Titanium Oxynitride Nanoparticles Anchored on Carbon Nanotubes as

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Titanium Oxynitride Nanoparticles Anchored on Carbon Nanotubes as Energy Storage Materials Litao Yan, Gen Chen, Shuai Tan, Meng Zhou, Guifu Zou, Shuguang Deng, Sergei N Smirnov, and Hongmei Luo ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.5b07630 • Publication Date (Web): 16 Oct 2015 Downloaded from http://pubs.acs.org on October 16, 2015

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ACS Applied Materials & Interfaces

Titanium Oxynitride Nanoparticles Anchored on Carbon Nanotubes as Energy Storage Materials Litao Yan,a+ Gen Chen,a+ Shuai Tan,b Meng Zhou,a Guifu Zou,c Shuguang Deng,a Sergei Smirnov,d and Hongmei Luo*,a

a

Department of Chemical and Materials Engineering, New Mexico State

University, New Mexico 88003, United States. b

School of Chemical & Biomolecular Engineering, Georgia Institute of

Technology, Atlanta, Georgia 30332, United States. c

College of Physics, Optoelectronics and Energy & Collaborative Innovation

Center of Suzhou Nano Science and Technology, Soochow University, Suzhou 215000, P. R. China. d

Department of Chemistry and Biochemistry, New Mexico State University, Las

Cruces, New Mexico 88003, United States.

+

Contribute equally to this work.

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ABSTRACT Sub-8 nm titanium oxynitride (TiON) nanoparticles are uniformly formed on the surface of carbon nanotubes (CNTs) by annealing amorphous TiO2 (a-TiO2) conformally coated CNTs (CNTs/a-TiO2) at 600 °C in ammonia gas. The novel CNTs/TiON nanocomposite is systematically characterized by X-ray diffraction (XRD), high resolution transmission electron image (HRTEM), scanning transmission electron microscopy-energy dispersive spectroscopy (STEM-EDX), and X-ray photoelectron spectroscopy (XPS). The results show that the elements of Ti, O and N are homogeneously distributed in TiON nanoparticles. The specific capacitance of CNTs/TiON exhibits 187 F g-1 at a current density of 0.5 A g-1, much higher than that of CNTs (33.4 F g-1) and CNTs/TiO2 (83.4 F g-1) obtained by annealing CNTs/a-TiO2 at 450 °C in nitrogen gas. CNTs/TiON also exhibits enhanced cycle durability, which enables its application as a promising candidate for supercapacitors.

KEYWORDS:

carbon

nanotubes;

conformal

coating;

titanium

oxynitride;

supercapacitor; electrochemical performance


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1. INTRODUCTION Compared with traditional secondary batteries, supercapacitors including pseudocapacitors and electrical double-layer capacitors offer the advantages of high power densities, fast dynamics of charge transport and excellent cyclic performance.1-3 The main classes of electrode materials for supercapacitors include various forms of carbon, conductive polymers, and transition metal oxides and sulfides.4-6 Among the carbon materials, carbon nanotubes (CNTs) are considered as a promising electrode material due to their high fraction of surface area accessible for the formation of electric double-layer.1,7,8 Recently, CNTs decorated with transition metal oxide/nitride nanoparticles emerged as an attractive opportunity in the development of supercapacitors. Such CNTs/metal oxide/nitride hybrids were reported to have high supercapacitance and superior electrochemical performance.6,9-12 For example, CNTs/TiO2 has been extensively investigated as an electrode material for supercapacitor since TiO2 has the advantages of stability, low cost, and environmental friendliness,13-16 and the CNTs/TiO2 hybrid exhibited enhanced supercapacitance compared with CNTs.15 In addition, titanium oxynitride (TiON) or titanium nitride (TiN) were proposed as novel electrode materials because of their high thermal and chemical stability and high electrical conductivity.10,17-22 TiON or TiN are usually prepared by a direct thermal nitridation of TiO2 in an ammonia gas atmosphere at elevated temperatures.21 Lee et al. successfully fabricated freestanding TiON sheets by nitridation of interlacing titanate fibers and achieved a specific capacitance of 120.9 F g-1 at a current density of 1.25 A 3 ACS Paragon Plus Environment


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g-1 in 2 M H2SO4.17 Assembly of free-standing TiN nanowires on carbon cloth was demonstrated as high-performance and flexible electrode of supercapacitors by Li et al.19 Graphene and TiN nanotube composite array electrode was synthesized by a simple adsorption-reduction process and demonstrated a higher capacitance than pure TiN.20 To our best knowledge, no relevant research on combining CNTs and TiON have been reported for supercapacitor application. In this paper, amorphous TiO2 was first conformally coated on the surface of CNTs.15 Then the CNTs/a-TiO2 was treated with ammonia gas to form the carbon nanotubes/titanium oxynitride composite (CNTs/TiON), while CNTs/a-TiO2 was treated under nitrogen gas to form the carbon nanotubes/titanium oxide composite (CNTs/TiO2), respectively. CNTs/TiON exhibited a higher capacitance than CNTs and CNTs/TiO2. The specific capacity of this novel supercapacitor reached as high as 187 F g-1 at a current density of 0.5 A g-1, and the enhanced electrochemical performance demonstrated that it is a promising supercapacitor electrode.

2. EXPERIMENTAL SECTION 2.1 Materials synthesis and characterization. 0.1 g of CNTs (from Pyrograf Products) was treated in a strong acid consisting of 20 mL of HNO3 and 60 mL of H2SO4 at 60 °C for 2 h to functionalize CNTs with hydrophilic groups. The modified CNTs were dispersed in a mixture of 20 mL of anhydrous ethanol and 1.5 mL of deionized (DI) water under ultrasonication and stirring. The preparation process of conformal amorphous TiO2 coating on the surface of CNTs is the same as our 4 ACS Paragon Plus Environment

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previously reported CNTs/a-TiO2.15,23 Briefly, 50 mg of functional CNTs was dispersed into DI water and ethanol solution. The diluted titanium isopropoxide was dropped into CNTs solution slowly. After string for 4 hours, the samples were collected by filtration and then dried in the vacuum oven. The amorphous CNTs/a-TiO2 hybrid was either annealed at 450 °C for 2 h in atmosphere of flowing N2 to get CNTs/TiO2, or in flowing ammonia gas at 600 °C for 2 h to form CNTs/TiON. The structure, composition, chemical bonding, valence state of the elements and morphology of the samples were characterized by X-ray diffraction (XRD, Rigaku Miniex-II with Cu Kα (1.5406 Å) radiation, 30 kV/15 mA), transmission electron microscope (TEM; JEOL-2010, 200 kV), Fourier transform infra-red (FTIR) spectroscopy (using KBr pellets, Bruker Vertex 80v optical bench), thermogravimetric analysis (TGA) (Perkin-Elmer Pyris 1 in the temperature range of 25-800 °C at a heating rate of 10 °C min-1 in air) and X-ray photoelectron spectroscopy (XPS,

Thermo K-Alpha spectrometer equipped with a monochromatic Al kα X-ray source), respectively. The high angle annular dark-ﬁeld scanning transmission electron microscopy (HAADF-STEM) image and energy dispersive spectroscopy (EDS) mapping images were taken on an atomic resolution analytical microscope (JEOL JEM-ARF200F). 2.2 Supercapacitor test. Electrode active materials (90 mg), isopropyl alcohol (400 µL), and PVDF (10 mg) were mixed and stirred overnight to get a homogeneous slurry ink. The carbon paper (Toray carbon paper 120, TORAYCA), used as electrochemical substrate, was coated carefully with the slurry ink and the weight 5 ACS Paragon Plus Environment


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loading of as-prepared electrodes was measured in the range 0.5-1.0 mg cm-2. The three-electrode cells were employed to examine the electrochemical performance including cyclic voltammetry (CV) measurements, electrochemical impedance spectra (EIS) and cyclic performance in 1 M H2SO4 electrolyte solution. Pt wire and an Ag/AgCl served as the counter electrode and the reference electrode, respectively. CV and galvanostatic charge/discharge measurements were performed in the cutoff voltage window of 0.1–1 V. EIS was measured using a CHI-680A workstation.

3. RESULTS AND DISCUSSION Figure 1a shows the typical XRD patterns of CNTs/TiO2 and CNTs/TiON. All the peaks for CNTs/TiO2 can be assigned to the anatase TiO2 phase. The peak around 2θ = 26° for CNTs could not be seen clearly since the lattice plane of (111) for anatase TiO2 phase is very close to that of (002) for graphite carbon. However, for CNTs/TiON, the peak of graphite carbon is clearly observed, indicating a phase and crystal structure change from that of TiO2. In fact, all peaks except for carbon’s at 26°, i.e., at 37, 42.9, 62.6, and 75 degrees, are corresponding to cubic phase of TiOxNy, which peaks are located between those of TiN and TiO. The calculated lattice constant of TiOxNy, a = 4.212 Å, is also between those of TiN (a = 4.241 Å) and TiO (a = 4.177 Å), again, demonstrating that the sample may be a solid solution between TiO and TiN, or titanium oxinitride (labeled TiOxNy). The values of x = 0.453 and y = 0.547 were calculated using Vegard’s law.24 The concentration of CNTs in the CNTs/TiON is determined to be 27% by TGA (Figure S1). Figure 1b and 1c show the 6 ACS Paragon Plus Environment

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typical TEM images of CNTs/TiON. We previously showed that amorphous TiO2 was uniformly coating the surface of CNTs after controllable hydrolysis of titanium precursor.15 Apparently, after heating in ammonia gas, amorphous TiO2 layer converted to TiON nanoparticles that remained on the surface of CNTs. Such successful preparation of uniform sub-8 nm TiON on the surface of CNTs was possible due to the initial conformal TiO2 coating on the surface of CNTs. The conversion of TiON nanoparticles from layered TiO2 could be ascribed to the high annealing temperature from TiO2 to TiON, which leads to the aggregation of TiON nanoparticles and results in the morphological evolution. High resolution transmission electron microscopy (HRTEM) image reveals that TiON has a lattice fringes of 0.245 nm, as shown in Figure 1d, which can be indexed as (111) lattice plane of TiON. Clear crystalline domains of TiON nanoparticles are highlighted with yellow circles, indicating the conversion of amorphous TiO2 to highly crystalline TiON phase. STEM-EDX (Figure 1e) was also used to investigate the chemical element distribution in CNTs/TiON. The elemental maps clearly show that Ti, O, and N are evenly distributed in the TiON nanoparticles on the surface of CNTs. HADDF-EELS also confirms that Ti, O, and N elements are uniformly distributed in the TiON particles, as shown in Figure 2. XPS, another powerful technique to examine the chemical composition and the oxidation state, was used to characterize the effect of nitridation. The XPS results for CNTs/TiON and CNTs/TiO2 are shown in Figure S2a and S2b, respectively. The two main peaks at around ~459 and ~465 eV are the characteristic peaks of Ti 2p3/2 and 7 ACS Paragon Plus Environment


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2p1/2. A peak at ~396 eV corresponding to N 1s can be also seen in the sample after nitridation at 600 °C, as shown in Figure S2a. Since such peak was not seen for the CNTs/TiO2 sample, it indicates again that N dopes into TiO2 to form TiON during nitridation process. High resolution XPS can further characterize the details about valence state of different elements. As shown in Figure 3, positions of the Ti 2p peak for CNTs/TiON is broader and shifted toward lower binding energies as compared to CNTs/TiO2, indicating that the oxidation state of Ti is reduced in the former. The Ti 2p broad peaks in nitridated sample can be represented by a superposition of peaks at 459, 456, 457 eV, assigned to Ti–O, Ti–N, and Ti–N–O, respectively. It was reported that H2 and N2 from the thermal decomposition of NH3 at high temperature introduced these three different bonding states of Ti in the nitridated sample.21 Partial reduction of Ti4+ by hydrogen can facilitate formation of oxygen vacancies and penetration of nitrogen into the TiO2 crystal structure.21 The peak for Ti–O–Ti bonding at around 530 eV appears in both CNTs/TiO2 and CNTs/TiON samples, suggesting that titanium oxide remains after our high temperature nitridation process. Figure S2c shows that the N 1s spectrum of CNTs/TiON can also be split into two primary components, at 396.5 eV and 397.3, corresponding to TiN and TiON, respectively. The minor peak at 398.5 eV could be assigned to the adsorbed N-contained species in CNTs/TiON sample. Figure S2d shows the C 1s fine scan of CNTs/TiON. Except for the main C-C peaks around 284.7 eV, two small peaks at 285.8 and 287.5 eV may be corresponding to bonding of C=N and C-N, respectively. FTIR was also used to investigate the chemical bonding information in the CNTs/TiON and CNTs/TiO2, as 8 ACS Paragon Plus Environment

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shown in Figure S3. It is clearly observed that the FTIR profile of CNTs/TiON is very different from that of CNTs/TiO2 with obvious peaks around 1420 cm-1 corresponding to C-N bond. Therefore, by annealing CNTs/a-TiO2 in ammonia gas flow at high temperature, nitrogen not only could be doped into CNTs but also can lead to the conversion from a-TiO2 to TiON. The charge–discharge curves for electrodes with CNTs/TiON, CNTs/TiO2, and the functionalized CNTs at the same current density in 1.0 M H2SO4 electrolyte are shown in Figure 5a. The calculated specific capacitances of the CNTs/TiON, CNTs/TiO2, and functionalized CNTs electrodes at the current density of 0.5 A g-1 were measured to be 187, 83.4, and 33.4 F g-1, respectively. The performance of our CNTs/TiON was higher that of others’ TiO2 based electrodes, as showed in Table S1. The CNTs/TiON electrode exhibits the highest specific capacitance among the three. It has been reported that the electrode with TiO2/TiN core-shell structure showed higher specific supercapacitance than that of pure TiO2 spheres.21 This was mainly attributed to the conducting TiN shell on TiO2 particles allowing fast transfer of electrons and resulting in improving the electronic transmission and substantially shortening the diffusion path of the ions. In our experiments, CNTs/TiO2 electrode exhibited a much better specific capacitance than CNTs. The reason may be that introduction of TiO2 on the surface of CNTs can modify their hydrophobic properties, and thus increase the specific capacitance.15 However, the low electronic conductivity of TiO2 thin coating on CNTs still limits the charge transfer and thus does not sufficiently improve the specific capacity, as shown in Figure 5. On the other hand, 9 ACS Paragon Plus Environment


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TiON improves electrical conductivity for nitrogen content greater that 40% and reaches close to metallic conductivity of TiN for nitrogen content exceeding 60%.25 During the nitridation process, exposed Ti serves as an active site to anchor ammonia molecules through Lewis-acid base interaction. Decomposition of ammonia at the surface of TiO2 promotes the O/N replacement in the TiO2 which eventually leads to its conversion into TiON. Ti gets partially reduced and Ti 2p in CNTs/TiON shifted to the lower energies compared with its oxidation state in TiO2. The excess electrons resulted from the reduction of Ti and introduction of nitrogen will cause doping effect and necessarily increase the conductivity. Besides, instead of thin TiO2 layer coating CNTs, TiON form nanoparticles on the surface of CNTs, which improves access of the electrolyte to CNTs and increases the overall contact area with electrolyte solution and electrode, as schematically illustrated in Figure 5. It is reported that the nitrogen doped CNTs can increase the capacitance comparing with pure CNTs due to the fact that it not only increases the capacitance and electrical conductivity but also improves the wettability of the material in the electrolyte and thus enhances the ion transfer efficiency.26 In addition, the excellent electronic conductivity of TiON could also increase the charge transfer. Consequently, introducing nitrogen into both CNTs and TiO2 may increase the wettability of the materials in the electrolyte and electronic conductivities. The electron and ion transfer efficiency were enhanced, resulting in the improved capacitance performance. Figure 4b shows the galvanostatic charge–discharge curves of CNTs/TiON supercapacitor in the same voltage window at different current densities. The typical 10 ACS Paragon Plus Environment

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characteristic of an ideal supercapacitor was obtained from linear and symmetrical charge–discharge curves at various current densities. The specific capacities of CNTs/TiON are 187, 142 and 122 F g-1 at current densities of 0.5, 1, and 2 A g-1, respectively, in agreement with typically observed decrease of the specific capacitance with increasing the current densities. The specific capacity of CNTs/TiON is higher than that of CNTs/TiO2, demonstrating that TiON is superior in enhancing the specific capacity of the CNT based supercapacitor. CV measurements of CNTs/TiON, CNTs/TiO2, and CNTs electrodes were performed in a potential window from 0.1 to 1.0 V at a scan rate of 100 mV s-1, as shown in Figure 4c. The CV curves for both CNTs/TiO2 and CNTs/TiON electrodes show a nearly symmetrical shape over the full scan range, even at a high scanning rate of 100 mV s-1, indicating a pseudo ideal double layer electrochemical behavior with typical supercapacitance. The functionalized CNT electrode, on the other hand, shows an obvious redox peak around 0.3 V apparently due to functional groups such as hydroxyl and carbonic functional active in this potential range.15 Normalized by the area, the CNTs/TiON electrode delivers the highest current response among the three electrodes. The high current response of CNTs/TiON may be attributed to its structure and enhanced electronic conductivity, as discussed above. The CV curves of these three electrodes in a potential window from 0.1 to 1.0 V at the scan rates of 5, 10, 20, 50, 100 mV s-1 are displayed in Figure 4d and Figure S4, respectively. As expected, the current responses for all the electrodes are increased with increasing of the scan rates. Again, CNTs/TiO2 and CNTs/TiON electrodes demonstrate approximately 11 ACS Paragon Plus Environment


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rectangular symmetric shape, while the functionalized CNTs electrode has redox peak around 0.3 V for at all scan rates. The cycling stability of the CNTs/TiON electrode was performed in a 1.0 M H2SO4 electrolyte at a current density of 1.0 A g-1 by repeating the charge-discharge processes. Figure 6 shows the specific capacitance of the CNTs/TiON electrode as function of cycle numbers. The specific capacitance decreases to 80% of the initial after 1000 cycles. No increased capacity was found from the reaction between water and TiON at the initial cycles, indicating good cycle stability at the operation conditions. The cycling results in our experiments were similar to those reported by Tian et al,20 for TiN/graphene composites and the fair cycle durability indicates a strong connection between TiON and the CNTs framework. EIS measurements were further applied to characterize the electrochemical properties of these three different electrodes. Figure 7 shows the Nyquist plots of the EIS of the three electrodes, CNTs/TiON, CNTs/TiO2 and CNTs. A similar bulk solution resistance Rs seen as the intercept in the EIS spectra in the high-frequency region could be observed among the three samples and so is Warburg resistance Rw caused by diffusion and appearing as the slope of line in the low-frequency region. On the other hand, a charge-transfer resistance Rct caused by kinetic processes at the electrode and corresponding to the diameter of a semi-circle in high-frequency region, is quite different. As can be seen, CNTs/TiON electrode exhibits overall lower resistance than that of functionalized CNTs and the CNTs/TiO2 electrode, resulting in faster charge transport.


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4. CONCLUSION In summary, we have successfully prepared sub-8 nm TiON nanoparticles anchored on the surface of CNTs after annealing amorphous TiO2 conformal coating of CNTs in ammonia gas. The material characterizations including XRD, HRTEM, STEM-EDX and HAADF-EELS revealed that the amorphous TiO2 was converted to crystalline TiON nanoparticles after the annealing process and elements of Ti, O and N are homogeneously distributed in the TiON nanoparticles. The specific capacity of the CNTs/TiON supercapacitor reached as high as 187 F g-1 at a current density of 0.5 A g-1, which is double of the value of CNTs/TiO2. The enhanced cycling performance of CNTs/TiON demonstrates that it is promising supercapacitor material, which may be attributed to its structure and enhanced electronic conductivity from TiON as compared to TiO2. AUTHOR INFORMATION Corresponding Author *Tel: 575-646-4204, Fax: 575-646-7706. E-mail: [email protected] (H. Luo) Notes The authors declare no competing financial interest.

ACKNOWLEDGEMENTS G.C. greatly thanks 2015 ECS Edward G. Weston Summer Fellowship. H.L. acknowledges the funding support from National Science Foundation DMR-1449035 and the Vice President for Research office at NMSU. G.Z acknowledges the support 13 ACS Paragon Plus Environment


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from 973 Program Special Funds for the Chief Young Scientist (2015CB358600), the Excellent Young Scholar Fund from National Natural Science Foundation of China (21422103), the Jiangsu Fund for Distinguished Young Scientist (BK20140010), and the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD). Supporting Information Available. TGA profile of CNTs/TiON. XPS spectra of CNTs/TiON, CNTs/TiO2, N 1s and C1s. FTIR profile of CNTs/TiO2 and CNTs/TiON. CV curves of CNTs and CNTs/TiO2 at different scanning rates. The performance of TiO2 based electrode. This material is available free of charge via the Internet at http://pubs.acs.org


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20. Tian, F.; Xie, Y.; Du, H.; Zhou, Y.; Xia, C.; Wang, W., Preparation and Electrochemical Capacitance of Graphene/titanium Nitride Nanotube Array. RSC Adv. 2014, 4, 41856-41863.

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Figure Caption Figure 1. XRD patterns of CNTs/TiON and CNTs/TiO2 (a); TEM images of CNTs/TiON (b, c); High resolution TEM images of TiON (d); STEM-EDX mapping of CNTs/TiON (e), and the element distribution of nitrogen (yellow), titanium (green), and oxygen (blue). Figure 2. HADDF-EELS images of TiON nanoparticles (a) with nitrogen (yellow) (b), titanium (green) (c), and oxygen (blue) (d). Figure 3. High resolution of XPS for Ti in CNTs/TiO2 (a) and CNTs/TiON (b). Figure 4. (a) Galvanostatic charge–discharge curves of CNTs, CNTs/TiO2, and CNTs/TiON at a current density of 0.5 A g-1; (b) Galvanostatic charge–discharge curves of CNTs/TiON at different current densities; (c) CV curves of CNTs, CNTs/TiO2 and CNTs/TiON at a scan rate of 100 mV s-1; and (d) CV curves of CNTs/TiON at different scanning rates. Figure 5. Schematic of charge transfer of CNTs (a), CNTs/TiO2 (b) and CNTs/TiON (c). Figure 6. The retention of the CNTs/TiON electrode as a function of cycle number. Figure 7. EIS of CNTs, CNTs/TiO2 and CNTs/TiON.


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Figure 1



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Figure 2


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Figure 3



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Figure 4


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Figure 6


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Titanium Oxynitride Nanoparticles Anchored on Carbon Nanotubes as

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