Efficient Photocatalytic Reduction Approach for Synthesizing

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An Efficient Photocatalytic Reduction Approach for Synthesizing Chemically Bonded N-Doped TiO2/Reduced Graphene Oxide Hybrid as a Freestanding Electrode for High-Performance Lithium Storage Yongzheng Shi, Dongzhi Yang, Ruomeng Yu, Yaxin Liu, Jin Qu, Bin Liu, and Zhong-Zhen Yu ACS Appl. Energy Mater., Just Accepted Manuscript • DOI: 10.1021/acsaem.8b00836 • Publication Date (Web): 05 Jul 2018 Downloaded from http://pubs.acs.org on July 10, 2018

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ACS Applied Energy Materials

An

Efficient

Photocatalytic

Reduction Approach for Synthesizing

Chemically Bonded N-Doped TiO2/Reduced Graphene Oxide Hybrid as a Freestanding Electrode for High-Performance Lithium Storage Yongzheng Shi,a,b Dongzhi Yang,a* Ruomeng Yu,a Yaxin Liu,a Jin Qu,a Bin Liu,b Zhong-Zhen Yua,b* a

Beijing Key Laboratory of Advanced Functional Polymer Composites, Beijing University of

Chemical Technology, Beijing 100029, China b

State Key Laboratory of Organic-Inorganic Composites, College of Materials Science and

Engineering, Beijing University of Chemical Technology, Beijing 100029, China E-mail: *[email protected] (D.-Z. Yang); *[email protected] (Z.-Z. Yu) ABSTRACT: Covalent bonds between active materials and conductive substrates significantly facilitate rapid interfacial charge transfer, thus enhancing the lithium-ion storage performances of freestanding electrodes. Herein, an efficient photocatalytic reduction approach is developed to synthesize chemically bonded N-doped TiO2 nanowire/reduced graphene oxide (N-TiO2/RGO) hybrid as a freestanding electrode for ultrafast lithium storage. Ti3+-C bonds are formed during the ultraviolet photocatalytic reduction process, as corroborated by Raman, electron paramagnetic resonance, and X-ray photoelectron spectra. The N-TiO2/RGO hybrid electrode exhibits a significantly higher rate capability than its counterpart without UV irradiation. After as long as 10,000 discharging/charging cycles, high capacities of 182.7, 125.1, and 101.8 mA h g-1 are retained at current rates of 5 C, 25 C, and even up to 50 C (1 C = 335 mA g-1), respectively. The excellent electrochemical 1

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performances of the N-TiO2/RGO hybrid are attributed to the enhanced electronic conductivity and lithium ion diffusion kinetics arising from the Ti3+-C bonds and the robust architecture of N-doped TiO2. KEYWORDS: lithium-ion batteries; photocatalytic reduction; freestanding anode; titanium dioxide; reduced graphene oxide

1. INTRODUCTION To meet the ever-growing power demand of the flexible and portable electronics, soft power systems are actively researched worldwide.1-4 Rechargeable lithium-ion batteries (LIBs) are the most promising technology to meet this power demand owing to their high energy density, long cycle life, and environmental benignity.5,6 Flexible LIBs must be lightweight, bendable, small in size, and have excellent mechanical properties while maintaining a fast and continuous power supply for long-term uses.7 However, the current flexible LIB electrodes usually suffer from insufficient rate capability and short cycle life.8 Therefore, it is imperative to develop highly durable freestanding electrodes with capacitor-like rate performances for portable and wearable electronic devices. With intrinsic safety, low volume change (4%) during lithiation/delithiation, and excellent rate capacity, titanium-based materials, especially titanium dioxide (TiO2), are preferred in fast rechargeable LIBs.9-11 Despite these advantages, the poor intrinsic ionic and electronic conductivities of TiO2 hamper its application for high-power anodes.12 Thus, enhancing the diffusion kinetics of lithium ions and electrons (Li+/e-) in electrode materials is highly crucial for obtaining high rate capabilities in LIBs.13 Nanostructuring of electrodes 2


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with

zero-dimensional

nanoparticles,14

one-dimensional

(1D)

nanotubes/wires,15,16

two-dimensional nanosheets,17 or three-dimensional (3D) architectures18 facilitates the contact of the high surface area active materials with electrolytes, thus shortening the diffusion length of Li+/e- and enhancing the interfacial electrochemical reactions. 1D elongated TiO2 nanowires

can

establish

a

3D

network

architecture

to

provide

(i)

adequate

electrode-electrolyte contact and (ii) direct and rapid Li+/e- transport pathways compared with other nanostructures.15 Moreover, the interpenetrating networks possess anti-agglomeration feature for long-term cycling of LIBs.19 For instance, excellent rate capability and ultralong cycling performance (6,000–10,000 cycles) were reported for interpenetrating networks of TiO2 on Cu foil.19,20 Doping of TiO2 with metals or nonmetals is another effective approach for enhancing the diffusivity of Li+/e- by lowering the energy barrier.13,21 For example, Nb-doped rutile TiO2 mesocrystals22 and N-doped TiO2@carbon microspheres23 with enhanced electronic conductivity and lithium ion diffusion kinetics were prepared for high-rate lithium storage. As another important component of freestanding electrodes, carbon nanomaterials including carbon nanotubes (CNTs), carbon nanofibers, reduced graphene oxide (RGO), and carbon textiles are widely used as conductive matrices due to their appropriate mechanical properties, excellent electrical conductivity, and high specific surface area.7 In contrast to conventional electrodes, freestanding electrodes avoid the use of polymer binders between active materials and conductive substrates, benefiting the rapid charge transfer and long cycle life.24 Till now, freestanding anodes including CNT/TiO2 nanoparticles,25 CNT sponge/TiO2 3



nanoparticles,26 electrospun nanofiber/TiO2 nanosheets,27 carbon textile/TiO2 nanosheets,28 RGO/TiO2 nanosheets/particles,29,30 and RGO foam/TiO2 nanosheets/particles24,31 have been fabricated for high-power lithium storage. Currently, the growth of various nanostructured TiO2 on conductive substrates is mainly motivated by non-covalent bonds. In traditional electrodes, varieties of covalent bonds,32-38 such as P-O-C,33 Ti-O-C,34 Sb-O-C,36 and Sn-O-C38 are constructed to enhance electrochemical performances of the electrodes. However, the Ti3+-C covalent bonds, which significantly facilitate rapid interface charge transfer, are essential but rarely studied for freestanding electrodes. It is therefore indispensable to establish Ti3+-C bonds between TiO2 and conductive substrates for ultrafast Li storage. Herein, we report an efficient photocatalytic reduction approach for synthesizing chemically bonded N-doped TiO2 nanowires/RGO sheets (N-TiO2/RGO) hybrid. With (NH4)2Ti3O7 (NHTO) as a photocatalyst and GO as a precursor of graphene, Ti3+-C bonds between NHTO and RGO are synthesized in ethanol by UV irradiation. The irradiation product is filtered to form a freestanding film, and the N-TiO2/RGO hybrid electrode is obtained after annealing in vacuum at 450 oC for 90 min. This freestanding electrode exhibits an excellent rate capability with a capacity of 198 mA h g-1 at a current density of 50 C (1 C = 335 mA g-1). After 10,000 cycles, the N-TiO2/RGO hybrid electrode yields high capacities of 182.7, 125.1, and 101.8 mA h g-1 at the current rates of 5 C, 25 C, and even up to 50 C, respectively. The lithium-ion storage mechanisms of the hybrid electrode are also explored.

2. EXPERIMENTAL SECTION 4


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2.1 Materials Graphite flakes (300 mesh) were purchased from Huadong Graphite Factory (China). Hydrogen peroxide (30%), hydrochloric acid (37%), sulfuric acid (98%), sodium nitrate, potassium permanganate, nitric acid (68%), ammonium sulfate ((NH4)2SO4), sodium hydroxide (NaOH) and ethanol were provided by Beijing Chemical Factory (China). P25 was supplied by Evonik Degussa GmbH (Germany). 2.2 Preparation of elongated ammonium titanate nanowires To prepare elongated NHTO nanowires, 0.2 g of P25 powder and 12 g of NaOH were added into 30 mL of deionized water with stirring for 10 min, and the mixture was then dispersed into two 25 mL Teflon-lined autoclaves and heated at 130 oC for 24 h with magnetic stirring at 550 rpm.19 NHTO nanowires were obtained by Na+-NH4+ ion exchange of gelatinous Na2Ti3O7 (NTO) in 1 M (NH4)2SO4, washed with deionized water and then ethanol for 5 times. A suspension of NHTO in ethanol (1 mg mL-1) was pre-irradiated by ultraviolet (UV) light (LT-400, Blue Sky Special Lamps Development Co., Ltd, UV band light source) for 30 min, and finally concentrated to 10 mg mL-1 for further use. 2.3 Synthesis of chemically bonded N-doped TiO2/RGO hybrids Graphene oxide (GO) was prepared by oxidizing graphite flakes using a modified Hummers’ method39 followed by ultrasonication. In a typical procedure of synthesizing a chemically bonded N-doped TiO2/RGO hybrid, 4.4 mg of GO and 15.6 mg of NHTO were added into 40 ml of ethanol and magnetically stirred for 6 h to form a homogeneous dispersion. The dispersion was then exposed to UV light with an intensity of 22.3 mW cm-2 for 10 min with 5



mild magnetic stirring. After drying in vacuum at 100 oC for 2 h, the product was heated at 450 oC for 90 min in vacuum at a ramp rate of 5 oC min-1 to obtain the N-TiO2/RGO hybrid. For comparison, an N-TiO2/RGO mixture was prepared under the similar conditions, but it was not irradiated by the UV light and denoted as N-TiO2/RGO-M. The abbreviations of the samples and their brief synthesis routes are listed in Table S1. 2.4 Characterization UV-vis diffuse reflectance absorption spectra were obtained with a Shimadzu UV-3100 spectrophotometer (Japan). The photoluminescence (PL) spectrum was recorded on a Hitachi F-7000 fluorescence spectrophotometer (Japan) with a PMT voltage of 840 V and a scanning speed of 120 nm min-1. X-ray diffraction (XRD) patterns and Raman spectra were measured on a Rigaku D/Max 2500 X-ray diffractometer (Japan) and a Renishaw inVia with an excitation wavelength of 514 nm (Britain), respectively. The microstructure and morphology were observed using a Hitachi S4700 field-emission scanning electron microscope (SEM) with an energy dispersive spectroscopy (EDS) and a JEOL JEM-3010 scanning TEM (STEM). The X-band electron paramagnetic resonance (EPR) of Ti3+ was investigated on a Bruker E500 spectrometer (Germany). Chemical bonds and compositions were characterized using a Nicolet Nexus 670 Fourier-transform infrared spectroscopy (FT-IR) and a Thermo VG RSCAKAB 250X high resolution X-ray photoelectron spectroscopy (XPS). The thermal stability of the samples was evaluated on a TA Instruments Q50 thermogravimetric analyzer (TGA) in air at a ramp rate of 10 oC min-1. Nitrogen isotherms were obtained using a Beishide 3H-2000PM2 equipment (China). 6


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2.5 Preparation of electrodes and measurements of electrochemical performances Electrochemical performances of the N-TiO2/RGO hybrid were measured using coin-type half-cells (CR 2032) with Celgard 2400 membrane as the separator, 1 M LiPF6 in a mixture of ethylene carbonate and diethyl carbonate (1/1, v/v) as the electrolyte, and Li-foil as the reference electrode. The working electrode was fabricated by filtering 5 mg of GO and 20 mg of NHTO/RGO dispersion in order using a filter paper and annealing the resultant film at 450 o

C in vacuum for 90 min at a heating speed of 5 oC min-1. The control electrode was prepared

under the same conditions, except that the NHTO/GO dispersion used was not subject to the UV irradiation. The mass loading range of the electrode was 1.0-1.2 mg cm-2. The cells were assembled in an Ar-filled glovebox (< 0.1 ppm of H2O and O2). Galvanostatic cycling performances of the electrodes were evaluated on LAND 2001A testers at room temperature. Electrochemical impedance spectroscopy (EIS) and cyclic voltammetry (CV) measurements were carried out on a Chenhua Instruments CHI760D electrochemical workstation (China).

3. RESULTS AND DISCUSSION 3.1. Synthesis and microstructure of N-TiO2/RGO hybrid Figure S1 shows the fabrication process of the N-TiO2/RGO hybrid electrode including (i) synthesizing elongated NHTO nanowires, (ii) preparing NHTO/RGO hybrid under UV irradiation, and (iii) fabrication of N-TiO2/RGO hybrid electrode by filtration and annealing. First, NTO nanowires are synthesized using P25 as the precursor with a modified hydrothermal reaction.19 As shown in Figure S2b, the elongated NTO nanowires (~37 µm) are interwoven into an interpenetrating network with an anti-agglomeration feature. NHTO 7



nanowires are then obtained by ion-exchanging of NTO with NH4+ ions, during which Na+ ions are completely replaced by NH4+ ions (Figure S3).

Figure 1. (a) UV-vis absorption spectrum, (b) (αhν)2-photon energy curve, and (c) PL spectrum of NHTO excited at 300 nm. The inset in Figure 1b shows the curves of (αhν)2-photon energy (the blue line for experimental data and the red line for a linear fit) and the curve of (αhν)1/2-photon energy (orange line) in the absorption edge region of NHTO. (d) Schematic illustrating the UV photocatalytic reduction approach for constructing Ti3+-C bonds. The NHTO dispersion in ethanol is pre-irradiated by the UV light for 30 min to make the Ti3+ ions more readily available for Ti3+-C bond formation. NHTO exhibits an onset absorption at 383.9 nm with a corresponding bandgap of 3.23 eV (Figure 1a, b). The absorption edge caused by indirect transitions rises rapidly in the UV region (Figure 1b inset).40 A wide PL signal from 320 to 550 nm is generated with the excitation wavelength of 300 nm (Figure 1c), and the strong PL peak at 480 nm possibly results from binding 8


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excitons.41 The photocatalytic reduction is schematically illustrated in Figure 1d: (i) UV irradiation of NHTO and GO dispersion in ethanol excites NHTO to generate electrons and holes; (ii) the holes are trapped by the α-H on ethanol, leaving electrons to reduce the surface Ti4+ of NHTO to Ti3+ and simultaneously partially reduce GO to RGO; (iii) the Ti3+ ions react with the epoxide and hydroxyl groups on the surface of GO sheets to form Ti3+-C bonds, resulting in chemically bonded NHTO/RGO hybrid. Under the premise of ensuring a thin and light freestanding electrode, a thin GO substrate is filtered under the NHTO/RGO hybrid layer to further enhance the electronic conductivity and structural stability of the electrode. After peeling off from the filter paper, the product is thermally annealed at 450 oC in vacuum for 90 min. Finally, the N-TiO2/RGO hybrid with 2.7 wt% of nitrogen is obtained (Figure S3c). The XRD pattern of as-prepared GO (Figure 2a) shows an intense peak at 12.5o corresponding to the (001) Bragg reflection with an interlayer spacing of 0.5 nm between GO sheets. The thickness of the as-prepared GO sheet is estimated to be 3.0 nm according to its AFM image in Figure S2c. The (002) Bragg reflection at 26.6o disappears, demonstrating the complete conversion of graphite to GO. After the ammonium ion replaces the sodium ions, the XRD curve of the ammonium titanate is shown in Figure S2a. The diffraction peaks around 9.9o, 12.0o, 16.5o, 24.9o, 29.2o, 33.4o, 41.8o, 44.5o, and 48.8o in the XRD pattern are ascribed to the formation of NHTO with NH4+ intercalated between TiO6 octahedra layers. 10,42-45

No obvious Bragg reflection peaks of GO and RGO are observed in the NHTO/RGO

and N-TiO2/RGO hybrids, as the tight bonding between the RGO sheets and Ti-based nanowires avoids the restacking of RGO sheets. Previous studies have verified that 9



TiO2(B)/anatase heterostructures in TiO2 (Figure 2a) could form disordered interfaces that increase the active sites for lithium-ion storage.46,47 By conducting Rietveld refinements in a two-phase analytical model, the contents of anatase (JCPDS No. 21-1272) and TiO2(B) (JCPDS No. 46-1237) in the N-TiO2 nanowires are found to be 54.9% and 45.1% (Figure S4), respectively, which can enhance the lithium storage of the N-TiO2/RGO hybrid by the disordered anatase/TiO2(B) interfaces.46-48

Figure 2. (a) XRD and (b) Raman curves of GO, NHTO, NHTO/RGO hybrid, N-TiO2, and N-TiO2/RGO hybrid. Figure 2b shows Raman curves of GO, NHTO, NHTO/RGO hybrid, N-TiO2, and N-TiO2/RGO hybrid. Typically, GO presents a D-peak at 1351 cm-1 and a G-peak at 1586 cm-1, corresponding to the defect sites induced by disordered sp3 carbon and the in-plane bond-stretching motion of sp2 carbon, respectively.49 After the photocatalytic reduction and subsequent vacuum annealing, the ID/IG ratio increases from 0.70 of GO to 0.99 of N-TiO2/RGO hybrid. The increase in ID/IG ratio is attributed to the decrease in the average size of the in-plane sp2 domains and defects, confirming the step by step reduction of GO.49,50 For N-TiO2, the Raman peaks centered at 146, 192, 244, 292, 398, 515, and 641 cm-1 are 10


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typical characteristics of anatase/TiO2(B) heterostructure,51,52 which is in good agreement with its XRD pattern. The presence of Raman peaks for NHTO/RGO close to those of N-TiO2 is sound evidence of NHTO conversion to TiO2 accelerated by photogenerated electrons.53 Whereas, the XRD patterns of NHTO/RGO hybrid and NHTO remain the same (Figure 2a), suggesting this conversion occurs only on the surface of NHTO nanowires. Some Raman peaks of NHTO/RGO and N-TiO2/RGO undergo redshifts, indicating the strong chemical bonding between RGO and the Ti-based nanowires (NHTO and N-TiO2).54

Figure 3. SEM images of (a) N-TiO2, (b) GO, and (c) N-TiO2/RGO hybrid. TEM images of (d) N-TiO2, (e) GO, and (f) N-TiO2/RGO hybrid. (g-k) EDS mapping images of the N-TiO2/RGO hybrid. 11



Consistent with the SEM image of NHTO in Figure S5a, the N-TiO2 nanowires in Figure 3a and d remain the interpenetrating network despite being subjected to the thermal annealing treatment at 450 oC for 90 min. The anti-aggregation feature is attributed to the facts: (i) the nanowires with a diameter of ~10 nm can be assembled into robust bundles (Figure S5c, d), and (ii) the nanowires are elongated to ~37 µm in length (Figure S2b), enabling them to form an interpenetrating network. The GO sheets are estimated to be 5-8 µm in size and highly agglomerated (Figure 3b, e). As shown in Figure S5b and 3c, Ti-based nanowires and RGO sheets exhibit chemical interactions, consistent with the Raman results (Figure 2b). At the same time, the RGO sheets overlap to form an interconnecting conduction network (Figure 3c, f) driven by π-π stacking interactions.55 The cross-section SEM of the electrode shows that the 34.8 µm thick electrode consists of a 33.7 µm thick N-TiO2/RGO hybrid and a 1.1 µm thick RGO current collector (Figure S6a, b). The TiO2/RGO hybrid is firmly anchored to the RGO current collector (Figure S6b), and the N-TiO2 nanowires tightly bind to RGO conductive linkers (Figure S6c) inside the electrode, both of which are beneficial for obtaining excellent structural stability and electronic conductivity of the freestanding electrode. The HRTEM image of N-TiO2 presents both anatase nanodomains and TiO2(B) nanodomains (Figure S7a) to form grain boundaries for additional lithium-ion storage.46,47 After the photocatalytic reduction and thermal reduction, the RGO sheets are obtained with an interlayer spacing of 0.35 nm (Figure S7b). To further investigate the nanostructure of the electrode, EDS elemental mapping images of N-TiO2/RGO hybrid are shown in Figure 3h-k. The carbon, nitrogen, oxygen and titanium elements are uniformly distributed in the hybrid, 12


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demonstrating a uniform distribution of N-TiO2 nanowires and RGO sheets. The nanostructure of the N-TiO2/RGO hybrid (Figure 3f) is similar to that of N-TiO2/RGO-M (Figure S8).

Figure 4. EPR curves of (a) NHTO, NHTO(UV), and NHTO/RGO hybrid, (b) N-TiO2/RGO-M, and N-TiO2/RGO hybrid. The chemical state of Ti is characterized by EPR (Figure 4). No obvious characteristic EPR signal is observed for NHTO and NHTO(UV) at 90 K, whereas an intense characteristic EPR signal is generated for NHTO/RGO hybrid at g = 2.04 (Figure 4a), indicating the presence of Ti3+ on the surface of NHTO nanowires. In fact, Ti4+ is easily converted to Ti3+ by the UV photocatalytic reduction in an ethanol dispersion. During the pre-UV irradiation, the color of the NHTO dispersion gradually turns from pale yellow to blue (Figure S9a, b), suggesting the presence of Ti3+.53 However, when GO is not involved in the reaction, the color of the NHTO dispersion rapidly changes back to pale yellow with a small amount of residual Ti3+ in the dispersion (Figures 4a, S9b, S9c) because most Ti3+ ions are oxidized by the oxygen from air that is dissolved in the dispersion.53 After annealing at 450 oC, the EPR

13



signal intensity of the N-TiO2/RGO hybrid is about 7.5 times that of the N-TiO2/RGO-M (Figure 4b) owing to the photocatalytic reduction. XPS spectra in Figure 5 further characterize the chemical bonds between N-TiO2 nanowires and RGO sheets formed by the efficient photocatalytic reduction. As depicted in Figure 5a, the characteristic peaks of hydroxyl, epoxide, and carboxyl of GO are observed at 284.5, 286.6 and 288.2 eV, respectively. After the UV irradiation, the peak intensities of the hydroxyl and epoxide groups are significantly weakened, while that of the carboxyl groups remains unchanged (Figure 5b), which is in line with the increased value of ID/IG in Figure 2b, demonstrating that GO is partially reduced to RGO. Simultaneously, an additional peak appears at 283.4 eV in the NHTO/RGO hybrid (Figure 5b), which is proved to be the characteristic peak of Ti3+-C bonds.15,54 After the thermal annealing, the Ti3+-C bonds remain in the N-TiO2/RGO hybrid, and the peak intensities of the hydroxyl and epoxide groups are rapidly weakened, indicating further reduction of RGO (Figure 5c). The formation of Ti3+-C bonds in NHTO/RGO and N-TiO2/RGO hybrids is corroborated by the redshift in the Raman spectra (Figure 2b) and the comparison of peak intensities in the EPR curves (Figure 4). The Ti 2p peaks of NHTO/RGO and N-TiO2/RGO hybrids appear at higher binding energies than those of NHTO/NHTO(UV) and N-TiO2 (Figure 5d), respectively, due to the electron-withdrawing inductive effect of the residual carboxylate on RGO.56 The major peaks at 457.6 and 463.5 eV in Figure 5e correspond to the characteristic peaks of Ti4+ 2p3/2 and Ti4+ 2p1/2, respectively. Two shoulder peaks centered at 459.1 and 464.9 eV (Figure 5e) are ascribed to Ti3+ 2p3/2 and Ti3+ 2p1/2, respectively, suggesting the presence of Ti3+ in the 14


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NHTO/RGO hybrid.57 A similar pair of shoulder peaks at 457.6 and 463.5 eV are found in Figure 5f, suggesting the retention of Ti3+ in the N-TiO2/RGO hybrid. N-doping is also realized in the N-TiO2/RGO hybrid, including the substituted atomic N species with a binding energy of 396.0 eV in the TiO2 lattice (Figure S10a, b).10 The N-doping enhances the electronic conductivity and ion diffusion kinetics of the nanostructured TiO2 by lowering its band gap.13,21

Figure 5. XPS spectra of C 1s for (a) GO, (b) NHTO/RGO hybrid, and (c) N-TiO2/RGO hybrid; XPS spectra of Ti 2p for (d) Ti-based nanomaterials, (e) NHTO/RGO hybrid, and (f) N-TiO2/RGO hybrid. The changes in the oxygen-containing groups of as-prepared GO are tracked by FT-IR spectra in Figure S11a. A broad peak at 3413 cm-1 and a sharp peak at 1726 cm-1 are assigned to the stretching vibrations of O-H and C=O, respectively. The characteristic peaks at 1384 and 1026 cm-1 are respectively ascribed to the stretching vibrations of epoxide (C-O) and 15



alkoxy (C-O). The skeletal vibration of C=C is located at 1623 cm-1. After the photocatalytic reduction and the annealing treatment, most characteristic peaks of the oxygen-containing functional groups disappear in the FT-IR spectrum of N-TiO2/RGO hybrid, with the exception of a weak carboxyl group peak at 805 cm-1. These changes indicate that the GO is substantially reduced, consistent with the abovementioned XPS results. The characteristic peak at 505 cm-1 represents the Ti-O stretching vibration in TiO2. No obvious differences in this region are observed for N-TiO2 and N-TiO2/RGO hybrid, confirming that Ti3+ forms only at the surface of N-TiO2 nanowires, and not within the nanowires.54 As estimated in Figure S11b, the contents of N-TiO2 in the N-TiO2/RGO hybrid and the hybrid electrode are as high as 81.2 and 73.4 wt.%, respectively. The RGO sheets enlarge the surface area of N-TiO2 nanowires from 142.5 to 174.7 m2 g-1 (Figure S11c, d). Thus, the Raman, EPR and XPS results reveal the formation of Ti3+-C bonds between the N-TiO2 nanowires and the RGO sheets by the efficient photocatalytic reduction approach. Although the photocatalytic formation of Ti3+-C bonds is proposed before,54 no semiconductor photocatalyst other than TiO2 has been reported. By using NHTO as a photocatalyst, our photocatalytic reduction approach exhibits the following advantages for high electrochemical performances of lithium-ion storage: (i) N-doping is achieved by the NHTO photocatalyst to compensate for the disadvantage of surface only Ti3+ formation, thus benefitting the rapid charge transfer in the electrode; and (ii) the NHTO nanowires facilitate the in situ construction of the 3D architecture for the long lifespan of LIBs by avoiding the damage of the nanostructure due to the ex situ construction after the annealing treatment. This 16


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photocatalytic reduction approach also provides a reference to improve the electrochemical performances of other titanate-based materials, such as lithium titanate and sodium titanate, by forming covalent bonds. 3.2. Electrochemical performances of the N-TiO2/RGO hybrid

Figure 6. (a) Rate performances of N-TiO2/RGO hybrid and N-TiO2/RGO-M. (b) The fourth galvanostatic discharge/charge profiles of N-TiO2/RGO hybrid electrode at different current rates. (c) CV curves of N-TiO2/RGO hybrid at a scan rate of 0.1 mV s-1. (d) Long-term galvanostatic cycling of N-TiO2/RGO hybrid at current densities of 5 C, 25 C, and 50 C for 10,000 cycles. (e) Cycling performances of N-TiO2/RGO hybrid at current densities of 50 C charge/5 C discharge for 1,000 cycles. The lithium-ion storage performances of the N-TiO2/RGO hybrid are evaluated by galvanostatic cycling at various rates (Figure 6a). The N-TiO2/RGO hybrid electrode exhibits an excellent rate performance compared with that of N-TiO2/RGO-M. Its specific capacity is 331 mA h g-1 for the second cycle at a current rate of 0.25 C, which is close to the theoretical 17



capacity of TiO2. As the rate increases to 50 C, the N-TiO2/RGO hybrid electrode presents a slight capacity loss and maintains a specific capacity of up to 198 mA h g-1; whereas the control electrode of N-TiO2/RGO-M suffers from a notable capacity fading, retaining only 42% of the specific capacity of the hybrid electrode. Figure 6b presents the corresponding galvanostatic discharge/charge profiles of the hybrid electrode. Three typical voltage regions occupy the galvanostatic discharge curves: region (I), lithium ion insertion into TiO2 by the solid-solution interface (> 1.75 V); region (II), lithium ion insertion into anatase phase by the diffusion control (≈ 1.75 V); and region (III), the long slope region corresponding to a pseudocapacitance and surface reactions of TiO2(B) phase (< 1.75 V).58 Since the anatase and TiO2(B) nanodomains are disorderly distributed in the TiO2 nanowires, the voltage plateaus corresponding to anatase TiO2 in region (II) is not very obvious. A dominant pair of S-peaks and a shoulder A-peak are observed in Figure 6c, assigned to pseudocapacitance/surface reactions of lithium-ion storage in TiO2(B) and diffusion-control lithium-ion storage in anatase, respectively.58 The N-TiO2/RGO hybrid exhibits excellent cycling stability as its CV curves are nearly overlapping after the second cycle. Long-term galvanostatic cycling up to 10,000 cycles is also carried out to determine the electrochemical stability of the N-TiO2/RGO hybrid (Figure 6d). The freestanding electrode reaches a steady Coulombic efficiency of nearly 100% after 10 cycles at current densities of 5 C, 25 C, and 50 C (Figure S12a-c). High capacities of 182.7, 125.1, and 101.8 mA h g-1 are retained at current densities of 5 C, 25 C, and 50 C, exhibiting capacity decay rates as low as 0.050‰, 0.045‰ and 0.033‰ per cycle, respectively. Simultaneously, Figure S13a and b show that the 18


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N-TiO2/RGO hybrid still retains its interpenetrating network after 10,000 cycles at the current rate of 50 C. In contrast, as shown in Figure S14, the N-TiO2/RGO-M electrode retains only 28.0% of the capacity at the current rate of 50 C. Intriguingly, the N-TiO2/RGO hybrid electrode still maintains a high discharge capacity of 180 mA h g-1 when subjected to the 50 C charge/5 C discharge for 1,000 cycles (Figure 6e). This result is of practical significance because it indicates that the fabrication of freestanding TiO2-based batteries for fast charging/durable discharging is feasible. Undoubtedly, such excellent electrochemical performances are related to the rapid interface charge transfer due to the formation of Ti3+-C covalent bonds. The well-constructed 3D architecture is also a crucial factor for the ultralong lifespan. To get an electrochemical insight into the reaction kinetics of the N-TiO2/RGO hybrid, Figure 7a shows the CV curves at various scan rates. The peak potential separation is small (0.53 V) as the scan rate increases from 0.1 to 1.1 mV s-1, implying a low polarization at high scan rates. The CV current (i) and scan rate (v) are related by the equation:15,59,60 i = avb where the b-value can be measured by the slope of the log10(ʋ)-log10(i) plot. The b-value indicates the lithium ion interaction behavior: the b-value of 0.5 represents the rigorous diffusion behavior; while the b-value of 1.0 implies a capacitive process. Figure 7b shows the b-value of the N-TiO2/RGO hybrid electrode is 0.84 in the scan rate ranges from 0.1 to 1.1 mV s-1, suggesting a rapid lithiation/delithiation process in the electrode even at high rates. This b value of 0.84 between 0.5 and 1, suggests combined contributions from 19



diffusion-controlled reactions and capacitive surface storage, benefiting from the Ti3+-C bonds as well as the N-doped anatase/TiO2(B) network. Furthermore, the interpenetrating network architecture of the N-TiO2/RGO hybrid electrode is beneficial for the good contact with the electrolyte. A contact angle close to 0o within 1 s is observed (Figure 7c), confirming the excellent wettability of the electrode.

Figure 7. (a) CV curves of N-TiO2/RGO hybrid at scan rates from 0.1 to 1.1 mV s-1. (b) The plot of log10(peak current)-log10(scan rate) for determining the b-value. (c) Optical pictures of the electrolyte droplets on the electrode surface at 0, 0.5, and 1 s. (d) CV curves of N-TiO2/RGO and N-TiO2/RGO-M at a scan rate of 0.1 mV s-1. (e) EIS curves of N-TiO2/RGO and N-TiO2/RGO-M. (f) Schematic illustration of rapid Li+/e- transport pathways in N-TiO2/RGO hybrid electrode. The CV curves of N-TiO2/RGO hybrid and N-TiO2/RGO-M at a scan rate of 0.1 mV s-1 are shown in Figure 7d. The S-peaks and A-peak are present in both the curves, revealing 20


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their similar lithium-ion storage behavior. However, the N-TiO2/RGO hybrid exhibits a larger integral area, indicating its higher Li storage capacity than that of N-TiO2/RGO-M. In addition, EIS is conducted on both electrodes (Figure 7e). Their Nyquist plots are composed of a semicircle in the high-frequency region and a sloping line in the low-frequency region, corresponding to the charge-transfer resistance (Rct) and the solid-state diffusion of lithium in this insertion material (ZW), respectively. The N-TiO2/RGO hybrid electrode has a smaller Rct than that of the N-TiO2/RGO-M electrode. The enhanced electronic conductivity and the short diffusion length of lithium ions (Figure 7f) enable the excellent electrochemical performances of the N-TiO2/RGO hybrid. To further demonstrate the importance of establishing chemical bonds between active materials and conductive substrates in freestanding anodes for lithium-ion storage, a comprehensive

comparison

is

performed

with

recently

reported

TiO2

nanostructure/conductive substrates (Figure 8). This N-TiO2/RGO hybrid electrode exhibits better rate performance and cycling stability than most TiO2 nanostructure/conductive substrates, including TiO2(B) nanosheets/carbon nanofibers,27 TiO2(B) nanosheets/RGO,29 3D carbon cloth/carbon nanowires@TiO2,61 TiO2 nanoparticles/3D carbon network,62 TiO2 nanoparticles/graphene foam,24 graphene/mesoporous TiO2,63 mesoporous TiO2/graphitized CNTs,64 TiO2 nanoparticles/carbon foam,65 and

N-doped

TiO2

nanocrystals/RGO

framework66. It is noted that some conductive substrates are graphene foam or carbon nanowires prepared by CVD,24,61 or graphitized graphene and carbon nanowires,63,64 which have superior conductivities as compared with that of RGO. Even so, the N-TiO2/RGO hybrid 21



electrode still has the superior electrochemical performances, especially at high rates, which is ascribed to the Ti3+-C bonds and the 3D robust architecture.

Figure 8. Comparisons of (a) rate performance and (b) cycling ability of the N-TiO2/RGO hybrid with other freestanding TiO2/RGO anodes that have been reported in recent years. 4. CONCLUSION An efficient photocatalytic reduction approach is developed to synthesize the N-TiO2/RGO hybrid with Ti3+-C bonds as a freestanding anode for high-performance lithium storage. The resultant N-TiO2/RGO hybrid electrode presents the key advantage of enhanced electronic conductivity owing to the covalent Ti3+-C bonds between N-TiO2 nanowires and RGO sheets, thus significantly facilitating the rapid interface charge transfer. Additionally, the N-doped TiO2 nanowires maintain an anti-agglomeration network, which effectively enhances the Li-ion diffusion kinetics by shortening the ion diffusion path and lowering the energy barrier, leading to excellent structural and cycling stabilities of the hybrid electrode. Therefore, the N-TiO2/RGO hybrid electrode achieves a high initial Coulombic efficiency of 88.7%, long-term cycling capacity of 101.8 mA h g-1 after 10,000 cycles at a high current rate of 50 C, 22


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and a high discharge capacity of 180 mA h g-1 after 1000 cycles of 50 C charge/5 C discharge, which is of practical significance for fast charging/durable discharging. It is expected this photocatalytic reduction approach can be applied to other titanate-based materials for enhancing Li-ion and Na-ion storage electrochemical performances.

ASSOCIATED CONTENT Supporting Information

Schematic illustrating the fabrication of N-TiO2/RGO hybrid anode; XRD patterns, EDS spectra, XPS curves, FT-IR spectra, TGA curves, and SEM and TEM images of NTO, NHTO, RGO, N-TiO2, NHTO/RGO, N-TiO2/RGO-M, and N-TiO2/RGO hybrid; digital pictures of NHTO suspensions; nitrogen adsorption and desorption isotherms; Coulombic efficiencies of N-TiO2/RGO hybrid; galvanostatic cycling of N-TiO2/RGO-M. AUTHOR INFORMATION Corresponding Authors E-mail: [email protected] (D.-Z. Yang); [email protected] (Z.-Z. Yu) ORCID Dongzhi Yang: 0000-0003-2592-7833 Zhong-Zhen Yu: 0000-0001-8357-3362 Notes The authors declare no competing financial interest. ACKNOWLEDGMENT

23



Financial support from the National Natural Science Foundation of China (51273015, 51521062, 51533001) and the National Key Research and Development Program of China (2016YFC0801302) is gratefully acknowledged. REFERENCES (1) Gelinck, G. H.; Huitema, H. E. A.; van Veenendaal, E.; Cantatore, E.; Schrijnemakers, L.; van der Putten, J. B.; Geuns, T. C.; Beenhakkers, M.; Giesbers, J. B.; Huisman, B. H. Flexible Active-Matrix Displays and Shift Registers Based on Solution-Processed Organic Transistors. Nature Mater. 2004, 3, 106. (2) Shao, Y.; El-Kady, M. F.; Wang, L. J.; Zhang, Q.; Li, Y.; Wang, H.; Mousavi, M. F.; Kaner, R. B. Graphene-Based Materials for Flexible Supercapacitors. Chem. Soc. Rev. 2015, 44, 3639-3665. (3) Bae, S.; Kim, H.; Lee, Y.; Xu, X.; Park, J. S.; Zheng, Y.; Balakrishnan, J.; Lei, T.; Kim, H. R.; Song, Y. I. Roll-to-Roll Production of 30-Inch Graphene Films for Transparent Electrodes. Nature Nanotech. 2010, 5, 574. (4) Xie, K.; Wei, B. Materials and Structures for Stretchable Energy Storage and Conversion Devices. Adv. Mater. 2014, 26, 3592-3617. (5) Tarascon, J. M.; Armand, M. Issues and Challenges Facing Rechargeable Lithium Batteries. Nature 2001, 414, 359. (6) Armand, M.; Tarascon, J. M. Building Better Batteries. Nature 2008, 451, 652. (7) Zhou, G.; Li, F.; Cheng, H. M. Progress in Flexible Lithium Batteries and Future Prospects. Energy Environ. Sci. 2014, 7, 1307-1338. 24


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Free-Standing Hybrid Film of Less Defective Graphene Coated with Mesoporous TiO2 for Flexible Lithium Ion Batteries with Fast Charging/Discharging Capabilities. 2D Mater. 2016, 4, 015011. (64) Liu, Y.; Elzatahry, A. A.; Luo, W.; Lan, K.; Zhang, P.; Fan, J.; Wei, Y.; Wang, C.; Deng, Y.; Zheng, G. Surfactant-Templating Strategy for Ultrathin Mesoporous TiO2 Coating on Flexible Graphitized Carbon Supports for High-Performance Lithium-Ion Battery. Nano Energy 2016, 25, 80-90. (65) Chu, S.; Zhong, Y.; Cai, R.; Zhang, Z.; Wei, S.; Shao, Z. Mesoporous and Nanostructured TiO2 Layer with Ultra-High Loading on Nitrogen-Doped Carbon Foams as Flexible and Free-Standing Electrodes for Lithium-Ion Batteries. Small 2016, 12, 6724-6734. (66) Liu, X. W.; Yang, Z. Z.; Pan, F. S.; Gu, L.; Yu, Y. Anchoring Nitrogen-Doped TiO2 Nanocrystals on Nitrogen-Doped 3D Graphene Frameworks for Enhanced Lithium Storage. Chem.-Eur. J. 2017, 23, 1757-1762.

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Efficient Photocatalytic Reduction Approach for Synthesizing

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