Preparation of Porous TiO2 from an Iso-Polyoxotitanate Cluster for

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C: Energy Conversion and Storage; Energy and Charge Transport 2

Preparation of Porous TiO from an Iso-Polyoxotitanate Cluster for Rechargeable Sodium Ion Batteries with High Performance Guanyun Zhang, Chenxiao Chu, Jian Yang, Chen-Ho Tung, and Yifeng Wang J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.9b00213 • Publication Date (Web): 05 Mar 2019 Downloaded from http://pubs.acs.org on March 6, 2019

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The Journal of Physical Chemistry

Preparation of Porous TiO2 from an IsoPolyoxotitanate Cluster for Rechargeable Sodium Ion Batteries with High Performance Guanyun Zhang, Chenxiao Chu, Jian Yang, Chen-Ho Tung and Yifeng Wang* Key Lab for Colloid and Interface Science of Ministry of Education, School of Chemistry and Chemical Engineering, Shandong University, Jinan 250100, P. R. China

Abstract. By aerobic calcination of a new titanium-oxide cluster (TOC) isolated from water, i.e., [Ti6O8(OH2)20]·(2,7-NDS)4·2HCl·14H2O (Ti6-NDS; 2,7-NDS = 2,7-naphthalenedisulfonate), porous anatase TiO2 was facially prepared. The sample obtained at 400 oC exhibited a surface area of 108 cm3 g-1 and an average pore diameter of 16 nm. When used as an anode material for sodium ion battery (SIB), it showed larger specific capacity and far superior rate performance than the other TiO2 samples, including those prepared from another three TOCs under the same conditions or from Ti6-NDS under different conditions and the one obtained from a commercial source. Mechanistic study indicates that the hierarchical structure surrounding the Ti-oxide cores of Ti6-NDS is pivotal for the formation and the SIB performance of the porous TiO2. This surfactant-free and catalyst-free strategy for the fabrication of TiO2 materials with special morphologies by thermal transformation of a TOC, provides a new optional protocol for the preparation of promising materials stable and efficient in SIBs.

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1. INTRODUCTION During the past decades, titanium dioxide nanomaterials have received intensive research interests of many scientists due to the promising and broad applications in pigments, solar energy harvesting and gas sensing.1 Recently, TiO2 has also been exploited as a promising anode material for energy storage, especially for sodium ion batteries (SIBs), because of its low cost, low-toxicity, high abundance, high theoretical capacity comparable to graphite (335 vs 372 mAh g-1), and high safety arising from both its stable structure and the high potential for Na+ intercalation (ca. 0.2–2.0 V vs Na/Na+).2-8 Among the various types of TiO2 nanomaterials, porous TiO2 is very attractive since it provides a large surface area and the greatly enriched active sites for catalysis. As for energy storage application of TiO2, a porous construction could effectively alleviate the volume expansion after intercalation of Na+ ions, shorten the diffusion path and enhance the mobility of Na+, and promote the permeation of electrolyte. These advantages may ultimately lead to remarkable rate performance of porous TiO2 in SIBs.3-4, 9-12 To date, fabrication of porous TiO2 materials mainly relies on template methods.10, 12-13 For example, the soft templating method which uses surfactants as templates is a common strategy and widely employed for preparation of porous TiO2 of tunable pore diameter by appropriate surfactant selection.14-15 During the synthesis, a Ti4+ precursor and a template form a hybrid inorganic-organic material in which the template molecules orderly organize into worm-like topography enclosed by the amorphous TiO2 hosts. The subsequent procedures (e.g., calcination and dissolution) to remove the templates leave the TiO2 hosts with tubular/porous structural features.15-16 Mono-nuclear TiIV precursors like TiCl4 and Ti(OiPr)4 are the most commonly used precursors. While there still remain many challenges in the preparation of nanostructured TiO2 with large surface area and controllable pore size, the use of discrete titanium-oxide clusters (TOCs) for preparation of porous TiO2 without additional templates has not been reported. Metal-oxide clusters are a class of discrete molecular materials with applications in many fields.1719

Among them, TOCs are very attractive for their roles as prenucleation clusters and as molecular

analogues of TiO2.20-21 They are also used as building blocks for construction of titania-based organicinorganic hybrid assemblies22-23 and in studying structures and electronic/photocatalytic properties of titania.24-25 Recently, we have isolated a few novel TOCs from highly acidic aqueous solutions of


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TiIV.26-29 Generally, in order to obtain a TOC, it is essential to use certain passivating ligands like alkoxide, amino acid and sulphate or stabilizing cations like tetrabutylammonium, which can form ordered organizations surrounding the Ti-oxide cores.29-30 The structure of a crystalline TOC may be viewed as many Ti-oxide cores distributed in a matrix of C, O, H and other atoms like S. Hence we envision that if such a crystalline material could be calcined to remove the light elements, the resultant TiO2 materials may exhibit some special morphology, like porous architectures (see Scheme 1). Herein, we report this new protocol could be used to prepare a porous TiO2 material by using an isopolyoxotitanate cluster as the precursor in a simple calcination process. The obtained TiO2 sample was used as an SIB anode material and showed outstanding rate performance.

Scheme 1. Schematic illustration for preparation of porous TiO2 via calcination of a crystalline TOC. The Ti-oxide core of a TOC is represented by a tetragon. A crystalline TOC sample may be viewed as many Ti-oxide cores embedded in the “sea” of C, O, H and other atoms drawn in grey, red, yellow, and blue, respectively. Four representative TOCs were chosen as the precursors for the preparation of TiO2 by aerobic calcination (Figure 1). Compound [Ti6O8(OH2)20]·(2,7-NDS)4·2HCl·14H2O (Ti6-NDS) is new, and contains the same [Ti6O8(OH2)20]8+ (Ti6) cluster as that of Ti6O8(OH2)20Cl8∙6TBAC∙4H2O (Ti6-TBAC) recently reported by us.27 Ti6 adopts μ2-O to connect all the Ti-ions and terminal -OH2 ligands to wrap the cluster’s core, and meanwhile it is a polyoxocationic cluster, all typical of the TOCs isolated from water.26-29 Compounds Ti17O24(OiPr)20 (Ti17) and Ti16O16(OEt)32 (Ti16) are typical titanium-oxo alkoxides which have been known for nearly 20 years. Importantly, although Ti6-NDS and Ti6-TBAC contain the same Ti6 cluster and meanwhile all the four compounds contain similar Ti-oxide cores, only Ti6-NDS formed porous TiO2 by aerobic calcination. The four compounds are realistic models for understanding how the hierarchical structures of the TOCs precursors affect the final TiO2 morphology and performance. This work provides a new strategy for preparation of TiO2 nanomaterials of special morphologies which are potential candidates in applications like SIBs and photocatalysis.



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Figure 1. Ball-and-stick structural views of [Ti6O8(OH2)20]8+ (Ti6), Ti17O24(OiPr)20 (Ti17) and Ti16O16(OEt)32 (Ti16). Color scheme: Ti, blue; O, red; C, grey. Hydrogen atoms are omitted for clarity. 2. EXPERIMENTAL SECTION 2.1 Materials. Disodium salt of 2,7-NDS was purchased from Aladdin Co. Ltd. Compounds Ti6TBAC, Ti16 and Ti17 were synthesized following the published protocols.27, 31-33 All of them were obtained as crystals and were characterized with single crystal X-ray diffraction analyses and IR spectroscopy. All other chemicals were of analytical grade, obtained from commercial sources and used as received. 2.3 Synthesis of Ti6-NDS. A mixture of TiCl4 (1.0 M; 10 mL) and 2,7-NDS disodium salt (0.4 M) was stirred in a 10 mL vial for 10 min under ambient conditions. After filtration through a 0.22 μm membrane, the solution was sealed in the vial and stored in a refrigerator at 4 °C. Colorless needlelike crystals of 1 formed in three days. The product was collected by filtration using a Buchner funnel under reduced pressure for ca. 10 min until dry. Phase purity was checked by IR, Raman and powder X-ray diffraction (PXRD; Figures S1-S3). The yield was ca. 64% based on Ti. Anal. Calcd (%): Ti, 12.79; C, 21.37; H, 4.19. Found (%): Ti, 13.38; C, 19.21; H, 3.65. Characteristic IR bands [cm-1]: 471 (w), 502 (w), 536 (w), 546 (w), 565 (w), 605 (m), 624 (w), 667 (w), 698 (s), 841 (s), 903 (s), 1026 (sh), 1104 (s), 1144 (m), 1163 (w), 1201 (m), 1268 (w), 1320 (w), 1413 (w), 1498 (w), 1584 (w), 1625 (b), 3105 (vb) (Figure S2). Characteristic Raman bands [cm-1]: 104 (s), 130 (s), 352 (w), 397 (w), 567 (w), 778 (sh), 913 (s), 1050 (sh), 1101 (sh), 1151 (w), 1204 (w), 1394 (sh), 1452 (m), 1583 (m), 1630 (w) (Figure S3). Crystallographic data for 1 (CCDC deposition no 1864981): Formula, C40H94Cl2O66S8Ti6; MW = 2245.69 g mol-1; T = 173 K; crystal system, triclinic; space group, P-1; a = 13.6291(9) Å, b = 19.1811(12) Å, c = 19.2371(13) Å; α = 73.182(4) °, β = 72.626(4) °, δ = 74.703(4) °; V = 4506.8(5) Å3; Z = 2; ρcald = 1.633 g cm-3 ; μ = 0.862 mm-1; F(000) = 2252.0; 2θmax = 55.168;


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52264 reflections; 20633 independent reflections (Rint = 0.0649); R1 = 0.0672, wR2 = 0.1788 for 20633 reflections (1182 parameters) with I > 2σ(I); R1 = 0.1174, wR2 = 0.2065 and GOF = 1.056 for all reflections; max/min residual electron density, 3.20/-1.26 e Å-3. 2.4 Preparation of the TiO2 Samples. The compounds were calcined under air atmosphere at a constant temperature for 6 h in a muffle furnace. The temperature ranged from 300 to 600 °C. For clarity, the samples prepared from Ti6-NDS at various temperatures are denoted as Ti6-temp and, those from Ti6-TBAC, Ti16 and Ti17 are denoted as Ti6TBAC-temp, Ti16–temp and Ti17–temp, respectively, where temp describes the calcination temperature. 2.2 Instruments. Single crystal X-ray diffraction data were collected using a Bruker SMART APEX II diffractometer (Mo Kα radiation, λ = 0.71073 Å) equipped with a CCD area detector. PXRD measurements were performed on a Bruker D8 Advance X-ray diffraction instrument using Cu-Kα radiation. IR spectra were measured using PerkinElmer Spectrum Two FT-IR. Raman spectra were obtained on a NEXUS 670 FT-IR Raman spectrometer with 473 nm laser. Elemental analyses of C, H, and N were performed on a FLASH EA1112 elemental analyzer. Content of Ti was determined using a colorimetric method as described previously.29 Transmission electron microscopy (TEM) was carried out using the JEM-2010F instrument. BET surface area measurements were performed by N2 adsorption at 77 K using an ASAP 2020 instrument. Thermogravimetric and differential scanning calorimetry analyses (TGA and DSC) were performed on a SDT Q600 instrument under air atmosphere. 2.5 Na-ion Battery Test. TiO2 powder, acetylene black and polyvinylidene fluoride (PVDF) were mixed in a weight ratio of 7:2:1 as the working electrodes. Na metal foil was the counter and reference electrode, glass microfibers of Whatman GF/F was the separator, and 1.0 M NaClO4 in ethylene carbonate (EC)/diethyl carbonate (DEC) (1:1 in volume) was the electrolyte. The above materials were assembled in coin-type cells (CR 2032) in a glove box under Ar atmosphere (Mikrouna Co). Charge/discharge tests were performed on an automatic battery tester (Land CT 2001A, China). 3. RESULTS AND DISCUSSION 3.1 The hierarchical structures of the TOCs



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Compound Ti6-NDS, with a formula [Ti6O8(OH2)20]·(2,7-NDS)4·2HCl·14H2O, containing a [Ti6O8(OH2)20]8+ polyoxocation (Ti6; Figure 1), was synthesized from a highly acidic, aqueous solution of TiCl4. 2,7-NDS was chosen for the synthesis since it is water-soluble under highly acidic conditions and meanwhile, it can interact with the aqua ligands of a TOC by both electrostatic and Hbonding interactions, both are beneficial for stabilizing and isolating TOCs from water.27 Previously, 2,7-NDS was also used to isolate Al-oxide clusters from water.34 The solution of 2,7-NDS and TiCl4 was initially clear and needle-like crystals slowly formed in the sealed vial at 4 oC after a few days. This phenomenon could be understood by the crystallographic structure of Ti6-NDS, in which 2,7NDS ions assemble by π-π stacking34-35 into one-dimensional chain-like fabrics along the [100] direction of the crystal (Figure 2A). Hence the slow precipitation process of crystals of Ti6-NDS corresponded to the slow self-assembly of both 2,7-NDS ions into the 1D chains and the formation of Ti6.

Figure 2. Periodic arrangements of the Ti-oxide cores and the organic fragments in crystals of (A) Ti6NDS, (B) Ti6-TBAC, (C) Ti16 and (D) Ti17. For clarity, Cl– ions, H2O molecules and other solvent molecules are omitted. In Ti6-NDS, every two 1D chains of 2,7-NDS ions are aligned back-to-back so that all the sulfonate groups could point to the outside (Figure 2A). In the channels between the chains of 2,7-NDS are located the Ti6 cations, Cl– ions, and solvent water. This special hierarcical structure maximizes


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hydrogen bonding and steric effect between Ti6 cations and 2,7-NDS anions.34 By contrast, other naphthalenedisulfonate acids, such as 1,6-, 1,5- or 2,6- NDS acid, did not produce any crystalline precipitate of TOCs under otherwise identical conditions. In compound Ti6-TBAC,27 the TBA+ cations are organized into hydrophobic shells and inside the shells are located the Ti6 cations, Cl– ions, and solvent water, quite different from the structure of 1 (Figure 2B). Different from both Ti6-NDS and Ti6-TBAC compounds which were synthesized from water, compounds Ti16 and Ti17 were synthesized via the solvothermal approaches in nonaqueous phases.33 The Ti-oxide cores of Ti16 and Ti17 are separated by the palisade layers consisted of the alkoxide ligands which are directly bonded on the surfaces of the Ti-oxide cores (Figures 2C and 2D). Both Ti16 and Ti17 clusters are neutral. 3.2 Characterization of the as-prepared TiO2 samples To prepare TiO2 from the above TOCs, they were calcined under air atmosphere for 6 h. Figure 3 shows the N2 adsorption/desorption isotherm of the as-prepared materials by calcination of Ti6-NDS at various temperatures. When the calcination temperature was below 350 °C, the adsorption and desorption curves are coincident, indicating the sample is nonporous (Figure 3A). When the calcination temperatures were more than 400 °C, the hysteresis loops which are indication of type-IV isotherm could be clearly seen in the N2 adsorption/desorption isotherm (Figures 3B-D). The pore-size distribution was calculated with the Barrett-Joyner-Halenda (BJH) method. From desorption branch of the isotherm, samples Ti6-400, Ti6-450 and Ti6-500 exhibit the most probable pore diameter of 16.2, 17.4 and 33.2 nm, respectively. By contrast, calcination of the other TOCs, including Ti6-TBAC, Ti16 and Ti17, did not produce porous TiO2 (see Figure S4 for N2 adsorption curves).



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Figure 3. Nitrogen adsorption and desorption isotherm of Ti6-350 (A), Ti6-400 (B), Ti6-450 (C) and Ti6-500 (D). Insets show the pore diameter distribution of the samples. The morphological features of the Ti6-400 sample were characterized by TEM (Figures 4). The particles are ca. 15 nm in diameter and most of them are inter-connected (Figure 4A). Pores with thin TiO2 walls may be discernible but regular pores could not be distinguished (Figure 4B). Hence, the porosity determined using N2 adsorption/desorption isotherm shown previously should be mainly attributed to the nanoparticle interconnections. A

B

Figure 4. The low resolution (A) and the high magnification (B) TEM images of the Ti6-400 sample. Bar = 50 nm.


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According to the PXRD data, when calcination temperature was between 200 and 300 °C, the samples obtained from Ti6-NDS were amorphous, indicating that the organic contents were not completely burned (data not shown). Broadened diffraction peaks exclusively assigned as TiO2 crystals with the tetragonal anatase phase (JCPDS No. 21-1272) began to emerge when calcination temperature was ca. 350 °C (Figure 5). The phase purity became higher as temperature increased since the peaks became sharper and more intensive. Both the Ti6-400 and Ti6-450 samples exhibit good crystallinity. Meanwhile, TGA data show that the weights of both Ti6-400 and Ti6-450 samples maintained upon combustion in air atmosphere, indicating both did not contain much oxidizable matter. When the temperature was above 500 °C, rutile phase began to emerge in the samples. 350 C 400 C 450 C

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Figure 5. PXRD patterns of the samples obtained by aerobic calcination of Ti6-NDS at various temperatures. Brunauer-Emmett-Teller (BET) surface area of the samples was also measured. It was found that the Ti6-400 sample exhibits the largest surface area, 107.5 m2 g-1 (Table 1), while those of Ti6-350, Ti6-450 and Ti6-500 are 61.0, 62.5 and 48.5 m2 g-1, respectively. In comparison, the BET surface area values of the Ti6TBAC-400 and Ti16-400 samples are 88.2 and 56.5 m2 g-1. A commercial anatase sample was also characterized and the BET surface area was 153 m2 g-1, larger than any of the samples obtained by calcination of the TOCs. Table 1. Characterization of the samples sample Ti6-300 Ti6-350 Ti6-400 Ti6-450

crystalline type amorphous 100% A 100% A 100% A

d (by XRD), nm 5.6 8.3 10

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total pore volume, cm3 g-1 ~0 0.050 0.22 0.14


Ti6-500 Ti6-600 Ti6TBAC-400 Ti16-400 Ti17-400 commercial anatase TiO2

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81% A, 19% R 52% A, 48% R 88% A, 12% R 100% A 100% A

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3.3 Mechanism for the formation of the porous TiO2 from Ti6-NDS One factor that is important for formation of the pores of the anatase TiO2 should be associated with the unique hierarchical structure of Ti6-NDS which is very different from those of Ti6-TBAC, Ti16, and Ti17 which did not form porous TiO2 upon calcination. Accordingly, a tentative mechanism for the formation of porous anatase TiO2 by calcination of Ti6-NDS is proposed as illustrated in Scheme 2.

Scheme 2. Schematic illustration of a tentative mechanism for the formation of porous anatase TiO2 by calcination of Ti6-NDS without additives. In TGA and DSC data of Ti6-NDS performed in a temperature ramp rate of 10 °C min-1 under high purity air flow (Figure S5), solvent water and HCl were found to be removed at below 300 °C while 2,7-NDS was removed at 400 – 500 °C. Removal of solvent water and HCl (Scheme 2, a → b) caused the Ti6 clusters to move along the [100] direction of the crystal lattice and to aggregate into larger clusters (Scheme 2, b → c). By contrast, the 2,7-NDS molecules could not coalesce along the [100]


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direction due to the π-π stacking structural feature; instead, the 1D chains of 2,7-NDS might coalesce perpendicular to the [100] direction of the crystal lattice. This enabled the 1D chains of 2,7-NDS to act as the template for the growth of the porous TiO2 during aggregation of the Ti6 clusters. To this regard, one of the roles of 2,7-NDS in formation of the porous TiO2 is just the same as that of surfactants in preparation of mesoporous zeolites. At a higher temperature when 2,7-NDS could react with atmospheric O2, removal of 2,7-NDS caused agglomeration of the Ti6 aggregates, and finally the porous TiO2 formed (Scheme 2, c → d). By contrast, for Ti6-TBAC, since the Ti6 clusters are spatially separated by the shells of tetrabutylammonium ions, removal of solvent H2O and HCl enclosed in these shells during calcination would lead TBA+ ions to pack more densely around the Ti6 clusters. Similarly, for Ti16 and Ti17, the palisade layers could also protect the Ti-oxide cores from aggregation before the oxidative decomposition of the isopropoxide ligands. Therefore, the organic species of Ti6-TBAC, Ti16 and Ti17 could not induce the growth of porous TiO2 during calcinations. In the DSC curve of Ti6-NDS, an intensive exothermic peak at 400 – 480 °C (centered at ca. 468 °C) is observed, corresponding to the oxidation of 2,7-NDS by O2 to form CO2 and formation of anatase TiO2. An additional broad exothermic peak emerges at ca. 460 – 550 °C, possibly attributed to anataseto-rutile phase transfer. Importantly, the TGA and DSC data clearly indicate that the temperature for combustion of 2,7-NDS to emit CO2 and that for formation of anatase TiO2 were overlapped. Therefore, it is believed that the released CO2 also facilitated the formation of pores of the as-prepared TiO2 by calcination of Ti6-NDS at ≥ 400 °C. 3.4 Electrochemical Properties TiO2 has been widely exploited as an electrode material since it exhibits excellent chemical stability, structural stability and extremely low volume effect.2-8 We took into consideration that the current porous TiO2 samples may be applied as an alternative SIB anode material. Therefore, the performance of Ti6-300, Ti6-400 and Ti6-500 as Na+ insertion hosts were investigated. For comparison, the anode performance of Ti6TBAC-400, Ti16-400, and the commercial anatase TiO2 were also examined. Performance of these samples at each current rate was examined for ten charge/discharge cycles from 0.1 C to 15 C (Figures 6A-B). Figure 6A reveals that the Ti6-400 electrode maintained good rate capability despite the increased current density by 150 times. Although all prepared from Ti6-NDS by



calcination, the specific capacity of Ti6-400 sample was always 3-5 times larger than those of Ti6-300 and Ti6-500 at any current rate (< 2 C). It was expected that Ti6-500 should exhibit a better lattice stability due to the larger particle size and a mixture of anatase-rutile phase. However, it indeed almost completely lose the specific capacity when the current rate was more than 2 C. On the one hand, it is obvious that the rate performances of Ti6TBAC-400 and Ti16-400 samples were poor (Figure 6B). Meanwhile, the rate performance of the Ti6-400 sample was also superior to that of the commercial anatase TiO2. Note that the performance of the commercial anatase TiO2 measured by us was quite similar to the previous reports and hence it can be used as a reference.36-37 Consequently, it is found out that the rate capability of the Ti6-400 anode is comparable to the outstanding TiO2 anode materials in the previously reports, such as porous carbon/TiO2 composite,38 F-doped TiO2,39 and Ni-TiO2 nanoarrays.40 In addition, a literature survey (see Table S1) also suggests the current protocol is of highly potential for preparation of TiO2 for SIBs’ anodes with high performance. Ti6-400, discharge Ti6-400, charge Ti6-500, discharge Ti6-500, charge Ti6-300, discharge Ti6-300, charge

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Figure 6. (A) Rate performance of the electrodes made of Ti6-300 (blue), Ti6-400 (black) and Ti6-500 (red). 1 C = 335 mA g−1. (B) Rate performance of the electrodes made of Ti6TBAC-400 (brown), Ti16400 (purple), and the commercial anatase TiO2 (pink). The data of Ti6-400 sample is included for comparison. (C) Discharge-charge profiles of the Ti6-400 electrode for the 1st (red) and 2nd (black)


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cycles. (D) Coulombic efficiency and long-term cycling performance of the Ti6-400 electrode (curves a and b) at 5 C and long-term cycling performance of the commercial anatase electrode (curve c). The discharge-charge profiles of the Ti6-400 sample were routinely measured at current rates of 0.5 C and 5 C, respectively (Figure 6C). This electrode delivered a 161 mAh g-1 discharge capacity and 101 mAh g-1 charge capacity at the initial cycle, corresponding to a coulombic efficiency of 63%. However, after a slow capacity fading in the initial dozens of cycles, a reversible capacity of 33 mAh g-1 was maintained during the subsequent cycles (Figure 6D), indicating an excellent long-term cyclability. Importantly, this value is twice of that of the electrode made of the commercial anatase. The capacity loss in the first cycle is presumably attributed to electrolyte decomposition and the formation of solid electrolyte interface layer, which is common for most of the sodium intercalation hosts.41-42 The average capacity fading per cycle of the Ti6-400 sample electrode within the first 100 cycles was close to 0.08%, and the coulombic efficiency was maintained nearly 100% at each cycle. The above information suggests that on one hand, the relative large specific surface area of Ti6-400 sample may be a reason for its large specific capacity. However, on the other hand, comparing to the commercial anatase TiO2 which exhibited a larger BET area and those samples prepared from various TOCs or from Ti6-NDS at various calcination temperatures, the specific capacity, the rate performance, and the long-term cycling performance of Ti6-400 were all better. Hence the superior properties of Ti6-400 could be attributed to its porous construction. Notably, the porous property of a material facilitates the permeation of electrolyte, shortens the diffusion path of Na+, increases the mobility of Na+, and maximizes the contact area of the electrode/electrolyte, all of which immensely contribute to enhancing its performance in battery.3-4, 10, 12 Moreover, the pores in the anatase material could promote the embedding of Na+, which acted to alleviate the volume expansion upon Na+ ions insertion. 4. CONCLUSIONS A facile and effective method for the fabrication of porous TiO2, involving the aerobic calcination of TOCs, has been demonstrated. Among the four representative cluster compounds used, only [Ti6O8(OH2)20]·(2,7-NDS)4·2HCl·14H2O (Ti6-NDS) in which the 2,7-NDS ions assemble into 1D chains to spatially separate the [Ti6O8(OH2)20]8+ polyoxocations formed the porous anatase TiO2 upon aerobic calcination. Based on the crystallographic structures of the TOC precursors and the TGA-DSC



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analyses, it was proposed that the 2,7-NDS ions acted as both the template and CO2 source to control porous TiO2 formation. The as-prepared porous TiO2 calcined at 400 oC exhibited a surface area of 108 cm3 g-1 and an average pore diameter of 16 nm. When used as an anode material for SIB, this sample showed larger specific capacity and remarkably better rate performance than the other reference TiO2 samples prepared using the same TOC under different conditions or other TOCs under the same conditions. This surfactant-free and catalyst-free strategy for the fabrication of TiO2 materials with special morphologies by using TOCs as precursors provides a new option for preparation of promising materials stable and efficient in SIBs. ASSOCIATED CONTENT Supporting Information The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/XXXXXXXX. Thermal gravimetric analysis (TGA), PXRD, spectroscopic data. X-ray crystallographic data (CIF) of 1. AUTHOR INFORMATION Corresponding Author *Email: [email protected]. Notes The authors declare no competing financial interest. Author contribution G.Z synthesized the materials, did the characterization and the relevant data analysis. C.C. and Prof. J.Y. studied the electrical properties. Y.W. directed the project and finalized the manuscript. ACKNOWLEDGMENT Y.W. gratefully acknowledges the financial supports by the National Natural Science Foundation of China (21401117), the NSF of Shandong Province (JQ201804), and the Fundamental Research Funds


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of Shandong University (2018JC026). J.Y. thanks Taishan Scholarship in Shandong Province for a financial support (ts201511004). REFERENCES (1) Many review papers and books are available, such as the special issue of "2014 Titanium Dioxide Nanomaterials Reviews" in Chem. Rev. 2014, Vol 114, Issue 19. (2) Huang, Y.; Zheng, Y.; Li, X.; Adams, F.; Luo, W.; Huang, Y.; Hu, L. Electrode Materials of Sodium-Ion Batteries toward Practical Application. ACS Energy Lett. 2018, 3, 1604-1612. (3) Hwang, J.-Y.; Myung, S.-T.; Sun, Y.-K. Sodium-Ion Batteries: Present and Future. Chem. Soc. Rev. 2017, 46, 3529-3614. (4) Yabuuchi, N.; Kubota, K.; Dahbi, M.; Komaba, S. Research Development on Sodium-Ion Batteries. Chem. Rev. 2014, 114, 11636-11682. (5) Kim, K.-T.; Ali, G.; Chung, K. Y.; Yoon, C. S.; Yashiro, H.; Sun, Y.-K.; Lu, J.; Amine, K.; Myung, S.-T. Anatase Titania Nanorods as an Intercalation Anode Material for Rechargeable Sodium Batteries. Nano Lett. 2014, 14, 416-422. (6) Yang, Z.; Zhang, J.; Kintner-Meyer, M. C. W.; Lu, X.; Choi, D.; Lemmon, J. P.; Liu, J. Electrochemical Energy Storage for Green Grid. Chem. Rev. 2011, 111, 3577-3613. (7) Weng, Z.; Guo, H.; Liu, X.; Wu, S.; Yeung, K. W. K.; Chu, P. K. Nanostructured TiO2 for Energy Conversion and Storage. RSC Adv. 2013, 3, 24758–24775. (8) Reddy, M. V.; Subba Rao, G. V.; Chowdari, B. V. R. Metal Oxides and Oxysalts as Anode Materials for Li Ion Batteries. Chem. Rev. 2013, 113, 5364-5457. (9) Hong, Z.; Kang, M.; Chen, X.; Zhou, K.; Huang, Z.; Wei, M. Synthesis of Mesoporous Co2+-Doped TiO2 Nanodisks Derived from Metal Organic Frameworks with Improved Sodium Storage Performance. ACS Appl. Mater. Interfaces 2017, 9, 32071-32079. (10) Li, W.; Liu, J.; Zhao, D. Mesoporous Materials for Energy Conversion and Storage Devices. Nat. Rev. Mater. 2016, 1, 16023. (11) Hong, Z.; Zhou, K.; Zhang, J.; Huang, Z.; Wei, M. Facile Synthesis of Rutile TiO2 Mesocrystals with Enhanced Sodium Storage Properties. J. Mater. Chem. A 2015, 3, 17412-17416. (12) Tachikawa, T.; Majima, T. Metal Oxide Mesocrystals with Tailored Structures and Properties for Energy Conversion and Storage Applications. Npg Asia Mater. 2014, 6, e100. (13) Fattakhova-Rohlfing, D.; Zaleska, A.; Bein, T. Three-Dimensional Titanium Dioxide Nanomaterials. Chem. Rev. 2014, 114, 9487-9558. (14) Jin, J.; Huang, S.-Z.; Liu, J.; Li, Y.; Chen, D.-S.; Wang, H.-E.; Yu, Y.; Chen, L.-H.; B-L., S. Design of New Anode Materials Based on Hierarchical, Three Dimensional Ordered



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TOC graphic


Preparation of Porous TiO2 from an Iso-Polyoxotitanate Cluster for

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