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TiO2 nanocomposite
Chemical Physics Letters 657 (2016) 167–171
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Research paper
Low turn-on field and high field emission current density from Ag/TiO2 nanocomposite Girish P. Patil a, Amol B. Deore c, Vivekanand S. Bagal a,b, Dattatray J. Late d, Mahendra A. More c, 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 Management & Engineering, Shirpur Campus 425405, India Center for Advanced Studies in Materials Science and Condensed Matter Physics, Department of Physics, Savitribai Phule Pune University, Pune 411007, India d Physical and Materials Chemistry Division, CSIR – National Chemical Laboratory, Dr. Homi Bhabha Road, Pune 411008, Maharashtra, India b c
a r t i c l e
i n f o
Article history: Received 26 April 2016 In final form 1 June 2016 Available online 2 June 2016
a b s t r a c t High current density of 1.24 mA/cm2 was drawn at an applied field of 4.4 V/lm from Ag/TiO2 nanocomposite. Also the turn-on field has been reduced from 3.9 V/lm to 2.7 V/lm for the emission current density of 10 lA/cm2. Ag/TiO2 nanocomposite was synthesized by using UV-switchable reducing agent. TiO2 nanotube wall was decorated by Ag nanoparticles with average diameter of 17 nm. To the best of our knowledge this is the first report on the field emission studies of Ag/TiO2 nanocomposite. Simple synthesis route coupled with superior field emission properties indicate the possible use of Ag/TiO2 nanocomposite for micro/nanoelectronic devices. Ó 2016 Elsevier B.V. All rights reserved.
1. Introduction A wide band gap semiconductor Titanium Dioxide (TiO2), exhibit a wide range of applications such as photocatalyst [1], solar cell [2] and field emitter [3]. For specific application, various OneDimensional (1D) TiO2 nanostructures such as nanobelts [4], nanowires [5], nanorods [6] and nanotubes [7] have been synthesized and characterized. To achieve the desired properties for specific application it is often found that the precise control over doping of foreign materials is not always possible. However, same desired properties may also achieved by surface decoration/modification. Attempts have been made by the researcher to decorate the surface of TiO2 by noble metal such as Au [8], Ag [9] and Pt [10] for photocatalysis and visible light-induced photoactivity application. However, after vast literature survey it has been observed that field emission studies of the Ag/TiO2 nanocomposite are not reported earlier. Field emission is purely quantum mechanical tunneling phenomenon where electrons are emitted from the surface of nanomaterial under the action of the strong electrostatic field. In the last few decades, various materials with dimensions in nano regime have attracted considerable attention due to their superior field emission behavior. Such behavior may be due to their high aspect ratio and alignment [11–13]. These materials with good electron
⇑ Corresponding author. E-mail address: [email protected] (P.G. Chavan). http://dx.doi.org/10.1016/j.cplett.2016.06.003 0009-2614/Ó 2016 Elsevier B.V. All rights reserved.
emissive potential are found to be suitable for their use in the display industry and micro/nanoelectronic devices. To enhance the field emission current density of these materials, different alternative techniques are explored, like coating with low work function semiconductor material [14] or attaching appropriate metal nanoparticles [8,15] on the emitter surface. Ag nanoparticles have proved itself for various applications such as catalytics [16], electronics [17] and plasmonics [18] due to non-toxicity, good electrical and optical properties. Since field emission is surface sensitive phenomenon, such ‘desirable’ emitter can be developed by growing a well adherent Ag nanoparticles on the surface of TiO2 nanotubes. A vast literature survey, including recent reports [19–21] indicate that, field emission studies of Ag/TiO2 nanocomposite are not found to be explored. Hence, for the scientific and technological advancement exploration of the field emission studies of the Ag/TiO2 nanocomposite is important. In present study, Ag nanoparticles with average diameter of 17 nm are decorated on surface of aligned TiO2 nanotubes by simple and economic method. The turn-on field is found to be quite lower than pristine aligned TiO2 nanotubes and other doped TiO2 nanotubes reported in the literature. A possible reason behind the observed low turn-on field is discussed in detail. 1.1. Experimental Anodization method was used to synthesize aligned TiO2 nanotubes [22]. Prior to anodization, high purity titanium foil (99.7% purity, 0.25 mm thickness, Sigma Aldrich) was degreased in an
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G.P. Patil et al. / Chemical Physics Letters 657 (2016) 167–171
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 vol.% HF (40%) in Dimethyl Sulfoxide (DMSO) at constant Direct Current (DC) voltage of 30 V for 22 h. The color of the titanium foil surface was found to be yellowish after anodization. Finally, the as-anodized Ti foil was rinsed in deionized water and used for further characterization. Annealing of Ti foil was carried out at 530 °C for 3 h in air. Synthesis of Ag/TiO2 nanocomposite was done by using Ultra Violet (UV) switchable reducing agent [23]. The synthesis procedure of Ag/TiO2 nanocomposite is shown schematically in Fig. 1. In a typical experiment, annealed TiO2 nanotubes was immersed in a solution containing 10 mL of 1 102 M phosphotungstic acid (PTA) for 12 h. The specimen was then removed from the PTA solution and washed three times with deionized water to remove any unbound PTA molecules. For the reduction of PTA, PTAfunctionalized TiO2 nanotubes were placed in separate quartz beaker and immersed in 1 mL of isopropanol and 4 mL of deionized water before purging with N2 gas for 15 min and photoexciting the solution for 2 h using a UV lamp (kex = 254 nm). An appropriate metal salt of Ag2SO4 (9 mL of 1 102 M) was then added to the reduced TiO2-PTA nanotubes and allowed to mature for 2 h, the Ag/TiO2 nanocomposite was removed from solution and washed three times with deionized water. 1.2. Characterizations The surface morphology of TiO2 nanotubes and Ag/TiO2 nanocomposite were studied by using Field Emission Scanning Electron Microscope (FESEM) (Model Hitachi S-4800), phase identification of the TiO2 nanotubes and Ag/TiO2 nanocomposite were made by X-ray Diffraction (XRD) by D8 Advance, Bruker instrument. The field emission current density–applied field (J–E) and current–time (I–t) measurements were carried out in all metal field emission microscope by using ‘close proximity’ (also termed as ‘planar diode’) configuration, wherein the aligned TiO2 nanotubes and Ag/TiO2 nanocomposite specimen served as a cathode and a semi-transparent cathodoluminescent phosphor screen as an anode. The area of both the specimens was 0.25 cm2. The cathode, pasted onto a copper rod using vacuum compatible conduct-
ing 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 J–E and I–t measurements were carried out at this base pressure using a Keithley Electrometer (6514) and a Spellman high voltage DC power supply (0–40 kV, U.S.A.). Special care was taken to avoid any leakage current by using shielded cables with proper grounding.
2. Results and discussion 2.1. Morphological study Fig. 2(a) and (b) depicts the FESEM images of the aligned TiO2 nanotubes. Fig. 2(a) indicates the formation of tube like morphology of TiO2 with an average diameter of 96 nm. Smooth walled TiO2 nanotubes with average length of 3 lm are shown in Fig. 2 (a). FESEM images of Ag/TiO2 nanocomposite are shown in Fig. 2 (c) and (d). Fig. 2(c) and (d) indicating the decoration of Ag nanoparticles on the entire surface of TiO2 nanotubes. A careful analysis of Fig. 2(c) denotes that the average diameter of Ag nanoparticles is 17 nm.
2.2. Crystal structural study XRD studies of the as-anodized TiO2 nanotubes, annealed TiO2 nanotubes and Ag/TiO2 nanocomposite have been shown in Fig. 3. From XRD spectrum it is clear that as-anodized TiO2 nanotubes are amorphous in nature. The XRD spectra of annealed TiO2 nanotubes and Ag/TiO2 nanocomposite shows a polycrystalline nature with predominantly TiO2 Anatase (A) peak centered at a 2H value of 25.31°. The reflections due to the TiO2 peaks can be indexed to Anatase TiO2 with lattice constants a = 3.782 Å and c = 9.502 Å, (JCPDS Card No. 84-1286). The observed other peaks such as 33.05, 40.13, 52.95, 70.62, 76.21, 77.36 are analyzed and index to Titanium (Ti) (JCPDS data Card No. 894893). The peaks due to Ti are further assigned to T. In the XRD analysis of Ag/TiO2 nanocomposite, no any peaks related to the Ag are observed. This may be due to small Ag concentration in relation to the TiO2 nanotubes and detection limit of equipment employed [8].
Fig. 1. Schematic of synthesis of Ag/TiO2 nanocomposite.
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Fig. 2. (a and b) FESEM images of TiO2 nanotubes and (c and d) FESEM images of Ag/TiO2 nanocomposite.
Fig. 3. XRD pattern of as-anodized aligned TiO2 nanotubes, annealed aligned TiO2 nanotubes and Ag/TiO2 nanocomposite.
2.3. Field emission studies Fig. 4(a) shows J–E plots of the aligned TiO2 nanotubes and Ag/ TiO2 nanocomposite. From Fig. 4(a) it is observed that, the field emission performance is drastically improved by decorating the surface of TiO2 nanotubes with Ag nanoparticles. Turn-on field defined at a current density of 10 lA/cm2 is found to be 3.9 V/ lm and 2.7 V/lm for aligned TiO2 nanotubes and Ag/TiO2 nanocomposite respectively. High emission current density 1.2 mA/cm2 has been achieved from Ag/TiO2 nanocomposite upon the application of applied electric field of 4.4 V/lm. In case of TiO2
Fig. 4. (a) J–E plots of aligned TiO2 nanotubes and Ag/TiO2 nanocomposite and (b) corresponding F–N plots.
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nanotubes the maximum current density of 400 lA/cm2 has been achieved upon the application of applied electric field of 6.2 V/lm. The low turn-on field of Ag/TiO2 nanocomposite is found to be quite superior than pristine TiO2 nanotubes as well as doped TiO2 nanotubes reported in the literature as summarized in Table 1 [8,24–27]. Since there is no report on field emission studies of the Ag/TiO2 nanocomposite comparison has been done with doped TiO2 nanotubes. The field emission phenomenon is depends upon various factors such as, density of emitters, electrical conductivity, geometry, alignment, screening effect and electron transfer mechanism from substrate to emitter etc. In present case TiO2 nanotubes were grown directly on the Ti foil itself and then further, Ag nanoparticles has been decorated on the surface of TiO2 nanotubes. Therefore, 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 [28]. Moreover, the Ag nanoparticles may responsible for the development of high local electric field due to nanometric dimension (average diameter = 17 nm) of the silver particles present entirely on the tube surface. 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 [29]. The field enhancement factor (b) has been estimated from the slope of the F–N plot, which is mathematically expressed as, 3
b¼
ð6:8 103 Þ£2 slope
where £ = work function. For the calculation of field enhancement factor (b) of aligned TiO2 nanotubes and Ag/TiO2 nanocomposites the work function of TiO2 i.e. 4.4 eV [3] and Ag i.e. 4.26 eV [30] was used. Thus, the field enhancement factor (b) of aligned TiO2 nanotubes and Ag/TiO2 nanocomposite are found to be 1335, and 1988 respectively. The observed low turn-on field of Ag/TiO2 nanocomposite may also attributed due to the high value of field enhancement factor (b). The field emission characteristic is further analyzed by the Fowler–Nordheim (F–N) plot. The F–N plot, i.e. ln(J/E2) versus (1/E), derived from the observed J–E characteristic is shown in Fig. 4 (b). F–N plot shows an overall linear behavior in case of aligned TiO2 nanotubes as well as Ag/TiO2 nanocomposite which is in good agreement with the literature reports [8,14]. The I–t plot at the preset of 1 lA emission current for the duration of 3 h is shown in Fig. 5. From Fig. 5 it is clear that the emission current is almost stable for TiO2 nanotubes and Ag/TiO2 nanocomposite. The small amount of instabilities/fluctuations in the form of ‘spikes’ are observed in the emission current of Ag/TiO2 nanocomposite which may occurs due to adsorption, desorption of the residual gas molecules and phenomenon of ion bombardment [31]. Large surface
Table 1 Turn-on field values of the doped TiO2 nanotubes reported in the literature. Materials
Turn-on field (V/lm) (for J = 10 lA/cm2)
Reference
TiO2 nanotubes Ag/TiO2 nanocomposite Au/TiO2 nanocomposite Carbon-doped TiO2 nanotubes arrays Nitrogen-doped TiO2 nanotubes arrays Nitrogen-doped TiO2 nanotubes Fe-doped TiO2 nanotubes
3.9 2.7 2.8 5 6.54 11.2 12
Present work [8] [24] [25] [26] [27]
Fig. 5. I–t plots of aligned TiO2 nanotubes and Ag/TiO2 nanocomposite.
area of Ag nanoparticles in case of Ag/TiO2 nanocomposite leads to the observation of instabilities in the initial emission current. It has been found that current remains constant over the entire duration of measurement and shows no sign of degradation in case of TiO2 nanotubes as well as Ag/TiO2 nanocomposite. 3. Conclusion Significant improvement in field emission characteristics in terms of low turn-on field and emission current density, was observed. The observed turn-on field of Ag/TiO2 nanocomposite has been found to be superior than pristine TiO2 nanotubes and doped TiO2 nanotubes reported in the literature. The high field emission current density 1.2 mA/cm2 has been achieved upon the application of low applied electric field of 4.4 V/lm. The present study shows that the Ag/TiO2 nanocomposite has potential for their use in micro/nano electronic devices. Acknowledgments GPP and PGC sincerely thank to SERB-DST, Govt. of India (Ref. No.: SB/EMEQ-208/2013 dated 23/08/2013) for financial support. References [1] M. Zlamal, J.M. Macak, P. Schmuki, J. Krysa, Electrochem. Commun. 9 (2007) 2822. [2] K. Zhu, N.R. Neale, A. Miedaner, A.J. Frank, Nano Lett. 7 (2007) 69. [3] G.P. Patil, V.S. Bagal, C.R. Mahajan, V.R. Chaudhari, S.R. Suryavanshi, M.A. More, P.G. Chavan, Vacuum 123 (2016) 167. [4] W. Zhou, H. Liu, J. Wang, D. Liu, G. Du, J. Cui, A.C.S. Appl, Mater. Interfaces 2 (2010) 2385. [5] S.H. Wang, T.Y. Tsai, S.J. Chang, W.Y. Weng, S.P. Chang, C.L. Hsu, I.E.E.E. Elect, Dev. Lett. 35 (2014) 123. [6] B. Liu, E.S. Aydil, J. Am. Chem. Soc. 131 (2009) 3985.
G.P. Patil et al. / Chemical Physics Letters 657 (2016) 167–171 [7] M.Y. Vasilkov, F.S. Fedorov, N.M. Ushakov, S.Yu. Suzdaltsev, Tech. Phys. Lett. 41 (2015) 29. [8] G.P. Patil, V.S. Bagal, S.R. Suryawanshi, D.J. Late, M.A. More, P.G. Chavan, Appl. Phys. A 122 (2016) 560. [9] X. He, Y. Cai, H. Zhang, C. Liang, J. Mater. Chem. 21 (2011) 475. [10] N.T. Nguyen, J.E. Yoo, M. Altomarea, P. Schmuki, Chem. Commun. 50 (2014) 9653. [11] P.G. Chavan, R.V. Kashid, S.S. Badhade, I.S. Mulla, M.A. More, D.S. Joag, Vacuum 101 (2014) 38. [12] D.R. Shinde, P.G. Chavan, S. Sen, D.S. Joag, M.A. More, S.C. Gadkari, S.K. Gupta, A.C.S. Appl, Mater. Interfaces 3 (2011) 4730. [13] 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. [14] S.R. Suryawanshi, A.K. Singh, M. Deo, D.J. Late, S. Sinha, M.A. More, Cryst. Eng. Commun. 17 (2015) 3936. [15] B. Das, D. Sarkar, S. Maity, K.K. Chattopadhyay, J. Mater. Chem. C 3 (2015) 1766. [16] D. Enache, J.K. Edwards, P. Landon, B. Solsona-Espriu, A.F. Carley, A.A. Herzing, M. Watanabe, C.J. Kiely, D.W. Knight, G.J. Hutchings, Science 311 (2006) 362. [17] X. Jiang, Q. Xiong, S. Nam, F. Qian, Y. Li, C.M. Lieber, Nano Lett. 7 (2007) 3214. [18] S.H. Choi, H.B. Na, Y.I. Park, K. An, S.G. Kwon, Y. Jang, M.H. Park, J. Moon, J.S. Son, I.C. Song, W.K. Moon, T. Hyeon, J. Am. Chem. Soc. 130 (2008) 15573. [19] G. Mittal, I. Lahiri, J. Phys. D Appl. Phys. 47 (2014) 323001.
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[20] X. Fang, Y. Bando, U.K. Gautam, C. Ye, D. Golberg, J. Mater. Chem. 18 (2008) 509. [21] T. Zhai, L. Li, Y. Ma, M. Liao, X. Wang, X. Fang, J. Yao, Y. Bando, D. Golberg, Chem. Soc. Rev. 40 (2011) 2986. [22] B.H. Ma, Y. Li, C.W. Wang, J. Wang, J.B. Chen, W.M. Liu, Acta Phys. Sin. 57 (2008) 5800. [23] A. Pearson, H. Zheng, K. Kalantar-zadeh, S.K. Bhargava, V. Bansal, Langmuir 28 (2012) 14470. [24] L.Q. Wang, C.W. Wang, J.B. Chen, R.S. Guo, F. Zhou, W.M. Liu, Thin Solid Films 519 (2011) 8173. [25] R.P. Antony, T. Mathews, K. Panda, B. Sundaravel, S. Dash, A.K. Tyagi, J. Phys. Chem. C 116 (2012) 16740. [26] G. Liu, F. Li, D.W. Wang, D.M. Tang, C. Liu, X. Ma, G.Q. Lu, H.M. Cheng, Nanotechnology 19 (2008) 025606. [27] C.C. Wang, K.W. Wang, T.P. Perng, Appl. Phys. Lett. 96 (2010) 143102. [28] J.B. Chen, C.W. Wang, B.H. Ma, Y. Li, J. Wang, R.S. Guo, W.M. Liu, Thin Solid Film 517 (2009) 4390. [29] R. Devarapalli, R. Kashid, A. Deshmukh, P. Sharma, M. Das, M. More, M. Shelke, J. Mater. Chem. C. 1 (2013) 5040. [30] M. Misra, P. Kapur, M.K. Nayak, M. Singla, New J. Chem. 38 (2014) 4197. [31] G. Sinha, S.K. Panda, A. Datta, P.G. Chavan, D.R. Shinde, M.A. More, D.S. Joag, A. Patra, ACS Appl. Mater. Interfaces 3 (2011) 2130.
Contents lists available at ScienceDirect
Chemical Physics Letters journal homepage: www.elsevier.com/locate/cplett
Research paper
Low turn-on field and high field emission current density from Ag/TiO2 nanocomposite Girish P. Patil a, Amol B. Deore c, Vivekanand S. Bagal a,b, Dattatray J. Late d, Mahendra A. More c, 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 Management & Engineering, Shirpur Campus 425405, India Center for Advanced Studies in Materials Science and Condensed Matter Physics, Department of Physics, Savitribai Phule Pune University, Pune 411007, India d Physical and Materials Chemistry Division, CSIR – National Chemical Laboratory, Dr. Homi Bhabha Road, Pune 411008, Maharashtra, India b c
a r t i c l e
i n f o
Article history: Received 26 April 2016 In final form 1 June 2016 Available online 2 June 2016
a b s t r a c t High current density of 1.24 mA/cm2 was drawn at an applied field of 4.4 V/lm from Ag/TiO2 nanocomposite. Also the turn-on field has been reduced from 3.9 V/lm to 2.7 V/lm for the emission current density of 10 lA/cm2. Ag/TiO2 nanocomposite was synthesized by using UV-switchable reducing agent. TiO2 nanotube wall was decorated by Ag nanoparticles with average diameter of 17 nm. To the best of our knowledge this is the first report on the field emission studies of Ag/TiO2 nanocomposite. Simple synthesis route coupled with superior field emission properties indicate the possible use of Ag/TiO2 nanocomposite for micro/nanoelectronic devices. Ó 2016 Elsevier B.V. All rights reserved.
1. Introduction A wide band gap semiconductor Titanium Dioxide (TiO2), exhibit a wide range of applications such as photocatalyst [1], solar cell [2] and field emitter [3]. For specific application, various OneDimensional (1D) TiO2 nanostructures such as nanobelts [4], nanowires [5], nanorods [6] and nanotubes [7] have been synthesized and characterized. To achieve the desired properties for specific application it is often found that the precise control over doping of foreign materials is not always possible. However, same desired properties may also achieved by surface decoration/modification. Attempts have been made by the researcher to decorate the surface of TiO2 by noble metal such as Au [8], Ag [9] and Pt [10] for photocatalysis and visible light-induced photoactivity application. However, after vast literature survey it has been observed that field emission studies of the Ag/TiO2 nanocomposite are not reported earlier. Field emission is purely quantum mechanical tunneling phenomenon where electrons are emitted from the surface of nanomaterial under the action of the strong electrostatic field. In the last few decades, various materials with dimensions in nano regime have attracted considerable attention due to their superior field emission behavior. Such behavior may be due to their high aspect ratio and alignment [11–13]. These materials with good electron
⇑ Corresponding author. E-mail address: [email protected] (P.G. Chavan). http://dx.doi.org/10.1016/j.cplett.2016.06.003 0009-2614/Ó 2016 Elsevier B.V. All rights reserved.
emissive potential are found to be suitable for their use in the display industry and micro/nanoelectronic devices. To enhance the field emission current density of these materials, different alternative techniques are explored, like coating with low work function semiconductor material [14] or attaching appropriate metal nanoparticles [8,15] on the emitter surface. Ag nanoparticles have proved itself for various applications such as catalytics [16], electronics [17] and plasmonics [18] due to non-toxicity, good electrical and optical properties. Since field emission is surface sensitive phenomenon, such ‘desirable’ emitter can be developed by growing a well adherent Ag nanoparticles on the surface of TiO2 nanotubes. A vast literature survey, including recent reports [19–21] indicate that, field emission studies of Ag/TiO2 nanocomposite are not found to be explored. Hence, for the scientific and technological advancement exploration of the field emission studies of the Ag/TiO2 nanocomposite is important. In present study, Ag nanoparticles with average diameter of 17 nm are decorated on surface of aligned TiO2 nanotubes by simple and economic method. The turn-on field is found to be quite lower than pristine aligned TiO2 nanotubes and other doped TiO2 nanotubes reported in the literature. A possible reason behind the observed low turn-on field is discussed in detail. 1.1. Experimental Anodization method was used to synthesize aligned TiO2 nanotubes [22]. Prior to anodization, high purity titanium foil (99.7% purity, 0.25 mm thickness, Sigma Aldrich) was degreased in an
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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 vol.% HF (40%) in Dimethyl Sulfoxide (DMSO) at constant Direct Current (DC) voltage of 30 V for 22 h. The color of the titanium foil surface was found to be yellowish after anodization. Finally, the as-anodized Ti foil was rinsed in deionized water and used for further characterization. Annealing of Ti foil was carried out at 530 °C for 3 h in air. Synthesis of Ag/TiO2 nanocomposite was done by using Ultra Violet (UV) switchable reducing agent [23]. The synthesis procedure of Ag/TiO2 nanocomposite is shown schematically in Fig. 1. In a typical experiment, annealed TiO2 nanotubes was immersed in a solution containing 10 mL of 1 102 M phosphotungstic acid (PTA) for 12 h. The specimen was then removed from the PTA solution and washed three times with deionized water to remove any unbound PTA molecules. For the reduction of PTA, PTAfunctionalized TiO2 nanotubes were placed in separate quartz beaker and immersed in 1 mL of isopropanol and 4 mL of deionized water before purging with N2 gas for 15 min and photoexciting the solution for 2 h using a UV lamp (kex = 254 nm). An appropriate metal salt of Ag2SO4 (9 mL of 1 102 M) was then added to the reduced TiO2-PTA nanotubes and allowed to mature for 2 h, the Ag/TiO2 nanocomposite was removed from solution and washed three times with deionized water. 1.2. Characterizations The surface morphology of TiO2 nanotubes and Ag/TiO2 nanocomposite were studied by using Field Emission Scanning Electron Microscope (FESEM) (Model Hitachi S-4800), phase identification of the TiO2 nanotubes and Ag/TiO2 nanocomposite were made by X-ray Diffraction (XRD) by D8 Advance, Bruker instrument. The field emission current density–applied field (J–E) and current–time (I–t) measurements were carried out in all metal field emission microscope by using ‘close proximity’ (also termed as ‘planar diode’) configuration, wherein the aligned TiO2 nanotubes and Ag/TiO2 nanocomposite specimen served as a cathode and a semi-transparent cathodoluminescent phosphor screen as an anode. The area of both the specimens was 0.25 cm2. The cathode, pasted onto a copper rod using vacuum compatible conduct-
ing 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 J–E and I–t measurements were carried out at this base pressure using a Keithley Electrometer (6514) and a Spellman high voltage DC power supply (0–40 kV, U.S.A.). Special care was taken to avoid any leakage current by using shielded cables with proper grounding.
2. Results and discussion 2.1. Morphological study Fig. 2(a) and (b) depicts the FESEM images of the aligned TiO2 nanotubes. Fig. 2(a) indicates the formation of tube like morphology of TiO2 with an average diameter of 96 nm. Smooth walled TiO2 nanotubes with average length of 3 lm are shown in Fig. 2 (a). FESEM images of Ag/TiO2 nanocomposite are shown in Fig. 2 (c) and (d). Fig. 2(c) and (d) indicating the decoration of Ag nanoparticles on the entire surface of TiO2 nanotubes. A careful analysis of Fig. 2(c) denotes that the average diameter of Ag nanoparticles is 17 nm.
2.2. Crystal structural study XRD studies of the as-anodized TiO2 nanotubes, annealed TiO2 nanotubes and Ag/TiO2 nanocomposite have been shown in Fig. 3. From XRD spectrum it is clear that as-anodized TiO2 nanotubes are amorphous in nature. The XRD spectra of annealed TiO2 nanotubes and Ag/TiO2 nanocomposite shows a polycrystalline nature with predominantly TiO2 Anatase (A) peak centered at a 2H value of 25.31°. The reflections due to the TiO2 peaks can be indexed to Anatase TiO2 with lattice constants a = 3.782 Å and c = 9.502 Å, (JCPDS Card No. 84-1286). The observed other peaks such as 33.05, 40.13, 52.95, 70.62, 76.21, 77.36 are analyzed and index to Titanium (Ti) (JCPDS data Card No. 894893). The peaks due to Ti are further assigned to T. In the XRD analysis of Ag/TiO2 nanocomposite, no any peaks related to the Ag are observed. This may be due to small Ag concentration in relation to the TiO2 nanotubes and detection limit of equipment employed [8].
Fig. 1. Schematic of synthesis of Ag/TiO2 nanocomposite.
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Fig. 2. (a and b) FESEM images of TiO2 nanotubes and (c and d) FESEM images of Ag/TiO2 nanocomposite.
Fig. 3. XRD pattern of as-anodized aligned TiO2 nanotubes, annealed aligned TiO2 nanotubes and Ag/TiO2 nanocomposite.
2.3. Field emission studies Fig. 4(a) shows J–E plots of the aligned TiO2 nanotubes and Ag/ TiO2 nanocomposite. From Fig. 4(a) it is observed that, the field emission performance is drastically improved by decorating the surface of TiO2 nanotubes with Ag nanoparticles. Turn-on field defined at a current density of 10 lA/cm2 is found to be 3.9 V/ lm and 2.7 V/lm for aligned TiO2 nanotubes and Ag/TiO2 nanocomposite respectively. High emission current density 1.2 mA/cm2 has been achieved from Ag/TiO2 nanocomposite upon the application of applied electric field of 4.4 V/lm. In case of TiO2
Fig. 4. (a) J–E plots of aligned TiO2 nanotubes and Ag/TiO2 nanocomposite and (b) corresponding F–N plots.
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nanotubes the maximum current density of 400 lA/cm2 has been achieved upon the application of applied electric field of 6.2 V/lm. The low turn-on field of Ag/TiO2 nanocomposite is found to be quite superior than pristine TiO2 nanotubes as well as doped TiO2 nanotubes reported in the literature as summarized in Table 1 [8,24–27]. Since there is no report on field emission studies of the Ag/TiO2 nanocomposite comparison has been done with doped TiO2 nanotubes. The field emission phenomenon is depends upon various factors such as, density of emitters, electrical conductivity, geometry, alignment, screening effect and electron transfer mechanism from substrate to emitter etc. In present case TiO2 nanotubes were grown directly on the Ti foil itself and then further, Ag nanoparticles has been decorated on the surface of TiO2 nanotubes. Therefore, 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 [28]. Moreover, the Ag nanoparticles may responsible for the development of high local electric field due to nanometric dimension (average diameter = 17 nm) of the silver particles present entirely on the tube surface. 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 [29]. The field enhancement factor (b) has been estimated from the slope of the F–N plot, which is mathematically expressed as, 3
b¼
ð6:8 103 Þ£2 slope
where £ = work function. For the calculation of field enhancement factor (b) of aligned TiO2 nanotubes and Ag/TiO2 nanocomposites the work function of TiO2 i.e. 4.4 eV [3] and Ag i.e. 4.26 eV [30] was used. Thus, the field enhancement factor (b) of aligned TiO2 nanotubes and Ag/TiO2 nanocomposite are found to be 1335, and 1988 respectively. The observed low turn-on field of Ag/TiO2 nanocomposite may also attributed due to the high value of field enhancement factor (b). The field emission characteristic is further analyzed by the Fowler–Nordheim (F–N) plot. The F–N plot, i.e. ln(J/E2) versus (1/E), derived from the observed J–E characteristic is shown in Fig. 4 (b). F–N plot shows an overall linear behavior in case of aligned TiO2 nanotubes as well as Ag/TiO2 nanocomposite which is in good agreement with the literature reports [8,14]. The I–t plot at the preset of 1 lA emission current for the duration of 3 h is shown in Fig. 5. From Fig. 5 it is clear that the emission current is almost stable for TiO2 nanotubes and Ag/TiO2 nanocomposite. The small amount of instabilities/fluctuations in the form of ‘spikes’ are observed in the emission current of Ag/TiO2 nanocomposite which may occurs due to adsorption, desorption of the residual gas molecules and phenomenon of ion bombardment [31]. Large surface
Table 1 Turn-on field values of the doped TiO2 nanotubes reported in the literature. Materials
Turn-on field (V/lm) (for J = 10 lA/cm2)
Reference
TiO2 nanotubes Ag/TiO2 nanocomposite Au/TiO2 nanocomposite Carbon-doped TiO2 nanotubes arrays Nitrogen-doped TiO2 nanotubes arrays Nitrogen-doped TiO2 nanotubes Fe-doped TiO2 nanotubes
3.9 2.7 2.8 5 6.54 11.2 12
Present work [8] [24] [25] [26] [27]
Fig. 5. I–t plots of aligned TiO2 nanotubes and Ag/TiO2 nanocomposite.
area of Ag nanoparticles in case of Ag/TiO2 nanocomposite leads to the observation of instabilities in the initial emission current. It has been found that current remains constant over the entire duration of measurement and shows no sign of degradation in case of TiO2 nanotubes as well as Ag/TiO2 nanocomposite. 3. Conclusion Significant improvement in field emission characteristics in terms of low turn-on field and emission current density, was observed. The observed turn-on field of Ag/TiO2 nanocomposite has been found to be superior than pristine TiO2 nanotubes and doped TiO2 nanotubes reported in the literature. The high field emission current density 1.2 mA/cm2 has been achieved upon the application of low applied electric field of 4.4 V/lm. The present study shows that the Ag/TiO2 nanocomposite has potential for their use in micro/nano electronic devices. Acknowledgments GPP and PGC sincerely thank to SERB-DST, Govt. of India (Ref. No.: SB/EMEQ-208/2013 dated 23/08/2013) for financial support. References [1] M. Zlamal, J.M. Macak, P. Schmuki, J. Krysa, Electrochem. Commun. 9 (2007) 2822. [2] K. Zhu, N.R. Neale, A. Miedaner, A.J. Frank, Nano Lett. 7 (2007) 69. [3] G.P. Patil, V.S. Bagal, C.R. Mahajan, V.R. Chaudhari, S.R. Suryavanshi, M.A. More, P.G. Chavan, Vacuum 123 (2016) 167. [4] W. Zhou, H. Liu, J. Wang, D. Liu, G. Du, J. Cui, A.C.S. Appl, Mater. Interfaces 2 (2010) 2385. [5] S.H. Wang, T.Y. Tsai, S.J. Chang, W.Y. Weng, S.P. Chang, C.L. Hsu, I.E.E.E. Elect, Dev. Lett. 35 (2014) 123. [6] B. Liu, E.S. Aydil, J. Am. Chem. Soc. 131 (2009) 3985.
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