancies and low coordination on M-O bonds. Thus, core level O 1s spectral features enable the understanding of oxygen vacancy creation when exposed to UV/O 3 treatment.
The typical plot of the emission current density ( J) as a function of the applied electric field (E) is shown in Fig. 4(a). From the ( J-E) plot, the turn on field, defined as the field required to draw emission current density of B1 mA cm À2 is found to be 5.69 and 4.38 V mm À1 for the pristine and UV/O 3 treated NiCo 2 O 4 , respectively. While the threshold field (the field at which the current density starts to shoot up exponentially is termed as threshold field at B10 mA cm À2 ) is 6.21 and 5.32 V mm À1 for the pristine and UV/O 3 treated NiCo 2 O 4 emitters, respectively. Interestingly a high emission current density of B386 mA cm À2 has been extracted from the UV treated NiCo 2 O 4 emitter at a relatively lower applied electric field of 8.40 V mm À1 , than the pristine (B168 mA cm À2 emission current density at 9.15 V mm À1 applied electric field). The comparative field emission properties of both the emitters are listed in Table 1.
The FE characteristics were further analysed using the Fowler-Nordheim (FN) theory. The relationship between the current density and applied electric field can be represented as, here, J is the emission current density, E is the applied electric field, l M is macroscopic pre-exponential correlation factor, a and b are the constants with the typical values of 1.54 Â 10 À6 A eV V À2 and 6.83 V eV À3/2 mm À1 , respectively, f is the work function of the emitter material, n F is the value of the principal Schottky-Nordheim barrier function (a correction factor), and b is the field enhancement factor. The FN plot i.e., ln( J/E 2 ) versus 1/E is depicted in Fig. 4(b), revealing the nonlinear nature, which is leaning towards saturation in the high field region. The deviation from linearity is ascribed to the semiconducting nature of the emitter. 46,47 Emission current stability is one of the essential parameters for a field emitter to be considered for practical application. A plot of the emission current (I) as a function of time (t) is depicted in Fig. 4(c). The emission current stability plots were recorded at a pre-set current of 5 mA for more than 3 hours. The emission current stability for the UV treated NiCo 2 O 4 emitter is found to be very good and characterized by a very low number of fluctuations about the mean current value (BmA) compared to the pristine one. The fluctuations can be attributed to adsorption/desorption of residual gaseous species on the emitter surface. 48 The FE image is shown in the inset of Fig. 4(c and d). The image displays a number of tiny spots, corresponding to the emission from the most protruding edges of the emitters.

Theoretical results and discussion
For this simulation, we have considered the spinel structure of NiCo 2 O 4 whose DFT optimized bulk structure is displayed in Fig. 5(a). The computed lattice parameter is 8.09 Å, agreeing well with the experimental value of 8.11 Å. 49 In spinel NiCo 2 O 4 , the Ni ions are octahedrally coordinated with oxygen ions and Co ions are tetrahedrally coordinated, as depicted in Fig. 5(a). DFT optimized Ni-O and Co-O bond lengths are 1.93 and 1.92 Å, respectively. Fig. 5(b) shows the relaxed structure of bulk NiCo 2 O 4 with two O vacancy introduced in it due to UV/O 3 treatment.
To acheive modification of the electronic properties, in Fig. 6 we have plotted electronic density of states for pristine NiCo 2 O 4 and NiCo 2 O 4 with O vacancy. We can see that the bulk NiCo 2 O 4 has only electronic states in the lower spin channel at the Fermi level. This signifies that bulk NiCo 2 O 4 is half-metallic in nature, agreeing with the results from the literature. 50 When O vacancy is introduced, half-metallicity is lost as we can see the electronic states in both spin channels are around the Fermi level in Fig. 6(b). Also, there is an enhancement in the states which may increase the conductivity of the material.
To calculate the work function of the material, we have generated surface of NiCo 2 O 4 from the relaxed bulk structure and performed geometry optimization. The relaxed structures of the NiCo 2 O 4 surface without and with O vacancy are depicted in Fig. 7(a).

Computation of work function
To provide theoretical support for enhanced field emission properties of UV/O 3 treated NiCo 2 O 4 , we computed the work function for the surface of NiCo 2 O 4 before and after O vacancy (UV/O 3 treatment) using the following expression, here E Vac indicates the converged electrostatic potential in a vacuum and E F denotes the Fermi level which can be from the valence band maximum (VBM) of the electronic density of states plot. For NiCo 2 O 4 , E F = 1.91776 eV and E Vac = 7.50 eV. So, the computed work function is found to be 5.59 eV, which is in good agreement with the earlier reported value of 5.53 eV. 16,51 For NiCo 2 O 4 with oxygen vacancy, E F = 1.85521 eV and f = 7.07 eV and the value of work function is 5.22 eV. So, we can see that there is substantial reduction in the value of work function which is one of the reasons for improved field emission performance for the UV treated NiCo 2 O 4 . A similar reduction in work function is obtained for the case of graphene oxide supported VSe 2 . 52 Oxygen vacancy leading to defects in the structure may be one of the reasons for the reduction in the work function.
In order to achieve modification of electronic orbitals, in Fig. 7(b-d), we have plotted the 3d orbital of Ni and Co and 2p orbital of O before and after the O vacancy in the NiCo 2 O 4 surface. We can see that for 3d orbitals of Ni and Co around the vacancy, there is an enhancement in the electronic states near the Fermi level due to the introduction of O vacancy, whereas for the O 2p orbital near the vacancy, there is a slight reduction in the states near the Fermi level. So, there is charge gain by the 3d orbitals of Ni and Co close to the vacancy and there is charge loss for the 2p orbital of the O atom near the vacancy. So, the UV/O 3 treatment leads to a rearrangement of the electronic charges near the O vacancy leading to a reduction in the work function which justifies the improved field emission properties after UV/O 3 treatment.

Conclusion
The UV/O 3 treated NiCo 2 O 4 emitter exhibits enhanced FE behaviour in contrast to the pristine NiCo 2 O 4 emitter, in terms of lower values of turn-on and threshold fields, and competency to deliver large emission current density at a relatively lower applied field. Using extensive DFT calculations, we have studied the modifications in electronic properties of spinel NiCo 2 O 4 due to the introduction of O vacancy as a result of UV/O 3 treatment. The O vacancy makes enhancement in electronic states near the Fermi level. There is a rearrangement of electronic charges near the vacancy attributing to a substantial reduction in work function which supports the improved field emission characteristics measured in the experiment after UV/O 3 treatment. The computed work function reduces when oxygen vacancy is introduced on the surface of pristine NiCo 2 O 4 which justifies the enhancement in the field electron emission after UV/O 3 treatment which supports the experimental findings. Thus, the material with induced oxygen vacancy defects can be further used for FE applications.

Conflicts of interest
Authors declare no conflict of interest.

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