Tunable Synthesis of Colorful Nitrogen-Doped Titanium Oxide and Its

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Tunable Synthesis of Colorful Nitrogen-doped Titanium Oxide and Its Application in Energy Storage Xin Wang, Xiaotao Yuan, Dong Wang, Wujie Dong, Chenlong Dong, Yajing Zhang, Tianquan Lin, and Fuqiang Huang ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.7b00308 • Publication Date (Web): 07 Feb 2018 Downloaded from http://pubs.acs.org on February 11, 2018

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ACS Applied Energy Materials

Tunable Synthesis of Colorful Nitrogen-doped Titanium Oxide and Its Application in Energy Storage Xin Wanga, ‡, Xiaotao Yuana, ‡, Dong Wangb, Wujie Donga, Chenlong Donga, Yajing Zhangb, Tianquan Linc,* and Fuqiang Huanga, c,* a

State Key Laboratory of Rare Earth Materials Chemistry and Applications and

National Laboratory, College of Chemistry and Molecular Engineering, Peking University, Beijing 100871, P.R. China. b

School of Science, China University of Geosciences, Beijing 100083, P.R. China.

c

State Key Laboratory of High Performance Ceramics and Superfine Microstructures,

Shanghai Institute of Ceramics, Chinese Academy of Sciences, Shanghai 200050, P.R. China. ‡ Xin Wang and Xiaotao Yuan contribute equally.

ABSTRACT: The one-pot synthesis of titania with diverse degrees of oxygen vacancies and nitrogen dopants through arc-discharge and nitridation process is first reported. The series of TiO2-x:N samples are prepared by tuning the ratio of CO2/H2 in the chamber. The chemical composition, microstructure and valence state of TiO2-x:N are characterized by a variety of measurements. Specifically, the as-prepared samples achieve the highest specific capacitance (210 F g−1 at 2 mV s−1), which is much higher than that of TiO2-x and commercial P25. Moreover, it exhibits good cycling stability with 9% attenuation of capacitance after 10,000 cycles. The capacitive enhancement can be attributed to more active pseudo-capacitive properties and improved electrical conductivity due to oxygen vacancies and nitrogen dopants. This work provides another feasible path to simplify the tunable synthesis of titania with different oxygen defects massively, and further optimize the degree of nitrogen dopants in order to realize better performance in the future.

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Keyword: arc-discharge method, oxygen vacancy, nitrogen dopant, titanium dioxide, supercapacitor Graphical Abstract:

1. Introduction Energy crisis is considered to be one of the greatest challenges in 21st century.1 Recently, supercapacitors, due to their larger power density than lithium-ion batteries and higher energy density than traditional dielectric capacitors, are extensively reported to be a promising candidate for the energy storage devices applicable for electric vehicle.2-4 Currently, transition metal oxides (for example RuO2, MnO2)5-6 have been vastly applied in supercapacitors owing to their high pseudocapacitance through surface redox reactions.7 However, these metal oxides are suffered from the poor electrical conductivity8-9 and the irreversibility of Faradic reactions on the surface of electrode,10 resulting in the low life cycle. Although the electrochemical stability can be enhanced by combining metal oxides with conductive materials (CNT, graphene, etc.),9,

11-12

it is desirable to improve the electrical conductivity and

capacitive behaviour of metal oxide. Titanium oxide, which has been widely applied as photocatalysts in the environmental purification, water splitting and dye-sensitised solar cell,13-19 is catching intensive interest in the energy storage, including lithium-ion batteries20 and supercapacitors.21 In comparison with state-of-the-art RuO2, titanium oxide is cheap,


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easy to synthesize with excellent chemical stability. However, current capacitances of titanium oxides are not comparable to other metal oxides (e.g. RuO2, MnO2)6 due to their poor electronic conductivity and inferior electrochemical activity.22, 23 Although many attempts have been explored to improve its electrochemical performance, such as

hydrogenation8,

24-26

and

nitridation,21,

27-29

the

process

are

usually

difficult-controlling and low-efficiency.30, 31 Herein, we proposed a fast, one-step and tunable approach to obtain titanium oxides with oxygen defect and nitrogen dopants via an arc-discharge process and subsequent nitridation.32-35 Through controlling the atmosphere in the arc-discharge process, the degree of oxygen vacancies of titania can be tuned with the different appearance. The electrical conductivity and electrochemical performance of these titanium oxides regulate along with the oxygen defects and nitrogen dopants. Specifically, the as-prepared TiO2-x:N achieves the impressive specific capacitance of 210 F g−1 at 2 mV s−1, which is significantly higher than that of nano-TiO2 (P25). Furthermore, the TiO2-x:N with great stability up to 10,000 cycles reveals its prospective candidate as one of energy storage materials in future.

2. Experimental Section Synthesis of nitrogen-doped titanium oxides: The titania samples were fabricated via arc-discharge method similar to what we reported previously.35-37 The anode and cathode were the titanium and graphite rod, respectively. The chamber was first vacuumed and filled with different gas to a certain pressure. The current kept at 40 A during the whole process. The distance between two rods kept less than 1.0 mm by adjusting the cathode. While the arc-discharge completed, the product was collected from the chamber wall, and that was the as-prepared TiO2 (TO-1) and TiO2-x(TO-2~TO-7). The corresponding specific synthesis conditions were shown in Table S1. The nitrogen-doped TiO2 was synthesized using the TO-6 sample by the ammonia heat treatment at different temperatures for 4 hours under the flow of 300 sccm, as shown in Table S2.



Material Characterization: Transmission electron microscopy (TEM) and the corresponding selected area electron diffraction (SAED) are conducted using a JEM-2100 electron microscope (JEOL Ltd.). Scanning electron microscopy (SEM) is conducted by a Supra 55 microscope (Carl Zeiss, Germany) at 1 kV. High-angle annular bright-field scanning transmission electron microscopy (HAABF-STEM) and corresponding elemental mapping were tested on a JEM-2100F using the STEM mode. X-ray diffraction (XRD) is measured at 6° min−1 on a Bruker D2 Focus diffractometer with Cu Kα radiation (λ=1.5418 Å). Raman spectra are collected by a Renishaw in Via Raman Microscope at 532 nm. X-ray photoelectron spectroscopy (XPS) is performed on an Axis Ultra imaging photoelectron spectrometer (Kratos Analytical Ltd.) using a Al Kα anode. Electrical conductivity is tested by the double-electrode system. The materials pressed into tablet are clamped using stainless sheet and indium to eliminate the contact resistance.

Electrochemical Characterization: Electrochemical performances are conducted using an electrochemical workstation (CHI760E). Properties of materials are measured using three-electrode configuration with a working electrode (embedded in a 3D graphene current collector), a Pt wire as the counter electrode and an Ag/AgCl as the reference electrode. 1 M H2SO4 and 3 M Li2SO4 solution are used as the electrolyte. Graphene foam was grown by chemical vapor deposition on a fugitive nickel foam38, which was later removed by 0.5M HCl etching. A mixture slurry of material, carbon black and PVDF (the weight ratio of 8:1:1) dispersed in the NMP solvent is coated onto a porous 3D-graphene sheet (loading mass of 0.5 mg cm−2). The electrochemical performances are carried out with cyclic voltammetry (CV), galvanostatic charge/discharge (CC) and corresponding electrochemical impedance spectroscopy (EIS). The EIS is tested at the open circuit voltage (vs. Ag/AgCl) with an amplitude (5 mV), and the frequency used in the study is 10−2 Hz~105 Hz. The ohmic contribution is obtained from its Nyquist plot. In order to investigate the durability, we probe the durability by CV at 100 mV s−1 for 10,000 cycles in 1 M H2SO4. The details about the thermodynamics and kinetics characteristic of TiO2-x:N see in the Supplementary information. ACS Paragon Plus Environment

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3. Results and discussions TiO2-x and TiO2-x:N samples are prepared via arc-discharging method and subsequent nitridation. The XRD patterns for the as-synthesized materials are shown in Figure 1a. The major reflections of TiO2-x and TiO2-x:N samples can be indexed with a P42/mnm space group corresponding to rutile TiO2, in good agreement with JCPDS No. 21−1276. For a comparison, TiO2-x is oxidized to TiO2, which possesses the phases of rutile and anatase TiO2. It can be seen that the phase of TiO2-x turned to rutile from anatase with the increase of reduced atmosphere (Figure S1). As for the TO-6 sample, the phase of TiO2-x:N samples with different nitridation temperatures turns from the dominant rutile TiO2 to dominant TiO (Figure S2). The crystallites size can be estimated from the XRD patterns by the Scherrer Equation (1):

‫=ܦ‬

௄×ఒ ఉ×௖௢௦ఏ

,

(1)

Where D is the average particle size, K is a shape factor (0.89), λ is the wavelength, β is the width at half maximum and θ represents the Bragg angle.39-41 The average particle sizes of TiO2, TiO2-x, TiO2-x:N and P25 calculated by Scherrer Equation are shown in Table S3. The titanium oxides with different oxygen vacancy and nitrogen dopant reveal different colours as shown in Figure 1c. P25 shows pure white, the titanium oxides synthesized by arc-discharge method show light grey to deep blue as the oxygen vacancy increase, and turn to black when the nitrogen dopant exists. The UV-vis spectra (Figure 1b) can also coincide with the colour transition. The absorption at the range of visible light apparently enhanced compared with P25, just as the reported properties of black titania.42



Figure 1. Structure and nanostructure of titania with tunable oxygen vacancy and nitrogen dopant. XRD patterns (a), UV-vis spectra (b) and digital photographs (c) of TiO2, TiO2-x, TiO2-x:N and P25 samples.

The morphologies of TiO2-x and TiO2-x:N are characterized by SEM and TEM. The TiO2-x is spherical nanoparticle with the average size of 45 nm (Figure 2a & 2b). The HRTEM image of TiO2-x shows the obvious lattice fringes corresponding to the (101) plane of anatase TiO2 and (110) plane of rutile TiO2. The HRTEM image of TiO2-x:N also shows the obvious lattice fringes corresponding to rutile TiO2. Another evidence of the phase for TiO2-x and TiO2-x:N is the selected area electron diffractions (Figure S3), which correspond with the rutile TiO2.


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Figure 2. Morphology of titania with oxygen vacancy and nitrogen dopant. (a) SEM of TiO2-x; (b) Transmission electron microscope of TiO2-x, inset: the particle diameter histogram of TiO2-x nanoparticles; HRTEM image of TiO2-x (c) and TiO2-x:N (d).

The change of diverse TiO2-x phases and valences state is supported by the Raman spectra (Figure 3a) clearly. The Raman peaks at 149, 398, 517 and 639 cm−1 can be considered respectively as the Eg (149 and 639 cm−1), B1g (398 cm−1), A1g and B1g (516 cm−1) modes of the anatase TiO2.43,44 As shown in the Raman patterns, the peaks at 424 cm−1 and 609 cm−1 correspond to Eg and A1g modes of rutile TiO2, which is in agreement with the XRD result. Moreover, with the increasing amount of reduced atmosphere, the intensity of rutile modes (Eg and A1g) tends to be stronger, while that of anatase modes (Eg at 149 cm−1) becomes weaker.45,46 The XPS of TiO2-x and TiO2-x:N are given in Figure 3 and Figure S4. The peaks located at ca. 457.1 and 463.2 eV are considered to the TiO 2p3/2 and 2p1/2, respectively. And the peaks located at ca. 458.8 and 464.2 eV are attributable to the TiO2 2p3/2 and 2p1/2, respectively. The additional peaks (455.3 and 461.1 eV) are attributable to TiN 2p3/2 and 2p1/2, respectively. Two peaks of N 1s at the binding energies of 396.4 and 398.1 eV are observed for TiO2-x:N (Figure 3c). The former is assigned to the substitutional



nitrogen (Ti-N) and the latter is attributable to interstitial nitrogen (Ti-O-N).47,48

Figure 3. Nanostructure and valence state of titania with oxygen vacancy and nitrogen dopant. (a) Raman result of TiO2, TiO2-x, TiO2-x:N and P25 samples; XPS spectrum of (b) Ti 3d and (c) N 1s of TiO2-x:N.

Electrochemical performances of the TiO2-x:N have been performed with a three-electrode system in 1 M H2SO4. Figure 4 show the CC and CV curves of TiO2-x:N electrode in 1 M H2SO4 at the different current densities and scan rates. The rectangle characteristic CV curves and linear CC curves indicate that the electrical double-layer capacitor (EDLC) dominates for TiO2-x:N. Compared with the P25 and blank, TiO2-x and TiO2-x:N electrode possess obvious larger capacitance, as shown in Figure 4b. And the specific capacitance can reach to 210 F g−1 for TiO2-x:N at 2 mV s−1, which is tenfold of TiO2 (Figure 4c). From the CV and CC data, TiO2-x:N provides great rate capability up to 500 mV s−1 or 50 A g−1. Superior stability is an important influence for practical supercapacitor electrode materials. Therefore, the TiO2-x:N electrode is evaluated using the CV mode at 50 mV s−1 for 10,000 cycles. The performance of the TiO2-x:N electrode gradually increases at 1,000 cycles because of an activation process. After 10,000 cycles, the capacitance maintains 91.6 % to the largest capacitance, and there is almost no obvious capacitance attenuation, indicating its superior stability. Electrochemical impedance spectroscopy is further performed to research the nature of the enhanced specific capacitance for the TiO2-x:N (Figure 4f). The intercept on the real axis indicates the equivalent series resistance (Rs), which contains the electroactive material’s resistances, electrolyte’s resistances and contact resistances. The Rs value of the TiO2-x:N is found to be 2.10 Ω in 1 M H2SO4. In addition to, the charge transfer resistance (Rct) is obtained from the semicircle’s


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diameter in the high-frequency range. Furthermore, the Nyquist plot of TiO2-x:N at the low frequency area displays a straight line perpendicular to the real axis, which shows its superb supercapacitor behavior. In order to research the effect of oxygen vacancies and nitrogen dopants to the conductivity, electrical conductivities of TiO2, TiO2-x and TiO2-x:N are evaluated (Figure S6). TiO2-x:N possesses the highest conductivity of 408.8 mS m−1 than that of other two samples, which is considered to the incorporation of the oxygen vacancy and nitrogen dopant.49

Figure 4. Electrochemical performance of TiO2-x:N in the three-electrode system. (a) The electrochemical performance of TiO2-x:N at different scan rates by the CV mode; (b) CV curves of TiO2-x:N, TiO2-x, TiO2 and P25 scanned at 2 mV s−1. TiO2 and P25 show no activity; (c) The comparison of specific capacitance and TiO2, TiO2-x, TiO2-x:N and P25 with scan rate; (d) CC curves of TiO2-x:N electrode tested at the different current densities; (e) Durability test of TiO2-x:N electrode for 10,000 cycles at 50 mV s−1 using CV mode; (f) Nyquist plots of the TiO2-x:N in 1 M H2SO4 at opening circuit voltage. Inset: zoom-in of the high frequency region.

In order to analyze the storage mechanism of TiO2-x:N, the corresponding Tafel plots are employed under the steady state case. Noting a strong sensitivity in room-temperature TiO2-x:N electrode potential to the steady-state current (Figure 5a) and pH (Figure 5b), we assume reversible proton incorporation/chemisorption29 to the TiO2-x:N causing charge compensation by electron inflow from the external circuit. Specifically, we envision that each nitrogen can absorb two protons via TiO2-xNy + zH+ + ze− ↔ TiO2-xNy-z/2(NH2)z/2

(2)

Thus, the potential ϕ providing a driving force ∆G = –zF + RTln(a(H+)) gives



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rise to a steady-state current i ∝exp(−∆G/RT) or i = io a ( H + ) exp( −

zF φ ) RT ,

(3)

Here, F is the Faraday constant and io is a proportionality constant. Comparing the slope values in Figure 3a with the theoretical value (2.3×RT/F=59.2 mV/decade), we determined z=2 for TiO2-x:N, confirming the beneficial two-proton absorption effect of N-doping that is available to TiO2-x:N. The Tafel plots provide thermodynamic understanding of the redox reactions at electrodes. To shed light onto the kinetics of the redox reaction, we analyzed the current/capacitance dependence on charging/discharging rate to distinguish the surface and the sub-surface contributions.30 In support of this and consistent with Fig. 3b, data from the same CV tests plotted as C against v−1/2 in Figure 5c show a very gradual transition instead of the bilinear behavior expected from Equation 4. Capacitance (C) is the time derivative of charge CV, obey the following relation: భ

C = ݇ଵ + ݇ଶ ߥ ିమ ,

(4)

In the above, k1 and k2 are constants for the surface and sub-surface charge storage, respectively. Therefore, the distinction between the two storage modes in TiO2-x:N is smeared. We suggest that this is because, at intermediate charging rates, some sub-surface regions are also available for charge storage in a rate-independent manner due to very rapid (proton) diffusion. As a result, some sub-surface regions become a part of the apparent surface, and the thickness of such region gradually increases with decreasing rates. If we assign the extrapolated C at infinite v (zero v−1/2) in Figure 5c to the apparent surface (rate-independent) capacitance, we obtain k1=150 F g−1, which is about 76 % of the capacitance value at the scan rate of 2 mV s−1. In addition to, plotting i against v in Figure 5d, we can find a power law i ~ vb with an exponent b of 0.82. This is in contrast to the two distinct exponents of 1 and 0.5 observed for lithium insertion into nanostructured Nb2O514, which is expected from Equation 5:

݅ = ܽߥ ௕ ,

(5)

Where ν is the scan rate, i is the current, a and b are constants. When the b value


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approaches 1, the rate-independent surface reaction (classically attributed to electrical double-layer capacitor) dominates the electrochemical reaction. While the b value approaches 0.5, it is dominated by the rate-dependent diffusion process (classically attributed to faradaic pseudocapacitance). The b value (0.82) indicates that the capacitance of TiO2-x:N is dominated by the electrical double-layer capacitor. Taken all the thermodynamic and kinetic evidence together, we suggest that TiO2-x:N owes its superior charge storage capacity and fast charging/discharging kinetics to its nanostructure, conductivity and strong N-enabled additional proton adsorption, which allow facile, almost rate-independent proton exchange to penetrate beneath the surface.

Figure 5. Mechanistic Studies of Electrochemical Reaction. (a) TiO2-x:N electrode potential against current density measured in same electrolyte at pH=6.8; (b) TiO2-x:N electrode potential against pH in KH2PO4/K2HPO4 electrolyte at steady-state current density of 1 µA cm−2; (c) TiO2-x:N electrode capacitance versus (scanning rate)−1/2 in 1 M H2SO4. Straight-line fit (dashed line) extrapolates to rate-independent capacitance in fast-charging limit; (d) Plot of TiO2-x:N electrode current density versus sweep rate at 0.35 VAg/AgCl in 1 M H2SO4 electrolyte.

In summary, we demonstrated the facile one-pot synthesis of colorful titanium oxides through an arc-discharge process and subsequent nitridation treatment for the



first time. More importantly, the degree of oxygen vacancies could be tuned by changing the oxidation of reaction atmosphere. Furthermore, the specific capacitance increased with the enhancement of oxygen vacancies density, while the over-reduction of titanium oxide will also decrease the materials’ electrochemical activities. Typically, the samples achieved the largest specific capacitance of 210 F g−1 at 2 mV s−1, which is significantly higher than P25. The improvement in capacity can be attributed to more active pseudocapacitive properties and increased electrical conductivity because of oxygen vacancies in the materials. Moreover, the TiO2-x:N exhibited good stability with 9 % reduction of capacitance after 10,000 cycles. This work may help to simplify the tunable synthesis of titanium oxides with oxygen defects, and further explore the proper degree of oxygen vacancies of TiO2-x in order to realize better performance of titanium oxides as the electrodes of supercapacitors in the future.

ASSOCIATED CONTENT

Supporting Information Available: The synthesis condition of TiO2 samples with different oxygen vacancy; The synthesis condition of nitrogen-doped TiO2 samples with different conditions; XRD patterns of different synthesis condition of TiO2 and TiO2-x samples; XRD pattern of TiO2-x:N sample with different nitridation temperature; The SAED image of TiO2-x and TiO2-x:N; XPS spectrum of TiO2-x and TiO2-x:N; The CV curves of P25, TiO2, TiO2-x and blank 3D graphene current collector in the 1M H2SO4 electrolyte at the different scan rates; Electrical conductivity of TiO2, TiO2-x and TiO2-x:N. See DOI: 10.1039/x0xx00000.

AUTHOR INFORMATION

Corresponding Author * Address correspondence to [email protected] and [email protected] Notes The authors declare no competing ﬁnancial interest. ACS Paragon Plus Environment

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ACKNOWLEDGMENT This work was financially supported from the National Key Research and

Development Program (Grant 2016YFB0901600), NSF of China (Grant 51672301), Science and Technology Commission of Shanghai (Grants 16JC1401700 and 16ZR1440500), the Key Research Program of Chinese Academy of Sciences (Grant QYZDJ-SSW-JSC013), Youth Innovation Promotion Association CAS and CAS Center for Excellence in Superconducting Electronics.

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Graphical Abstract 483x206mm (150 x 150 DPI)


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Figure 1 275x175mm (150 x 150 DPI)



Figure 2 246x239mm (150 x 150 DPI)


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Figure 3 363x110mm (150 x 150 DPI)



FIgure 4 344x177mm (150 x 150 DPI)


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Figure 5 244x171mm (150 x 150 DPI)


Tunable Synthesis of Colorful Nitrogen-Doped Titanium Oxide and Its

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