Synthesis of High-Performance Titanium Sub-Oxides for

Research Article Cite This: ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

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Synthesis of High-Performance Titanium Sub-Oxides for Electrochemical Applications Using Combination of Sol−Gel and Vacuum-Carbothermic Processes Sheng-Siang Huang,† Yu-Hsiang Lin,† Wesley Chuang,† Pei-Sian Shao,† Chung-Hsien Chuang,† Jyh-Fu Lee,‡ Meng-Lin Lu,‡ Yu-Ting Weng,*,† and Nae-Lih Wu*,† †

Department of Chemical Engineering, National Taiwan University, Taipei 106, Taiwan National Synchrotron Radiation Research Center, Hsinchu 30076, Taiwan

‡

S Supporting Information *

ABSTRACT: A series of nanocrystalline titanium (Ti) sub-oxides, including TiO, Ti2O3, Ti3O5, and Ti4O7, with high surface area and activity are successively synthesized using a facile synthesis method that combines the sol−gel and the energy-efficient vacuum-carbothermic (SG-VC) processes. The combination results in synergy in producing nanomaterials with high surface area (>100 m2 g−1), good conductivity, and rich intra-grain defect features, giving the oxides unique surface activities suitable for particular electrochemical applications. The phase compositions of the resulting powders are primarily determined by two process parameters, including the carbothermic carbon (C) content, expressed as the C-to-Ti molar ratio of the reactant powder, and the cooling protocol. Carbothermic C contents exceeding a threshold of C/Ti ∼ 3.7 exclusively produced non-Magnéli phase (MP) oxides including TiO and Ti2O3, while the MP oxides, Ti3O5 and Ti4O7, can be formed only with lower C contents combined with selected quenching protocols that kinetically limit oxygen replenishment during cooling. Examples of the resulting MP Ti4O7 powder exhibiting outstanding pseudocapacitive and oxygen evolution reaction catalytic behaviors are demonstrated. KEYWORDS: Magnéli phase, Titanium oxide, Carbothermic synthesis, Supercapacitor, Oxygen evolution reaction catalyst

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INTRODUCTION

resulting in the change of the oxygen arrangement from cornerand edge-sharing to face-sharing.8−12 Depending on oxygen deficiency, the MP Ti sub-oxides can exhibit substantially higher electronic conductivities than TiO2 while retaining the nature of high chemical robustness.13−20 Among them, Ti4O7 has been commercially applied (under the brand name of Ebonex) in electrochemical processes, including fuel cells, bipolar lead-acid batteries, and waste treatment.21−30 Ti4O7 has been synthesized in the literature in two ways, including calcination in hydrogen-containing atmosphere31−34 and carbothermal treatment under vacuum.31,35−37 The latter consistently shows substantially shorter calcination time and is

One unique feature of titanium (Ti) oxides is the existence of a wide range of titanium−oxygen (Ti−O) stoichiometries. These oxides, having various crystalline structures, exhibit a broad range of electrical, optical, and electrochemical properties that may be suitable for different applications. In particular, the socalled Magnéli phase (MP) Ti sub-oxides, having a general formula TinO2n‑1, where n is an integer number ranging from 3 to 10, have drawn considerable attention due to their high electronic conductivities.1−7 The structures of the MP Ti suboxides can be considered as deviations from ideal rutile TiO2 with oxygen deficiency. Rutile TiO2 is made up of TiO6 octahedra with corner sharing and edge sharing oxygen atoms (Figure SI1), while the MP Ti sub-oxides contain shear planes for every nth layer to accommodate the oxygen deficiency, © XXXX American Chemical Society

Received: September 9, 2017 Revised: December 8, 2017 Published: January 11, 2018 A

DOI: 10.1021/acssuschemeng.7b03189 ACS Sustainable Chem. Eng. XXXX, XXX, XXX−XXX

Research Article

ACS Sustainable Chemistry & Engineering Table 1. Synthesis Conditions and Microstructural Properties of Synthesized Ti Oxides Predominant Ti oxides

TiO

TiO

Ti2O3

Ti2O3

Ti3O5

TiO2

Ti4O7

TiO2

C/Ti molar ratioa Calcination temperature (°C) Holding time (h) Cooling condition BET specific surface area (m2 g−1)c Residue carbon (wt %)d Powder conductivity (S cm−1)e

6.49 1000 4 Qb − − −

6.49 1000 4 NCb 259 18.4 1.78

3.67 1000 4 Q − − −

3.67 1000 4 NC 230 14.9 2.51

2.17 1000 2.5 Q from 650 °C 192 4.6 2.01

2.17 1000 2.5 NC − 10.9 0.24

1.07 1000 2.5 Q from 500 °C 123 3.8 1.18

1.07 1000 2.5 NC − − −

a

The initial C content is expressed in terms of the molar ratio between the amount of C in the added glucose and the amount of Ti in the alkoxide introduced during the sol−gel process. bQ: quenched by directly removing the vacuum calcination reactor out of the furnace to the ambient. NC: naturally cooled in the furnace for ∼8 h. cMeasured by N2 adsorption at 77 K. dDetermined by TGA analysis. eMeasured at 5.7 kgf cm−2. FEG-TEM). Brunauer−Emmett−Teller (BET) specific surface area was determined by nitrogen adsorption (Micrometrics, ASAP 2010). The conductivity measurement of the Ti oxide powders was carried out using a homemade apparatus (see Figure SI2 for setup) that consisted of a hollow Teflon cylinder and two Cu pressing pistons connected to an ohmmeter (Chen Hwa, 502BC). During measurement, the prepared powder was put into the chamber between the two pistons, and the electrical resistance between the pistons was directly measured under increasing pressing pressure. Electrochemical Measurements. The electrode, on a dry basis, was composed of 75 wt % active material (Ti oxides), 8 wt % C black (XC72), 5 wt % C nanotube, and 12 wt % poly(1,1,2,2-tetrafluoroethylene) as binder. Ethanol was used as a solvent to make electrode paste, and Ti foil (1 cm × 1 cm) was used as the current collector. The paste was then coated on the current collector and heated at 60 °C for 30 min. Cyclic voltammetry measurements were carried out on a potentiostat (627E, CH Instruments). And the experiments were performed in the three-electrode configuration comprising the Ti oxides working electrode, a platinum foil counter electrode, and an Ag/AgCl/saturated KCl reference electrode. The electrolyte is 2 M Li2SO4 aqueous solution. The specific capacitance (Csp) is calculated according to Csp = (∫ I dV)/(2m ν ΔV) where I is the measured current, m the loading of active material, ν, the scanning rate, and ΔV the potential window. The linear sweep voltammetry study was conducted with a potential scan rate of 5 mV s−1 using aqueous 0.1 M KOH electrolyte.

more energy efficient. Nevertheless, it remains a challenge to synthesize Ti oxides with tailored oxygen stoichiometry using this method. Previous studies typically used crystalline TiO2 as precursors and the resulting powders contained primarily micron-sized particles or had low surface areas less than 10 m2 g−1.36,37 Nanocrystallinity, giving active surface and high surface area, is important for certain applications, such as catalysis, lithium-ion batteries, sodium-ion batteries, and supercapacitors.38−45 In this study, it is demonstrated that a series of nanocrystalline Ti sub-oxides having a wide range of oxygen stoichiometries and high surface areas can be successfully synthesized using a combination of the sol−gel and vacuumcarbothermic (SG-VC) processes. Although either SG or VC has been used for synthesizing Ti sub-oxides, the combination has resulted in synergy in producing unique powder characteristics that may open up new applications.

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MATERIALS AND METHODS

Powder Synthesis. A beaker containing Ti(IV) tetra isopropoxide (TTIP, 97% Sigma-Aldrich) was covered with perforated Al foil and placed with constant stirring in an humid atmosphere containing HCl vapor to enable the sol−gel transformation through condensation reaction. The resulting Ti hydroxy oxide (Ti(OH)xOy) precipitate was then redispersed in an aqueous solution of glucose. The solution was finally dried at 50 °C. The glucose was used as the carbon (C) source for the carbothermic reaction, and the glucose concentration was adjusted according to the selected C content for the subsequent VC treatment. The carbothermic C content is characterized in terms of the molar ratio between the amount of C in the added glucose and the amount of Ti in the alkoxide introduced for the sol−gel process. The C/Ti ratios varied from 6.49 to 1.07 (Table 1). The dried glucose-Ti hydroxy oxides precursor powders having various C/Ti ratios were then calcined at 1000 °C in a vacuum (3.67. Their oxide compositions were insensitive to the cooling protocol: either slow cooling in the furnace or rapid cooling by quenching produced the same oxide phase (Table 1). By contrast, for the MP sub-oxides, including Ti3O5 and Ti4O7, the cooling protocol played an important role. Slow cooling always produced TiO2 phase, while the MP oxides could be obtained only by first slow cooling to an intermediate temperature (650 °C for Ti3O5 and 500 °C for Ti4O7) followed by quenching. The lower the intermediate quenching temperature is, the higher the oxygen stoichiometry of the final oxide phase is. It is inferred that a high carbothermic C content lead to sufficiently low residue oxygen partial pressure in the vacuum reactor, and the resulting sub-oxide phases, such as TiO and Ti2O3, are essentially in equilibrium with the oxygen partial pressure so that they are insensitive to the cooling protocol. On the other hand, a low C content corresponds to a high residue oxygen partial pressure, and oxygen replenishes into the oxide with descending temperature during cooling. As a result, slow cooling always results in the oxygen-enriched oxide phase, namely, TiO2, while the sub-oxides, such as Ti3O5 and Ti4O7, can only be obtained with fast cooling (quenching in the ambient), which kinetically limits oxygen replenishment. The nature of the oxygen replenishing process can be revealed by the TGA analysis. Figure 3 compares the TGA

Figure 1. X-ray diffraction (XRD) patterns of a series of as-prepared Ti oxides. The reference powder diffraction data are also listed.

Figure 3. Thermal gravimetric analysis of as-prepared Ti4O7 and TiO2.

curves of the Ti4O7 and TiO2 powders. For the TiO2 sample, monotonous weight loss started from nearly 350 °C and ended around 600 °C due to oxidation of the residue C (Table 1). By contrast, Ti4O7 powder showed a weight gain between 100 and 350 °C, followed by monotonous weight loss at higher temperatures. Other Ti sub-oxide powders also exhibited the weight gain behavior (Figure SI3). The weight gain within the low temperature range indicates oxygen replenishment of Ti4O7 to induce transformation into the oxygen-enriched oxides. These results indicate that the Ti sub-oxides can readily be oxygenated by residue oxygen inside the calcination vacuum reactor during cooling.

Figure 2. X-ray absorption near-edged spectrum (k-edge) analysis of as-prepared Ti oxides. The dotted line represents the absorption spectrum of Ti foil.

As summarized in Table 1, there are primarily two important processing parameters that determined the predominant oxide phases; they are the carbothermic C content (i.e., C/Ti molar C


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For brevity, only the conductivity at the maximum pressure (5.7 kgf cm−2) is hereafter discussed. As shown, the measured Ti sub-oxide powders exhibited powder conductivities were in the range of 1−3 S cm−1, which were 5−10 times that of the TiO2 (with C) powder. Since all the samples contained residue C, estimation of the oxide conductivity was made as follows. As a reference, the powder conductivity of pure (C-free) TiO2 was found to be below the instrument sensitivity (∼10−8 S cm−1). On the other hand, the synthesized C-containing (10.9 wt.%; Table 1) TiO2 powder showed a conductivity of 0.24 S cm−1, which translates into a bulk conductivity of 2.2 S cm−1 for the C component. The bulk conductivity of the oxide component alone was thus estimated by calculation assuming the oxide and C components to be in a either serial or parallel configuration (Figure SI4). It was found that the oxide conductivities in either configuration were in fact very close, within 5% difference, to those of the composite powders. It is worth mentioning that the powder conductivities of the present MP oxides are not to be compared with those reported for the single-crystal or thinfilm samples of the corresponding oxides because of the large contact resistance between the nanosized MP particles for the present powder samples. On the other hand, the powder conductivities of the MP samples are comparable with that of a commercially conductive C black (Black Pearls 2000), as shown in Figure 6, indicating the potential of the MP oxide powders for the applications as granular electrode materials.

SEM analysis showed that the residue C existed in two kinds of morphologies, including agglomerates of C nanoparticles (Figure 4a) or thin coating on the oxide grains (Figure 4b).

Figure 4. Microscopy analyses: SEM micrographs of powders with different C contents, including (a) TiO and (b) Ti4O7. TEM micrographs of (c and d) Ti4O7.

There were more C agglomerates found in the C-rich samples, including the TiO and Ti2O3 powders. The oxide grains exhibited sizes ranging from a few tens to approximately 200 nm. TEM analyses detected size distributions of primary particles in the similar range and confirmed the presence of the C coating as well as particulate deposit on the surface of the oxide surfaces (Figure 4c). Moreover, high resolution TEM revealed the abundance of defect structures of the particles (Figure 4d). The structural defects, when extending to the surface, may give the particles high surface activity. Figure 5 shows the powder conductivity data. For every tested powder, the conductivity always increased with increasing pressing pressure due to increasing compactness.

Figure 6. Electrochemical assessment of SG-VC derived Ti-oxide powder electrodes. (a) CV plots (I: current; ν: potential scan rate; m: oxide weight; BP: Black Pearls 2000 carbon black). (b) Specific capacitance versus scan rate for Ti4O7 and BP electrodes. (c) Linear sweep voltammetry curve of Ti4O7 in 0.1 M KOH solution.

The BET specific surface area (SSA) of the Ti oxide powders were in the range of 260−120 m2 g−1, which were more than 5 to 20 times higher than those reported in the literature, some of which also contained residue C.33,34,36,37,46 The fact that the SSAs of the powders increased with increasing C content (Table 1) suggests that the residue C contributed to the powder SSA. However, it is worth noting that the two MP powders, Ti3O5 and Ti4O7, in particular, contained relatively

Figure 5. Powder conductivity of the as-synthesized Ti oxide powders and a commercial C black (Black Pearls 2000). D


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ACS Sustainable Chemistry & Engineering low C contents (5 wt.%) but possessed SSAs well above 100 m2 g−1. High-temperature oxidation to remove the residue C runs the risk of changing the surface structures of the oxide components. Assumption of a C SSA of 2000 m2 g−1, which to our knowledge is close to the highest SSAs of activated carbons, should give a conservative estimate for the SSAs of the oxide phases in the two MP powders. Accordingly, we obtained an estimated SSA of 100 m2 g−1 for Ti3O5 and of 47 m2 g−1 for Ti4O7. For comparison, the SSAs of spherical particles having a size in the range of 10−200 nm with a density of 4.23 g cm−3 (TiO2) are in the range of 142.0−7.1 m2 g−1. The estimated SSAs for Ti3O5 and Ti4O7 fall within this theoretical range. On the other hand, the assumed C SSA is apparently too high for the TiO and Ti2O3 powders, which contained significant amounts of C agglomerates, in contrast to the C thin coating and nanoparticles in the MP powders (Figure 4). Electrochemical Performance. Due to their high chemical stability, conductive Ti oxides have been suggested for various electrochemical applications. Figure 6a compares the cyclic voltammograms (CVs) of the electrodes made of the derived Ti oxide powders in aqueous electrolyte of Li2SO4. Other than an oxidation event attributable to oxygen evolution from OH− oxidation above ∼0.5 V (versus Ag/AgCl), there is no redox reaction due either to the phase transformation or to corrosion of the oxides over a wide potential range of 2 V. In particular, their extraordinary electrochemical stability under the negatively bias potential region in the aqueous electrolyte is a unique advantage over other transitional-metal-oxide (such as Mn and vanadium oxides) electrode materials, which often undergo irreversible reduction and phase transformation, along with dissolution, in aqueous electrolytes under reducing conditions. For assessing their potential in supercapacitor applications, the TiO2 electrode had a very small specific capacitance (5.3 F g−1). For the sub-oxide electrodes, on the other hand, the specific capacitance increases in the order of Ti3O5 (26 F g−1) < Ti2O3 (35 F g−1) ≪ Ti4O7 (140 F g−1). The Ti4O7 electrode exhibited a high oxygen oxidation current above 0.5 V, which led to an operating potential window of 1.5 V between −1.0 and 0.5 V (Figure 6a). For comparison, Figure 6a also shows the CV plot of a conductive carbon electrode made of Black Pearls 2000, which has a specific area of ∼2000 m2 g−1. The carbon electrode has a specific capacitance of 61 F g−1, and the capacitance is expected to originate mainly from the electricdouble-layer capacitance. In spite of having a substantially smaller SSA than the C material (123 versus ∼2000 m2 g−1), the Ti4O7 electrode exhibited a higher specific capacitance, suggesting that its capacitance is primarily pseudocapacitance in nature. Moreover, the Ti4O7 electrode also showed higher capacitances at high scanning rates (Figure 6b). The large specific capacitance and oxygen evolution reaction (OER) current indicate high surface activity of the resulting Ti4O7 material. The prominent oxidation current observed for the Ti4O7 electrode over the high-potential side of the CV plots led us to assess its potential application for OER in the alkaline electrolyte. Figure 6c plots the linear sweep voltammetry curve of the Ti4O7 electrode subjected to a scan rate of 5 mV s−1 in a 0.1 M KOH solution, showing an OER onset potential of 0.41 V versus Ag/AgCl or 1.37 V versus RHE. As shown in Table 2, this onset potential rivals many electrodes employing specially designed OER catalysts in the literature. The underlying electrochemical mechanisms for the observed

Table 2. Comparison of OER Potential Electrode

OER potential vs RHE (V)/electrolyte pH

ref

FeCo oxide nanosheet Na0.08Ni0.9Fe0.1O2 and NiP CoFe2O4/C nanorod array N-doped Co3O4 nanosheet Activated Mn−Co oxyphosphide Plasma-engraved Co3O4 nanosheet Ni−P porous nanoplate SG-VC Ti4O7

1.46/13 1.40/14 1.45/14 1.53/13 1.54/14 1.49/13 1.48/14 1.37/13

47 48 49 50 51 52 53 this study

pseudocapacitance and OER catalysis activities are under study. At this moment, it is sufficient to say that the combined properties of high electronic conductivity, large SSA, and high surface activity make the SG-VC derived Ti sub-oxides, in particular, Ti4O7, promising electrochemical electrode materials.

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CONCLUSIONS A series of nanocrystalline Ti sub-oxides, including TiO, Ti2O3, Ti3O5, and Ti4O7, with high surface areas and electrochemical activities were successively synthesized using a facile SG-VC synthesis method. The phase composition was primarily determined by two process parameters, including the initial C content and cooling protocol. High C contents exceeding a threshold of C/Ti molar ratio ∼3.7 produced non-MP oxides including TiO and Ti2O3, while the MP oxides, Ti3O5 and Ti4O7, were formed with lower C contents combined with selected quenching protocols that kinetically limited oxygen replenishment upon cooling. The resulting oxide powders, containing residue C, showed the characteristics of high surface area (>100 m2 g−1), high intra-grain defect structure, and good powder conductivity (>1.0 S cm−1). The oxides have shown outstanding electrochemical stability suitable for electrochemical applications. In particular, the Ti4O7 powder exhibited very promising pseudocapacitive and OER catalytic properties.

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ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acssuschemeng.7b03189. Additional information on Ti oxides involving crystal structure, apparatus for powder conductivity measurement, thermal gravimetric analysis, and conductivity calculation (PDF)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected] (Y.-T. Weng). *E-mail: [email protected] (N.-L.Wu). ORCID

Nae-Lih Wu: 0000-0001-6545-8790 Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This study was supported by the Ministry of Science and Technology (MOST 104-2815-C-002-149-E and 104-2221-EE


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Phase Titanium Sub-Oxides Conductive Ceramic TinO2n‑1. ECS Electrochem. Lett. 2013, 2 (2), A21−A24. (18) Wang, L.; Lettenmeier, P.; Golla-Schindler, U.; Gazdzicki, P.; Cañas, N. A.; Morawietz, T.; Hiesgen, R.; Hosseiny, S. S.; Gago, A. S.; Friedrich, K. A. Nanostructured Ir-supported on Ti4O7 as a costeffective anode for proton exchange membrane (PEM) electrolyzers. Phys. Chem. Chem. Phys. 2016, 18 (6), 4487−4495. (19) Yao, C. H.; Li, F.; Li, X.; Xia, D. G. Fiber-like nanostructured Ti4O7 used as durable fuel cell catalyst support in oxygen reduction catalysis. J. Mater. Chem. 2012, 22 (32), 16560−16565. (20) Shen, P. K.; He, C.; Chang, S.; Huang, X.; Tian, Z. Magnéli phase Ti8O15 nanowires as conductive carbon-free energy materials to enhance the electrochemical activity of palladium nanoparticles for direct ethanol oxidation. J. Mater. Chem. A 2015, 3 (27), 14416− 14423. (21) Adamaki, V.; Clemens, F.; Ragulis, P.; Pennock, S. R.; Taylor, J.; Bowen, C. R. Manufacturing and characterization of Magnéli phase conductive fibres. J. Mater. Chem. A 2014, 2 (22), 8328−8333. (22) Alegre, C.; Modica, E.; Lo Vecchio, C.; Siracusano, S.; Arico, A. S.; Baglio, V. Pd supported on Ti-sub-oxides as bifunctional catalyst for air electrodes of metal-air batteries. Int. J. Hydrogen Energy 2016, 41 (43), 19579−19586. (23) Geng, P.; Chen, G. H. Magnéli Ti4O7 modified ceramic membrane for electrically-assisted filtration with antifouling property. J. Membr. Sci. 2016, 498, 302−314. (24) Yoon, K. J.; Song, S. J.; Seok, J. Y.; Yoon, J. H.; Park, T. H.; Kwon, D. E.; Hwang, C. S. Evolution of the shape of the conducting channel in complementary resistive switching transition metal oxides. Nanoscale 2014, 6 (4), 2161−2169. (25) Kasian, O. I.; Luk’yanenko, T. V.; Demchenko, P.; Gladyshevskii, R. E.; Amadelli, R.; Velichenko, A. B. Electrochemical properties of thermally treated platinized Ebonex® with low content of Pt. Electrochim. Electrochim. Acta 2013, 109, 630−637. (26) Padilha, A. C. M.; Raebiger, H.; Rocha, A. R.; Dalpian, G. M. Charge storage in oxygen deficient phases of TiO2: defect Physics without defects. Sci. Rep. 2016, 6, 28871−28877. (27) Santos, M. C.; Elabd, Y. A.; Jing, Y.; Chaplin, B. P.; Fang, L. Highly porous Ti4O7 reactive electrochemical water filtration membranes fabricated via electrospinning/electrospraying. AIChE J. 2016, 62 (2), 508−524. (28) Velichenko, A. B.; Kasian, O. I.; Luk’yanenko, T. V.; Amadelli, R.; Demchenko, P. Y.; Gladyshevskii, R. E. Physicochemical Properties and Electrochemical Behavior of Ebonex/Pt-Based Materials. Prot. Met. Phys. Chem. Surf. 2013, 49 (6), 705−711. (29) Wu, Q. M.; Ruan, J. M.; Zhou, Z. C.; Sang, S. B. Magnéli phase titanium sub-oxide conductive ceramic TinO2n‑1 as support for electrocatalyst toward oxygen reduction reaction with high activity and stability. J. Cent. South Univ. 2015, 22 (4), 1212−1219. (30) You, S. J.; Liu, B.; Gao, Y. F.; Wang, Y.; Tang, C. Y. Y.; Huang, Y. B.; Ren, N. Q. Monolithic Porous Magnéli-phase Ti4O7 for Electrooxidation Treatment of Industrial Wastewater. Electrochim. Electrochim. Acta 2016, 214, 326−335. (31) Walsh, F. C.; Wills, R. G. A. The continuing development of Magnéli phase titanium sub-oxides and Ebonex® electrodes. Electrochim. Acta 2010, 55 (22), 6342−6351. (32) Radecka, M.; Trenczek-Zajac, A.; Zakrzewska, K.; Rekas, M. Effect of oxygen nonstoichiometry on photo-electrochemical properties of TiO2‑x. J. Power Sources 2007, 173 (2), 816−821. (33) Ioroi, T.; Senoh, H.; Yamazaki, S. I.; Siroma, Z.; Fujiwara, N.; Yasuda, K. Stability of corrosion-resistant Magnéli-phase Ti4O7supported PEMFC catalysts at high potentials. J. Electrochem. Soc. 2008, 155 (4), B321−B326. (34) Tang, C.; Zhou, D. B.; Zhang, Q. Synthesis and characterization of Magnéli phases: Reduction of TiO2 in a decomposed NH3 atmosphere. Mater. Lett. 2012, 79, 42−44. (35) Andersson, S.; Collén, B.; Kuylenstierna, U.; Magnéli, A.; et al. Phase Analysis Studies on the Titanium-Oxgen System. Acta Chem. Scand. 1957, 11 (10), 1641−1652.

002−183 -MY3. S.J. Ji of MOST (National Taiwan University) is acknowledged for microscopy analysis.

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REFERENCES

(1) Arif, A. F.; Balgis, R.; Ogi, T.; Iskandar, F.; Kinoshita, A.; Nakamura, K.; Okuyama, K. Highly conductive nano-sized Magnéli phases titanium oxide (TiOx). Sci. Rep. 2017, 7, 3646−3654. (2) Ganiyu, S. O.; Oturan, N.; Raffy, S.; Cretin, M.; Esmilaire, R.; van Hullebusch, E.; Esposito, G.; Oturan, M. A. Sub-stoichiometric titanium oxide (Ti4O7) as a suitable ceramic anode for electrooxidation of organic pollutants: A case study of kinetics, mineralization and toxicity assessment of amoxicillin. Water Res. 2016, 106, 171−182. (3) Kundu, D.; Black, R.; Berg, E. J.; Nazar, L. F. A highly active nanostructured metallic oxide cathode for aprotic Li-O2 batteries. Energy Environ. Sci. 2015, 8 (4), 1292−1298. (4) Li, L. H.; Deng, Z. X.; Xiao, J. X.; Yang, G. W. A metallic metal oxide (Ti5O9)-metal oxide (TiO2) nanocomposite as the heterojunction to enhance visible-light photocatalytic activity. Nanotechnology 2015, 26 (25), 255705−255713. (5) Wang, G. R.; Liu, Y.; Ye, J. W.; Qiu, W. B. Synthesis, microstructural characterization, and electrochemical performance of novel rod-like Ti4O7 powders. J. Alloys Compd. 2017, 704, 18−25. (6) Shao, X. L.; Kim, K. M.; Yoon, K. J.; Song, S. J.; Yoon, J. H.; Kim, H. J.; Park, T. H.; Kwon, D. E.; Kwon, Y. J.; Kim, Y. M.; Hu, X. W.; Zhao, J. S.; Hwang, C. S. A study of the transition between the nonpolar and bipolar resistance switching mechanisms in the TiN/TiO2/ Al memory. Nanoscale 2016, 8 (36), 16455−16466. (7) He, C.; Chang, S.; Huang, X.; Wang, Q.; Mei, A.; Shen, P. K. Direct synthesis of pure single-crystalline Magnéli phase Ti8O15 nanowires as conductive carbon-free materials for electrocatalysis. Nanoscale 2015, 7 (7), 2856−2861. (8) Liborio, L.; Mallia, G.; Harrison, N. Electronic structure of the Ti4O7 Magnéli phase. Phys. Rev. B: Condens. Matter Mater. Phys. 2009, 79 (24), 245133−245142. (9) Ma, M.; You, S. J.; Liu, G. S.; Qu, J. H.; Ren, N. Q. Macroporous monolithic Magnéli-phase titanium sub-oxides as anode material for effective bioelectricity generation in microbial fuel cells. J. Mater. Chem. A 2016, 4 (46), 18002−18007. (10) Smith, J.; Walsh, F.; Clarke, R. Electrodes based on Magnéli phase titanium oxides: the properties and applications of Ebonex® materials. J. Appl. Electrochem. 1998, 28 (10), 1021−1033. (11) Xu, B. Q.; Sohn, H. Y.; Mohassab, Y.; Lan, Y. P. Structures, preparation and applications of titanium suboxides. RSC Adv. 2016, 6 (83), 79706−79722. (12) Andersson, S.; Collén, B.; Kruuse, G.; Kuylenstierna, U.; Magnéli, A.; Pestmalis, H.; Asbrink, S. Identification of Titanium Oxides by X-ray Powder Patterns. Acta Chem. Scand. 1957, 11 (10), 1653−1657. (13) Ganiyu, S. O.; Oturan, N.; Raffy, S.; Esposito, G.; van Hullebusch, E. D.; Cretin, M.; Oturan, M. A. Use of Substoichiometric Titanium Oxide as a Ceramic Electrode in Anodic Oxidation and Electro-Fenton Degradation of the Beta-blocker Propranolol: Degradation Kinetics and Mineralization Pathway. Electrochim. Electrochim. Acta 2017, 242, 344−354. (14) Geng, P.; Su, J. Y.; Miles, C.; Comninellis, C.; Chen, G. H. Highly-Ordered Magnéli Ti4O7 Nanotube Arrays as Effective Anodic Material for Electro-oxidation. Electrochim. Electrochim. Acta 2015, 153, 316−324. (15) Hasegawa, G.; Sato, T.; Kanamori, K.; Nakano, K.; Yajima, T.; Kobayashi, Y.; Kageyama, H.; Abe, T.; Nakanishi, K. Hierarchically Porous Monoliths Based on N-Doped Reduced Titanium Oxides and Their Electric and Electrochemical Properties. Chem. Mater. 2013, 25 (17), 3504−3512. (16) Kitada, A.; Hasegawa, G.; Kobayashi, Y.; Kanamori, K.; Nakanishi, K.; Kageyama, H. Selective Preparation of Macroporous Monoliths of Conductive Titanium Oxides TinO2n‑1 (n = 2, 3, 4, 6). J. Am. Chem. Soc. 2012, 134 (26), 10894−10898. (17) Luo, Z. U.; Sang, S. B.; Wu, Q. M.; Liu, S. Y. A Conductive Additive for Zn Electrodes in Secondary Ni/Zn Batteries: The Magnéli F


Research Article

ACS Sustainable Chemistry & Engineering (36) Zhu, R. J.; Liu, Y.; Ye, J. W.; Zhang, X. Y. Magnéli phase Ti4O7 powder from carbothermal reduction method: formation, conductivity and optical properties. J. Mater. Sci.: Mater. Electron. 2013, 24 (12), 4853−4856. (37) Zhang, X. Y.; Lin, Y. H.; Zhong, X. X.; Wang, L. J.; Liu, W. Y.; Singh, A.; Zhao, Q. Fabrication and characterization of Magnéli phase Ti4O7 submicron rods. J. Mater. Sci.: Mater. Electron. 2016, 27 (5), 4861−4865. (38) Chen, X.; Mao, S. S. Titanium Dioxide Nanomaterials: Synthesis, Properties, Modifications, and Applications. Chem. Rev. 2007, 107 (7), 2891−2959. (39) Fröschl, T.; Hörmann, U.; Kubiak, P.; Kučerová, G.; Pfanzelt, M.; Weiss, C. K.; Behm, R. J.; Hüsing, N.; Kaiser, U.; Landfester, K.; Wohlfahrt-Mehrens, M. High surface area crystalline titanium dioxide: potential and limits in electrochemical energy storage and catalysis. Chem. Soc. Rev. 2012, 41 (15), 5313−5360. (40) Lan, T.; Liu, Y.; Dou, J.; Hong, Z.; Wei, M. Hierarchically porous TiO2 microspheres as a high performance anode for lithiumion batteries. J. Mater. Chem. A 2014, 2 (4), 1102−1106. (41) Liu, Y.; Lan, T.; Zhang, W.; Ding, X.; Wei, M. Hierarchically porous anatase TiO2 microspheres composed of tiny octahedra with enhanced electrochemical properties in lithium-ion batteries. J. Mater. Chem. A 2014, 2 (47), 20133−20138. (42) 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 (33), 17412−17416. (43) Lan, T.; Dou, J.; Xie, F.; Xiong, P.; Wei, M. Ultrathin TiO2-B nanowires with enhanced electrochemical performance for Li-ion batteries. J. Mater. Chem. A 2015, 3 (18), 10038−10044. (44) Liu, Y.; Liu, M.; Lan, T.; Dou, J.; Wei, M. One-step hydrothermal synthesis of Nb doped brookite TiO2 nanosheets with enhanced lithium-ion intercalation properties. J. Mater. Chem. A 2015, 3 (37), 18882−18888. (45) Zhang, W.; Lan, T.; Ding, T.; Wu, N.-L.; Wei, M. Carbon coated anatase TiO2 mesocrystals enabling ultrastable and robust sodium storage. J. Power Sources 2017, 359, 64−70. (46) Krishnan, P.; Advani, S. G.; Prasad, A. K. Magnéli phase TinO2n‑1 as corrosion-resistant PEM fuel cell catalyst support. J. Solid State Electrochem. 2012, 16 (7), 2515−2521. (47) Zhuang, L.; Ge, L.; Yang, Y.; Li, M.; Jia, Y.; Yao, X.; Zhu, Z. Ultrathin Iron-Cobalt Oxide Nanosheets with Abundant Oxygen Vacancies for the Oxygen Evolution Reaction. Adv. Mater. 2017, 29 (17), 1606793−1606799. (48) Weng, B.; Xu, F.; Wang, C.; Meng, W.; Grice, C. R.; Yan, Y. A layered Na1‑xNiyFe1‑yO2 double oxide oxygen evolution reaction electrocatalyst for highly efficient water-splitting. Energy Environ. Sci. 2017, 10 (1), 121−128. (49) Lu, X.-F.; Gu, L.-F.; Wang, J.-W.; Wu, J.-X.; Liao, P.-Q.; Li, G.-R. Bimetal-Organic Framework Derived CoFe2O4/C Porous Hybrid Nanorod Arrays as High-Performance Electrocatalysts for Oxygen Evolution Reaction. Adv. Mater. 2017, 29 (3), 1604437−1604443. (50) Xu, L.; Wang, Z.; Wang, J.; Xiao, Z.; Huang, X.; Liu, Z.; Wang, S. N-doped nanoporous Co3O4 nanosheets with oxygen vacancies as oxygen evolving electrocatalysts. Nanotechnology 2017, 28 (16), 165402−165409. (51) Guan, B. Y.; Yu, L.; Lou, X. W. General Synthesis of Multishell Mixed-Metal Oxyphosphide Particles with Enhanced Electrocatalytic Activity in the Oxygen Evolution Reaction. Angew. Chem., Int. Ed. 2017, 56 (9), 2386−2389. (52) Xu, L.; Jiang, Q.; Xiao, Z.; Li, X.; Huo, J.; Wang, S.; Dai, L. Plasma-Engraved Co3O4 Nanosheets with Oxygen Vacancies and High Surface Area for the Oxygen Evolution Reaction. Angew. Chem. 2016, 128 (17), 5363−5367. (53) Yu, X.-Y.; Feng, Y.; Guan, B.; Lou, X. W.; Paik, U. Carbon coated porous nickel phosphides nanoplates for highly efficient oxygen evolution reaction. Energy Environ. Sci. 2016, 9 (4), 1246−1250.

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Synthesis of High-Performance Titanium Sub-Oxides for

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