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Review

A Review: Synthesis and Applications of Titanium Sub-Oxides

by
Xiaoping Wu
1,*,
Haibo Wang
1 and
Yu Wang
2
1
State Key Laboratory of V and Ti Resources Comprehensive Utilization, Ansteel Research Institute of Vanadium & Titanium (Iron & Steele), Panzhihua 617000, China
2
The School of Chemistry and Chemical Engineering, State Key Laboratory of Power Transmission Equipment & System Security and New Technology, Chongqing University, 174 Shazheng Street, Shapingba District, Chongqing 400044, China
*
Author to whom correspondence should be addressed.
Materials 2023, 16(21), 6874; https://doi.org/10.3390/ma16216874
Submission received: 10 September 2023 / Revised: 16 October 2023 / Accepted: 20 October 2023 / Published: 26 October 2023
Browse Figures
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Abstract

:
Magnéli phase titanium oxides, also called titanium sub-oxides (TinO2n−1, 4 < n < 9), are a series of electrically conducting ceramic materials. The synthesis and applications of these materials have recently attracted tremendous attention because of their applications in a number of existing and emerging areas. Titanium sub-oxides are generally synthesized through the reduction of titanium dioxide using hydrogen, carbon, metals or metal hydrides as reduction agents. More recently, the synthesis of nanostructured titanium sub-oxides has been making progress through optimizing thermal reduction processes or using new titanium-containing precursors. Titanium sub-oxides have attractive properties such as electrical conductivity, corrosion resistance and optical properties. Titanium sub-oxides have played important roles in a number of areas such as conducting materials, fuel cells and organic degradation. Titanium sub-oxides also show promising applications in batteries, solar energy, coatings and electronic and optoelectronic devices. Titanium sub-oxides are expected to become more important materials in the future. In this review, the recent progress in the synthesis methods and applications of titanium sub-oxides in the existing and emerging areas are reviewed.

1. Introduction

Titanium sub-oxides, often referred to as Magnéli phase TiOx, comprise a series of different titanium oxides that have the general formula TinO2n−1 (4 ≤ n ≤ 10) [1,2,3,4,5]. It is well-known that titanium dioxide (TiO2) is an electrical insulator, as it has a large band gap (anatase: 3.2 eV; 3.0: rutile) [6]. However, Magnéli phase titanium oxides are electrically conducting, and the value of electrical resistivity decreases with the increase in the oxygen deficiency [7]. Furthermore, Magnéli phase TiOx are found to be more stable than carbon in electrochemically oxidizing conditions [1,8]. These titanium sub-oxides have attracted much recent attention as promising new conducting materials because they are electrically conducting, and are highly stable towards chemical corrosion.
The earliest phase analysis using X-ray methods on the oxygen–titanium system was carried out by Ehrlich, who reported the existence of three intermediary titanium oxides [9,10]. In the 1950s, a phase diagram of a titanium–oxygen system was constructed from data in the literature by DeVries et al. [11]; later, a comprehensive phase analysis of TiOx was studied by the group of Arne Magnéli, and a number of phases in the titanium–oxygen system were reported, including Ti4O7, Ti5O9, Ti6O11, Ti7O13, Ti8O15, Ti9O17 and Ti10O19 [12]. The electrical properties of these titanium oxides were studied by Bartholomew et al., and it was found that titanium sub-oxides have semiconductor-to-metal transitions at certain temperatures and their electrical conductivities change with the oxygen content of those materials [13]. Ti4O7 has the highest electrical conductivity among Magnéli phase TiOx at room temperature [1].
Magnéli phase TiOx are generally synthesized by the reduction of TiO2. The reduction sequence is as follows: TiO2 → TinO2n−1 (n > 10) → TinO2n−1 (4 < n < 10) → Ti3O5 → Ti2O3 → TiO → Ti2O [14]. Oxygen defect formations and its concentration depend on the synthesis conditions. Ti4O7 can be expressed as TiO1.75, as the ratio of O/Ti of Ti4O7 is 1.75. The Magnéli phase TinO2n−1 are the intermediate products. It is critical to have well-controlled preparation conditions to synthesize each Magnéli phase, with high chemical and phase purities that have significant effects on the intrinsic properties such as electrical, optical behavior and their catalytic activity.
It is increasingly important to synthesize nanostructured titanium sub-oxides because of their particular properties resulting from the high surface areas of the materials [15,16,17,18]. Progress has been made in applying nanostructured titanium sub-oxides in the areas of fuel cells, water treatment, batteries and so on. To update the recent research progress of synthesis and applications of titanium sub-oxides, this review will discuss and highlight the recent developments in the synthesis and applications of Ti4O7 in the fields of energy, environment, catalysts and others, as well as future directions for research.

2. Synthesis Methods

The phase diagram of the Ti–O system (Figure 1) shows various stable phases at different O/Ti ratios. The region at the right of the diagram contains the discrete Magnéli phases of TinO2n−1 (n = 4–10) and TiO2. At sufficiently elevated temperatures, TiO2 can be reduced to a lower oxidation state such as Magnéli series, including Ti4O7. To obtain individual phases such as Ti4O7 in the Magnéli series, the condition for reduction of TiO2 needs to be carefully controlled. The key parameters for the synthesis of Ti4O7, as well as for other Magnéli phases, include temperature, time, reducing atmosphere and reducing agents. Hydrogen, carbon, metal and hydride can be used as reducing agents.

2.1. Reduction of TiO2 by Hydrogen

At sufficiently elevated temperatures, hydrogen (H2) or a mixture hydrogen–inert gas such as argon (Ar) is used to reduce TiO2 into the titanium sub-oxides [1]. The reduction process can be considered to be a reaction of oxygen being removed progressively from TiO2. The reaction for the synthesis of Ti4O7 through hydrogen reduction is shown in Equation (1):
4TiO2 + H2 = Ti4O7 + H2O
The reduction reaction to produce Ti4O7 is carried out at sufficiently elevated temperature, generally higher than 1000 °C. The sequence of the formation of Magnéli series TiOx in the hydrogen reduction reactions of TiO2 is Ti9O17, Ti8O15, Ti7O13, Ti6O11, Ti5O9 and Ti4O7. The reduction reaction of TiO2 is carried out in a flow of hydrogen in a reactor that is heated externally to maintain a high temperature. As Ti4O7 is the last in the reduction sequence, the synthesis of Ti4O7 needs higher temperature, longer reduction time or a combination of two, compared with the parameters to the formation of other Magnéli series. The reaction temperature, reaction time, gas composition and size of TiO2 particles are important factors for the synthesis of Magnéli phases [2,3]. A summary of the synthesis of Ti4O7 through hydrogen reduction can be found in Table 1.

2.2. Reduction by Carbon

Titanium dioxide can be reduced by carbon in an inert atmosphere to produce various titanium sub-oxides, as shown in Equation (2):
nTiO2 (s) + C (s) = TinO2n−1 (s) + CO (g)
The carbothermal reduction of TiO2 is a complex process in which the oxygen in TiO2 is progressively removed by carbon. TiO2 is initially reduced to TinO2n−1, Ti3O5, Ti2O3 and TiCxOy [25,37,38,39], but an over stoichiometric carbon/TiO2 ratio may lead to the formation of TiCxOy, not titanium sub-oxides. TinO2n−1 phases are only formed as intermediates [25,37]. To prepare titanium sub-oxides through the carbothermal reduction of TiO2, the stoichiometric carbon/TiO2 ratio is important for the control of the phases formed. The carbothermal reduction of TiO2 can be carried out in different gas atmospheres or in a vacuum. Li et al. synthesized Ti4O7 by reacting TiO2 anatase (100 nm) with carbon black at 1020 °C for 0.5–2 h in argon and in a vacuum [26]. The study indicated that at the same temperature, the extent of carbothermal reduction of titanium dioxide is dependent on the molar ratio of TiO2/C, and excessive carbon may lead to over reduction down the sequence of titanium sub-oxides. Ti4O7 with a purity of 98.5% was obtained in argon at 1100 °C. Dewan et al. studied the carbothermal reduction of TiO2 in hydrogen, helium and argon through temperature-programmed reduction experiments [40]. In argon and helium, the carbothermal reduction of TiO2 started at 850 °C. In hydrogen, they found that the phases in a sample after being reduced to 915 °C were Ti8O15 and unreacted TiO2, and the phase in a sample after being reduced to 975 °C was only Ti4O7. Ti4O7 and Ti3O5 phases were found at 1035 °C.
Titanium sub-oxide fibers with high electrical conductivity have been prepared by reducing TiO2 in a carbon black micro-environment [29]. Organic polymers or compounds can be used to synthesize titanium sub-oxides. The carbon in the organic polymers is used as a carbon source for reducing TiO2 or other titanium-containing compounds. These organic polymers or compounds include poly (ethyleneimine), polyethyleneglycol [41], poly (styrene-b-2-vinylpyridine) [42], resol [43], glucose [44] and poly (vinyl alcohol) [27]. A summary of the various preparation methods for titanium sub-oxides using the carbon reduction method can be found in Table 1.

2.3. Reduction by Metals

Metals can be used to reduce TiO2 to form titanium sub-oxides [31,32]. Calcium, aluminum, sodium, silicon and titanium have been used to reduce TiO2. For example, by controlling the ratio of metallic titanium and TiO2, metallic titanium (Ti) can be used to reduce TiO2 to obtain various titanium sub-oxides through a reaction shown in Equation (3).
(2n−1)TiO2(s) + Ti(s) = 2TinO2n−1(s)
Andersson et al. synthesized various titanium sub-oxides by the reduction of TiO2 with titanium metal under argon, and established the different phases from X-ray diffraction determinations [9]. Strobel et al. used Ti and TiO2 to react in situ in carefully out-gassed transport tubes. Cl2 and tellurium tetrachloride were used as transporting agents to synthesize crystals of TinO2n−1 with n = 2 to 9 [45]. Gusev et al. developed a method for the synthesis of titanium sub-oxides by reducing TiO2 with titanium. This method involved the mechanical activation and annealing in argon at temperatures of 1333–1353 K for 4 h [46]. It is worth noting that the synthesis of titanium sub-oxides using TiO2 and Ti can be viewed to be an oxidation reaction in which Ti is oxidized by TiO2. Theoretically, oxidation of Ti is one of the possible ways to obtain titanium suboxides. However, oxygen is highly reactive and can oxidize Ti directly to TiO2 easily. Fine Ti powder is far more difficult to prepare than TiO2. A summary of the synthesis of titanium sub-oxides by metal reduction can be found in Table 1.

2.4. Reduction by Hydride

Metal hydrides have strong reducing reactivity, even at low temperatures. The reduction of TiO2 to titanium sub-oxides could occur at low temperatures to avoid significant sintering and crystal growth of particles in the formation process of titanium sub-oxides. Therefore, metal hydrides could be used to synthesize nanostructured titanium sub-oxides using nanostructured TiO2 as a starting material. Nagao et al. synthesized titanium sub-oxides by reacting TiO2 with TiH2 at 550 °C [34]. The nanoparticles of a series of phases of titanium sub-oxide including Ti2O3, Ti3O5, Ti4O7 and Ti8O15 were obtained by changing the molar ratios of TiO2/TiH2. Other hydride reduction methods for titanium sub-oxide synthesis can be found in Table 1.

2.5. Synthesis of Nanostructured Titanium Sub-Oxides

It has become increasingly important to synthesize nanostructured titanium sub-oxides because of their particular properties resulting from the high surface areas of the materials. Nanostructured non-stoichiometric TiO2−x titanium sub-oxides, titanium sub-oxides TinO2n−1 in particular, have emerged as alternatives to TiO2 in applications of clean energy generation, and as catalysts for degrading harmful compounds and others [47,48,49,50,51,52]. Although titanium sub-oxides can be synthesized by the reduction of TiO2 using hydrogen or carbon, the sizes of synthesized titanium sub-oxide particles are usually in the order of micrometers, because these reduction reactions occur at high temperatures (generally over 1000 °C) and proceed for hours. Under these conditions, TiO2 and formed titanium sub-oxide particles undergo sintering and crystal growth, leading to the formation of much larger particles. To synthesize nanostructured titanium sub-oxides, more reactive titanium-containing starting materials, stronger reducing agents or alternative reaction techniques are required for the reduction reactions to be carried out under milder reaction conditions such as lower temperatures or short reaction times.
Han et al. prepared Ti8O15 nanowires and Ti4O7 fibers by heating H2Ti3O7 nanowires in hydrogen at 850 °C and 1050 °C [53]. Hydrogen trititanate H2Ti3O7 is one of the compounds in the series of titanates (M2TinO2n+1, M = H, Na, or K). The synthesis process has two steps. Firstly, H2Ti3O7 nanowires are prepared by reacting TiO2 particles with NaOH in an autoclave at 150–180 °C for 2–5 days, and then purified using the acid washing method [54,55,56]. Scanning electron microscopy (SEM) and transmission electron microscopy (TEM) studies showed that prepared H2Ti3O7 were nanowires of 30–200 nm in diameter and up to 10 µm long. Secondly, prepared H2Ti3O7 nanowires were reduced in hydrogen for 1–4 h at 800–1050 °C. In the hydrogen reduction reaction at 850 °C, H2Ti3O7 nanowires changed into Ti8O15 nanorods or nanoparticles, as shown in Figure 2. By heating H2Ti3O7 in hydrogen at 1050 °C, the product formed was Ti4O7. The TEM image shows that most products are in the form of fibers, with diameters of approximately 1 µm.
He et al. fabricated Ti8O15 nanowires using an evaporation–deposition synthesis method [15]. The synthesized Ti8O15 nanowires were ∼30 nm in diameter (as shown in Figure 3), and were found to have an electrical conductivity of 20.6 S cm−1.
Zhang et al. prepared pure Ti4O7 particles with diameters of 200–500 nm in hydrogen at 850 °C using peroxotitanium acid H4TiO5 (Ti(OH)3O-O-H) as a starting material [57]. H4TiO5 was prepared by treating titanium powder with NH3·H2O and H2O2. Pang et al. synthesized Ti4O7, using a simple polymer-mediated route in which the cross-linked titanium ethoxide with polyethylene glycol was treated by carbothermal reduction at ~950 °C in an Ar stream. TEM images revealed that the material primarily comprises ~8–20 nm Ti4O7 crystals. The sulfur composites Ti4O7/S-60 or Ti4O7/S-70 were prepared with either 60 or 70 wt% sulfur using a melt-diffusion method at 155 °C [58]. Ti4O7 was used to prepare Ti4O7/S cathodes for lithium–sulfur cells [59,60,61].
Portehault et al. developed a new bottom-up approach to synthesize various nano-scaled Magnéli phases under mild conditions [41]. In this method, titanium (IV) ethoxide was reacted with amino- or ethoxy-containing oligomers or polymers. The resulting clear gels were heated at different temperatures under N2 or Ar. TinO2n−1 compounds (n = 3, 4, 5, 6, 8) were obtained for the first time as nano-Magnéli phases with specific surface areas from 55 to 300 m2 g−1. The synthesis steps for the Magnéli/carbon nanocomposites are illustrated in Figure 4.
Huang et al. synthesized nanocrystalline Ti2O3, Ti3O5 and Ti4O7 using a synthesis method that combines sol-gel and vacuum-carbothermic processes [44]. Yao et al. successfully synthesized Ti4O7 using TiO(NO3)2 as a starting material in a hydrogen atmosphere at 1000 °C for 6 h [22]. The SEM images clearly showed that the synthesized titanium sub-oxides are spherical particles with an average particle size of approximately 250 nm. Davydov synthesized Ti4O7 nanopowder with an average size of 115 ± 30 nm using a two-step procedure. In the first step, titanium (III) oxalate particles with controlled sizes were produced by reacting metallic Ti with oxalic acid in a heated aqueous solution. In the second step, Ti4O7 was prepared through high-temperature calcination of titanium (III) oxalate particles in a flowing hydrogen gas [62]. This synthesis process is similar to the process that has been used to prepare titanium oxycarbide nanoparticles [63]. Tominaka et al. synthesized Ti2O3 nanoparticles by heating TiO2 nanoparticles (10–30 nm) and CaH2 powder at 350 °C [35].
Ioroi et al. synthesized nanoparticles of titanium sub-oxides by irradiating TiO2 particles dispersed in liquid with a pulsed UV laser [64]. Xu et al. developed a synthesis process to prepare titanium sub-oxide nanoparticles via a thermal plasma method, using metatitanic acid H2TiO3 (TiO(OH)2) as a starting material. The prepared titanium sub-oxides nanoparticles are spherical, with particle sizes in the range of 20–100 nm [23]. Fukushima et al. synthesized Ti4O7 nanoparticles with different sizes by carbothermal reduction using a multimode microwave apparatus [28]. Takeuchi et al. synthesized 60 nm Ti4O7 nanoparticles via carbothermal reduction of TiO2 nanoparticles using polyvinylpyrrolidone as the carbon source. The carbothermal reduction was carried out using 2.45 GHz microwave irradiation at 950 °C for 30 min. The results of this study demonstrate that microwave heating can drastically reduce the heating time to avoid excessive sintering and crystal growth of Ti4O7 in a conventional carbothermal reduction process [65]. Arif et al. prepared chain-structured titanium sub-oxides with diameters under 30 nm using a thermal-induced plasma process. The synthesized titanium sub-oxide nanoparticles consisted of a mixture of several Magnéli phases. After a heat treatment, as-synthesized titanium sub-oxides nanoparticles were found to have low electrical resistivity [66].
A summary of synthetic methods for nanostructured titanium sub-oxides is reported in Table 2. A comparison among the synthesis methods to highlight the advantages, limitations and characteristics of the prepared sub-oxides is presented in Table 3.

3. Applications of Titanium Sub-Oxides

The structures of Magnéli phase titanium oxides are based on the rutile TiO2 crystal lattice. Rutile TiO2 is made up of octahedra having a titanium atom in the center and oxygen atoms at each corner. Shared edge or corner oxygen atoms link adjacent octahedra, as shown in Figure 5. The crystal structure of titanium sub-oxides can be described as a structure having a two-dimensional chain of titanium dioxide in which titanium atoms locate at the center and oxygen atoms locate at the corners in an octahedral structure [67,68]. In TinO2n−1, every nth layer has an oxygen deficiency, which leads to shear planes in the crystal structure. The Ti4O7 crystal has three octahedral TiO2 layers and one TiO layer. As a result of the vacancy of oxygen atoms, the TiO layer causes titanium atoms to be closer together.
The unique crystal structure makes titanium sub-oxide materials have attractive properties, such as high conductivity, superior chemical stability and electrochemical stability [69]. As shown in Table 4, the conductivity of Ti4O7 material is the highest among the Magnéli phase materials. Research shows that Ti4O7 is highly stable in acidic or alkali conditions. Some studies indicated that the expected half-life of Ti4O7 is 50 years in 1.0 M H2SO4 at room temperature [70].
As a result of their remarkable electrical conductivity, electrochemical stability, cost-effectiveness and environmentally friendly natures, titanium sub-oxides are also considered to have potential as a superior anode material for wider electrochemical applications [67,68]. Research has shown that Magnéli phases have a catalytic property. Among the Magnéli phases, Ti4O7 exhibits the greatest catalytic property [45,69]. It has a wide electrochemical window with regard to water oxidation and reduction [70,71,72]; thus, it can be used for electrochemical treatment of pollutants in water. Titanium sub-oxides are generally prepared in the form of powders. More recently, two-dimensional films and three-dimensional porous materials of titanium sub-oxides have been successfully fabricated. Advances in the development of multiple dimensional titanium sub-oxide materials has led to new applications.

3.1. Catalysis Support in Fuel Cells

Proton exchange membrane fuel cells (PEMFCs) are a clean energy technology that has made significant advances in recent decades [73,74]. However, the high cost of the component materials and the low stability of the electrodes are major barriers for their large-scale commercial applications in some areas [75]. PEMFCs use Pt catalysts in the form of nanoparticles dispersed on a support material. The nature of the support materials can have a significant influence on the electro-activity and durability of the Pt catalysts [76,77,78]. Carbon materials are the most common support material for PEMFCs. However, carbon-supported Pt catalysts are prone to corrosion under the harsh operating conditions [79,80], which can severely affect the performance of PEMFCs and reduce the operational lifetime of the fuel cell electrodes. Titanium sub-oxides are considered to be promising support materials for PEMFCs due to the high thermal and oxidative stability, electronic conductivity and strong interactions between Pt nanoparticles and titanium sub-oxide support [81]. Chisaka et al. synthesized Ti4O7 particles via carbothermal reduction, using titanium oxysulfate (TiOSO4) and polyethylene glycol as precursors. The Pt catalyst using Ti4O7 as support exhibited excellent load cycle durability, which was the highest among the state-of-the-art platinum/oxide catalysts, with no change in the cell performance after 10,000 voltage cycles [82]. Esfahani et al. synthesized doped titanium sub-oxide Ti3O5Mo0.2Si0.4 (TOMS) as a novel fuel cell catalyst support. Ti3O5Mo0.2Si0.4 (TOMS) support exhibited remarkably high electronic conductivity and high stability. The fuel cell devices that used the Pt/TOMS catalyst achieved high performance, better than that of commercial catalysts [83]. Nguyen et al. demonstrated the excellent durability of titanium sub-oxide as a catalyst support for Pd in alkaline direct ethanol fuel cells [84]. Won et al. developed Ti4O7-supported Pt-based catalysts for a bifunctional oxygen catalyst in a unitized regenerative fuel cell for both the oxygen reduction reaction (ORR) and the oxygen evolution reaction (OER) to enhance their activity and stability [85]. Zhang et al. developed an ordered Ag@Pd alloy supported on Ti4O7. The ordered characteristics of the Ag@Pd alloy and its strong electron transfer with the corrosion-resistant Ti4O7 improved the catalytic activity and stability [86].

3.2. Electrocatalytic Degradation for Wastewater Treatment

Titanium sub-oxides have been characterized as an ideal choice of anode for the electrochemical treatment of many pollutants. Chen et al. decomposed trichloroethylene (TCE) and chloroform (CF) in an electrochemical cell using a titanium sub-oxide ceramic sheet plated with Pt or Pd as the working electrode. The decomposition kinetics was found to be of the first order for TCE and CF [87]. Kearney et al. used Ebonex (a titanium sub-oxide ceramic) electrodes for treating nitrate-contaminated water. Complete de-nitrification was achieved using an Ebonex cathode and a stable anode based on Ti/IrO2 or Ti/RuO2 [88]. Yang et al. examined the degradation of perfluorooctanesulfonate in electrochemical oxidation processes, using an anode made from Ti4O7. The decomposition rate of perfluorooctanesulfonate was shown to be pseudo-first-order. This study illustrates the promise of Ti4O7 electrodes for degrading per- and polyfluoroalkyl compounds and co-contaminants in groundwater [89]. Ganiyu et al. reported a study of the electrochemical degradation of the antibiotic amoxicillin in aqueous solution. The Ti4O7 anode of the cell was prepared using plasma spraying technology. The oxidative degradation of amoxicillin by hydroxyl radicals was assessed as a function of the applied current, and was found to follow pseudo-first-order kinetics. Comparative studies of mineralization efficiency showed that a Ti4O7 anode performed better for the removal of total organic carbon (TOC) than the classical dimensional stable anode and Pt anode. Ti4O7 anodes could provide a cost-effective alternative to boron doped diamond anodes in electro-oxidation processes [90]. Teng et al. investigated the electrochemical oxidation of sulfadiazine using a Ti/Ti4O7 mesh anode. Their results showed that electrochemical oxidation could achieve almost 100% removal of sulfadiazine in 60 min under the conditions of 0.05 mol L−1 Na2SO4, pH = 6.33 and current density of 10 mA cm−2. It was found that Ti/Ti4O7 mesh anodes were very stable in the treatment of actual pharmaceutical wastewater, and had a large electrochemically active surface area due to the network structure of the Ti/Ti4O7 mesh anode [91]. Further research will continue to improve the performance of titanium sub-oxide electrodes through optimizing the fabrication process of the electrodes and further integrating them with other technologies for more efficient applications.

3.3. Reactive Electrochemical Membrane

One recent research advancement in water treatment concerns the development of technologies that incorporate multiple treatment methods into a single technology to increase the efficiency and reduce the complexity of water treatment. A novel technology known as reactive electrochemical membranes (REM) combines membrane filtration with electrochemically advanced oxidation processes. In this REM technology, titanium sub-oxide materials serve as both a ceramic membrane for filtration and a reactive electrode surface for oxidizing contaminants [92]. Zaky et al. used Ti4O7 REM to investigate the removal of p-substituted phenolic compounds in water. They demonstrated that the REM was active for both direct anodic oxidation and production of OH• radicals to degrade phenolic compounds [93]. Guo et al. synthesized a novel REMs for water treatment using tubular asymmetric TiO2 ultrafiltration membranes as precursors. REMs composed of high purity Ti4O7 showed optimal reactivity. The performance of REMs was assessed by measuring the outer-sphere charge transfer (Fe(CN)64−) and oxidation of organic compounds through both direct oxidation and generation of OH•. In an optimal condition, the removal rate for oxalic acid was determined to be 401.5 ± 18.1 mmol h−1 m−2 at 793 L m−2 h−1. The current efficiency was approximately 84%. These results show the high promise of REMs in applications of water treatment [94]. Qi et al. prepared Ti4O7 REM by thermal reduction of mechanically pressed TiO2 powders, using the Ti powder as the reducing agent. The prepared Ti4O7 REMs show high oxygen evolution potential and electrocatalytic activity for the generation of OH• [95]. You et al. fabricated a monolithic porous Ti4O7 electrode for electrochemical oxidation of industrial dyeing and finishing wastewater. The electrochemical oxidation using porous Ti4O7 electrode produced efficient and stable reduction of recalcitrant organic pollutants onsite, without any extra addition of chemicals [96]. Geng et al. fabricated tubular Ti4O7/Al2O3 composite microfiltration membranes for electrically-assisted antifouling filtrations. The tubular Ti4O7/Al2O3 membrane was tested for its antifouling performance by treating different feed solutions that are known to foul easily in an electrically-assisted membrane filtration module. The results demonstrated that the Ti4O7/Al2O3 composite membranes showed much better antifouling performance than uncoated Al2O3 membranes. The incorporation of a Ti4O7-modified membrane into the electrically-assisted filtration process provides a potential alternative for ceramic membrane filtrations to have antifouling properties for maintaining long-lasting permeate quality and simplifying the filtration operation [97]. Liang et al. developed a REM system using a Ti4O7 microfiltration membrane as the filter and the anode. The REM system was evaluated for the performance in deactivating Escherichia coli (E. coli) in water at various current densities. The results showed that the concentration of E. coli was reduced from 6.46 log CFU/mL to 0.18 log CFU/mL, after passing through the Ti4O7 microfiltration membrane filter. The scanning electron microscope and extracellular protein analysis showed that the membrane filtration effect and direct oxidation generated from the REM system are responsible for the observed bacteria removal and inactivation [98]. Research is continuing to optimize the electrode fabrication process, and to develop titanium sub-oxide electrodes doped with active electrode materials to further increase the efficiency of water treatment processes, prolonging electrode working life and expanding the degradation of complex pollutants.

3.4. Batteries

The lithium–sulfur battery (LSB) is considered to be one of the next-generation technologies for future batteries because of its remarkable specific capacity of 1675 mA h g−1 and the availability of low-cost sulfur [60,99]. However, the development of commercial LSBs needs to resolve the issues of low sulfur utilization and poor cyclability, which are caused by a number of factors, such as the low conductivity of sulfur, the high solubility of the lithium polysulfides, passivation of the reactive surface of lithium anodes, and so on. To address these issues, one of the research efforts is to develop host materials to limit the movement of the lithium polysulfides in the sulfur cathode. Tao et al. discovered that conductive Ti4O7 was a highly effective matrix to bind with sulfur species. Ti4O7–S cathodes exhibit higher reversible capacity and improve cycling performance over previously developed TiO2–S cathodes. The strong adsorption of sulfur species on the low-coordinated Ti sites of Ti4O7 was attributed to the improved performance of Ti4O7–S cathodes [100]. Wei et al. prepared mesoporous Ti4O7 microspheres that exhibit interconnected mesopores (20.4 nm), large pore volume (0.39 cm3 g−1), and a high surface area (197.2 m2 g−1). The sulfur cathode embedded with a matrix of mesoporous Ti4O7 microspheres exhibits a superior reversible capacity and a low decay in capacity. The improved electrochemical performance is due to the strong chemical bonding of the lithium polysulfides to Ti4O7, and trapping in the mesopores and voids of the matrix [43]. Zhang et al. reported a facile approach to prepare nanostructured Ti4O7 with different morphologies. Ti4O7 nanorods and nanoparticles were prepared. The as-prepared Ti4O7 nanorods and nanoparticles were examined as a sulfur host for Li–S batteries. The electrochemical tests showed that the Ti4O7 nanorods exhibited better performance in cycle stability and rate capacity compared with Ti4O7 nanoparticles. This confirmed that the morphology of Ti4O7 could influence its electrochemical performance for lithium sulfur batteries [101]. Wu et al. synthesized a composite containing carbon nanotubes and nanosized Ti4O7 (oCNTs-Ti4O7), and coated the composite on the surface of the separator. Compared with a common separator, the separator modified with the oCNTs-Ti4O7 layer exhibited significantly improvement in the utilization of active substances, and restrained the shuttling effect of polysulfides. The Li–S battery fabricated using the separator modified with the oCNTs-Ti4O7 layer showed great enhancements in cycle and rate performance, as well as other in electrochemical properties [102]. Yu et al. fabricated a lithium–sulfur battery cathode containing 7.5 wt% to 10 wt% Ti4O7. The addition of Ti4O7 as a conductive additive into the cathode resulted in better rate capability and reversible cycling performance. The high electronic conductivity and surface adsorption of the polysulfides of Ti4O7 were attributed to the improvement in the electrochemical performance. This research also showed an effective way to improve the performance of lithium–sulfur batteries [103]. Titanium sub-oxides have also been used to improve the performance of other types of batteries such as lead–acid batteries and Zn–air batteries [104,105].

3.5. Other Applications

Titanium sub-oxides are also considered to be attractive for solar cells [106,107,108,109,110], sensors [111,112,113,114,115], electronic and photonic materials [3,7,16,116], and biological applications [117]. A summary of application areas of these materials in existing and emerging areas of research is listed in Table 5.

4. Summary and Outlook

In this review, recent progress in the synthesis and applications of Magnéli phase titanium oxides was reviewed. Titanium sub-oxides are synthesized through the reduction of titanium dioxide (TiO2) using hydrogen, carbon, metals or metal hydrides as reduction agents. The particle sizes of as-synthesized titanium sub-oxides are generally in the micrometer range, based on the conventional synthesis methods. However, progress has been made to synthesize nanostructured titanium sub-oxides through optimizing thermal reduction processes, using more powerful reduction agents or using new titanium-containing precursors [15,23,28,62,65,66]. Magnéli phase titanium oxides have numerous applications in electrodes, fuel cells, degradation of pollutants, batteries and coatings. Among these compounds, Ti4O7 has received the most widespread attention due to its excellent electrical conductivity, and chemical and electrochemical stability. More recently, Magnéli phase titanium oxides as functional materials or additives have been used to enhance the performance of electro-catalysts, cathodes in batteries, advanced electrochemical oxidation processes, solar cells, electronic materials, sensors and coatings [95,110,111,118,119,120,121,122]. It is expected that further research will be continue to optimize synthesis processes of Magnéli phase titanium oxides to further increase the electrochemical and catalytic properties, and to improve the performance of devices containing Magnéli phase titanium oxides through optimizing the fabrication process and further integrating with other technologies for more efficient applications. Titanium sub-oxides are expected to become more important materials for sustainability in the future.

Author Contributions

Conceptualization, X.W.; writing—original draft preparation, X.W. and H.W.; writing—review and editing, X.W., H.W. and Y.W.; project administration, Y.W.; funding acquisition, Y.W. All authors have read and agreed to the published version of the manuscript.

Funding

This research was financially supported by the National Natural Science Foundation of China (U19A20100).

Institutional Review Board Statement

Not applicable.

Informed Consent Statement

Not applicable.

Data Availability Statement

Not applicable.

Acknowledgments

Authors would like to thank R. Lu and Q. Miao for the technical assistance and administrative support in the preparation of the review.

Conflicts of Interest

The authors declare no conflict of interest.

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Materials 16 06874 g001
Figure 1. Phase diagram of the Ti–O system (Reprinted from ref. [14], copyright 2018, MDPI).
Figure 1. Phase diagram of the Ti–O system (Reprinted from ref. [14], copyright 2018, MDPI).
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Materials 16 06874 g002
Figure 2. Ti8O15 nanorods prepared by reducing H2Ti3O7 at 850 °C in hydrogen; (a) XRD pattern of the product; (b) SEM image of the product; (c) low magnification TEM image of the product; and (d) a high-magnification TEM image of part of a nanorod (Reprinted from ref. [53], copyright 2008, American Institute of Physics).
Figure 2. Ti8O15 nanorods prepared by reducing H2Ti3O7 at 850 °C in hydrogen; (a) XRD pattern of the product; (b) SEM image of the product; (c) low magnification TEM image of the product; and (d) a high-magnification TEM image of part of a nanorod (Reprinted from ref. [53], copyright 2008, American Institute of Physics).
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Materials 16 06874 g003
Figure 3. (A,B) SEM images of the Ti8O15 nanowires; (C) SEM image of the cross-section of the Ti8O15 nanowires; (D,E) TEM images of the Ti8O15 nanowires; (F) HRTEM image of Ti8O15 nanowires (Reprinted from ref. [15], copyright 2015, The Royal Society of Chemistry).
Figure 3. (A,B) SEM images of the Ti8O15 nanowires; (C) SEM image of the cross-section of the Ti8O15 nanowires; (D,E) TEM images of the Ti8O15 nanowires; (F) HRTEM image of Ti8O15 nanowires (Reprinted from ref. [15], copyright 2015, The Royal Society of Chemistry).
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Materials 16 06874 g004
Figure 4. Synthesis steps for Magnéli/carbon nanocomposites (Reprinted from ref. [41], copyright 2011, American Chemical Society).
Figure 4. Synthesis steps for Magnéli/carbon nanocomposites (Reprinted from ref. [41], copyright 2011, American Chemical Society).
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Materials 16 06874 g005
Figure 5. Edge-sharing TiO2 and Ti4O7 octahedra sheets showing the face-sharing shear plane in Ti4O7 (Reprinted from ref. [68], copyright 2010, Elsevier).
Figure 5. Edge-sharing TiO2 and Ti4O7 octahedra sheets showing the face-sharing shear plane in Ti4O7 (Reprinted from ref. [68], copyright 2010, Elsevier).
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Table 1. Summary of synthesis of Ti4O7 and other titanium sub-oxides.
Table 1. Summary of synthesis of Ti4O7 and other titanium sub-oxides.
Synthesis MethodProcess ConditionsCharacterizationRef.
H2 reductionPigmentary TiO2 reacts with H2 at 1050 °CMonophasic Ti4O7[4]
H2 reductionAnatase TiO2 reacts with H2 (99.99%) at 950 °CPure triclinic phase of Ti4O7, 0.5–1 µm[2]
H2 reductionTiO2 nanotube arrays react with H2 at 850 °C for two hoursTi4O7 nanotube arrays[19]
H2 reductionTiO2 reacts with H270% Ti4O7, 30% Ti5O9[20]
H2 reductionRutile TiO2 was reduced in a mixture of N2 and H2 gasesTi4O7, Ti5O9 and Ti6O11[21]
H2 reductionTiO(NO3)2 reacts with H2 at 1000 °C for 6 hTi4O7, 250 nm[22]
H2 reductionH2TiO3 reacts with H2/Ar in a thermal plasma reactorTinO2n−1 nanoparticles,
20–100 nm		[23]
H2 reductionTiO2 + H2 in a combined catalytic and thermal reduction reactionTi8O15, Ti4O7, Ti3O5[24]
C reductionThe reduction of TiO2 by graphite or metallic titaniumVarious phases[25]
C reductionTiO2 anatase (100 nm) reacts with carbon black at 1020 °C for 0.5–2 hTi4O7, 98.5%[26]
C reductionTiO2 reacts with poly(vinyl alcohol) at 1100 °CTi4O7, a few hundreds of nm in size[27]
C reductionTiO2 reacts with polymer PVP at 925 °C in a microwave furnaceTi4O7 nanoparticles (25, 60, and 125 nm)[28]
C reductionReduction of rutile TiO2 in a carbon black micro-environmentTitanium sub-oxide fibers[29]
Metal reductionHeating Ti and TiO2 in an electric arc furnaceTitanium sub-oxides[12]
Metal reductionHeating TiO2 and Ti metal in an evacuated silica tube at 1150 °CTi4O7 crystals[30]
Metal reductionHeating Ti and TiO2 in H2Ti4O7[31]
Metal reductionHeating TiO2 and silicon
powder or silicon/CaCl2 powder		Various titanium sub-oxide powders[32]
Metal reductionReducing macroporous anatase TiO2 using a zirconium getterTinO2n−1 (n = 2, 3, 4, 6) [33]
Hydride reductionSolid-phase reaction of TiO2 with TiH2 at relatively low temperatureTitanium sub-oxide nanoparticles[34]
Hydride reductionHeating TiO2 nanoparticles and CaH2 powder at 350 °CTi2O3 nanoparticles [35]
Hydride reductionTiO2 was embedded with CaH2 and heated at 360 to 500 °C.TiOx thin films[36]
Table 2. A summary of methods for synthesis of nanostructured titanium sub-oxides.
Table 2. A summary of methods for synthesis of nanostructured titanium sub-oxides.
MaterialsMethodRef.
Ti8O15 nanowiresHeating H2Ti3O7 nanowires in hydrogen at 850 °C[53]
Ti8O15 nanowiresAn evaporation–deposition synthesis method[15]
Ti4O7 particles with diameters of 200–500 nmReduction of H4TiO5 with hydrogen at 850 °C [57]
Ti4O7 crystals (8–20 nm)Carbothermal reduction of cross-linked titanium ethoxide with polyethylene glycol at ~950 °C in Ar stream[58]
Magnéli phases with specific surface areas from 55 to 300 m2 g−1The gels made from titanium (IV) ethoxide and amino- or ethoxy-containing oligomers or polymers were heated at different temperatures under N2 or Ar[41]
Nanocrystalline Ti2O3, Ti3O5 and Ti4O7A combined sol-gel and vacuum-carbothermic processes[44]
Ti4O7 particles (around 250 nm)Reduction of TiO(NO3)2 in hydrogen at 1000 °C for 6 h [22]
Ti4O7 nanopowder (115 ± 30 nm)Reduction of titanium (III) oxalate particles in hydrogen[62]
Ti2O3 nanoparticlesHeating TiO2 nanoparticles (10–30 nm) and CaH2 powder at 350 °C [35]
Titanium sub-oxide nanoparticlesIrradiation of TiO2 particles dispersed in liquid with a pulsed UV laser [64]
Titanium sub-oxide nanoparticles (20–100 nm)By a thermal plasma method, using metatitanic acid (H2TiO3) as a starting material[23]
Ti4O7 nanoparticlesCarbothermal reduction using a multimode microwave apparatus [28]
Ti4O7 nanoparticles (60 nm)Carbothermal reduction of TiO2 nanoparticles using microwave irradiation at 950 °C for 30 min[65]
Titanium sub-oxides (30 nm)A thermal-induced plasma process[66]
Table 3. A comparative table among the synthesis methods.
Table 3. A comparative table among the synthesis methods.
Synthesis MethodsAdvantages/LimitationsCharacteristics
Hydrogen reductionA simple, well established/handling reactive gasFor synthesis of multi-dimensional pure titanium sub-oxides
Carbon reductionUse of various of carbon sources/uniform mixing of the reactantsFor synthesis of various titanium sub-oxides by controlling mole ratio of carbon and TiO2
Metal reductionWithout handling reactive gas/controlling reaction processUsually a mixture of different titanium sub-oxides
Hydride reductionReaction at relatively low temperature/handling reactive starting reactantFor synthesis of titanium sub-oxides with smaller particle sizes
Table 4. Electrical conductivity for single Magnéli phase materials *.
Table 4. Electrical conductivity for single Magnéli phase materials *.
TinO2n−1 PhaseElectrical Conductivity (σ/S cm−1)Log10 (σ/S cm−1)
Ti4O719953.3
Ti5O96312.8
Ti6O11631.8
Ti8O15251.4
* Adopted from ref. [68], copyright 2010, Elsevier.
Table 5. Summary of application areas of Magnéli phases.
Table 5. Summary of application areas of Magnéli phases.
AreaExamplesRef.
ElectrodesElectrodes for lead–acid batteries[67]
Fuel cellsConductive titanium sub-oxide support materials in fuel cells[73,74,75,76,77,78,79,80,81,82,83,84,85,86]
Remediation of aqueous waste and contaminated waterElectrocatalytic degradation for wastewater treatment[87,88,89,90,91]
Ti4O7 reactive membranesMembranes for
advanced electrochemical oxidation processes		[92,93,94,95,96,97,98]
BatteriesAs a sulfur host in Li2S battery, and conductive additive for improving performance of Li2S battery [43,60,99,100,101,102,103,104,105]
Solar cellsThe TiO/TiOx layer can enhance the absorption of sunlight, thus increasing solar conversion efficiency[106,107,108,109,110]
SensorsInvestigation of using nanostructured titanium sub-oxides as sensor materials for the determination of gaseous materials[111,112,113,114,115]
Electronic and photonic materialsNanostructured Ti4O7 in TiO2 resistive switching memory. Ti3O5 for light-triggered metal semiconductor transition. Titanium sub-oxides as thermoelectric materials[3,7,16,116]
Biological applicationsCoating material for medical devices[117]
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Wu, X.; Wang, H.; Wang, Y. A Review: Synthesis and Applications of Titanium Sub-Oxides. Materials 2023, 16, 6874. https://doi.org/10.3390/ma16216874

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Wu X, Wang H, Wang Y. A Review: Synthesis and Applications of Titanium Sub-Oxides. Materials. 2023; 16(21):6874. https://doi.org/10.3390/ma16216874

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Wu, Xiaoping, Haibo Wang, and Yu Wang. 2023. "A Review: Synthesis and Applications of Titanium Sub-Oxides" Materials 16, no. 21: 6874. https://doi.org/10.3390/ma16216874

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