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TiO2
Vacuum 123 (2016) 167e174
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Observation of low turn-on field emission from nanocomposites of GO/TiO2 and RGO/TiO2 Girish P. Patil a, Vivekanand S. Bagal a, b, Chetan R. Mahajan c, Vijay R. Chaudhari c, Sachin R. Suryawanshi d, Mahendra A. More d, Padmakar G. Chavan a, * a
Department of Physics, School of Physical Sciences, North Maharashtra University, Jalgaon 425001, India Department of Applied Sciences & Humanities, SVKM's NMIMS, Mukesh Patel School of Technology Manangement & Engineering, Shirpur Campus, 425405, India c University Institute of Chemical Technology, North Maharashtra University, Jalgaon 425001, India d Center for Advanced Studies in Materials Science and Condensed Matter Physics, Department of Physics, Savitribai Phule Pune University, Pune 411007, India b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 26 March 2015 Received in revised form 8 October 2015 Accepted 28 October 2015 Available online 3 November 2015
Highly dense TiO2 nanotubes were synthesized by anodization of titanium foil. The TiO2 nanotubes were characterized by using XRD, FESEM, TEM and HRTEM. GO/TiO2 and RGO/TiO2 nanocomposites were synthesized by electrophoresis method using de-ionised water. FESEM images confirm complete covering of the GO and RGO nanosheets on the entire surface of TiO2 nanotubes. Field emission studies of TiO2 nanotubes, GO/TiO2 and RGO/TiO2 nanocomposites have been carried out. Turn-on field for the emission current density of 10 mA/cm2, has been observed to be 2.9 V/mm, 3.3 V/mm and 2.6 V/mm for TiO2 nanotubes, GO/TiO2 and RGO/TiO2 nanocomposites respectively. For the TiO2 nanotubes, GO/TiO2 and RGO/TiO2 nanocomposites, the field enhancement factor (b) were calculated and are found to be 2059, 1482 and 2300. The turn-on field of the RGO/TiO2 nanocomposite has been found to be superior than other reported values of TiO2 nanostructures. To the best of our knowledge this is the first report on field emission studies of GO/TiO2 and RGO/TiO2 nanocomposites. Simple synthesis route coupled with the superior field emission behavior makes the present emitter more suitable for nano-electronics application. © 2015 Elsevier Ltd. All rights reserved.
Keywords: Synthesis Field emission Current stability
1. Introduction Various One-Dimensional (1D) nanostructures such as nanowires, nanorods, nanosheets and nanotubes have been investigated for their applications in solar cell, gas sensing, biomedical, photodetector and field emission properties [1e3]. Titanium Dioxide (TiO2) is a promising material which exhibits quite superior properties such as non-toxicity, chemical stability and high photo catalytic activity. Due to this novel properties it has been used in solar cell [4], photocatalysis [5], gas sensing [6] and biomedical applications [7]. Band gap energy of TiO2 is 3.2 eV and has work function of 4.4 eV lower than other materials used for field emission application such as, CNTs (5.0 eV), ZnO (5.3 eV), ZnS (7.0 eV) etc [8]. Field emission is a purely quantum mechanical phenomenon where
* Corresponding author. E-mail address: [email protected] (P.G. Chavan). http://dx.doi.org/10.1016/j.vacuum.2015.10.028 0042-207X/© 2015 Elsevier Ltd. All rights reserved.
electrons are emitted from the surface of nanomaterials under the action of the strong electrostatic field. Field emission is geometry dependent phenomenon. TiO2 nanotubes array possesses geometrical/morphological features similar to highly aligned CNTs. TiO2 nanotubes also can made as a tightly adherent layer on the surface of Ti foil/substrate by simple anodization process. Such tight layer of the TiO2 nanotubes with substrate makes them suitable as electron percolation pathways for free electron transfer between interfaces [9]. Aligned TiO2 nanotubes array with high packing density can significantly enhance field emission properties of the materials [10] and it can be a suitable candidate for field emission application. High emission current density with low turn-on field and good current stability are the properties of good emitter and are very much desirable for application in a range of field emission based devices [11]. Nanomaterials with unique properties could further modify by altering their electronic properties via chemical doping or changing their surface topography. However, nanomaterials with dissimilar
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and unique properties if coupled together then one could expect the superior properties. In this regards, the surface of the aligned TiO2 nanotubes could be modify by the material which is superior for the field emission application. Graphene shows similar or identical properties as carbon nanotubes. Graphene has high mechanical strength [12], excellent electrical conductivity [13] and high aspect ratio (size to thickness) which makes them as good field emitter. Graphene has various applications in nanoelectronic devices including sensor [14], supercapacitor [15], hydrogen storage [16] field emission [17], and bioelectronics [18,19]. In this report, attempt has been made to utilize the novel properties of graphene by coupling it with TiO2 nanotubes. Synthesis of nanocomposites of TiO2 with Graphene Oxide (GO) and Reduced Graphene Oxide (RGO) has been done by electrophoresis technique. Field emission results of the aligned TiO2 nanotubes, GO/ TiO2 and RGO/TiO2 nanocomposites are discussed in detail. 2. Experiment 2.1. Synthesis of TiO2 nanotubes Anodization method was used to synthesize the aligned TiO2 nanotubes [20]. Prior to anodization, high purity titanium foil (99.7% purity, 0.25 mm thickness, Sigma Aldrich) was degreased in an ultrasonic bath for 10 min with ethanol and acetone sequentially. Anodization was performed in a two electrode configuration with titanium foil (1 cm 2 cm) as the working electrode and platinum foil (1 cm 2 cm) as the counter electrode under constant potential at room temperature (25 C). The reaction was carried out by adding 3 volume % HF (40%) in Dimethyl Sulfoxide (DMSO) at constant DC voltage of 30 V for 18 h. The schematic of synthesis of aligned TiO2 nanotubes is shown in Fig. 1. The color of the titanium foil surface was found to be yellowish after anodization. Finally, the as-anodized Ti foil was rinsed in de-ionized water and used for further characterization. Annealing of the as-anodized Ti foil was carried out at 530 C for 3 h. 2.2. Synthesis of GO Graphite powder and potassium permanganate (KMnO4, 99.9%) were purchased from LOBA Chemie and Qualigens respectively. Petroleum ether and hydrogen peroxide (H2O2, 30%) were purchased from Ranken. Sulfuric acid, ortho phosphoric acid,
hydroquinone, potassium nitrate and hydrazine hydrate were purchased from Merck India and ethanol from Changshu Yangyua Chemical, China. All the chemicals were used without further purification. All the solutions were prepared using de-ionized water. Graphite flakes were subjected to oxidative treatment by potassium permanganate, concentrated sulfuric acid and ortho phosphoric acid [21]. Briefly, 3 gm of natural graphite flakes and 9:1 mixture of sulfuric acid and ortho phosphoric acid (360 ml H2SO4:40 ml H3PO4) was taken in three neck round bottom flask and stirred for 15 min. 18 gm of potassium permanganate (KMnO4) was gradually added to the suspension over a period of 60 min with continuous stirring. Temperature of reaction mixture was increased up to 35e40 C. The reaction mixture was stirred for 12 h at 45e50 C. It was allowed to cool at room temperature and then transferred to a beaker containing 400 gm ice and 3 ml hydrogen peroxide (30%). The suspension was filtered through a polyester cloth and centrifuged the filtrate at 5000 rpm for 20 min. The precipitate was washed twice in series with 200 ml water, 200 ml of 30% HCl and 200 ml of ethanol, respectively. The resultant product was coagulated using 200 ml ether. The final solid product was dried under vacuum for 24 h. 2.3. Synthesis of RGO As-prepared GO was further reduced using hydrazine hydrate [22]. Typically, 100 mg of GO was dispersed in 100 ml water by sonication, to obtain an optically clear yellow-brown color dispersion. Hydrazine hydrate (1 ml) was added to the dispersion and refluxed for 24 h. Black color precipitate was seen to be settled at the bottom of the flask and it was isolated by filtration through polyester cloth. Repeated washing of the product with copious amount of water and methanol was done. The final product was dried under vacuum. The product obtained by reduction of GO is named as RGO. 2.4. Preparation of GO/TiO2 and RGO/TiO2 nanocomposites GO and RGO nanosheets were deposited on aligned TiO2 nanotubes by electrophoretic deposition. 25 mg of as-prepared GO/ RGO were dispersed in de-ionized water through sonication and used as an electrolytic medium for the electrophoretic deposition. Aligned TiO2 nanotubes were used as working electrode and Platinum foil as counter electrode. Constant potential of 30 V was applied for 30 min across the electrodes using programmable
Fig. 1. Schematic of synthesis approach of the GO/TiO2 and RGO/TiO2 nanocomposites.
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power supply (Scientific PSD9003 30 V, 2 Amp.). The schematic assembly of electrophoretic deposition is shown in Fig. 1. Finally, specimens were dried in air and used for further characterizations.
2.5. Characterizations The phase identification of the as-anodized and annealed product was done by X-ray diffraction (XRD) using D8 Advance, Bruker instrument. The surface morphology of the as-anodized and annealed TiO2 nanotubes has been studied using Field Emission Scanning Electron Microscope (FESEM) (Model Hitachi S-4800) and the elemental composition has been obtained using EnergyDispersive X-ray Spectrometer (EDS) attached to the FESEM. The morphology and crystalline nature of the annealed TiO2 nanotubes were further studied by Transmission Electron Microscope (TEM), High Resolution Transmission Electron Microscope (HRTEM) and Selected Area Energy Diffraction (model number, Tecnai G2 20 Twin, FEI). Raman spectra were recorded at 514.5 nm using Jobin YVON HORRIBA HR 800 UV detector. Investigation of surface chemical nature of GO and RGO was done by 8400 Shimadzu, Japan Fourier Transform Infrared Spectroscopy (FTIR) with a resolution of 4 cm1, over the frequency range 400e4000 cm1. The specimens were prepared by mixing the ground samples with powdered potassium bromide and pressing the mixture under high pressure to obtain the pellet. Keithley 2611A source meter four probes IeV instrument was used to determine an electrical conductivity. The field emission current density-applied field (JeE) and current-time (Iet) measurements were carried at in all metal field emission microscope. The field emission studies were carried out in a ‘close proximity’ (also termed as ‘planar diode’) configuration, wherein the annealed aligned TiO2 nanotubes, GO/TiO2 and RGO/TiO2 nanocomposite films served as a cathode and a semi-transparent cathodoluminescent phosphor screen as an anode. Fig. 2 is a schematic of field emission diode assembly. The area of TiO2, GO/ TiO2 and RGO/TiO2 nanocomposite specimens was kept similar (0.25 cm2). The cathode, pasted onto a copper rod using vacuum compatible conducting silver paste, was held in front of the anode screen at a distance of ~1 mm. The cathode did not show any appreciable degassing and vacuum was obtained with usual speed. After baking the system at 150 C for 12 h, pressure of ~1 108 mbar was obtained. The JeE and Iet measurements were carried out at this base pressure using a Keithley Electrometer (6514) and a Spellman high voltage DC power supply (0e40 kV, U. S. A.). Special care was taken to avoid any leakage current by using shielded cables with proper grounding.
Fig. 2. Schematic of the field emission diode assembly.
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3. Results and discussion Fig. 3 depicts the FESEM images of the aligned TiO2 nanotubes, GO/TiO2 and RGO/TiO2 specimens. Fig. 3a indicating large coverage of well aligned TiO2 nanotubes. Aligned TiO2 nanotubes with an average diameter of 90 nm, wall thickness of 14 nm and average length 1.5 mm have been recorded from Fig. 3b. An effect of applied potential on the morphology of the aligned TiO2 nanotubes has been investigated by electrophoretic technique, where using deionized water in the presence of TiO2 nanotubes only. FESEM images in Fig. 3c and d depict no morphological change of aligned TiO2 nanotubes which undergo electrophoresis treatment. Fig. 3e is a low magnification image of the GO/TiO2 nanocomposite. Formation of nanocomposite of GO and aligned TiO2 nanotubes has been observed. Close magnification image (Fig. 3f) confirms that aligned TiO2 nanotubes are covered completely by GO nanosheets. Fig. 3f shows most protruding TiO2 nanotubes and are denoted by arrow. Detailed analysis of FESEM results indicates that coating of few GO sheets on the aligned TiO2 nanotubes. TiO2 nanotubes are clearly seen even of the coating of GO nanosheets (Fig. 3f). Similar types of observations have been recorded for nanocomposite of RGO/TiO2. Fig. 3g and h indicate low magnification as well as close-up image of RGO/TiO2 nanocomposite. Since the RGO has a higher electrical conductivity as compared to the GO (Table 1) the thickness of sheets has been seen to be higher than the GO/TiO2 nanocomposite and are shown in Fig. 3g and h. More protruding or tapered RGO sheets have been seen to be dominated in case of RGO/TiO2 nanocomposite. XRD studies of the as-anodized TiO2 nanotubes, annealed TiO2 nanotubes and water treated TiO2 nanotubes have been recorded (not shown here). As-anodized TiO2 nanotubes are found to be amorphous in nature. The XRD of the annealed TiO2 nanotubes shows a polycrystalline nature with predominantly TiO2 Anatase (A) peak centered at 2Ɵ value of 25.31. The reflections due to the TiO2 peaks can be indexed as Anatase TiO2 with lattice constants a ¼ 3.782 Å and c ¼ 9.502 Å, which is consistent with the literature (JCPDS Card No. 84-1286). The other peaks are related to the Titanium (Ti) substrate and assigned those as Ti. The XRD pattern gives no indication of TiOxFy or TiOxSy and other impurity phases in the specimen, indicating that a considerable amount of the solvent may be trapped in the amorphous anodic film but do not enter at the Anatase lattice [23]. The XRD pattern of water treated TiO2 does not show any change in the lattice constant or ‘d’ spacing and/or any shift in the 2Ɵ value. Fig. 4 represent the XRD pattern of GO and RGO. [002] diffraction line for graphite is observed at 7.8 and 24 for GO and RGO, respectively. The diffraction line at a small angle for GO is attributed to large inter layer distance (d-spacing) between graphite sheets which may be due to the incorporation of functional groups by oxidation. For RGO [002] diffraction line is shifted to higher diffraction which is indicative of decrease in dspacing due to removal of functional groups after reduction. For the purpose of crystallinity of the aligned TiO2 nanotubes detail TEM and HRTEM study have been done. Bright field images of TiO2 nanotubes are shown in Fig. 5a and b. Fig. 5a and b indicates, hollow tube like morphology with an average diameter ~90 nm. Individual nanotube is shown as inset of Fig. 5b. Fig. 5c is the HRTEM image of an annealed TiO2 nanotube which indicate single crystalline nature of the TiO2 nanotube with fringe spacing of 0.35 nm along (101) plane [24]. SAED pattern shown as inset of Fig. 5c indicates polycrystalline nature of the TiO2 nanotubes. In Raman spectra of GO and RGO (Fig. 6), the characteristic peaks of graphite ca. 1597 cm1 (G-Band) and 1353 cm1 (D-band) are observed. D-band is an indicative of defects in graphite structure, however G-band is for its sp2 hybridized structure. Compared to GO, red shifting in G and D-bands of RGO suggests the large
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Fig. 3. FESEM images: (a) and (b) TiO2 nanotubes, (c) and (d) TiO2 nanotubes after water treatment, (e) low and (f) high magnification GO/TiO2 nanocomposite and (g) low and (h) high magnification RGO/TiO2 nanocomposite.
Table 1 Electrical conductivity measurements of GO and RGO. Sr no.
1 2
Specimen
GO RGO
Slope (A/V)
R(U) ¼ 1/slope (V/A)
7.40504 107 0.03228
1350431.5 30.9789
W ¼ thickness (meter)
W/S S ¼ 0.002M
0.00231 0.00170
1.155 0.85
proportions of sp2 structure due to removal of oxygenated functionalities from graphite surface [25]. Moreover, decrease in fullwidth at-half-maximum for RGO indicates the repairing of defects in its graphitic network [26]. Fig. 7 depicts the FTIR spectra of GO and RGO. Major bands at
G7 (W/S)
r0 (ohm meters)
r ¼ r0/G7 (W/S) (ohm meters)
s ¼ 1/r
1.373 1.714
16963.16 0.3892
12354.814 0.22734
8.09401 105 4.398667
(conductivity) (mhos/meter) or (Siemens/meter)
3350 cm1, 1750 cm1 and 1172 cm1 observed for GO can be attributed due to the stretching frequency of hydroxyl, carbonyl and CeO functionalities. All these vibrations were disappeared for RGO, due to the reduction by hydrazine. Moreover, intensity of bands at 1512 cm1 (for C]C) increases for RGO due to the re-
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Fig. 4. X-Ray diffractograms of GO & RGO.
establishment of sp2 network. IeV plots of GO and RGO are shown in Fig. 8. Electrical conductivity measurements of GO and RGO are shown in Table 1.
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The JeE characteristic of the aligned TiO2 nanotubes, GO/TiO2 and RGO/TiO2 nanocomposites is depicted in Fig. 9a. Turn-on field defined as the field required to draw an emission current density of 10 mA/cm2 is found to be 2.9 V/mm, 3.3 V/mm and 2.6 V/mm for aligned TiO2 nanotubes, GO/TiO2 and RGO/TiO2 nanocomposites respectively. As the field was increased further to 4.2 V/mm, 4.9 V/ mm, 3.6 V/mm for aligned TiO2 nanotubes, GO/TiO2 and RGO/TiO2 nanocomposites the emission current density of 80 mA/cm2 has been recorded. The turn-on field values of the aforesaid specimens have been seen to be quite superior as compared to the other TiO2 nanostructures as summarized in Table 2 [9,27e30]. Since there is no report on the field emission studies of GO/TiO2 or RGO/TiO2 nanocomposite the comparison has been done with TiO2 nanostructures. From Table 2 it is cleared that the RGO/TiO2 nanocomposite show superior field emission properties in terms of turnon field than the aligned TiO2 nanotubes and GO/TiO2 as well as other TiO2 nanostructures [9,27e30]. The observed field emission
Fig. 5. (a) and (b) Low magnification TEM images of the TiO2 nanotubes, TEM image of individual nanotube with high magnification is shown as inset of (b) (c) HRTEM image of the TiO2 nanotube with SAED pattern as inset.
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Fig. 6. Raman spectra of GO & RGO.
Fig. 9. (a) JeE plots of the aligned TiO2 nanotubes, GO/TiO2 and RGO/TiO2 nanocomposites and (b) corresponding FeN plots.
Fig. 7. FTIR spectra of GO & RGO.
properties of aligned TiO2 nanotubes, GO/TiO2 and RGO/TiO2 nanocomposites could be attributed due to the following reasons. (a) Field emission studies of TiO2 The turn-on field for aligned TiO2 nanotubes has been found to be 2.9 V/mm, for the emission current density of 10 mA/cm2. Iet plot
has been recorded for the preset value of 10 mA for the duration of 2 h. The above results show that the aligned TiO2 nanotube possesses better field emission properties in terms of turn-on field, emission current density and fairly good emission stability for the entire duration. The field emission properties are related to the morphology/geometry of the emitter/s. The observed turn-on field could be attributed due very sharp wall thickness of 14 nm of the aligned TiO2 nanotubes and also a large density of the aligned TiO2 nanotubes (~240 107/cm2). In addition, emission current may suppress due to screening effect. Finally, well adherent layer of TiO2 nanotubes film on Ti surface provides the easy electron percolation pathways that transport the electrons efficiently from the substrate to the emission sites [9], since aligned TiO2 nanotubes were grown directly on the Ti foil itself. (b) Field emission studies of GO/TiO2
Fig. 8. IeV plots of GO and RGO.
The turn-on field for GO/TiO2 nanocomposite has been found to be 3.3 V/mm, for the emission current density of 10 mA/cm2. The observed turn-on field is seen to be higher than aligned TiO2 nanotubes. From the FESEM images (Fig. 3b and e of the aligned TiO2 nanotubes and GO/TiO2) it has been clear that open and aligned ends of the TiO2 nanotubes have been completely covered with GO nanosheets. A high field emission current can be induced from the emitter only when it must have conical or very sharp protrusions. In case of GO/TiO2 nanocomposite, the more possible emitters of TiO2 i.e. nanotubes have been seen to be blocked by a few layer of GO sheets. GO is known to be an insulating material [31] which leads to the observation of high turn-on field as
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Table 2 Turn-on field values of the TiO2 nanostructures reported in the literature. Specimen
Turn-on field (V/mm) (for J ¼ 10 mA/cm2)
Reference
Aligned TiO2 nanotubes GO/TiO2 nanocomposite RGO/TiO2 nanocomposite Aligned TiO2 nanotubes N2 doped TiO2 TiO2 nanowires Quasi aligned TiO2 nanowires TiO2 nanotubes
2.9 3.3 2.6 7.8 11.2 5.7 3.1 23.8
Present work
compared to aligned TiO2 nanotubes. All covered nanosheets are tend to be lie more flatter on TiO2 nanotubes. Very few GO nanosheets are seen to be protruded due to very sharp edges marked by arrows (Fig. 3f) and seen to be responsible for the observation of field emission result. Hence, the turn-on field of GO/TiO2 nanocomposite has been seen to be increased as compared to the aligned TiO2 nanotubes specimen. GO is a sheet like structure having sp2 networks discontinued by oxygenated functionalities. Despite discontinuity, it possesses a large number of tiny, isolated sp2 domains those are individually conducting in nature due to persevered C]C skeleton. In fact 1512 cm1 band in FTIR and presence of G-band in Raman spectra clearly support the existence of sp2 domain in GO, those can impart the slight but non-zero conductivity. Concerning the practical evidence of conductivity we have given the IeV characteristics of GO, which clearly show the non-zero and finite conductivity (Table 1). Iet plot of GO/TiO2 nanocomposite was recorded at the preset current of 10 mA for the duration of 2 h and is shown in Fig. 10. The observed fluctuations in the Iet plot may be attributed due to the adsorption and diffusion of residual gas molecules on the emitter surface [32]. The striking feature of the field emission behavior of the GO/TiO2 nanocomposite is that the average emission current remains nearly constant over the entire duration and show no sign of degradation.
Fig. 10. Iet plots of the aligned TiO2, GO/TiO2 and RGO/TiO2 nanocomposites.
9 27 28 29 30
(c) Field emission studies of RGO/TiO2 The turn-on field for RGO/TiO2 nanocomposite has been found to be 2.6 V/mm, for the emission current density of 10 mA/cm2. The observed turn-on field is found to be lower than the aligned TiO2 nanotubes and GO/TiO2 nanocomposite. As seen from the FESEM images Fig. 3g and h it has been cleared that during formation of the nanocomposite of RGO/TiO2, the RGO nanosheets tend to lie tapered (are marked by arrows in FESEM image Fig. 3h) on the aligned TiO2 nanotubes instead of appearing flat with respect to the substrate. The emission current density of an emitter exponentially depends on the field enhancement factor b, which is defined by the shape and size (aspect ratio) of the emitter [33]. In the present study, we have estimated the field enhancement factor (b) from the slope of the FowlereNordheim (FeN) plot, which is mathematically expressed as,
b¼
3 6:8 103 ∅2 slope
Where ∅ ¼ Work function. By considering the work function of TiO2, GO and RGO as 4.4 eV, 4.9 eV [34] and 4.5 eV [35] the field enhancement factor b was calculated and found to be 2059, 1482 and 2300 for aligned TiO2 nanotubes, GO/TiO2 and RGO/TiO2 nanocomposite. The high value of field enhancement factor b for RGO/TiO2 may be attributed due to high density of protruded RGO nanosheets observed in the FESEM results (Fig. 3h). Estimation of the density of the nanoprotrusions/nanostructures has been done by calculating the number of the protruded emission sites observed in the FESEM images (Fig. 3f and h) of aligned TiO2 nanotubes, GO/TiO2 and RGO/TiO2 nanocomposites. Density of the nanoprotrusions and corresponding turn-on field of aligned TiO2 nanotubes, GO/TiO2 and RGO/TiO2 nanocomposites have been summarized in Table 3. A good correlation has been found between density of nanoprotrusions and the turn-on field of aligned TiO2 nanotubes, GO/TiO2 and RGO/TiO2 nanocomposites. Iet plot of RGO/TiO2 nanocomposite was recorded at the preset value of 10 mA for the duration of 2 h and is shown in Fig. 10. The small amount of increment or decrement in the emission current in the form of fluctuations may be attributed due to the adsorption and diffusion of residual gas molecules on the emitter surface [32]. The important feature of the field emission behavior of the RGO/ TiO2 nanocomposite is that the average emission current remains nearly constant over the entire duration and shows no sign of degradation. This is a very important feature, particularly from the practical application of the emitter material as an electron source [32]. The FeN plot, i.e., ln (J/E2) versus (1/E), derived from the observed JeE characteristic is shown in Fig. 9b. FeN plots of the aligned aligned TiO2 nanotubes, GO/TiO2 and RGO/TiO2
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G.P. Patil et al. / Vacuum 123 (2016) 167e174 Table 3 The density of fabricated nanostructures with turn-on field. Specimen
Nanoprotrusions/cm2
Turn-on field (V/mm) (for J ¼ 10 mA/cm2)
Aligned TiO2 nanotubes GO/TiO2 nanocomposite RGO/TiO2 nanocomposite
240 107 1.4 107 3.2 107
2.9 3.3 2.6
nanocomposites exhibit non-linear nature over entire range of applied field with two distinct linear lines (shown in Fig. 9b). In principle, the field emitted electrons from a semiconducting emitter originate from both the conduction and the valence bands, as reported in our earlier studies [36e38]. The quantification of contribution of the valence and the conduction bands to the emission current mainly depends on their density of states and band gap energy. One of the striking features of wide band gap semiconducting emitters is observation of non-linear FeN plot over the entire applied field strength. 4. Conclusions GO/TiO2 and RGO/TiO2 nanocomposites have been synthesized by the simple technique of electrophoresis. FESEM images of the nanocomposite of GO/TiO2 and RGO/TiO2 indicate the formation of protruded emitters. RGO/TiO2 nanocomposite show the promising field emission behavior in terms of low turn-on field. Observation of low turn-on field may be attributed due to the high value of the field enhancement factor b as well as high electrical conductivity. The simplicity of the synthesis route with better field emission characteristics indicates that the RGO/TiO2 nanocomposite may be the promising candidate for field emission based applications. Acknowledgments GPP and PGC sincerely thank to SERB DST, Government of India (Ref. No.: SB/EMEQ-208/2013, dated 23/08/2013) for financial support. GPP and PGC also thank to UGCSAP-BSR Phase-III project for financial support. SRS gratefully acknowledges BARC, Mumbai, for the award of SRF under BARC-UoP memorandum (Grant No: GOI-E-153). References [1] S. Iijima, Nature 56 (1991) 354. [2] M.D. Shsinde, P.G. Chavan, G.G. Umarji, S.S. Arbuj, S.B. Rane, M.A. More, D.S. Joag, D.P. Amalnerkar, J. Nanosci. Nanotechnol. 12 (2012) 3788. [3] B.A. Kakade, V.K. Pillai, D.J. Late, P.G. Chavan, F.J. Sheini, M.A. More, D.S. Joag, Appl. Phys. Lett. 97 (2010) 073102. [4] K. Zhu, N.R. Neale, A. Miedaner, A.J. Frank, Nano. Lett. 7 (2007) 69. [5] M. Zlamal, J.M. Macak, P. Schmuki, J. Krysa, Electrochem. Commun. 9 (2007) 2822. [6] M. Paulose, O.K. Varghese, G.K. Mor, C.A. Grimes, K.G. Ong, Nanotechnology 17
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Observation of low turn-on field emission from nanocomposites of GO/TiO2 and RGO/TiO2 Girish P. Patil a, Vivekanand S. Bagal a, b, Chetan R. Mahajan c, Vijay R. Chaudhari c, Sachin R. Suryawanshi d, Mahendra A. More d, Padmakar G. Chavan a, * a
Department of Physics, School of Physical Sciences, North Maharashtra University, Jalgaon 425001, India Department of Applied Sciences & Humanities, SVKM's NMIMS, Mukesh Patel School of Technology Manangement & Engineering, Shirpur Campus, 425405, India c University Institute of Chemical Technology, North Maharashtra University, Jalgaon 425001, India d Center for Advanced Studies in Materials Science and Condensed Matter Physics, Department of Physics, Savitribai Phule Pune University, Pune 411007, India b
a r t i c l e i n f o
a b s t r a c t
Article history: Received 26 March 2015 Received in revised form 8 October 2015 Accepted 28 October 2015 Available online 3 November 2015
Highly dense TiO2 nanotubes were synthesized by anodization of titanium foil. The TiO2 nanotubes were characterized by using XRD, FESEM, TEM and HRTEM. GO/TiO2 and RGO/TiO2 nanocomposites were synthesized by electrophoresis method using de-ionised water. FESEM images confirm complete covering of the GO and RGO nanosheets on the entire surface of TiO2 nanotubes. Field emission studies of TiO2 nanotubes, GO/TiO2 and RGO/TiO2 nanocomposites have been carried out. Turn-on field for the emission current density of 10 mA/cm2, has been observed to be 2.9 V/mm, 3.3 V/mm and 2.6 V/mm for TiO2 nanotubes, GO/TiO2 and RGO/TiO2 nanocomposites respectively. For the TiO2 nanotubes, GO/TiO2 and RGO/TiO2 nanocomposites, the field enhancement factor (b) were calculated and are found to be 2059, 1482 and 2300. The turn-on field of the RGO/TiO2 nanocomposite has been found to be superior than other reported values of TiO2 nanostructures. To the best of our knowledge this is the first report on field emission studies of GO/TiO2 and RGO/TiO2 nanocomposites. Simple synthesis route coupled with the superior field emission behavior makes the present emitter more suitable for nano-electronics application. © 2015 Elsevier Ltd. All rights reserved.
Keywords: Synthesis Field emission Current stability
1. Introduction Various One-Dimensional (1D) nanostructures such as nanowires, nanorods, nanosheets and nanotubes have been investigated for their applications in solar cell, gas sensing, biomedical, photodetector and field emission properties [1e3]. Titanium Dioxide (TiO2) is a promising material which exhibits quite superior properties such as non-toxicity, chemical stability and high photo catalytic activity. Due to this novel properties it has been used in solar cell [4], photocatalysis [5], gas sensing [6] and biomedical applications [7]. Band gap energy of TiO2 is 3.2 eV and has work function of 4.4 eV lower than other materials used for field emission application such as, CNTs (5.0 eV), ZnO (5.3 eV), ZnS (7.0 eV) etc [8]. Field emission is a purely quantum mechanical phenomenon where
* Corresponding author. E-mail address: [email protected] (P.G. Chavan). http://dx.doi.org/10.1016/j.vacuum.2015.10.028 0042-207X/© 2015 Elsevier Ltd. All rights reserved.
electrons are emitted from the surface of nanomaterials under the action of the strong electrostatic field. Field emission is geometry dependent phenomenon. TiO2 nanotubes array possesses geometrical/morphological features similar to highly aligned CNTs. TiO2 nanotubes also can made as a tightly adherent layer on the surface of Ti foil/substrate by simple anodization process. Such tight layer of the TiO2 nanotubes with substrate makes them suitable as electron percolation pathways for free electron transfer between interfaces [9]. Aligned TiO2 nanotubes array with high packing density can significantly enhance field emission properties of the materials [10] and it can be a suitable candidate for field emission application. High emission current density with low turn-on field and good current stability are the properties of good emitter and are very much desirable for application in a range of field emission based devices [11]. Nanomaterials with unique properties could further modify by altering their electronic properties via chemical doping or changing their surface topography. However, nanomaterials with dissimilar
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and unique properties if coupled together then one could expect the superior properties. In this regards, the surface of the aligned TiO2 nanotubes could be modify by the material which is superior for the field emission application. Graphene shows similar or identical properties as carbon nanotubes. Graphene has high mechanical strength [12], excellent electrical conductivity [13] and high aspect ratio (size to thickness) which makes them as good field emitter. Graphene has various applications in nanoelectronic devices including sensor [14], supercapacitor [15], hydrogen storage [16] field emission [17], and bioelectronics [18,19]. In this report, attempt has been made to utilize the novel properties of graphene by coupling it with TiO2 nanotubes. Synthesis of nanocomposites of TiO2 with Graphene Oxide (GO) and Reduced Graphene Oxide (RGO) has been done by electrophoresis technique. Field emission results of the aligned TiO2 nanotubes, GO/ TiO2 and RGO/TiO2 nanocomposites are discussed in detail. 2. Experiment 2.1. Synthesis of TiO2 nanotubes Anodization method was used to synthesize the aligned TiO2 nanotubes [20]. Prior to anodization, high purity titanium foil (99.7% purity, 0.25 mm thickness, Sigma Aldrich) was degreased in an ultrasonic bath for 10 min with ethanol and acetone sequentially. Anodization was performed in a two electrode configuration with titanium foil (1 cm 2 cm) as the working electrode and platinum foil (1 cm 2 cm) as the counter electrode under constant potential at room temperature (25 C). The reaction was carried out by adding 3 volume % HF (40%) in Dimethyl Sulfoxide (DMSO) at constant DC voltage of 30 V for 18 h. The schematic of synthesis of aligned TiO2 nanotubes is shown in Fig. 1. The color of the titanium foil surface was found to be yellowish after anodization. Finally, the as-anodized Ti foil was rinsed in de-ionized water and used for further characterization. Annealing of the as-anodized Ti foil was carried out at 530 C for 3 h. 2.2. Synthesis of GO Graphite powder and potassium permanganate (KMnO4, 99.9%) were purchased from LOBA Chemie and Qualigens respectively. Petroleum ether and hydrogen peroxide (H2O2, 30%) were purchased from Ranken. Sulfuric acid, ortho phosphoric acid,
hydroquinone, potassium nitrate and hydrazine hydrate were purchased from Merck India and ethanol from Changshu Yangyua Chemical, China. All the chemicals were used without further purification. All the solutions were prepared using de-ionized water. Graphite flakes were subjected to oxidative treatment by potassium permanganate, concentrated sulfuric acid and ortho phosphoric acid [21]. Briefly, 3 gm of natural graphite flakes and 9:1 mixture of sulfuric acid and ortho phosphoric acid (360 ml H2SO4:40 ml H3PO4) was taken in three neck round bottom flask and stirred for 15 min. 18 gm of potassium permanganate (KMnO4) was gradually added to the suspension over a period of 60 min with continuous stirring. Temperature of reaction mixture was increased up to 35e40 C. The reaction mixture was stirred for 12 h at 45e50 C. It was allowed to cool at room temperature and then transferred to a beaker containing 400 gm ice and 3 ml hydrogen peroxide (30%). The suspension was filtered through a polyester cloth and centrifuged the filtrate at 5000 rpm for 20 min. The precipitate was washed twice in series with 200 ml water, 200 ml of 30% HCl and 200 ml of ethanol, respectively. The resultant product was coagulated using 200 ml ether. The final solid product was dried under vacuum for 24 h. 2.3. Synthesis of RGO As-prepared GO was further reduced using hydrazine hydrate [22]. Typically, 100 mg of GO was dispersed in 100 ml water by sonication, to obtain an optically clear yellow-brown color dispersion. Hydrazine hydrate (1 ml) was added to the dispersion and refluxed for 24 h. Black color precipitate was seen to be settled at the bottom of the flask and it was isolated by filtration through polyester cloth. Repeated washing of the product with copious amount of water and methanol was done. The final product was dried under vacuum. The product obtained by reduction of GO is named as RGO. 2.4. Preparation of GO/TiO2 and RGO/TiO2 nanocomposites GO and RGO nanosheets were deposited on aligned TiO2 nanotubes by electrophoretic deposition. 25 mg of as-prepared GO/ RGO were dispersed in de-ionized water through sonication and used as an electrolytic medium for the electrophoretic deposition. Aligned TiO2 nanotubes were used as working electrode and Platinum foil as counter electrode. Constant potential of 30 V was applied for 30 min across the electrodes using programmable
Fig. 1. Schematic of synthesis approach of the GO/TiO2 and RGO/TiO2 nanocomposites.
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power supply (Scientific PSD9003 30 V, 2 Amp.). The schematic assembly of electrophoretic deposition is shown in Fig. 1. Finally, specimens were dried in air and used for further characterizations.
2.5. Characterizations The phase identification of the as-anodized and annealed product was done by X-ray diffraction (XRD) using D8 Advance, Bruker instrument. The surface morphology of the as-anodized and annealed TiO2 nanotubes has been studied using Field Emission Scanning Electron Microscope (FESEM) (Model Hitachi S-4800) and the elemental composition has been obtained using EnergyDispersive X-ray Spectrometer (EDS) attached to the FESEM. The morphology and crystalline nature of the annealed TiO2 nanotubes were further studied by Transmission Electron Microscope (TEM), High Resolution Transmission Electron Microscope (HRTEM) and Selected Area Energy Diffraction (model number, Tecnai G2 20 Twin, FEI). Raman spectra were recorded at 514.5 nm using Jobin YVON HORRIBA HR 800 UV detector. Investigation of surface chemical nature of GO and RGO was done by 8400 Shimadzu, Japan Fourier Transform Infrared Spectroscopy (FTIR) with a resolution of 4 cm1, over the frequency range 400e4000 cm1. The specimens were prepared by mixing the ground samples with powdered potassium bromide and pressing the mixture under high pressure to obtain the pellet. Keithley 2611A source meter four probes IeV instrument was used to determine an electrical conductivity. The field emission current density-applied field (JeE) and current-time (Iet) measurements were carried at in all metal field emission microscope. The field emission studies were carried out in a ‘close proximity’ (also termed as ‘planar diode’) configuration, wherein the annealed aligned TiO2 nanotubes, GO/TiO2 and RGO/TiO2 nanocomposite films served as a cathode and a semi-transparent cathodoluminescent phosphor screen as an anode. Fig. 2 is a schematic of field emission diode assembly. The area of TiO2, GO/ TiO2 and RGO/TiO2 nanocomposite specimens was kept similar (0.25 cm2). The cathode, pasted onto a copper rod using vacuum compatible conducting silver paste, was held in front of the anode screen at a distance of ~1 mm. The cathode did not show any appreciable degassing and vacuum was obtained with usual speed. After baking the system at 150 C for 12 h, pressure of ~1 108 mbar was obtained. The JeE and Iet measurements were carried out at this base pressure using a Keithley Electrometer (6514) and a Spellman high voltage DC power supply (0e40 kV, U. S. A.). Special care was taken to avoid any leakage current by using shielded cables with proper grounding.
Fig. 2. Schematic of the field emission diode assembly.
169
3. Results and discussion Fig. 3 depicts the FESEM images of the aligned TiO2 nanotubes, GO/TiO2 and RGO/TiO2 specimens. Fig. 3a indicating large coverage of well aligned TiO2 nanotubes. Aligned TiO2 nanotubes with an average diameter of 90 nm, wall thickness of 14 nm and average length 1.5 mm have been recorded from Fig. 3b. An effect of applied potential on the morphology of the aligned TiO2 nanotubes has been investigated by electrophoretic technique, where using deionized water in the presence of TiO2 nanotubes only. FESEM images in Fig. 3c and d depict no morphological change of aligned TiO2 nanotubes which undergo electrophoresis treatment. Fig. 3e is a low magnification image of the GO/TiO2 nanocomposite. Formation of nanocomposite of GO and aligned TiO2 nanotubes has been observed. Close magnification image (Fig. 3f) confirms that aligned TiO2 nanotubes are covered completely by GO nanosheets. Fig. 3f shows most protruding TiO2 nanotubes and are denoted by arrow. Detailed analysis of FESEM results indicates that coating of few GO sheets on the aligned TiO2 nanotubes. TiO2 nanotubes are clearly seen even of the coating of GO nanosheets (Fig. 3f). Similar types of observations have been recorded for nanocomposite of RGO/TiO2. Fig. 3g and h indicate low magnification as well as close-up image of RGO/TiO2 nanocomposite. Since the RGO has a higher electrical conductivity as compared to the GO (Table 1) the thickness of sheets has been seen to be higher than the GO/TiO2 nanocomposite and are shown in Fig. 3g and h. More protruding or tapered RGO sheets have been seen to be dominated in case of RGO/TiO2 nanocomposite. XRD studies of the as-anodized TiO2 nanotubes, annealed TiO2 nanotubes and water treated TiO2 nanotubes have been recorded (not shown here). As-anodized TiO2 nanotubes are found to be amorphous in nature. The XRD of the annealed TiO2 nanotubes shows a polycrystalline nature with predominantly TiO2 Anatase (A) peak centered at 2Ɵ value of 25.31. The reflections due to the TiO2 peaks can be indexed as Anatase TiO2 with lattice constants a ¼ 3.782 Å and c ¼ 9.502 Å, which is consistent with the literature (JCPDS Card No. 84-1286). The other peaks are related to the Titanium (Ti) substrate and assigned those as Ti. The XRD pattern gives no indication of TiOxFy or TiOxSy and other impurity phases in the specimen, indicating that a considerable amount of the solvent may be trapped in the amorphous anodic film but do not enter at the Anatase lattice [23]. The XRD pattern of water treated TiO2 does not show any change in the lattice constant or ‘d’ spacing and/or any shift in the 2Ɵ value. Fig. 4 represent the XRD pattern of GO and RGO. [002] diffraction line for graphite is observed at 7.8 and 24 for GO and RGO, respectively. The diffraction line at a small angle for GO is attributed to large inter layer distance (d-spacing) between graphite sheets which may be due to the incorporation of functional groups by oxidation. For RGO [002] diffraction line is shifted to higher diffraction which is indicative of decrease in dspacing due to removal of functional groups after reduction. For the purpose of crystallinity of the aligned TiO2 nanotubes detail TEM and HRTEM study have been done. Bright field images of TiO2 nanotubes are shown in Fig. 5a and b. Fig. 5a and b indicates, hollow tube like morphology with an average diameter ~90 nm. Individual nanotube is shown as inset of Fig. 5b. Fig. 5c is the HRTEM image of an annealed TiO2 nanotube which indicate single crystalline nature of the TiO2 nanotube with fringe spacing of 0.35 nm along (101) plane [24]. SAED pattern shown as inset of Fig. 5c indicates polycrystalline nature of the TiO2 nanotubes. In Raman spectra of GO and RGO (Fig. 6), the characteristic peaks of graphite ca. 1597 cm1 (G-Band) and 1353 cm1 (D-band) are observed. D-band is an indicative of defects in graphite structure, however G-band is for its sp2 hybridized structure. Compared to GO, red shifting in G and D-bands of RGO suggests the large
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Fig. 3. FESEM images: (a) and (b) TiO2 nanotubes, (c) and (d) TiO2 nanotubes after water treatment, (e) low and (f) high magnification GO/TiO2 nanocomposite and (g) low and (h) high magnification RGO/TiO2 nanocomposite.
Table 1 Electrical conductivity measurements of GO and RGO. Sr no.
1 2
Specimen
GO RGO
Slope (A/V)
R(U) ¼ 1/slope (V/A)
7.40504 107 0.03228
1350431.5 30.9789
W ¼ thickness (meter)
W/S S ¼ 0.002M
0.00231 0.00170
1.155 0.85
proportions of sp2 structure due to removal of oxygenated functionalities from graphite surface [25]. Moreover, decrease in fullwidth at-half-maximum for RGO indicates the repairing of defects in its graphitic network [26]. Fig. 7 depicts the FTIR spectra of GO and RGO. Major bands at
G7 (W/S)
r0 (ohm meters)
r ¼ r0/G7 (W/S) (ohm meters)
s ¼ 1/r
1.373 1.714
16963.16 0.3892
12354.814 0.22734
8.09401 105 4.398667
(conductivity) (mhos/meter) or (Siemens/meter)
3350 cm1, 1750 cm1 and 1172 cm1 observed for GO can be attributed due to the stretching frequency of hydroxyl, carbonyl and CeO functionalities. All these vibrations were disappeared for RGO, due to the reduction by hydrazine. Moreover, intensity of bands at 1512 cm1 (for C]C) increases for RGO due to the re-
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Fig. 4. X-Ray diffractograms of GO & RGO.
establishment of sp2 network. IeV plots of GO and RGO are shown in Fig. 8. Electrical conductivity measurements of GO and RGO are shown in Table 1.
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The JeE characteristic of the aligned TiO2 nanotubes, GO/TiO2 and RGO/TiO2 nanocomposites is depicted in Fig. 9a. Turn-on field defined as the field required to draw an emission current density of 10 mA/cm2 is found to be 2.9 V/mm, 3.3 V/mm and 2.6 V/mm for aligned TiO2 nanotubes, GO/TiO2 and RGO/TiO2 nanocomposites respectively. As the field was increased further to 4.2 V/mm, 4.9 V/ mm, 3.6 V/mm for aligned TiO2 nanotubes, GO/TiO2 and RGO/TiO2 nanocomposites the emission current density of 80 mA/cm2 has been recorded. The turn-on field values of the aforesaid specimens have been seen to be quite superior as compared to the other TiO2 nanostructures as summarized in Table 2 [9,27e30]. Since there is no report on the field emission studies of GO/TiO2 or RGO/TiO2 nanocomposite the comparison has been done with TiO2 nanostructures. From Table 2 it is cleared that the RGO/TiO2 nanocomposite show superior field emission properties in terms of turnon field than the aligned TiO2 nanotubes and GO/TiO2 as well as other TiO2 nanostructures [9,27e30]. The observed field emission
Fig. 5. (a) and (b) Low magnification TEM images of the TiO2 nanotubes, TEM image of individual nanotube with high magnification is shown as inset of (b) (c) HRTEM image of the TiO2 nanotube with SAED pattern as inset.
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Fig. 6. Raman spectra of GO & RGO.
Fig. 9. (a) JeE plots of the aligned TiO2 nanotubes, GO/TiO2 and RGO/TiO2 nanocomposites and (b) corresponding FeN plots.
Fig. 7. FTIR spectra of GO & RGO.
properties of aligned TiO2 nanotubes, GO/TiO2 and RGO/TiO2 nanocomposites could be attributed due to the following reasons. (a) Field emission studies of TiO2 The turn-on field for aligned TiO2 nanotubes has been found to be 2.9 V/mm, for the emission current density of 10 mA/cm2. Iet plot
has been recorded for the preset value of 10 mA for the duration of 2 h. The above results show that the aligned TiO2 nanotube possesses better field emission properties in terms of turn-on field, emission current density and fairly good emission stability for the entire duration. The field emission properties are related to the morphology/geometry of the emitter/s. The observed turn-on field could be attributed due very sharp wall thickness of 14 nm of the aligned TiO2 nanotubes and also a large density of the aligned TiO2 nanotubes (~240 107/cm2). In addition, emission current may suppress due to screening effect. Finally, well adherent layer of TiO2 nanotubes film on Ti surface provides the easy electron percolation pathways that transport the electrons efficiently from the substrate to the emission sites [9], since aligned TiO2 nanotubes were grown directly on the Ti foil itself. (b) Field emission studies of GO/TiO2
Fig. 8. IeV plots of GO and RGO.
The turn-on field for GO/TiO2 nanocomposite has been found to be 3.3 V/mm, for the emission current density of 10 mA/cm2. The observed turn-on field is seen to be higher than aligned TiO2 nanotubes. From the FESEM images (Fig. 3b and e of the aligned TiO2 nanotubes and GO/TiO2) it has been clear that open and aligned ends of the TiO2 nanotubes have been completely covered with GO nanosheets. A high field emission current can be induced from the emitter only when it must have conical or very sharp protrusions. In case of GO/TiO2 nanocomposite, the more possible emitters of TiO2 i.e. nanotubes have been seen to be blocked by a few layer of GO sheets. GO is known to be an insulating material [31] which leads to the observation of high turn-on field as
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173
Table 2 Turn-on field values of the TiO2 nanostructures reported in the literature. Specimen
Turn-on field (V/mm) (for J ¼ 10 mA/cm2)
Reference
Aligned TiO2 nanotubes GO/TiO2 nanocomposite RGO/TiO2 nanocomposite Aligned TiO2 nanotubes N2 doped TiO2 TiO2 nanowires Quasi aligned TiO2 nanowires TiO2 nanotubes
2.9 3.3 2.6 7.8 11.2 5.7 3.1 23.8
Present work
compared to aligned TiO2 nanotubes. All covered nanosheets are tend to be lie more flatter on TiO2 nanotubes. Very few GO nanosheets are seen to be protruded due to very sharp edges marked by arrows (Fig. 3f) and seen to be responsible for the observation of field emission result. Hence, the turn-on field of GO/TiO2 nanocomposite has been seen to be increased as compared to the aligned TiO2 nanotubes specimen. GO is a sheet like structure having sp2 networks discontinued by oxygenated functionalities. Despite discontinuity, it possesses a large number of tiny, isolated sp2 domains those are individually conducting in nature due to persevered C]C skeleton. In fact 1512 cm1 band in FTIR and presence of G-band in Raman spectra clearly support the existence of sp2 domain in GO, those can impart the slight but non-zero conductivity. Concerning the practical evidence of conductivity we have given the IeV characteristics of GO, which clearly show the non-zero and finite conductivity (Table 1). Iet plot of GO/TiO2 nanocomposite was recorded at the preset current of 10 mA for the duration of 2 h and is shown in Fig. 10. The observed fluctuations in the Iet plot may be attributed due to the adsorption and diffusion of residual gas molecules on the emitter surface [32]. The striking feature of the field emission behavior of the GO/TiO2 nanocomposite is that the average emission current remains nearly constant over the entire duration and show no sign of degradation.
Fig. 10. Iet plots of the aligned TiO2, GO/TiO2 and RGO/TiO2 nanocomposites.
9 27 28 29 30
(c) Field emission studies of RGO/TiO2 The turn-on field for RGO/TiO2 nanocomposite has been found to be 2.6 V/mm, for the emission current density of 10 mA/cm2. The observed turn-on field is found to be lower than the aligned TiO2 nanotubes and GO/TiO2 nanocomposite. As seen from the FESEM images Fig. 3g and h it has been cleared that during formation of the nanocomposite of RGO/TiO2, the RGO nanosheets tend to lie tapered (are marked by arrows in FESEM image Fig. 3h) on the aligned TiO2 nanotubes instead of appearing flat with respect to the substrate. The emission current density of an emitter exponentially depends on the field enhancement factor b, which is defined by the shape and size (aspect ratio) of the emitter [33]. In the present study, we have estimated the field enhancement factor (b) from the slope of the FowlereNordheim (FeN) plot, which is mathematically expressed as,
b¼
3 6:8 103 ∅2 slope
Where ∅ ¼ Work function. By considering the work function of TiO2, GO and RGO as 4.4 eV, 4.9 eV [34] and 4.5 eV [35] the field enhancement factor b was calculated and found to be 2059, 1482 and 2300 for aligned TiO2 nanotubes, GO/TiO2 and RGO/TiO2 nanocomposite. The high value of field enhancement factor b for RGO/TiO2 may be attributed due to high density of protruded RGO nanosheets observed in the FESEM results (Fig. 3h). Estimation of the density of the nanoprotrusions/nanostructures has been done by calculating the number of the protruded emission sites observed in the FESEM images (Fig. 3f and h) of aligned TiO2 nanotubes, GO/TiO2 and RGO/TiO2 nanocomposites. Density of the nanoprotrusions and corresponding turn-on field of aligned TiO2 nanotubes, GO/TiO2 and RGO/TiO2 nanocomposites have been summarized in Table 3. A good correlation has been found between density of nanoprotrusions and the turn-on field of aligned TiO2 nanotubes, GO/TiO2 and RGO/TiO2 nanocomposites. Iet plot of RGO/TiO2 nanocomposite was recorded at the preset value of 10 mA for the duration of 2 h and is shown in Fig. 10. The small amount of increment or decrement in the emission current in the form of fluctuations may be attributed due to the adsorption and diffusion of residual gas molecules on the emitter surface [32]. The important feature of the field emission behavior of the RGO/ TiO2 nanocomposite is that the average emission current remains nearly constant over the entire duration and shows no sign of degradation. This is a very important feature, particularly from the practical application of the emitter material as an electron source [32]. The FeN plot, i.e., ln (J/E2) versus (1/E), derived from the observed JeE characteristic is shown in Fig. 9b. FeN plots of the aligned aligned TiO2 nanotubes, GO/TiO2 and RGO/TiO2
174
G.P. Patil et al. / Vacuum 123 (2016) 167e174 Table 3 The density of fabricated nanostructures with turn-on field. Specimen
Nanoprotrusions/cm2
Turn-on field (V/mm) (for J ¼ 10 mA/cm2)
Aligned TiO2 nanotubes GO/TiO2 nanocomposite RGO/TiO2 nanocomposite
240 107 1.4 107 3.2 107
2.9 3.3 2.6
nanocomposites exhibit non-linear nature over entire range of applied field with two distinct linear lines (shown in Fig. 9b). In principle, the field emitted electrons from a semiconducting emitter originate from both the conduction and the valence bands, as reported in our earlier studies [36e38]. The quantification of contribution of the valence and the conduction bands to the emission current mainly depends on their density of states and band gap energy. One of the striking features of wide band gap semiconducting emitters is observation of non-linear FeN plot over the entire applied field strength. 4. Conclusions GO/TiO2 and RGO/TiO2 nanocomposites have been synthesized by the simple technique of electrophoresis. FESEM images of the nanocomposite of GO/TiO2 and RGO/TiO2 indicate the formation of protruded emitters. RGO/TiO2 nanocomposite show the promising field emission behavior in terms of low turn-on field. Observation of low turn-on field may be attributed due to the high value of the field enhancement factor b as well as high electrical conductivity. The simplicity of the synthesis route with better field emission characteristics indicates that the RGO/TiO2 nanocomposite may be the promising candidate for field emission based applications. Acknowledgments GPP and PGC sincerely thank to SERB DST, Government of India (Ref. No.: SB/EMEQ-208/2013, dated 23/08/2013) for financial support. GPP and PGC also thank to UGCSAP-BSR Phase-III project for financial support. SRS gratefully acknowledges BARC, Mumbai, for the award of SRF under BARC-UoP memorandum (Grant No: GOI-E-153). References [1] S. Iijima, Nature 56 (1991) 354. [2] M.D. Shsinde, P.G. Chavan, G.G. Umarji, S.S. Arbuj, S.B. Rane, M.A. More, D.S. Joag, D.P. Amalnerkar, J. Nanosci. Nanotechnol. 12 (2012) 3788. [3] B.A. Kakade, V.K. Pillai, D.J. Late, P.G. Chavan, F.J. Sheini, M.A. More, D.S. Joag, Appl. Phys. Lett. 97 (2010) 073102. [4] K. Zhu, N.R. Neale, A. Miedaner, A.J. Frank, Nano. Lett. 7 (2007) 69. [5] M. Zlamal, J.M. Macak, P. Schmuki, J. Krysa, Electrochem. Commun. 9 (2007) 2822. [6] M. Paulose, O.K. Varghese, G.K. Mor, C.A. Grimes, K.G. Ong, Nanotechnology 17
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