Uncovering the origin of enhanced field emission properties of rGO–MnO2 heterostructures: a synergistic experimental and computational investigation

The unique structural merits of heterostructured nanomaterials including the electronic interaction, interfacial bonding and synergistic effects make them attractive for fabricating highly efficient optoelectronic devices. Herein, we report the synthesis of MnO2 nanorods and a rGO/MnO2 nano-heterostructure using low-cost hydrothermal and modified Hummers' methods, respectively. Detailed characterization and confirmation of the structural and morphological properties are done via X-ray Diffraction (XRD), Field Emission Scanning Electron Microscopy (FESEM) and Transmission Electron Microscopy (TEM). Compared to the isolated MnO2 nanorods, the rGO/MnO2 nano-heterostructure exhibits impressive field emission (FE) performance in terms of the low turn-on field of 1.4 V μm−1 for an emission current density of 10 μA cm−2 and a high current density of 600 μA cm−2 at a relatively very low applied electric field of 3.1 V μm−1. The isolated MnO2 nanorods display a high turn-on field of 7.1 for an emission current density of 10 μA cm−2 and a low current density of 221 μA cm−2 at an applied field of 8.1 V μm−1. Besides the superior FE characteristics of the rGO/MnO2 nano-heterostructure, the emission current remains quite stable over the continuous 2 h period of measurement. The improvement of the FE characteristics of the rGO/MnO2 nano-heterostructure can be ascribed to the nanometric features and the lower work function (6.01 and 6.12 eV for the rGO with 8% and 16% oxygen content) compared to the isolated α-MnO2(100) surface (Φ = 7.22 eV) as predicted from complementary first-principles electronic structure calculations based on density functional theory (DFT) methods. These results suggest that an appropriate coupling of rGO with MnO2 nanorods would have a synergistic effect of lowering the electronic work function, resulting in a beneficial tuning of the FE characteristics.


Introduction
Nanoscale heterostructure design comprising different material compositions is emerging as an attractive strategy and essential building block for functional devices to achieve improved performance. The desired physicochemical properties of the participating nanomaterials in nanostructured hybrids/ composites complement each other by tuning their electronic properties to meet the requirements for the fabrication of efficient electronic devices. 1 Reduced Graphene Oxide (rGO) is an attractive and ideal nanomaterial to be paired with another suitable semiconductor for the development of multifunctional heterostructures because of its unique electronic properties, high electrical conductivity (5 Â 10 À3 S cm À1 ), exible structure, high aspect ratio, and high specic surface area (2630 m 2 g À1 ). [2][3][4][5] Owing to its unique physicochemical properties, rGO is being recognized as a material of great interest for potential applications in nanoelectronics, 6 nanoelectromechanical systems, 7 sensors, 8 catalysis, 9 energy storage devices, 10,11 optics, 12 and eld emission (FE). [13][14][15] There exist several reports of the successful synthesis of rGO or modied graphene heterostructures with various semiconducting nanomaterials such as TiO 2 , SnO 2 , ZnO, Si, CdSe, etc. in the literature. [16][17][18] For eld emission applications, where electrons are extracted from the surface of a metal/semiconductor by an electrostatic eld through quantum mechanical tunneling, rGO-based nanocomposites such are rGO-Bi 2 S 19 and WS 2 -RGO, 20 have demonstrated superior eld emission properties. Among transition metal oxides, manganese dioxide (MnO 2 ) has attracted increasing interest for eld emission applications, owing to their wide structural diversity combined with unique chemical and physical properties. 21,22 The advantages of MnO 2 as eld emitter are the lower cost for raw materials and the fact that manganese is more environmentally friendly than other metal oxide. 23,24 MnO 2 has also attracted a lot of attention as an electrochemical pseudocapacitor material due to its high theoretical capacitance (1370 F g À1 ). [25][26][27] Wu et al. reported inspiring results such as low turn-on eld value of 8.4 V mm À1 at current density of 1 mA cm À2 and maximum emission current density of 160 mA cm À2 at an applied eld 18 V mm À1 . 21 The eld emission applications of MnO 2 is, however, limited by its low specic surface area and poor electrical conductivity (10 À5 to 10 À6 S cm À1 ). Compared to its at lms, by fabricating rGO/ MnO 2 nanocomposite the interface area can be signicantly enlarged, which is desirable for eld emission application. 28 Besides, rGO is solution-processable and thus can be deposited in large areas onto different kinds of substrates enabling simple and cost-effective fabrication of eld electron emitters for display applications. The formation of rGO-MnO 2 nanostructures and their electrochemical performance have been extensively investigated and it was demonstrated that compared to the single metal-oxide, rGO/MnO 2 nanocomposites show superior electric conductivity, electric capacity and charge/ discharge efficiency for supercapacitor performance. [29][30][31][32][33][34] These characteristics make rGO/metal-oxide nanocomposites promising materials for energy applications. Considering that eld emission is geometry (shape, size, aspect ratio, alignment, and areal density of the nanostructure) and work function dependent phenomenon, well-aligned rGO/MnO 2 nanostructures is promising for enhancement of eld emission characteristics. 35 Herein, we report a simple and cost-effective solution-based method to prepared MnO 2 nanorods and rGO/MnO 2 nanoheterostructure. The structural and morphological verications have been done by using X-ray diffraction (XRD), Field Emission Scanning Electron Microscope (FESEM), and Transmission Electron Microscopy (TEM). Finally, the eld emission properties of the as-prepared MnO 2 nanorods and rGO/MnO 2 nano-heterostructure was systematically characterized and compared. The rGO/MnO 2 nano-heterostructure exhibits superior eld emission characteristics compared to the MnO 2 nanorod. The rGO/MnO 2 nano-heterostructure demonstrates a low turn-on eld of 1.4 V mm À1 for an emission current density of 10 mA cm À2 compared to 7.1 V mm À1 for MnO 2 nanorod. The combined contribution of the sharp edges of the thin rGO sheets and high aspect ratio of the MnO 2 nanorods, coupled with synergetic effect in the rGO/MnO 2 nano-heterostructure are responsible for the observed enhanced eld emission behavior. Consistent with the experimental data, our complementary rst-principles DFT calculations predict lower work function for the rGO/MnO 2 nano-heterostructure compared to the isolated MnO 2 as the primary origin for improved eld emission.

Synthesis of MnO 2 and rGO/MnO 2
The rGO has been synthesized by modied Hummer's method 36 whereas the MnO 2 nanorods has been synthesized by the hydrothermal method. 37 For the preparation of the rGO/MnO 2 composite, 1 mg ml À1 rGO was dispersed in the 100 ml of DI water in a beaker. Later, 10 mM of KMnO 4 and 10 mM of MnSO 4 were added into the rGO solution and stirred for 30 min form a homogenous solution. The prepared solution was transferred into a stainless steel autoclave and kept at 160 C for 24 h. Aer cooling to room temperature, the material was lter washed with DI water and ethanol to obtian rGO/MnO 2 composite, which was dried in an oven at 80 C for 12 hours and used for various characterizations presented next. Scheme 1 represents the synthesis steps which were followed for synthesis of rGO, MnO 2 and rGO/MnO 2 heterostructure.

Materials characterization
The MnO 2 nanorods and rGO/MnO 2 nano-heterostructure were characterized by various complementary experimental methods. The XRD patterns were obtained with a Bruker D8 Advance X-ray diffractometer using the Cu Ka line (l ¼ 1.54 A) at 1 grazing angle. The HR-TEM micrographs and selected area electron diffraction (SAED) patterns were obtained with JEOL-JEM 2100 microscope operating at 200 kV. The samples were dry dispersed over 300 mesh copper grids coated with holey carbon lm. A Field Emission Scanning Electron Microscope (FEG-SEM Model -Tescan MAIA3) was used to examine the morphology and surface topography of the MnO 2 nanorods and rGOs/MnO 2 composite. The accelerating voltage was 15 kV. Xray spectroscopy (EDS) measurements were done using Oxford Instruments X-Max N 80 detector and analyzed using Aztec soware. X-ray Photoelectron Spectroscopy (XPS) was carried on the samples using a Kratos Axis Ultra DLD photoelectron spectrometer utilizing monochromatic AlKa radiation operating at an energy of 120 W (10 Â 12 kV). Data were analyzed using Casa XPS and modied Wagner sensitivity factors as supplied by the instrument manufacturer aer subtraction of a Shirley background. All spectra were calibrated to the C(1s) line taken to be 284.8 eV.

Field emission
The eld emission studies of the MnO 2 nanorods and rGO/ MnO 2 heterostructure were carried out in the Ultra-High Vacuum (UHV) chamber at a base pressure of $1 Â 10 À8 mbar (Excel Instruments model: I-100). Detail experimental procedure may found in our earlier paper. 38 The conguration of the eld emission experiment steps up is shown in ESI (scheme S1). † The distance between inter-electrode was maintained at 1 mm. The area of both specimens (MnO 2 nanorods and rGOs/MnO 2 heterostructure) was 0.25 cm 2 .

Computational methods
The rst-principles spin polarized density functional theory (DFT) calculations were performed using the Vienna Ab initio Simulation Package (VASP), 39-41 a periodic plane wave DFT code which includes the interactions between the core and valence elections using the Project Augmented Wave (PAW) method. 42 An energy cut-off of 600 eV, and Monkhorst-Pack 43 k-point mesh of 7 Â 7 Â 3 was used to sample the sample the Brillouin zone of bulk a-MnO 2 . Geometry optimizations were performed based on the conjugate-gradient algorithm until the residual Hellmann-Feynman forces on all relaxed atoms reached 10 À3 eV A À1 . The electronic exchange-correlation potential was calculated using the Perdew-Burke-Ernzerhof (PBE) generalized gradient approximation (GGA) functional. 44 To accurately reproduce the experimentally known band gaps and density of states features of a-MnO 2 and rGO, the screened hybrid functional HSE06 45 was used with the exchange value of 25%. The projected density of states (PDOS) was calculated using tetrahedron method with Bloch correction. 46 The most stable a-MnO 2 (100) surface 47 was employed to form the nano-heterostructure with rGO (rGO/a-MnO 2 ). The a-MnO 2 (100) surface was created from the optimized bulk material using the METADISE code, which ensures the creation of surfaces with zero dipole moment perpendicular to the surface plane. The rGO/a-MnO 2 nano-heterostructure was constructed with (2 Â 4)-a-MnO 2 (100) and (5 Â 5)-rGO supercells. We used k-point meshes of 9 Â 9 Â 1 for the rGO monolayer, 5 Â 5 Â 1 for the a-MnO 2 (100) surface, and 5 Â 5 Â 1 for the rGO/ a-MnO 2 composite. In each simulation cell, a vacuum region of length 20 A was added perpendicular to the surface to avoid interactions between periodic slabs. The electrostatic potential of each surface was averaged along the c-direction, using the Macro Density package. [48][49][50] The work function (F) was calculated as F ¼ V vacuum À E F , where V vacuum and E F are the vacuum and Fermi level, respectively. Dipole correction perpendicular to all surfaces was accounted for, which ensured that there is no net dipole perpendicular to the surfaces that may affect the potential in the vacuum level. 51-53

Characterization of MnO 2 and rGO/MnO 2
The crystalline structures of the MnO 2 nanorods and rGO/MnO 2 nano-heterostructure were conrmed by XRD and the corresponding results are presented in Fig. S1. † All the diffraction peaks in Fig. S1 † can be indexed to the tetragonal crystal structure of MnO 2 (ICDD card no.  with lattice constant a ¼ b ¼ 9.815 A and c ¼ 2.847 A. We have observed the highest growth of a-MnO 2 in the (211) plane. 54 The XRD diffraction pattern of the rGO-MnO 2 nanostructure is shown in inset of Fig. S1. † The broad peak at 2q around 26 corresponds to the (002) plane of the reduced graphene oxide. 55 The eld emission scanning electron microscope (FESEM) images in Fig. 1, reveal the morphological properties of the MnO 2 nanorods and rGO/ MnO 2 nano-heterostructure. The FESEM images recorded at different magnications (panels a-c of Fig. 1) show the formation of randomly distributed MnO 2 nanorods. The low magni-cation image shown in Fig. 1a depicts large coverage of the MnO 2 nanorods. The diameter of the formed ultra-long nanorods is estimated in the range of 140-150 nm as revealed by the high magnication FESEM images analyses (Fig. 1b and c). It is evident from the FESEM image of the rGO/MnO 2 nanoheterostructure that the MnO 2 nanorods are embedded in the rGO network (Fig. 1b-d). The high magnication image in Fig. 1f reveals the enormous coverage of the rGO/MnO 2 nanoheterostructure.
Energy dispersive X-ray spectroscopy (EDX) composition analysis in the 0-10 keV energy range (ESI, Fig. S2 †) (Fig. 2c) clearly reveals its crystalline nature. The lattice fringes are clearly observed in the HR-TEM image (Fig. 2c and d) and a 0.69 nm interplanar This journal is © The Royal Society of Chemistry 2020 RSC Adv., 2020, 10, 25988-25998 | 25991 distance indicates these planes to be of the (110) character. 56 The inverse FFT HR-TEM image and the corresponding prole plot of the MnO 2 nanorod are shown in Fig. S3(a and b). † A number of rmly attached a-MnO 2 nanorods onto the rGO sheets can be clearly seen from Fig. 2e and f. X-ray photoelectron spectroscopy (XPS) analysis was applied to determine the oxidation state and elemental composition of the prepared MnO 2 nanorods and rGO/MnO 2 nano-heterostructure. The XPS results for the MnO 2 nanorods are shown in Fig. S4(a and b) † and the rGO/MnO 2 nano-heterostructure in Fig. S4(c-e). † The peaks centered at 642.63 and 654.53 eV in the high-resolution spectrum of Mn 2p (Fig. S4a †) can be assigned to the Mn 2p 3/ 1s shown in Fig. S4c and S4d, † respectively, for rGO/MnO 2 nano-heterostructure, conrm the presence of MnO 2 , whereas the C (1s) spectra (Fig. S4e †) exhibit peaks that originates from the rGO sheets. Fig. 3(a and b) shows the emission current density as a function of the applied electrical eld (J-E curves) for the MnO 2 nanorods and rGO/MnO 2 nano-heterostructure. In this work, the turn-on eld dened as the eld required to draw an emission current density (J) of 10 mA cm À2 is found to be 7.1 and 1.4 V mm À1 for the MnO 2 nanorods and the rGO/MnO 2 nano-heterostructure, respectively. The rGO/MnO 2 nano-heterostructure also attains an impressive current density of 600 mA cm À2 at an applied eld of 3.1 V mm À1 compared to the isolated MnO 2 which displayed a lower current density of 221 mA cm À2 at a relatively higher applied eld of 8.1 V mm À1 . A comparison between the turn-on eld values obtained in the present study and previously reported MnO 2 nanostructures and the MnO 2 /rGO nanoheterostructures is provided in Table 1. 21,22,[61][62][63][64][65][66] The Fowler-Nordheim (F-N) plot for MnO 2 nanorods and rGO/MnO 2 nano-heterostructure obtained from ln(J/E 2 ) verses (1/E) is shown in Fig. 3(c and d). Consistent previous reports, 21,22 the FN plots of the MnO 2 nanorods and rGO/MnO 2 nanoheterostructure exhibit linear behavior in good agreement with their semiconducting nature. The nanometric features of the rGO/MnO 2 nano-heterostructure coupled with the high electrical conductivity of rGO are suggested as the key factors behind the observed superior eld emission properties. [2][3][4][5] Apart from the improved eld emission performance, the electron emission current stability is an important parameter for device fabrication considerations. We have therefore measured the emission current as a function of time in order to ascertain the robustness of the rGO/MnO 2 nano-heterostructure. The Table 1 Comparison of the turn-on field with reported MnO 2 nanomaterials and the MnO 2 /rGO nano-heterostructures in the present study

Sr. no. Specimen
Turn-on eld (V mm À1 ) (for J ¼ 10 mA cm À2 ) Reference  emission current versus time (I-t) plot of the MnO 2 nanorods and rGO/MnO 2 nano-heterostructure were recorded continuously for 2 h at a preset value of emission current of $1 mA as shown in Fig. 3(e and f). Generally, the results show that the emission current remains quite stable without showing any sign of diminishing over the 2 h period of continuous testing. Instabilities in the form of spikes can be attributed to the presence of residual gas molecules across the emitter surface.

Density functional theory analyses
Considering that eld emission is a geometry and work function (F) dependent phenomenon, with lower F enhancing the eld emission characteristics, we have carried out rstprinciples density functional theory calculations to gain atomic-level insight into the electronic structure and work function of the isolated rGO, a-MnO 2 (100) surface, and the rGO/a-MnO 2 (100) nanocomposite. As the electronic band gap of rGO vary depending on the degree of reduction, the rGO monolayer with two concentrations of epoxide functional groups with 8% and 16% oxygen contents were modelled as shown in Fig. 4(a and b). The band gap of the rGO with 8% and 16% oxygen contents are predicted at 0.48 eV (Fig. 4d) and 0.81 eV (Fig. 4e). This is consistent with experimental data that showed that the band gap of rGO can be tuned from 0.264-0.786 eV by controlling the surface concentration of epoxide groups. 67 The bulk a-MnO 2 was modelled tetragonal crystal structure (space group -I4/m, no. 87) in the antiferromagnetic AFM-C2 conguration 68 as shown in Fig. 4c. A full unit cell relaxation yielded a strain-free a-MnO 2 with lattice parameters a ¼ b ¼ 9.763 A, c ¼ 2.872 A, which compares closely with known experimental data (a ¼ b ¼ 9.71 A and c ¼ 2.88 A). 69,70 The electronic band gap of a-MnO 2 is predicted at 2.42 eV (Fig. 4f), in close agreement with an experimental estimate of 2.23 eV 71 and previous theoretical prediction of 2.7 eV. 72 The valence band edge of a-MnO 2 is demonstrated to consist mainly of O-p states whereas the conduction band edge is dominated by Mn-d states. The smaller band gap of the rGO compared to the MnO 2 suggests that the rGO has better electrical conductivity than the metal oxide. The optimized structures of the rGO/a-MnO 2 (100) nanocomposite formed by rGO with epoxide functional group with 8% and 16% oxygen contents are shown in Fig. 5a and d, respectively. The rGO is stabilized on the MnO 2 surface via C-O and C-Mn chemical bonds through the terminal C atoms. The interactions of the rGO with the MnO 2 surface gave rise to electron density redistribution within the rGO/a-MnO 2 (100) nanocomposite, which was analyzed by determining the threedimensional-charge density difference iso surface contours as shown in Fig. 5b for the rGO with 8% oxygen content and Fig. 5e for rGO with 16% oxygen content. The yellow and cyan regions represent charge depletion and accumulation in the space, respectively. We observe electron density accumulations mainly in the interfacial bonding regions in the rGO/a-MnO 2 (100) nanocomposite, suggesting strong interactions between rGO and MnO 2 (100) surface. Consistent with the strong interaction, we observe strong hybridization between the C-p orbitals of the rGO with the Mn-d and O-p of the MnO 2 surface as shown in Fig. 5c and f, resulting in metallic conductivity of the composite systems.
The analysis of the work function (F) for the rGO monolayer, a-MnO 2 (100) and the rGO/a-MnO 2 (100) nanocomposite can help us to understand the origin/direction of charge transfer at the rGO/a-MnO 2 (100) interface. The F for the isolated rGO is predicted at 5.21 and 5.85 eV for the 8% and 16% oxygen contents, respectively ( Fig. 6a and b). This is consistent with a previous theoretical investigation which predicted a work function of rGO with epoxy groups to be 4.35 eV for 1.5% oxygen content and 5.6 eV for 20% oxygen content. 73 The F for the a-MnO 2 (100) surface is predicted at 7.22 eV (Fig. 6c), also in good agreement with earlier theoretical pH-corrected work function of 7.7 eV for the a-MnO 2 (110) surface. 72 The work function of the rGO/a-MnO 2 (100) nanocomposite is predicted at 6.01 and 6.12 eV for the rGO with 8% and 16% oxygen contents (Fig. 6d  and e), both of which are lower than that of the isolated a-MnO 2 (100) surface (Fig. 6c). The reduction in the work function of rGO/a-MnO 2 (100) nanocomposite relative to the isolated a-MnO 2 (100) surface can be ascribed to the interfacial bonding, electronic interaction and synergistic effects. Considering that the electron emission capability of a material is dictated by its work function, the observed superior eld emission characteristics of the rGO/a-MnO 2 nanocomposite compared to the isolated a-MnO 2 material can be attributed to the predicted lower work function for the rGO/a-MnO 2 (100) nanocomposite. Reduction in the work function has been observed in other composite materials compared to the isolated materials. 52,53 For instance, Susaki et al., have shown that the deposition of a single unit cell of MgO on an Nb:SrTiO 3 substrate reduces the work function by about 0.8 eV. 74 Similarly, by decorating SnSe nanosheets with Au nanoparticles (Au/SnSe) and porous ZnO nanosheets with CuSCN nanocoins (CuSCN/ZnO) resulted in signicant improvements in the FE characteristics owing to predicted lower functions. 52,53 The higher work function predicted for the a-MnO 2 (100) surface compared to rGO monolayer (Fig. 6) suggests that spontaneous electrons transfer will ow from the rGO monolayer to the a-MnO 2 (100) aer the two are coupled together. Besides the reduction of the work function, the formation of nano-protrusions (denoted by red circles) in the rGO/MnO 2 nano-heterostructure (Fig. S5 †) can act as effective emission sites. In addition, the better electrical conductivity (5 Â 10 À3 S cm À1 ) of rGO is expected to plays an important role in electron transportation. The rGO as backbone may results in easy and high percolation of electrons from the rGO to MnO 2 nanorods giving rise to the observed superior eld emission behavior of the rGO/MnO 2 nano-heterostructure.

Conclusion
In summary, we report the successful synthesis of MnO 2 nanorods and rGO/MnO 2 nano-heterostructure using cost effective hydrothermal and modied Hummer's methods, respectively. The coupling of rGO sheets with MnO 2 nanorods is demonstrated to have a synergistic effect in improving the FE characteristics of formed rGO/MnO 2 nano-heterostructure. The dramatic reduction of the turn-on eld by 5.7 V mm À1 for an emission current density of 10 mA cm À2 and the achieved high current density of 600 mA cm À2 with an applied eld of 3.1 V mm À1 demonstrate the superior FE characteristics of the rGO/ MnO 2 nano-heterostructure compared to the isolated porous MnO 2 nanorods. The results are corroborated by rst principles DFT calculations, which predict lower work function for the rGO/ MnO 2 nano-heterostructure (6.01 and 6.12 eV for the rGO with 8% and 16% oxygen contents, respectively) compared to the isolated MnO 2 (7.22 eV) as the primary origin for the improved eld emission of the rGO/MnO 2 nanocomposite. The controlled nanofabrication of rGO/MnO 2 heterostructure reported here provides a promising approach for designing highly efficient MnO 2 -based next generation FE electron sources and extend their practical applications in micro/nano electronic devices.

Conflicts of interest
There are no conicts to declare.