Plasma-Induced Oxygen Vacancies in Urchin-Like Anatase Titania

Subscriber access provided by READING UNIV

Article

Plasma-Induced Oxygen Vacancies in Urchin-Like Anatase Titania Coated by Carbon for Excellent Sodium-Ion Battery Anodes Qingmeng Gan, Hanna He, Kuangmin Zhao, Zhen He, Suqin Liu, and Shuping Yang ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.7b13760 • Publication Date (Web): 17 Jan 2018 Downloaded from http://pubs.acs.org on January 17, 2018

Just Accepted “Just Accepted” manuscripts have been peer-reviewed and accepted for publication. They are posted online prior to technical editing, formatting for publication and author proofing. The American Chemical Society provides “Just Accepted” as a free service to the research community to expedite the dissemination of scientific material as soon as possible after acceptance. “Just Accepted” manuscripts appear in full in PDF format accompanied by an HTML abstract. “Just Accepted” manuscripts have been fully peer reviewed, but should not be considered the official version of record. They are accessible to all readers and citable by the Digital Object Identifier (DOI®). “Just Accepted” is an optional service offered to authors. Therefore, the “Just Accepted” Web site may not include all articles that will be published in the journal. After a manuscript is technically edited and formatted, it will be removed from the “Just Accepted” Web site and published as an ASAP article. Note that technical editing may introduce minor changes to the manuscript text and/or graphics which could affect content, and all legal disclaimers and ethical guidelines that apply to the journal pertain. ACS cannot be held responsible for errors or consequences arising from the use of information contained in these “Just Accepted” manuscripts.

ACS Applied Materials & Interfaces is published by the American Chemical Society. 1155 Sixteenth Street N.W., Washington, DC 20036 Published by American Chemical Society. Copyright © American Chemical Society. However, no copyright claim is made to original U.S. Government works, or works produced by employees of any Commonwealth realm Crown government in the course of their duties.

Page 1 of 52 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

ACS Applied Materials & Interfaces

Plasma-Induced Oxygen Vacancies in Urchin-Like Anatase Titania Coated by Carbon for Excellent Sodium-Ion Battery Anodes Qingmeng Gan†‡, Hanna He†‡, Kuangmin Zhao†‡, Zhen He*†, Suqin Liu*† and Shuping Yang§ †

College of Chemistry and Chemical Engineering, Central South University, Changsha, 410083,

P.R. China.

‡

Innovation Base of Energy and Chemical Materials for Graduate Students Training, Central

South University, Changsha, 410083, P.R. China. §

School of Mathematics and Statistics, Central South University, Changsha, 410083, P.R. China.

KEYWORDS: oxygen vacancies, sodium-ion batteries, anatase TiO2, plasma; carbon

ABSTRACT: The incorporation of oxygen vacancies in anatase TiO2 has been studied as a promising way to accelerate the transport of electrons and Na+ ions, which is important for achieving excellent electrochemical properties for anatase TiO2. However, wittingly introducing oxygen vacancies in anatase TiO2 for sodium-ion anode by a facile and effective method is still a challenge. In this work, we report an innovative method to introduce oxygen vacancies into the 1 ACS Paragon Plus Environment

ACS Applied Materials & Interfaces 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60

Page 2 of 52

urchin-like N-doped carbon coated anatase TiO2 (NC-DTO) by a facile plasma treatment. The superiorities of the oxygen vacancies combined with the conductive N-doped carbon coating enable the obtained NC-DTO of greatly improved sodium storage performance. When served as the anode for SIBs, the NC-DTO electrode shows superior electrochemical performance (capacity: 272 mA h g-1 at 0.25 C, capacity retention: 98.8% after 5000 cycles at 10 C, as well as ultrahigh capacity: 150 mA h g-1 at 15 C). Density functional theory (DFT) calculations combined with experimental results suggest that considerably improved sodium storage performance of NC-DTO is because of enhanced electronic conductivity from N-doped carbon layer as well as narrowed band gap and lowered sodiation energy barrier from the introduction of oxygen vacancies. This work highlights that introducing oxygen vacancies into TiO2 by plasma is a promising method to enhance the electrochemical propety of TiO2, which also can be applied to different metal oxides for energy storage devices.

2 ACS Paragon Plus Environment

Page 3 of 52 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


INTRODUCTION Over the past few decades, lithium-ion batteries (LIBs) have been considered a prospective energy device in various electronic devices such as electricals and mobile phones owing to its high energy density as well as environmentally friendly.1-3 Nonetheless, with increasing demand for LIBs, the limited resources and enhanced price of lithium restrain the further development of LIBs. As a substitute, sodium-ion batteries (SIBs) have been researched greatly in recent years due to their abundant resources and low costs.4 However, the relatively large radius of Na+ (1.02 Å) makes it difficult to find appropriate materials that can accommodate sodium ions efficiently. As a common anode in LIBs, graphite only exhibits an ultralow capacity in SIBs (~35 mA h g-1) due to the unmatched interlayer spaces.5-6 The alloy-type materials (e.g., Na15Sn4,7 Na3Sb,8 and Na3P9) and the conversion-type materials (e.g., metal oxides and sulfides) have been proved as high-capacity anodes for SIBs, however, the severe volumetric expansion during the charge-discharge process results in unsatisfied cycling stability. Titanium dioxide (TiO2) with different crystal forms (e.g., amorphous,10 anatase,11 rutile,12 and TiO2(B)13) has drawn much attention for sodium storage because of their abundant sources, low costs, environmentally friendliness, and especially stable structures. Among these crystal forms of TiO2, anatase TiO2 has been proved to have most electrochemical activity in SIBs due to its two-dimensional diffusion routes as well as abundant accommodation sites for Na+ intercalation.14-15 Nevertheless, the low diffusivity of Na+ in anatase TiO2 leads to a low capacity 3 ACS Paragon Plus Environment


Page 4 of 52

of anatase TiO2, especially at high rates. In addition, the band gap of anatase TiO2 is up to ~3.2 eV, resulting in its terrible electronic conductivity (~10-13 S cm-1).16-17 Extensive research, including optimizing nanostructures and morphologies,18-20 coating carbon layers,21 and importing defects,22-24 has been devoted to addressing the above drawbacks of anatase TiO2.

Among various strategies, the incorporation of defects (e.g. oxygen vacancies) can not only change the electronic structures but also provide more open channels for the diffusion of Na+.25-26 Up to now, numerous methods have been developed to introduce oxygen vacancies into TiO2. For example, Chen group synthesized black TiO2 nanoparticles through handling pure white TiO2 in 20.0-bar H2 atmosphere at 200 oC for 5 days.27 However, the ultrahigh hydrogen pressure is hard to reach and the synthesis is time-consuming. Ji et al. mixed NaBH4 (as the reductant) with TiO2 and then obtained black TiO2 powder through a thermal treatment.28 Wang’s group grounded Mg powder with TiO2 and then obtained TiO2 with oxygen vacancies after the thermal treatment.29 Although these methods could produce TiO2, the involved reductants are difficult to remove and thus might affect the purity of the produced TiO2. The plasma method has been proved as a brilliant strategy to effectively import oxygen vacancies into metal oxides (e.g., α-Fe2O330 and Co3O431) without destroying the morphologies and structures within a short time. In addition, no impurity needs to be removed after the treatment, which largely simplifies the synthetic process and improves the purity of the products. Previous work demonstrated that the structure stability of TiO2 might decline because of presence of 4 ACS Paragon Plus Environment

Page 5 of 52 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


oxygen vacancies in the bulk of TiO2 and this drawback could be settled by carbon coating.26 Therefore, the strategy to synthesize defect-rich anatase TiO2 by Ar/H2 plasma treatment together with surface N-doped porous carbon coating could be an effective way to achieve great rate performance and excellent cycling durability.

In this work, we synthesize a N-doped carbon coated anatase TiO2 enriched with defect (oxygen vacancies) (NC-DTO) through a facile hydrothermal method followed by an Ar/H2 plasma treatment during the annealing process. The oxygen vacancies are efficiently incorporated into the anatase TiO2 bulk and the N-doped carbon-coating layer by the Ar/H2 plasma treatment. When tested as the anode in SIBs, the NC-DTO shows a highly boosted capacity and high-rate capability. Density functional theory (DFT) calculations and experimental results demonstrate that oxygen vacancies in TiO2 bulk can effectively enhance intrinsic electronic conductivity of anatase TiO2 as well as accelerate Na+ mobility in the TiO2 host. Moreover, the ultrathin nanoflakes of the stable three-dimensional (3D) urchin-like TiO2 can provide shortened paths for Na+ diffusion. The N-doped porous carbon outside the TiO2 with oxygen vacancies can substantially improve the electronic conductivity as well as restrict volumetric expansion during sodiation/desodiation process. This work offers a new plasma-assistant method to induce oxygen vacancies for constructing advanced electrode materials for energy storage.

METHODS 5 ACS Paragon Plus Environment


Page 6 of 52

Material synthesis: TiO2 powder was synthesized via a facile hydrothermal strategy.34 In detailed, 1.5 mL tetrabutyl titanate (TBOT) was added drop by drop into 40 mL acetic acid (CH3COOH) under magnetic stirring. Then, obtained homogeneous solution was moved to a Teflon-lined stainless-steel autoclave (100 mL). After heating at 150 oC for 10 h, the obtained white TiO2 precursor was washed 3 times by ethanol. After annealing TiO2 precursor under 600 oC for 2 h at 10% Ar/H2 atmosphere, the urchin-like anatase TiO2 (donated as TO) was obtained. The oxygen vacancy-riched urchin-like anatase TiO2 (donated as DTO) was obtained by treating the TiO2 precursor in a 10% Ar/H2 plasma condition at 600 oC for 2 h.

The N-doped carbon coated oxygen vacancy-riched urchin-like anatase TiO2 (donated as NC-DTO) was prepared through a facile strategy. First, 300 mg of the TiO2 precursor was well dispersed in 50 mL of Tris-buffer (pH=8.5) through ultrasonic treatment. Then, 50 mg of dopamine hydrochloride was dispersed into the mixture under stirring at 30 oC. After stirring for 12 h, the yellow powder was washed 3 times through ethanol. Finally, yellow powder was treated in a 10% Ar/H2 plasma atmosphere at 600 oC for 2 h. The plasma input power was 150 W (Figure S1). For comparison, the N-doped carbon coated urchin-like anatase TiO2 (donated as NC-TO) was obtained following the same method as NC-DTO but without the plasma treatment.

Characterizations: The crystalline structures of TiO2-based materials were characterized through using an X-ray diffraction (XRD) on a Dandong Haoyuan X-ray diffractometer (DX-2700) with a Cu-Kα1 source (λ=1.5418 Å) operated at 40 kV. The Raman spectra were obtained on a LabRAM Aramis spectrometer (HORIBA Jobin Yvon, λ=633 nm). The X-ray photoelectron spectroscopy (XPS) measurements were carried out on an ESCALAB 250Xi spectrometer using monochromatic Al Kα radiation (1486.6 eV). The scanning


Page 7 of 52 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


electron microscopy (SEM) was performed on an FEI Nova NanoSEM 230 microscope (accelerating voltage: 10 kV). Transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) were performed on a Tecnai G2 F20 S-TWIX microscope (300 kV). The electron spin resonance (ESR) was operated through using a JES-FA200 instrument (modulation frequency: 100 kHz). The UV-vis spectra were conducted on a UV-2600 spectrometer (Shimadzu, Japan). The thermogravimetric analysis (TGA) was carried out on a NETZSCH STA 449C differential scanning calorimeter. DC electrical conductivity was studied through a direct four-point probe technique (RTS-9, Guangzhou, China). The specific surface area was calculated by using the Brunauer-Emmett-Teller (BET) equation based on the N2 adsorption-desorption isotherms which were recorded using a Builder SSA-4200 instrument.

Electrochemical measurements: For the preparation of electrodes, the as-prepared samples, super P carbon, and polyvinylidene fluoride (PVDF) in weight ratios of 70:20:10 were mixed to form a homogeneous slurry and then casted onto a Cu foil. Then, the Cu foil loaded with the active materials was transferred to vacuum oven and dried at 100 oC for 6 h. The cells were assembled in an Ar-filled glovebox (MIKROUNA, China) by using a piece of sodium metal as the counter electrode, a solution of 1 M NaClO4 in a mixture of propylene carbonate (PC) and fluoroethylene carbonate (FEC) (V/V=95:5) as the electrolyte, and the active materials as the work electrode. The mass loading of the active materials is 1.3-1.4 mg cm-2. The evaluation of sodium storage performance was performed on a LAND CT2001C battery test system from 0.01 to 3.0 V (vs. Na/ Na+) at room temperature. The cyclic voltammetry (CV) measurements of the cells were conducted on a CHI660D electrochemical workstation (Shanghai Chenhua Instruments Co.) (potential window of 0.01 to 3.0 V (vs. Na/



Page 8 of 52

Na+)). The electrochemical impedance spectroscopy (EIS) was performed on an EG&G Princeton Applied Research PARSTAT2273 potentiostat/galvanostat (frequency range: 100 kHz-0.01 Hz).

Calculation methods: The calculations were conducted by utilizing the density functional theory (DFT) as implemented in Material Studio (MS). The exchange-correlation functions were taken as the generalized gradient approximation (GGA) with the Perdew-Burke-Ernzerhof (PBE) function. The valence electrons were depicted through a plane-wave basis set with an energy cutoff of 370 eV to ensure the precision of the calculations. A supercell of anatase TiO2 with 2 × 2 × 1 unit cells was applied to the calculations. For calculations of the electronic properties and equilibrium geometries, the integrations over the Brillouin zone were performed with a 3 × 3 × 3 special k-point mesh.

Results and discussion Crystalline structures of as-synthesized products (i.e., TO, DTO, NC-TO, and NC-DTO) were studied through XRD. Figure 1a illustrate the XRD patterns of these samples, which show nearly identical diffraction peaks at 25.3o, 37.8o, 48.0o, 53.9o, 55.1o, and 62.7o, assigning to (101), (004), (200), (105), (201), and (204) crystal planes of the anatase TiO2 (PDF#71-1166). XRD pattern of the NC-DTO remains the same as that of the TO after the plasma treatment and carbon coating, indicating that the N-doped carbon coating and oxygen vacancy incorporation have no damage to the structure of the anatase TiO2. The insets of Figure 1a show the optical photographs of the TiO2 products. The TO is purely white and after the plasma treatment the obtained DTO is 8 ACS Paragon Plus Environment

Page 9 of 52 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


brownish black, corresponding to the incorporation of oxygen vacancies into the TiO2 lattices. After the carbon coating, the colors of NC-TO and NC-DTO turn to black due to the surface carbon layers.

Figure 1. (a) XRD patterns of TO, DTO, NC-TO, and NC-DTO with corresponding optical photographs (as the insets). (b) Raman spectra of TO, DTO, NC-TO, and NC-DTO (inset figure magnified view of the most intense Eg peaks of the four samples).



Page 10 of 52

The structural properties of obtained samples were further investigated through Raman spectra. As shown in Figure 1b, the four samples all show bands at about 150 cm-1 (Eg), 402 cm-1 (B1g), 523 cm-1 (B1g) and 643 cm-1 (Eg), which can be well indexed to anatase TiO2. Notably, the peaks of DTO, NC-TO, and NC-DTO become weaker and wider than that of TO, which could be due to the improved electronic conductivities of DTO, NC-TO, and NC-DTO. According to previous literature, the improved electronic conductivity can lower the skin depth of the incident photons, which is reflected by the reduction of peak intensity in the Raman spectra.26 In addition, the slightly blue-shifted peaks of DTO and NC-DTO in the range of 100 to 800 cm-1 imply the existence of a certain amount of oxygen vacancies, consistent well with the following XPS, UV-vis, and ESR results.26 The graphitic quality of the carbon on the NC-TO and NC-DTO was also studied by Raman spectra. Figure S2a illustrates the two obvious peaks at 1339.2 and 1585.1 cm-1, assigning to the disordered carbon (D-band) and the ordered graphitic carbon (G-band), respectively.32 Through calculation, the intensity ratios of the D band to G band (ID/IG) in NC-TO and NC-DTO is 1.03 and 1.08, respectively. This result demonstrates the amophous and defective features of the N-doped carbon layers on NC-TO and NC-DTO.

The XPS measurements were employed to deeply prove the presence of oxygen vacancies. As shown in Figure 2a, the peaks of Ti (2s, 2p, 3s, and 3p), O 1s, and C 1s can be observed in the survey spectra of the four samples and an additional N 1s peak arises in the spectra of NC-TO and NC-DTO because of the N-doped carbon layers. As shown in Figure 2b, the O 1s peak of 10 ACS Paragon Plus Environment

Page 11 of 52 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


TO can be deconvoluted to two peaks at 529.9 and 531.3 eV, assigning to the Ti-O and surface -OH bands, respectively.33 After importing oxygen vacancies, these two O peaks of NC-DTO shift to 529.6 and 531.0 eV, respectively. Meanwhile, the percentage of the peak from surface -OH in NC-DTO is increased compared to that in TO. This phenomenon is probably because more oxygen vacancies tend to bind with H atoms and form more surface -OH groups.26 Figure 2c exhibits the high-resolution Ti 2p spectra of TO and NC-DTO. For TO, the two peaks at 458.6 and 464.3 eV, which are ascribed to Ti 2p3/2 and Ti 2p1/2, can be clearly observed. In contrast, the Ti 2p peaks of NC-DTO move to a lower binding energy of 458.3 and 463,9 eV, respectively. These results demonstrate the successful introduction of oxygen vacancies in NC-DTO according to previous work.35 Figure 2d shows the peaks of N 1s at 398.6, 400.6, and 401.3 eV, corresponding to N-6, N-5, and N-Q, respectively.36 This result proves the existence of N element in the carbon layer. Extensive previous reports have proved that N-doped carbon layer may further enhance the electronic conductivity of TiO2.37 The nitrogen content in the carbon layers of NC-DTO and NC-TO are 2.2% and 1.9%, respectively. The high-resolution XPS C 1s peak can be deconvolute to four peaks at 284.5, 285.1, 286.1, and 288.9 eV, assigning to the C=C, C-O, C-N, and C-O-Ti bonds, respectively (Figure S2b).38-39 The presence of C-O-Ti bonds suggests that the N-doped carbon layer tightly binds with TiO2, which could substantially improve the electronic conductivity according to previous reports.40 As shown in Figure S2c, the weight content of the carbon layer of NC-DTO based on the TGA result is only 8.6%, which is similar to that of NC-TO (6.7%). 11 ACS Paragon Plus Environment


Page 12 of 52

Figure 2. (a) XPS survey, (b) high-resolution O 1s, and (c) high-resolution Ti 2p spectra of TO and NC-DTO. (d) High-resolution N 1s spectrum of NC-DTO.

The UV-vis absorption spectra of TO, DTO, NC-TO, and NC-DTO are shown in Figure 3a. The TO shows a typical spectrum of anatase TiO2 with a sharp absorption edge at about 388 nm, which can be ascribed to the band gap of 3.2 eV.41-42 The absorption edge appears slight red shifts for DTO (at 410 nm) and NC-TO (at 428 nm). The NC-DTO shows an absorption edge at 12 ACS Paragon Plus Environment

Page 13 of 52 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


the largest wavelength of 446 nm, corresponding to a band gap of 2.8 eV. Previous reports have reported that a certain amount of oxygen vacancies and the carbon layer can narrow band gap of anatase TiO2 and result in an improved electronic conductivity of anatase TiO2,28 which we will analyze in detail in the following sections. The EPR was utilized to directly demonstrate the existence of oxygen vacancies due to its high sensitivity to detect paramagnetic species with unpaired electrons. As shown in Figure 3b, the TO without oxygen vacancies does not show any characteristic EPR response, suggesting may be no oxygen vacancies in TO. Apparently, an obvious signal at around g = 2.009 can be observed for other three samples (i.e., DTO, NC-TO, and NC-DTO), indicating the existence of oxygen vacancies which might be derived from the plasma treatment and/or N-doped carbon layers.43-44 It is well-known that the N-doped carbon always has unpair electrons due to the existence of N-5 and N-6, which could be easily observed by EPR.44 Moreover, the EPR of the N-doped carbon exhibits a similar signal at g = 2.009 (Figure S2d), demonstrating the existence of defect of N-5 or N-6. Note that the signal intensity of NC-DTO is the highest among these samples, suggesting more defects in NC-DTO. The EPR results demonstrate the effective introduction of oxygen vacancies by the plasma treatment.



Page 14 of 52

Figure 3. (a) UV-vis absorption spectra and (b) EPR spectra of TO, DTO, NC-TO, and NC-DTO. SEM was utilized to investigate the morphologies of the as-prepared products. As shown in Figure 4a and b, the TO displays a well-defined urchin-like morphology with a uniform size of 2-3 µm in diameter. The SEM image of TO with a higher magnification (Figure 4c) reveals that the urchin-like TO is assembled of numerous nanosheets. Figure 4g-i exhibit the urchin-like morphology of NC-TO, suggesting the carbon layer is well coated outside the TO. After plasma treatment, the urchin-like morphologies of DTO and NC-DTO can be well maintained (Figure 4d-f and 4j-l, respectively). The TEM and HRTEM analyses were conducted to thoroughly investigate the detailed morphologies and structures of TO, DTO, NC-TO, and NC-DTO. Their TEM images (Figure 5a, d, g, and j) all show the urchin-like morphologies consisting of extensive branches emanating from the core of the urchin, which are consistent well with the SEM results. As shown in Figure 5b, e, h and k, these subordinate structures are integrated from ultrathin nanosheets. HRTEM image of TO shows a lattice fringe spacing of about 0.35 nm, 14 ACS Paragon Plus Environment

Page 15 of 52 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


assigning to the (101) lattice plane of the anatase TiO2 (Figure 5c). Similarly, the NC-TO shows the same lattice fringe spacing with an obvious carbon layer of 3-5 nm on the surface of crystal anatase TiO2, demonstrating the successful and uniform coating of carbon layers. Interestingly, the HRTEM images of both DTO and NC-DTO show discontinued crystal fringes (marked by the dotted circles in Figure 5f and l), suggesting the existence of oxygen vacancies in DTO and NC-DTO. Besides, the NC-DTO also exhibits an ultrathin carbon layer coating the surface of anatase TiO2, which can improve the electronic conductivity and stabilize the oxygen vacancies in NC-DTO. The elemental mapping analysis (Figure 5m-m4) demonstrates presence of titanium (Ti), oxygen (O), nitrogen (N), as well as carbon (C) in NC-DTO. According to the N2 absorption-desorption isotherms, BET surface areas of TO, DTO, NC-TO, and NC-DTO are 113, 115, 152, and 161 m2 g-1, respectively (Figure S3). BET surface areas of DTO and NC-DTO are close to those of TO and NC-TO, respectively, suggesting that the plasma treatment would not significantly change the structure of the samples. The higher BET surface areas of NC-TO and NC-DTO are mainly attributed to their more numerous pores in the carbon layer, which can shorten the diffusion pathways and facilitate the transport of Na+ during the charge-discharge process.



Page 16 of 52

Figure 4. SEM images of (a-c) TO, (d-f) DTO, (g-i) NC-TO, and (j-l) NC-TO at different magnifications.


Page 17 of 52 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 5. TEM images of (a, b) TO, (d, e) DTO, (g, h) NC-TO, and (j-k) NC-DTO. HRTEM images of (c) TO, (f) DTO, (i) NC-TO, and (l) NC-DTO, and the corresponding elemental mapping of (m) NC-DTO, (m1) C, (m2) Ti, (m3) O, and (m4) N.

To further study the inﬂuences of the oxygen vacancies and carbon coating on sodium storage performance of the TiO2 electrodes, the TO, DTO, NC-TO, and NC-DTO are assembled 17 ACS Paragon Plus Environment


to half-cells as the working electrodes and first evaluated by galvanostatic charge-discharge cycling tests at 0.25 C (1 C = 336 mA g-1). Figure 6a illustrates the cycling performance of TO, DTO, NC-TO, and NC-DTO. In the first cycle, the discharge capacities for TO, DTO, NC-TO, and NC-DTO are 577, 520, 510, and 564 mA h g-1, respectively. During cycling, the capacities of all four products appear a slightly increased tendency in the initial 20 cycles owing to the activation process of anodes, which has been reported on TiO2-based materials.25, 45-46 After 200 cycles, capacities of 167, 207, 199, and 272 mA h g-1 are obtained for TO, DTO, NC-TO, as well as NC-DTO, respectively. Figure 6b reveals the coulombic efficiencies of these four samples at 0.25 C for 200 cycles. The TO, DTO, NC-TO, and NC-DTO exhibit initial coulombic efficiencies of 36.5%, 39.6%, 43.7%, and 52.7%, respectively, which are similar to the typical values reported for anatase TiO2 in many previous reports.47-49 Obviously, the NC-DTO presents the highest initial coulombic efficiency compared to other samples. The higher initial coulombic efficiency of NC-DTO might be due to its improved electronic conductivity as well as lower sodiated energy barriers from the oxygen vacancies, resulting in more reversible trapping of Na+ inside the NC-DTO. Meanwhile, the outside N-doped carbon layer can restrict the electrolytes from directly contacting with the electrode surfaces, leading to less irreversible side reactions and preventing the excessive formation of solid-electrolyte-interface (SEI), and thus improving electrode reversibility during initial charge-discharge process.50-51 After the initial cycle, the coulombic efficiency of NC-DTO rapidly increases to 90.6% in the seventh cycle and then reaches and maintains nearly 100% after the nineteenth cycle. Among these samples, the 18 ACS Paragon Plus Environment

Page 18 of 52

Page 19 of 52 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


NC-DTO shows the largest initial coulombic efficiency and needs the least cycles to reach a high coulombic efficiency (i.e., nearly 100%), suggesting an excellent reversibility in sodium storage for the NC-DTO electrode.

Figure 6c, d and S4 illustrate the charge-discharge profiles of the four samples in various cycles at 0.25 C. As shown in Figure S5, initial discharge curves of the four samples deliver a similar tendency, which can be separated into three regions. The voltage in region A drops rapidly from the open-circuit voltage to 1.25 V because of the pseudocapacitive Na+ adsorption.29 In region B, the voltage decreases from 1.25 V to 1.0 V alongside with an evident plateau because of the formation of the SEI film and irreversible sodiation,12 corresponding to the irreversible capacity loss in the initial CV cycle (which will be discussed in the later section). In region C, the voltage drop becomes slower below 1.0 V, which mainly comes from the reversible sodiation/desodiation process according to the previous reports on anatase TiO2-based materials.50-51 In the subsequent cycles, the discharge voltage plateau shift to about 0.7 V for the four samples, which can be assigned to the insertion/extraction process of Na+ into/from anatase hosts. Among four samples, the charge-discharge curves of NC-DTO from 5th to 200th are almost overlapped, suggesting the stable electrochemical properties of the NC-DTO anode during the successive sodiation/desodiation processes.



Page 20 of 52

Figure 6. (a) Cycling performance and (b) the corresponding coulombic efficiencies of TO, DTO, NC-TO, and NC-DTO at 0.25C (1C = 336 mA g-1). (c) Galvanostatic charge and discharge voltage profiles for NC-DTO and (d) TO of various cycles at the rate of 0.25 C.

The superior electrochemical properties of the NC-DTO electrode was further highlighted through the increased rate from 0.1 to 15 C, which are exhibited in Figure 7a. Regarding TO, the capacity is about 252 mA h g-1 at 0.1 C and decreases rapidly to 201, 177, 155, 126, 103, 83, and 60 mA h g-1 when current densities increase to 0.25, 0.5, 1, 2.5, 5, 10, and 15 C, respectively.


Page 21 of 52 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


The capacities of DTO and NC-TO are almost the same at low rates, but at the high rates of 5, 10, and 15 C, the DTO delivers higher capacities of 144, 123, and 112 mA h g-1, respectively. Clearly, the existence of oxygen vacancies can substantially facilitate the transport of Na+, leading to superior rate performance. For the NC-DTO electrode, the capacities of 197, 226, 258, 280, 301, and 338 mA h g-1 are reached at 0.1, 0.25, 0.5, 1, 2.5, and 5 C, respectively. Even at the high rates of 10 and 15 C, the NC-DTO can exhibit high capacities of 170 and 150 mA h g-1, respectively. NC-DTO shows the highest capacities among the four samples, which is much higher than that of the recently reported nanostructured TiO2-based materials.28,

47-48, 52

The

corresponding charge-discharge profiles at various rates are shown in Figure 7b, 7c, S6a, and S6b. Clearly, even at high rates, the charge-discharge profiles of the NC-DTO anode maintain well without any evident change. In contrast, charge-discharge profiles of TO begin to appear significant variant at 5C, while this phenomenon appear a certain improvement of DTO and NC-TO. These results are ascribed to enhanced electronic conductivity originated from the existence of oxygen vacancies and carbon layer, which can facilitate the transport of electrons and diffusion of Na+ at high rates and thus lead to an improved high-rate capability. Furthermore, the EIS was performed to study the charge transfer characteristics of the four samples by using the half-cells, which were tested at 0.25 C for 10 cycles before testing. As shown in Figure 7d, the semicircle in the high frequency region represents the impedance of the charge transfer.53-54 Fitting value of the charge-transfer resistance for NT-DTO is about 30.5 Ω, which is much lower than those of TO (243.7 Ω), DTO (50.9 Ω), and NC-TO (63.9 Ω) (Table S1) because of the 21 ACS Paragon Plus Environment


Page 22 of 52

improved electronic conductivity due to the introduced oxygen vacancies and N-doped carbon coating. According to the four-point probe results, the NC-DTO shows the highest electronic conductivity (1.7 × 10-3 S cm-1) among the four samples (8.3 × 10-4 S cm-1 for DTO, 3.1 × 10-4 S cm-1 for NC-TO, as well as 3.3 × 10-8 S cm-1 for TO). Diffusion coefficient (D) of Na+ is calculated by using following equation,37 D = (R2T2)/(2A2n4F4C2σ2)

where A, n, F, C, R, and T represent the electrode area, electron number, Faraday constant, molar concentration of Na-ions, universal gas constant, and the absolute temperature, respectively. The σ could be obtained from the slope of the linear fit (Z’∝σω-1/2, Figure S6c), where Z’ and ω represent the real part of the impedance and angular frequency, respectively. The diffusion coefficients (D) of Na+ for the TO, DTO, NC-TO and NC-DTO electrodes are calculated to be 1.763 × 10-16 cm2 s-1, 8.973 × 10-16 cm2 s-1, 1.235 × 10-15 cm2 s-1, and 1.464 × 10-15 cm2 s-1, respectively. According to these results, the NC-DTO has the highest electronic conductivity as well as the largest diffusion coefficient of Na+.

Another significant property of the NT-DTO anode is its excellent long-term cycling stability at a high rate of 10 C (3360 mA h g-1) (Figure 7e). The capacities of all samples appear a slightly increase tendency in the initial cycles because of the electrochemical activation which has been discussed above. For TO, the capacity increases from 85 to 93 mA h g-1 in the initial 150 cycles, begins to decline after 1000 cycles, and reaches 77 mA h g-1 after 5000 cycles, with a 22 ACS Paragon Plus Environment

Page 23 of 52 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


retention of 82.8% (compared to the highest capacity) at 10 C. After the introduction of oxygen vacancies, the DTO shows a higher capacity of 132 mA h g-1 at 10 C after 170 cycles, but a worse retention of 79.5%. As for NC-TO, its cycling stability becomes better due to the carbon coating (with a retention of 97.4%). These results suggest that importing oxygen vacancies can greatly improve capacity of DTO but result in an unstable cycling property, which can be effectively solved by carbon coating. Therefore, by combining the oxygen vacancies and carbon layer, the NC-DTO presents the highest capacity of about 160 to 170 mA h g-1 in the first 200 cycles. After 5000 cycles, NC-DTO still remains a capacity of 168 mA h g-1, holding the greatest retention of as high as 98.8%. Compared to the reported TiO2-based anodes (Table S2), the obtained NC-DTO electrode shows much better electrochemical performance. To further demonstrate the stability of NC-DTO, the ex-situ XRD patterns of NC-DTO after different charge-discharge cycles were obtained (Figure S7). The main diffraction peaks of (101), (004), and (200) of NC-DTO after different cycles are almost the same, suggesting the excellent structure stability of NC-DTO. In addition, the morphology of NC-DTO after 200 cycles at 10 C were studied through SEM and TEM. SEM images show that the NC-DTO after the cycling still exhibits a well-defined urchin-like morphology without evident morphology destruction (Figure S8a and b). Moreover, the carbon layer still tightly coats on the TiO2 surface, suggesting that the clustering and structural strain of TiO2 are well confined through the carbon layer in sodiation/desodiation process (Figure S8c and d).



Page 24 of 52


Page 25 of 52 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


Figure 7. (a) Rate performance of the TO, DTO, NC-TO, and NC-DTO electrodes at various rates. The corresponding voltage profiles at various rates of (b) NC-DTO and (c) TO. (d) Electrochemical impedance spectra of the TO, DTO, NC-TO, and NC-DTO electrodes after 10 cycles at the rate of 0.25C. (e) Cycling performance of the TO, DTO, NC-TO, and NC-DTO electrodes at 10 C.

To acquire insights into the effects of the oxygen vacancies and carbon coating on the sodium storage behaviors of TiO2, the electrochemical kinetics related to the faradaic redox processes of the TO, DTO, NC-TO, and NC-DTO electrodes were studied based on the CV tests. Figure 8a shows typical CV curves of the NC-DTO electrode in the initial five cycles at 0.2 mV s-1. The almost overlapped curves with a pair of well-defined cathodic/anodic peaks (at about ~0.77/0.87 V with a peak deviation (∆E) of 0.10 V) can be observed, which presents the stable sodiation/desodiation process of anatase TiO2.55-56 As the scan rate increases from 0.2 to 4.0 mV s-1 (Figure 8b), the ∆E of the NC-DTO electrode becomes smaller due to the smaller polarization at high rates. Even at higher scan rates of 6 to 60 mV s-1 (Figure 8c), the peak shapes in the CVs of NC-DTO do not appear obviously variations. As for TO, less overlapped CVs with a couple of redox peaks (at ~0.53/0.83 V) can be observed (Figure 8d) with a ∆E of 0.30 V. In addition, as shown in Figure 8e and f, the redox peaks in the CVs become more and more ambiguous and deflected and the ∆E increases from 0.2 to 60 mV s-1, implying a large polarization of the TO electrode at high rates. As for NC-TO and DTO, the ∆E of them are 0.20 and 0.24 V (Figure S9a 25 ACS Paragon Plus Environment


and d), respectively, demonstrating that the carbon layer and oxygen vacancies can be benefitial for the diffusion of Na ions. Moreover, the peak shapes of NC-TO and DTO do not appear obvious changes as the scan rate increases (Figure S9b, c, e and f). Clearly, the ∆E of the NC-DTO electrode is the smallest, suggesting more feasible sodiation/desodiation and better reversibility of NC-DTO because of the enhanced kinetics after the introduction of oxygen vacancies and carbon coating (Figure 8a, d, S9a and d).57 It is well known that the charge-discharge process can be separated into two typical types, i.e., faradaic behavior from Na+ insertion and extraction mechanisms and pseudocapacitive process from surface faradic redox reactions. Through the CV analysis at various sweep rates, it is possible to investigate the electrode reaction mechanisms for Na+ storage. The relationship between the current (i) and scan rate (v), i = avb, can be depicted by the log(v)-log(i) plot.56 a and b are both adjustable parameters, where with the value of b determined from the slope of log(v)-log(i) plot. A value of b of 0.5 means a fully diffusion-controlled behavior, whereas a value of b of 1.0 means a fully surface-controlled process. As shown in Figure 8g, the slope of NC-DTO is up to 0.918 in the range of 0.2-6 mV s-1, which can be assigned to surface pseudocapacitive process. The decrease of the slope from 0.995 to 0.567 takes place above 6 mV s-1, suggesting the diffusion-controlled behavior at higher scan rates. As for NC-TO and DTO, they both exhibit the surface pseudocapacitive process at the scan rates below 2.0 mV s-1 and diffusion-controlled process at the scan rates above 2.0 mV s-1. It is clear that both carbon coating and oxygen vacancies can improve the diffusion of Na ions. In contrast, the slope of TO changes from 0.872 to 0.575 over 26 ACS Paragon Plus Environment

Page 26 of 52

Page 27 of 52 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39 40 41 42 43 44 45 46 47 48 49 50 51 52 53 54 55 56 57 58 59 60


0.8 mV s-1, showing that the diffusion of Na+ is the rate-controlling process except at low sweep rates (

Plasma-Induced Oxygen Vacancies in Urchin-Like Anatase Titania

Recommend Documents