Cellulose Tailored Anatase TiO2 Nanospindles in Three-Dimensional

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Cellulose Tailored Anatase TiO Nanospindles in Three-Dimensional Graphene Composites for High-Performance Supercapacitors Yangbin Ding, Wei Bai, Jinhua Sun, Yu Wu, Mushtaque A. Memon, Chao Wang, Chengbin Liu, Yong Huang, and Jianxin Geng ACS Appl. Mater. Interfaces, Just Accepted Manuscript • DOI: 10.1021/acsami.6b02164 • Publication Date (Web): 26 Apr 2016 Downloaded from http://pubs.acs.org on April 30, 2016

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ACS Applied Materials & Interfaces

Cellulose Tailored Anatase TiO2 Nanospindles in Three-Dimensional Graphene Composites for High-Performance Supercapacitors

Yangbin Ding,†,‡ Wei Bai,† Jinhua Sun,† Yu Wu,† Mushtaque A. Memon,† Chao Wang,† Chengbin Liu,‡,* Yong Huang,† Jianxin Geng†,*

†

Technical Institute of Physics and Chemistry, Chinese Academy of Sciences, 29 Zhongguancun East Road, Haidian District, Beijing 100190, China, Email: [email protected]

‡

State Key Laboratory of Chemo/Biosensing and Chemometrics, College of Chemistry and Chemical Engineering, Hunan University, 2 Lushan South Road, Yuelu District, Changsha 410082, China

1 ACS Paragon Plus Environment


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Abstract: The morphologies of transition metal oxides have decisive impact on the performance of their applications. Here, we report a new and facile strategy for in situ preparation of anatase TiO2 nanospindles in three-dimensional reduced graphene oxide (RGO) structure (3D TiO2@RGO) using cellulose as both an intermediate agent eliminating the negative effect of graphene oxide (GO) on the growth of TiO2 crystals and as a structure-directing agent for the shape-controlled synthesis of TiO2 crystals. High-resolution transmission electron microscopy and X-ray diffractometer analysis indicated that the spindle shape of TiO2 crystals was formed through the restriction of the growth of high energy {010} facets due to preferential adsorption of cellulose on these facets. Because of the 3D structure of the composite, the large aspect ratio of the TiO2 nanospindles, and the exposed high-energy {010} facets of the TiO2 crystals, the 3D TiO2@RGO(Ce 1.7) exhibited excellent capacitive performance as an electrode material for supercapacitors, with a high specific capacitance (ca. 397 F g−1), a high energy density (55.7 Wh kg−1), and a high power density (1327 W kg−1) on the basis of the masses of RGO and TiO2. These levels of capacitive performance far exceed those of previously reported TiO2-based composites.

Keywords: Anatase TiO2, shape-controlled synthesis, cellulose, three-dimensional graphene composites, supercapacitors

Introduction Composites of graphene and transition metal oxides (TMOs) as energy-storage materials have attracted enormous attention among scientists because of their unique combined features of graphene and TMOs, such as large surface area, chemical stability, high electrical conductivity, and electrochemical activity.1−7 However, the synthesis of shape-controlled TMOs in the assembled


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structures of chemically derived graphene is challenging because of the following limitations: (i) the precursors of TMOs can be vigorously hydrolyzed in aqueous suspensions of graphene oxide (GO), leading to severe aggregation of the resultant TMO particles, and (ii) the strong interactions between the TMO nanocrystals and GO are not compatible with morphology-controlled growth of the inorganic nanocrystals. Recently, several groups have reported the synthesis of inorganic compounds in the presence of GO or functionalized GO.8,9 The abundance of functional groups and defects on the surfaces of GO sheets afford strong interactions with the inorganic species coated on GO surfaces, which provide pinning forces that hinder the diffusion of small particles and recrystallization and do not allow for tuning of the morphologies of the nanocrystals.8 Loh et al.10 have reported the synthesis of metal−organic framework nanowires using benzoic acid-functionalized graphene as a structure-directing template, in which the high density of carboxylic groups plays a decisive role in overcoming the intrinsic limitation of GO with few carboxylate functionalities in terms of the metal-chelation abilities. Recently, we have found that sheets of Ni−Al layered double hydroxide (LDH) preferentially attach to the surfaces of reduced GO (RGO) sheets when synthesized in the presence of GO because of strong interactions between the materials and that RGO/Ni−Al LDH nanowires can be obtained by adjusting the quantity of urea in the hydrothermal reactions.11 In the aforementioned studies,10,11 the obtained nanowires have exhibited higher electrical conductivities than the compounds with other morphologies. Given that controlled morphologies can enhance a material’s particular properties,11−13 developing an efficient approach for synthesis of morphology-controlled TMOs in assembled structures of graphene is essential for advanced applications of the graphene/TMO composites. Among the traditional semiconductor materials, TiO2 has been widely used in environmental and energy applications.14,15 The incorporation of TiO2 in graphene materials results in a combination of the features of the two components, e.g., the electrochemical activity of TiO2 and the electrical conductivity



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of graphene, and has led to notable applications in solar energy conversion,16−18 electrocatalysis,19−21 photocatalysis,22 and lithium-ion batteries (LIBs).12,13,23−25 In the aforementioned applications, the performance of graphene/TiO2 composites has been demonstrated to be closely dependent on the nanostructure and morphology of the TiO2 particles.12,13,16,17,23,24 To date, several strategies for synthesizing graphene/TiO2 composites have been reported, and they typically include two steps: synthesis of TiO2 nanoparticles and incorporation of the TiO2 nanoparticles into graphene materials.16,17,20,21,23−28 Although the two-step procedure avoids the interference of the functional groups of GO sheets with the nucleation and growth of TiO2 nanocrystals, it may have the drawback of low efficiency in terms of loading the TiO2 nanoparticles on the RGO sheets. In contrast, in situ methods have recently been reported for the synthesis of TiO2 in the presence of GO with the aid of surfactants, which facilitate the stable dispersion of graphene sheets in water and the efficient loading of TiO2 nanoparticles on the surfaces of RGO sheets.29,30 However, the TiO2 nanocrystals synthesized with these in situ methods have irregular morphologies.31−33 Therefore, the interfacial interactions between TiO2 nanocrystals and graphene materials should be tuned to obtain TiO2 nanocrystals with controlled morphologies through in situ synthesis methods. Here, we report a new and scalable strategy for the in situ preparation of anatase TiO2 nanospindles in a three-dimensional RGO structure (3D TiO2@RGO) using cellulose as a structure-directing agent to control the growth of the TiO2 crystals. In this strategy, compared with previously reported methodologies,28,31 the use of cellulose allows the simultaneous growth of the TiO2 nanospindles and the formation of the 3D RGO structures.11 Transmission electron microscopy (TEM) and X-ray diffraction (XRD) data indicated that the spindle shape of the TiO2 crystals is formed through the preferential growth along the [001] direction, whereas growth in the [010] direction is restricted by the adsorption of cellulose. Through this growth mechanism, the TiO2 crystals form a spindle shape with


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the {010} facets preserved. Because of the 3D RGO structure, spindle-shaped TiO2 crystals, and preserved high-energy {010} facets, our 3D TiO2@RGO(Ce 1.7) composite exhibits significantly improved performance as an electrode material for supercapacitors. In particular, this material has a high specific capacitance (ca. 397 F g−1), a high energy density (55.7 Wh kg−1), and a high power density (1327 W kg−1) relative to the masses of RGO and TiO2.

Experimental Section Chemicals. The GO used in this study was synthesized through a modified Hummers’ method.34 After cycles of centrifugation/dispersion in DI water, a GO hydrogel with concentration of ca. 3.4 wt% was obtained. Cotton linters were supplied by Hebei Jigao Chemical Fiber Co., Ltd. A NaOH/urea aqueous solution (7 wt% NaOH, 12 wt% urea, and 81 wt% deionized water) was used to dissolve the cellulose.35,36 Cellulose solutions with different concentrations, i.e., 0.4, 0.9, 1.7, 2.5, 3.4, and 6.8 wt%, were prepared to investigate the effect of cellulose on the morphologies of the TiO2 crystals. Hydrazine hydrate (50−60% in H2O) was purchased from Sigma–Aldrich. All other chemicals (extra purity grade) were obtained from Sinopharm Chemical Co., Ltd. Preparation of 3D TiO2@RGO composites. The 3D TiO2@RGO composites were synthesized by ball milling and hydrothermal processing. In a typical synthesis, the aforementioned GO hydrogel (4 mg) and cellulose solution (C = 3.4 wt%, 40 mg) were pre-mixed to obtain a GO/cellulose mixture (GO/cellulose ratio = 1:10). Subsequently, hydrazine hydrate solution (2 mL) and titanium tetrabutoxide (TTBO, 1.36 mL) were added to the mixture, which was immediately subjected to ball milling at 532 rpm for 6 h. The obtained homogenous mixture was transferred into a 50-mL Teflon-lined stainless steel autoclave. After being maintained at 180 °C for 10 h, the autoclave was allowed to cool to room temperature. The obtained monolith was washed with deionized water. Finally, the porous 3D



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TiO2@RGO composite was obtained by freeze-drying for 48 h and designated as 3D TiO2@RGO(Ce 3.4). The aforementioned cellulose solutions with different concentrations (C = 0.4, 0.9, 1.7, 2.5, 3.4, and 6.8 wt%) were also used in the synthesis, and the initial GO-to-cellulose ratios were designed to be 1:1.2, 1:2.5, 1:5, 1:7.5, 1:10, and 1:20, respectively. The resultant composites were designated as 3D TiO2@RGO(Ce x), wherein x represents the percentage of cellulose in the solutions used. In addition, different amounts of TTBO (i.e., 0.68, 1.36, 2.06, and 2.72 mL), corresponding to different TTBO-toGO mass ratios (i.e., 5:1, 10:1, 15:1, and 20:1), were used in the synthesis. Preparation

of

supercapacitors

and

electrochemical

characterization.

Electrodes

for

supercapacitors were prepared by mixing the porous 3D TiO2@RGO composites, acetylene black, and poly(tetrafluoroethylene) (PTFE, 60% dispersion in H2O) in a mass ratio of 70:25:5 with ethanol using a mortar and pestle. Acetylene black and PTFE were used as a conductive agent and binder, respectively. The mixture was rolled into 10−20-µm-thick sheets and then punched into 5-mm-diameter discs. After drying for 12 h under vacuum at 60 °C, each pair of discs was weighed, and their masses were determined to be ca. 2 mg. Each pair of dried electrodes was soaked in 6 M KOH aqueous solution for at least 24 h. Finally, the two electrodes and a porous separator (Celgard 3501) were impregnated with 6 M KOH electrolyte and sandwiched between a pair of stainless steel plates. This two-electrode cell configuration was used to measure the supercapacitance.37 Electrochemical measurements were performed on a Zennium 40088 electrochemical workstation. Cyclic voltammetry (CV) curves were collected over a voltage range from −0.5 to 0.5 V at various scanning rates: 1, 5, 10, 50, 100, and 500 mV s−1. Electrical impedance spectroscopy (EIS) data were collected with an AC perturbation of 5 mV over the frequency range of 100 kHz to 0.01 Hz. The specific capacitance, Cs (F g−1), was estimated from the galvanostatic charging/discharging curves using equation (1):


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Cs =

4 I∆t m∆U

(1)

where I is the discharge current (A), ∆t is the discharge time (s), ∆U is the potential window (V), and m is the total mass (g) of the active material in the two electrodes.38 The energy density, E (Wh kg−1), and the power density, P (W kg−1), of the electrode material were derived from the specific capacitance using equations (2) and (3), respectively.38

C s (∆U ) 2 E= 2 × 3.6

(2)

3600 × E t

(3)

P=

where t is the discharge time (s) of the charge/discharge curves.

General characterization. Scanning electron microscopy (SEM) observations were performed with a field-emission SEM (Hitachi S-4800) operated at an acceleration voltage of 5 kV. TEM images were collected on a JEOL-2100F microscope operated at an acceleration voltage of 200 kV. The crystallographic structures of the composites were determined by their powder XRD patterns collected on a Bruker D8 Focus diffractometer with an incident wavelength of 0.154 nm (Cu Kα radiation) and a Lynx Eye detector. Raman spectra were recorded on a Renishaw in Via-Reflex confocal Raman microscope with an excitation wavelength of 532 nm. TGA measurements were conducted with a TGA Q50 at a scanning rate of 5 °C min−1 in an air atmosphere.



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Results and Discussion To control the morphologies of TiO2 crystals in the graphene-based composites during in situ synthesis, a compound needs to function as both a structure-directing agent to control the morphologies of the TiO2 crystals and an isolating agent to eliminate the negative effect of GO on the growth of the TiO2 crystals. In this study, cellulose was used in the in situ synthesis of TiO2 crystals in the presence of GO (Figure 1a). The abundant hydroxyl groups in cellulose form hydrogen bonds with the polar groups of GO, thereby facilitating the construction of 3D porous structures in the resultant RGO composites.35 Cellulose also acts as a structure-directing agent for the morphology-controlled synthesis of TiO2 crystals because its monomer, glucose, functions as a crystal-growth-directing agent in the synthesis of faceted TiO2 crystals.31,39 GO is the precursor of RGO in the 3D porous structures, which allocate conductive frameworks for supporting TiO2 crystals and are currently predominant in electrode materials for energy-storage devices.40,41


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Figure 1. Schematic illustration of synthesis of 3D TiO2@RGO composites using cellulose as a structure-directing agent. (a) Synthesis route. (b−d) Optical images of (b) the monolith of 3D TiO2@RGO(Ce 3.4) composite obtained after hydrothermal processing, (c) a heart-shaped sample of the 3D TiO2@RGO(Ce 3.4) obtained by cutting the as-prepared monolith, and (d) the preserved heart shape of the 3D TiO2@RGO(Ce 3.4) after removal of water by freeze-drying.

Figure 1a illustrates the synthesis process, which includes ball milling and hydrothermal processing, used to synthesize the 3D porous composites of TiO2 crystals and RGO. Ball milling played a key role in maintaining the homogeneous dispersion of RGO sheets, which were chemically converted from GO 9 ACS Paragon Plus Environment


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sheets by hydrazine in cellulose.35 Titanic hydroxide, the hydrolysis product of TTBO with an initial TTBO/GO ratio of 10:1 and a GO/cellulose ratio of 1:10, was found to be homogeneously dispersed in the 3D porous RGO structure because of its interactions with cellulose (Figure S1a). Analyzing the XRD pattern of the aforementioned composite indicated that the titanic hydroxide particles were amorphous because only the Bragg reflections of cellulose were detected (Figure S1b). In the hydrothermal process, TiO2 crystals were formed through hydrolysis of the titanic hydroxide and were found to attach to the walls of the 3D RGO structures. The 3D TiO2@RGO(Ce 3.4) composite remained monolith-shaped because of the hydrogen bond interactions between cellulose and the residual polar groups on the RGO sheets (Figure 1b).33,42−45 Here, Ce 3.4 represents the concentration of cellulose solution used in the synthesis (for more detail, refer to the Experimental Section). The obtained monolith was flexible and robust enough to be cut into various shapes (Figure 1c). This result also indicated the robust attachment of TiO2 crystals to the walls of the RGO structures via cellulose. Finally, the monolith of the 3D porous 3D TiO2@RGO(Ce 3.4) composite was preserved after the removal of water by freeze-drying (Figure 1d).


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Figure 2. SEM images of the 3D TiO2@RGO composites obtained with different amounts of cellulose: (a, b) 3D TiO2@RGO(Ce 0.4), (c, d) 3D TiO2@RGO(Ce 0.9), (e, f) 3D TiO2@RGO(Ce 1.7), (g, h) 3D TiO2@RGO(Ce 2.5), (i, j) 3D TiO2@RGO(Ce 3.4), and (k, l) 3D TiO2@RGO(Ce 6.8). The SEM images shown in (b, d, f, h, j, and l) were taken at a higher magnification.



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To investigate the effect of cellulose on the morphology of the TiO2 crystals, a series of 3D TiO2@RGO composites were synthesized using different amounts of cellulose and maintaining the TTBO/GO ratio at 10:1. The amount of cellulose was varied by using cellulose solutions with different concentrations. When low-concentration cellulose solutions were used (C = 0.4 and 0.9 wt%)—the ratios of GO to cellulose were calculated to be 1:1.2 and 1:2.5—no monoliths were formed after hydrothermal processing. The failure to form monoliths was ascribed to insufficient hydrogen bond interactions because of the low cellulose contents of the composites. SEM observations revealed that the 3D TiO2@RGO(Ce 0.4) composite showed porous structures and that the hydrolysis product of titanic hydroxide after the hydrothermal process exhibited irregularly shaped aggregates (Figure 2a, b), which were later confirmed to be amorphous by XRD. In contrast, when a cellulose solution of C = 0.9 wt% was used, the TiO2 crystals showed spindle-like shapes in the 3D TiO2@RGO(Ce 0.9) composite (Figure 2c, d). Copious SEM observations indicated that the nanosized spindles had coarse surfaces and relatively uniform sizes, with diameters ranging from 100 to 200 nm and lengths ranging from 600 to 700 nm. The coarse surfaces may have originated from the attached cellulose, which would facilitate electron deposition because of its polar functional groups. When higher-concentration cellulose solutions (C = 1.7, 2.5, and 3.4 wt%, i.e., GO/cellulose = 1:5, 1:7.5, and 1:10) were used, the 3D TiO2@RGO composites exhibited monoliths after hydrothermal processing, which were found to have robust mechanical properties and were capable of being cut into various shapes. SEM observations indicated that these 3D TiO2@RGO composites exhibited porous structures (Figure 2e, g, i), and the TiO2 crystals showed spindle-like shapes (Figure 2f, h, j). Upon careful inspection of the TiO2 crystals obtained with different amounts of cellulose, the aspect ratios of the spindles were found to increase as the GO/cellulose ratio changed from 1:2.5 to 1:10. Further increasing the concentration of cellulose in solution (C = 6.8 wt%, i.e., GO/cellulose = 1:20) resulted in TiO2 crystals with spindles with a higher


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aspect ratio (Figure 2k, l). Based on the aforementioned data, it can be concluded that the existence of cellulose not only favors the formation of porous structures in the 3D TiO2@RGO composites but also functions as a structure-directing agent, controlling the growth of the TiO2 crystals.

Figure 3. TEM and HRTEM images of the TiO2 spindles obtained by including different amounts of cellulose in the synthesis: (a, b) 3D TiO2@RGO(Ce 1.7), (c, d) 3D TiO2@RGO(Ce 2.5), (e, f) 3D TiO2@RGO(Ce 3.4), and (g, h) 3D TiO2@RGO(Ce 6.8). The HRTEM images were taken from the indicated areas of the TiO2 spindles shown in the respective TEM images.

To elucidate the structure of the TiO2 spindles and explore how cellulose controlled their growth, individual spindles were studied using TEM. The TiO2 spindles in the 3D TiO2@RGO composites prepared with different amounts of cellulose (concentrations of cellulose solutions = 1.7, 2.5, 3.4, and 6.8 wt%) showed similar morphological features (Figure 3a, c, e, g). Each TiO2 spindle consisted of a trunk and two triangle-shaped ends. Careful inspection of the TEM images revealed that the truck areas were coated with a layer of cellulose and that the triangle-shaped ends had smooth surfaces. These 13 ACS Paragon Plus Environment


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findings indicated that cellulose had been preferentially adsorbed on the facets of the trunks of the TiO2 spindles, thereby reducing the surface energy. In contrast, because of the high surface energy of the exposed {001} facets, the growth along the [001] direction was faster, and the TiO2 crystals exhibited a spindle shape with the long axis pointing in the [001] direction. In agreement with the SEM data, TEM observations also indicated that the specific ratio of the TiO2 spindles increased as the cellulose content used in the synthesis increased. Thus, higher amounts of cellulose adsorbed on the {010} facets of the TiO2 spindles enhanced the restriction on the growth of these facets. High-resolution TEM (HRTEM) images of TiO2 crystals were taken from the tips of the respective spindles (Figure 3b, d, f, h). In these HRTEM images, three sets of lattice fringes were identified with spacings of 3.5, 3.5, and 2.4 Å, which were assigned to the (101), (−101), and (002) planes of the anatase phase of the TiO2 crystals.46 The angle between the (101) and (−101) planes was measured to be 43.4°, which is in agreement with the value calculated from the lattice constants of anatase TiO2 (space group I41/amd, a = b = 3.7852 Å, c = 9.5139 Å).47,48 Based on the assignment of the lattice fringes shown above, the two side edges and the front edge of the triangle-shaped tips were recognized as the {101}, {−101}, and {001} facets, respectively.49 Because the lattice fringes of the (101), (−101), and (002) planes were noted in the HRTEM images, the incident electron beam shone on the TiO2 crystals along the [010] zone. Therefore, the exposed facets of the trunk parts of the TiO2 spindles were identified as {010} facets. Thus, the lattice fringes of the (010) planes are not readily observed because these TiO2 spindles tend to lie on their {010} facets.45 In agreement with the HRTEM data, selected area electron diffraction (SAED) patterns confirmed the crystal orientation of the TiO2 spindles (Figure S2). Therefore, on the basis of the TEM images of the TiO2 spindles, the crystal structures illustrated in the HRTEM images, and the SAED patterns, it can be concluded that cellulose molecules are preferentially absorbed on the {010} facets of the TiO2 spindles.


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Figure 4. XRD characterization of the crystal structure of the TiO2 spindles: (a) XRD patterns of 3D TiO2@RGO composites, (b) extended XRD patterns that were normalized with respect to the intensity of the (200) Bragg reflection.

The crystal structures of the 3D TiO2@RGO composites were further characterized using XRD (Figure 4a). The 3D TiO2@RGO(Ce 0.4) composite did not contain crystalline TiO2 crystals because no Bragg reflections corresponding to TiO2 crystals were detected in its XRD pattern. This finding was ascribed to the existence of plentiful oxygen-containing groups on the surfaces of the GO sheets, which interfered with the growth of TiO2 crystals. In contrast, when high-concentration cellulose solutions



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were used (C = 0.9, 1.7, 2.5, 3.4, and 6.8 wt%), the Bragg reflections of TiO2 crystals were readily detected and were indexed based on anatase TiO2.46 The broad reflection peaks at ca. 20.2 and 22.1° were assigned to the (110) and (200) Bragg reflections of the cellulose II structure, respectively.50 Figure 4b shows the enlarged XRD patterns that were normalized with respect to the intensity of the (200) Bragg reflection. It can be clearly seen that the intensity of the (004) Bragg reflection relative to that of the (200) Bragg reflection increased with the cellulose concentration, suggesting that the evolution of the TiO2 crystals along the [001] direction was more progressive than that along the [200] direction.45,51 This preferential growth eventually resulted in the spindle-shape TiO2 crystals, and the aspect ratio increased with the cellulose concentration used in the synthesis. The XRD findings were in agreement with the conclusions drawn from the SEM and TEM observations. Therefore, the presence of cellulose not only served as an intermediate agent to eliminate the negative effect of GO on the growth of TiO2 crystals but also as a structure-directing agent for the shape-controlled growth of TiO2 crystals. A series of 3D TiO2@RGO composites was also synthesized by varying the TTBO/GO ratios, i.e., 5:1, 10:1, 15:1, and 20:1. SEM and XRD observations indicated that the TiO2 crystals retained their spindle shapes and the anatase phase at all tested TTBO/GO ratios (Figure S3a−e). These results suggested that the shape-controlled growth of the TiO2 crystals was primarily determined by cellulose and not the TTBO/GO ratio. Furthermore, energy-dispersive X-ray spectroscopy analysis showed that the Ti content in the 3D TiO2@RGO composites increased linearly as function of the initial amount of TTBO used in the synthesis (Figure S3f). This result was superior to those of previous studies in which the shapes of TiO2 particles were influenced by the amount of precursor.45,52−55 The advantage of the TiO2 crystals in terms of good retention of the spindle shape was ascribed to the dual functions of cellulose in the synthesis of the 3D TiO2@RGO composites.


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Figure 5. Compositions, interfacial interactions, and electrical properties of the 3D TiO2@RGO composites. (a) TGA curves of the composites. (b) Raman spectra of the composites. (c) I−V curves of the composites. The inset shows the electrical conductivities of the 3D TiO2@RGO composites. 17 ACS Paragon Plus Environment


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To investigate the compositions of the 3D TiO2@RGO composites, thermogravimetric analysis (TGA) curves were collected in an air atmosphere (Figure 5a). These curves revealed that the composites experienced two significant weight losses, which were ascribed to the thermal decompositions of cellulose and RGO, respectively, according to our previous reports.11,35 From the differential TGA curve of the 3D TiO2@RGO(Ce 0.4) composite (Figure S4a), thermal decomposition of the cellulose and the RGO were detected at ca. 320 and 500 °C, respectively. Increasing the quantity of cellulose in the composites led to decreased thermal decomposition temperatures of RGO, eventually to 430 °C for the 3D TiO2@RGO(Ce 6.8) composite (Figure S4b). This occurrence might be caused due to the improved dispersability of RGO sheets in the presence of high contents of cellulose, resulting in an improved accessibility of the RGO sheets to the atmospheric air after the thermal degradation of cellulose. In addition, the gradual weight loss between the temperatures of the two significant weight losses was also assigned to the thermal decomposition of the RGO sheets, in agreement with the literature.11 Thus, the contents of RGO and TiO2 as a whole were respectively estimated to be ca. 68.7, 62.4, 53.1, 45.6, 36.6, and 33.4 wt% for the 3D TiO2@RGO(Ce x) composites (x=0.4, 0.9, 1.7, 2.5, 3.4, and 6.8). Elemental compositions of the composites were obtained from XPS analysis (Figure S5 and Table S1) and the variation of Ti content in the composites was consistent with the TGA data. Raman spectra indicated that the presence of the TiO2 spindles in the 3D TiO2@RGO composites shifted the G band towards lower frequency (i.e., 1598 cm−1 for RGO and 1587 cm−1 for the composites) (Figure 5b), conforming the electron transfer from TiO2 spindles to RGO sheets in the composites.41 The electrical conductivities of the 3D TiO2@RGO composites were measured using tablet samples.11 I–V curves of the composites were collected, and the electrical conductivities were obtained from the slopes of the I–V curves (Figure 5c). The 3D TiO2@RGO(Ce 0.4) composite contained the lowest amount of cellulose among the tested samples, but its electrical conductivity was rather low for a 3D TiO2@RGO composite.


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This result was ascribed to the amorphous character of the Ti-containing compound. In contrast, upon formation of the TiO2 spindles, the 3D TiO2@RGO(Ce 0.9) composite showed a markedly enhanced electrical conductivity. This finding was in agreement with our previous report showing that high-aspect ratio spindles facilitate efficient electrical transport along the axial direction.11 The RGO sheets attached to the TiO2 spindles created a robust 3D network, which resulted in composites with high electrical conductivities. However, further increasing the amount of cellulose in the synthesis decreased the electrical conductivities of the corresponding composites because of the insulating properties cellulose.

Figure 6. Electrochemical characterization of 3D TiO2@RGO composites as electrode materials for supercapacitors. (a) CV curves for 3D TiO2@RGO composites obtained at a scan rate of 50 mV s−1. (b) CV curves collected for a 3D TiO2@RGO(Ce 1.7) composite at different scan rates. (c) GV charge/discharge profiles for 3D TiO2@RGO composites obtained at a current density of 0.36 A g−1. (d) Nyquist plots for 3D TiO2@RGO composites measured over a frequency range of 100 kHz to 0.01 Hz with an AC perturbation of 5 mV; the fit curves (solid lines) were obtained using the equivalent circuit



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shown in Figure S7. (e) GV charge/discharge profiles collected for the 3D TiO2@RGO(Ce 1.7) composite at different current densities (left Y axis), and the long-term stability measured over nearly 5000 cycles at a current density of 1 A g−1 (right Y axis). (f) The relationship between power density and energy density for the 3D TiO2@RGO(Ce 1.7) composite.

After obtaining the 3D TiO2@RGO composites, we next explored their capacitive performances. Figure 6a shows the CV curves collected from the supercapacitor cells prepared using the 3D TiO2@RGO(Ce x) composites (x=0.4, 0.9, 1.7, 2.5, 3.4, and 6.8) as electrode materials at a scan rate of 50 mV s−1 in a potential window from −0.5 to 0.5 V. Except for the 3D TiO2@RGO(Ce 0.4) composite, all of the composites exhibited CV loops with rectangular shapes, indicating ideal electrochemical double-layer capacitive behavior. The distinctly different CV loops indicated that crystalline TiO2 is required to achieve good capacitive performance. Among the 3D TiO2@RGO composites that contained TiO2 crystals, the 3D TiO2@RGO(Ce 1.7) composite’s CV loop had the largest integrated area, indicating that this composite had the highest capacity. This result was attributed to the tradeoff between the 3D porous structure, the contents of RGO and TiO2, and the electrical conductivity of the 3D TiO2@RGO(Ce 1.7) composite. Meanwhile, inspection of the CV loops revealed humps, implying pseudocapacitance behaviors attributable to the electrochemical adsorption of cations on the surface of the TiO2 crystals through a charge-transfer process.28,56 Investigation of the CV loops of bare TiO2 crystals clarified the pseudocapacitance of TiO2 (Figure S6). As such, the 3D TiO2@RGO(Ce 1.7) composite was identified as an excellent electrode material for supercapacitors because of the contributions from both electrochemical double-layer capacitance and pseudocapacitance. In addition, the 3D TiO2@RGO(Ce 1.7) composite showed good retention of the ideal capacitive behavior, because the rectangular shape of the CV loop was maintained at high scan rates of up to 500 mV·s−1 (Figure 6b).


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This high capacitive performance was attributed to the conjunction of RGO with the TiO2 spindles, which resulted in a relatively high electrical conductivity of the 3D TiO2@RGO(Ce 1.7) composite (Figure 5c), and to the porous structure of the composite, which facilitated readily access of the electrode materials by the electrolyte. Figure 6c shows the galvanostatic (GV) charge/discharge profiles collected at 0.36 A g−1 for the supercapacitor cells prepared using 3D TiO2@RGO composites as electrode materials. Consistent with the CV results, the GV profiles also indicated that the 3D TiO2@RGO(Ce 1.7) composite exhibited the best capacitive performance among the 3D TiO2@RGO composites. The specific capacitance was calculated to be ca. 214 F g−1 on the basis of the mass of the 3D TiO2@RGO(Ce 1.7) composite, i.e., ca. 397 F g−1 (on the basis of the masses of RGO and TiO2). The energy density and power density were calculated to be 55.7 Wh kg−1 and 1327 W kg−1, respectively, on the basis of the masses of RGO and TiO2. These levels of performance as electrode materials for supercapacitors far exceed those of previously reported TiO2-based composites.28,57−61 EIS was used to investigate the electrode dynamics of the supercapacitor cells (Figure 6d). Each Nyquist plot consisted of a squeezed semicircle at high frequencies and a straight line at low frequencies. The Nyquist plots were fit using a commonly adopted equivalent circuit (Figure S7).62−64 As summarized in Table S2, all of the 3D TiO2@RGO composites showed low serial resistances (Rs, from 0.30 to 0.49 Ω), which comprise the electrolyte solution resistance and contact resistance at the interface of active material/current collector and are recognized as the intercepts between the impedance spectra and real impedance axis (Z'). The 3D TiO2@RGO(Ce 1.7) composite exhibited the lowest chargetransfer resistance (Rct1, 0.52 Ω) among the 3D TiO2@RGO composites. The low Rct1 reflected a rapid electron transfer at the electrode/electrolyte interface, which could be attributed to the polarized electrode material surface due to electron transfer from TiO2 spindles to RGO (Figure 5b) and the robust 3D porous structure of the 3D TiO2@RGO(Ce 1.7) composite. Moreover, the 3D TiO2@RGO(Ce 1.7)



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composite exhibited a straight line with the highest slope at low frequencies, which is related to Warburg impedance (W) and indicated the highest diffusion of ions into the electrode materials.62 Consistent with the literature,62−64 an additional capacitance Cdl2 was recognized in the equivalent circuit. The Cdl2 is commonly ascribed to the leakage resistance or some blocking oxide film.62,63 In this research, the Cdl2 may reflect an additional interface in the electrode material because of its composite system. The 3D TiO2@RGO(Ce 1.7) was found to exhibit the lowest Rct2 among the 3D TiO2@RGO composites. The supercapacitor cells prepared with the 3D TiO2@RGO(Ce 1.7) composite were also measured at different charge/discharge current densities. These cells exhibited reversible charge/discharge behavior at all of the current densities tested from 0.36 to 3.57 A g−1 (Figure 6e, left Y axis). This finding was ascribed to the rapid formation of electrochemical double-layer charges and the fast Faradaic process of the surface atoms of the TiO2 crystals.65−67 Accordingly, the energy densities and power densities of the 3D TiO2@RGO(Ce 1.7) composite were calculated at the various current densities. Figure 6f shows the relationship between the power density and energy density. Notably, the 3D TiO2@RGO(Ce 1.7) composite exhibited higher power density and energy density in a new area in the well-accepted Ragone plot of various electrical energy-storage devices (Figure S8).68 The outstanding energy storage performance of the 3D TiO2@RGO(Ce 1.7) composite was ascribed to its unique structures. First, the cellulose-modulated porous structure provided a large specific contact area between the composite and electrolyte, thus leading to high electrochemical double-layer capacitance. Second, the nanosized distribution of the TiO2 spindles and their faceted shape played an important role in achieving the high capacitance. The high-energy {010} facets, as reported previously,60,69,70 may improve the charge deposition because of the presence of unsaturated coordinated surficial atoms (4fold-coordinated Ti and 2-fold-coordinated O). The two aforementioned features resulted in high


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specific capacitance and high energy density. Third, the conjugation of the RGO network with the spindle-shaped TiO2 crystals resulted in high electrical conductivity of the composite, which guaranteed high power density at higher current rates. Finally, the stability of the supercapacitor cells prepared with the 3D TiO2@RGO(Ce 1.7) composite was tested by GV charge/discharge cycling at a current density of 1 A g−1. Figure 6e (right Y axis) shows that the specific capacitance exhibited a slight drop during the first 2300 cycles, followed by a relatively stable maintenance of the specific capacitance. Further cycling confirmed the long-term cycling stability of the supercapacitor cell because the specific capacitance remained ca. 75% of its initial value after 5000 cycles.

Conclusions We report a new methodology that is facile and scalable and allows the in situ preparation of shape-controlled TiO2 crystals in 3D RGO structures. The cellulose used in the synthesis functions as both an intermediate agent eliminating the negative effect of GO on the growth of TiO2 crystals and as a structure-directing agent for the synthesis of TiO2 nanospindles. Analysis of the HRTEM images revealed that the TiO2 nanospindles formed through preferential growth along the [001] direction, whereas the growth of {010} facets was restricted by cellulose adsorption. Supercapacitors prepared using the 3D TiO2@RGO(Ce 1.7) composite as an electrode material exhibited excellent electrochemical performance, i.e., high specific capacity (ca. 397 F g−1), high energy density (55.7 Wh kg−1), and high power density (1327 W kg−1) based on the masses of RGO and TiO2. Overall, the methodology described herein offers a new avenue for the synthesis of TMO-graphene hybridized nanostructures for use in energy-storage devices. We believe that this strategy may also inspire the preparation of other 3D porous structures for applications in other areas, including catalysis, photocatalysis, selective adsorption, and separations.



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Acknowledgements. This research was supported by the “Hundred Talents Program” of Chinese Academy of Sciences and the National Natural Science Foundation of China (U1362106).

Supporting Information: SEM image and XRD pattern of the titanic hydroxide/RGO composite, TEM images and SAED pattern of the TiO2 spindles in 3D TiO2@RGO(Ce 1.7), characterization of the 3D TiO2@RGO(Ce 1.7) composites synthesized with different TTBO/GO ratios, TGA and DTGA curves of TiO2@RGO(Ce 0.4) and TiO2@RGO(Ce 6.8) composites, XPS analysis and elemental composition of 3D TiO2@RGO composites, A CV curve of bare TiO2 nanospindles, the equivalent circuit used for fitting the Nyquist plots and summary of the kinetic parameters of the 3D TiO2@RGO composites, the Ragone plot of power density versus energy density for various electrical energy-storage devices and the 3D TiO2@RGO(Ce 1.7) composite. This material is available free of charge via the Internet at http://pubs.acs.org.

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Cellulose Tailored Anatase TiO2 Nanospindles in Three-Dimensional

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