Self-modification of titanium dioxide materials by Ti3+ and/or oxygen vacancies: new insights into defect chemistry of metal oxides

Juan Su a, Xiaoxin Zou *b and Jie-Sheng Chen *a
aSchool of Chemistry and Chemical Engineering, Shanghai Jiao Tong University, Shanghai 200240, China. E-mail: chemcj@sjtu.edu.cn
bState Key Lab of Inorganic Synthesis & Preparative Chemistry, College of Chemistry, Jilin University, Changchun 130012, China. E-mail: chemistryzouxx@gmail.com

Received 18th December 2013 , Accepted 30th January 2014

First published on 31st January 2014


The defect chemistry of metal oxides is a very important research aspect of inorganic solid-state materials. This is because (i) a certain amount of defects or imperfections are always present in metal oxide materials; (ii) the presence of defects affects, and even sometimes determines, the physical and chemical properties of the materials; and (iii) more importantly, defects do not necessarily have adverse effects on the properties of materials, and judicious “defect engineering” can bring about improved properties desired in material systems, and even some new useful functionalities that are not available to the “perfect” material. In this review, we specially highlight the recent research efforts toward understanding the defect chemistry of titanium dioxide (also known as titania, TiO2), a widely-studied multifunctional metal oxide. In the discussion, particular attention is paid to the synthesis of Ti3+/oxygen vacancy self-modified TiO2 materials and the favorable effects of these defects on the materials' properties and applications. This review, focusing on a representative metal oxide (i.e., TiO2), is anticipated to provide some new insights into the general defect chemistry of metal oxides, and to give impetus to the development of the “defect engineering” of metal oxide materials.


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Juan Su

Juan Su completed each of her academic degrees at the Department of Chemistry of Jilin University from 2004 to 2013, receiving her PhD in 2013 with Professor Jie-Sheng Chen on porous functional semiconductor materials. In 2013, she joined Prof. Kai-Xue Wang group as a postdoctor at the school of chemistry and chemical engineering of Shanghai Jiao Tong University in Shanghai. Her current scientific interest is mainly focused on the controllable preparation of porous reduced TiO2 materials and the microstructure-regulating of their functions for energy and environmental science.

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Xiaoxin Zou

Xiaoxin Zou was awarded a PhD in Inorganic Chemistry from Jilin University (China) in 06/2011, and then moved to the University of California, Riverside and Rutgers, The State University of New Jersey as a Postdoctoral Scholar from 07/2011 to 10/2013. He is currently an associate professor at Jilin University. His research interests focus on the design and synthesis of noble metal-free, nanostructured and/or nanoporous materials for water splitting and renewable energy applications.

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Jie-Sheng Chen

Jie-Sheng Chen received his BSc (1983) and MSc (1986) degrees from Sun Yat-sen University in Guangzhou and his PhD degree (1989) from Jilin University. He worked briefly from 1989 to 1990 as a lecturer at Jilin University until moving to the Royal Institution of Great Britain in London as a postdoctor. In 1994 he returned to Jilin University and was promoted to a full professor later on. In 2008, he joined the faculty of School of Chemistry and Chemical Engineering at Shanghai Jiao Tong University. His research interests are focused on the interfacial interactions of material components and the electrochemical and catalytic performances of solid materials.


1. Introduction

Titanium (Ti) is the ninth most abundant element (0.63%) in the Earth's crust, and it is only exceeded in abundance by O, Si, Al, Fe, Ca, Na, K and Mg.1 As the most important oxide of titanium, titanium dioxide (also known as titania, TiO2), which mainly exists in three polymorphs (i.e., anatase, rutile and brookite), is a multifunctional metal oxide material.2–8 Since the early twentieth century, TiO2 has been commercially used as a white pigment, sunscreen additive etc. These conventional applications of TiO2 mainly benefit from its basic physical and chemical properties, such as a high refractive index, strong UV-light absorbing capabilities, excellent chemical stability and abundance.2–8

In 1972, Fujishima and Honda discovered that the water splitting took place on a TiO2 electrode under ultraviolet (UV) light irradiation.9 This pioneering work immediately evoked great interest among chemical researchers, and simultaneously enormous effort was devoted to the research of TiO2 materials.1–8 This has resulted in many promising TiO2-based applications, ranging from photovoltaics, photocatalysis and self-cleaning techniques to sensors and photo-/electrochromics.1–15 TiO2 is generally the core component in these applications, and the properties of TiO2 mainly determine the efficiency of these applications, and the operating circumstances that we finally use. Thus, judiciously tuning the structure of TiO2 to optimize its properties/functions and further understanding the structure–property/function correlations has been actively pursued.

The main structure parameters of TiO2, which are strongly related to its properties/functions, typically include the crystal phase, crystallinity, shape, size, surface structure and defects.16–20 While many strategies have been developed to optimize the properties/functions of TiO2 by tuning its crystal phase, crystallinity, shape, size, and/or surface structure, the defect tuning of TiO2 remained elusive for a long time. However, breakthroughs have recently been made in the synthesis and applications of TiO2 materials with a large amount of defects (i.e., Ti3+ and oxygen vacancies), and specifically the favorable effects of these defects on materials' properties has attracted wide attention.

In this review, the defects in TiO2 are particularly designated as Ti3+ ions and oxygen vacancies, and the methods that are used to create these defects in TiO2 are described as “self-modification”. The most important characteristic of the self-modification method is that the improvement of the properties of TiO2 originates from the deliberately-introduced defects in it. In other words, through modifying the atomic structures of TiO2 by deliberately introducing defects (e.g., removing or rearranging titanium/oxygen atoms), the properties of TiO2 can be enhanced to a great extent. Herein, we summarize recent developments in the synthesis of self-modified TiO2 materials with Ti3+ ions and/or oxygen vacancies, and the favorable effects of these defects on the materials' properties and applications.

2. Synthetic strategies for self-modified TiO2

The synthetic strategies to self-modify TiO2 can be roughly divided into the categories “partial reduction method” and “partial oxidation method”. The former is more frequently used to prepare self-modified TiO2 materials.

X-ray photoelectron spectroscopy (XPS) and electron paramagnetic resonance spectroscopy (EPR) are generally applied to study the Ti3+ ions and oxygen vacancies in TiO2. XPS is an effective technique to determine the presence and amount of Ti3+ ions in TiO2, but it does not give any information about oxygen vacancies. In addition, because the detection depth of the XPS measurements is <10 nm, they can only provide the structural information of Ti3+ species at the surface and subsurface of the TiO2 particles. As a complementary technique of XPS, EPR can give some valuable information about Ti3+ species and single-electron-trapped oxygen vacancies (Vo˙), and can quantitatively analyse Ti3+ and Vo˙ using 2-diphenyl-1-picrylhydrazyl (DPPH) and a manganese (Mn) marker as the standards.

It is worth noting that the relationships between Ti3+ and oxygen vacancies are very complex in a solid TiO2 material. There are mainly three situations: (1) when the electric charges of Ti3+ species in TiO2 can be totally balanced by oxygen vacancies, and Ti3+ and oxygen vacancies should appear/disappear simultaneously; (2) besides Ti3+ and oxygen vacancies, a certain amount of structural defects (mainly referring to the local structural rearrangements in TiO2) are present in the TiO2 materials. This will lead to inequalities between Ti3+ and oxygen vacancies in the material; (3) when the electric charges of Ti3+ species in TiO2 are balanced by protons, Ti3+ has no direct connection with the oxygen vacancies.

2.1. Partial reduction method

The partial reduction method starts from a Ti(IV)-containing precursor (including TiO2), which is partially reduced by a suitable reductant under certain conditions to finally create a self-modified (or oxygen-deficient) TiO2 material. The specific means that were used in previously-reported partial reduction methods are summarized as below:
(i). High-temperature hydrogenation. Typically, TiO2 samples are treated by H2 at elevated temperatures to form self-modified TiO2 materials.21–26 During the hydrogenation process, H2 will react with the lattice oxygen, leading to the formation of oxygen vacancies in TiO2, and simultaneously one oxygen vacancy leaves behind two excess electrons. These electrons can locate at titanium positions (i.e., forming Ti3+), and can also remain at the positions of oxygen vacancies (i.e., forming electron-containing oxygen vacancies). It should be pointed out that the defect types are very sensitive to their formation conditions, and different defect types might exist and coexist in the same sample. For example, H. Liu et al. showed that the H2-treated samples obtained at < 450 °C possessed only single-electron-trapped oxygen vacancies (Vo˙), whereas the samples obtained at >450 °C possessed both Vo˙ and Ti3+.21 In another example, X. Yu et al. showed that the defect types and their distribution between the surface and bulk strongly depended on the hydrogenation temperature and time.22 Longer hydrogenation at 600–700 °C induced the attenuation of Ti3+, and the increase of O species (Fig. 1). The reason behind this phenomenon proposed by the authors is that bulk Ti3+ defects might diffuse to the surface and react with surface oxygen vacancies and absorbed oxygen molecules during longer hydrogenation, finally resulting in the formation of O species.
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Fig. 1 Digital images and the electron paramagnetic resonance (EPR) spectra of H2-treated TiO2 samples with different reaction times.22 Reprinted with permission from ref. 22. Copyright 2013 American Chemical Society.
(ii). Reduction of Ti4+ by other reductants. In addition to H2, other reductants, including metallic zinc, Al, diethylene glycol (DEG), NaBH4 and CO, were also employed to produce self-modified TiO2 materials.27–32 Using the redox reaction between Zn and Ti4+ (Zn + Ti4+ → Zn2+ + Ti3+), Zheng et al. prepared stable Ti3+ self-doped TiO2 with tunable phase composition.27 In addition, Sayed and co-workers reported that TiO2 was refluxed at 220 °C in DEG (a reducing agent and solvent) to form Ti3+-contianing TiO2 materials.29 Kang et al. demonstrated that TiO2 nanotube arrays were reduced by NaBH4, a strong reducing agent.30 The reaction was proposed as follows:
NaBH4 + 8OH → NaBO2 +8e + 6H2O

Ti4+ + e → Ti3+

In another study, a combustion method was developed by Feng’s group to synthesize self-modified TiO2 materials using in situ generated reducing gases (CO and NO) as the reductant. For instance, when a mixture containing ethanol, hydrochloric acid, titanium(IV) isopropoxide and ethylimidazole was introduced into a preheated oven (500–600 °C), bulk Ti3+ self-doped TiO2 was directly obtained.31 Using a similar combustion method, but replacing titanium(IV) isopropoxide with porous amorphous TiO2, the same group successfully prepared stable titania with a large number of Vo˙ defects (Fig. 2).32


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Fig. 2 Schematic representation of the synthesis of Vo˙–TiO2 from porous amorphous TiO2 in the presence of imidazole and HCl at 450 °C. The combustion of imidazole in the presence of HCl can release reducing gases such as CO and NO.32 Copyright 2013 WILEY-VCH Verlag GmbH & Co. KGaA Weinheim.
(iii). Hydro(solvo)thermal synthesis. The hydro(solvo)thermal technique was reported to synthesize self-modified TiO2 materials by different groups.33,34 Cheng’s group synthesized Ti3+-doped anatase TiO2 with {001} active facets by using TiB2 as the precursor in a HF-containing hydrothermal system (Fig. 3),33 whereas Zhao’s group synthesized Ti3+-doped anatase mesocrystals with {001} and {101} active facets by using titanium isopropoxide as the titanium source and formic acid as the solvent.34 Although the formation mechanism is still not clear in the above reaction systems, the hydro(solvo)thermal technique has proved to be very advantageous in combining defect engineering and facet control.
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Fig. 3 Optical photo (A), typical SEM and TEM images (B and C), SAED patterns (D) and high-resolution TEM image (E) of oxygen-deficient anatase TiO2 sheets.33 Reprinted with permission from ref. 33. Copyright 2009 American Chemical Society.
(iv). Plasma treatment, vacuum activation and e-beam irradiation. In low-temperature plasma, high-energy electrons and atoms are available to reduce the surface/subsurface layer of TiO2 materials, finally leading to the formation of self-modified TiO2 materials.35–38 This method has been employed by some Japanese researchers, and TiO2 materials with Vo˙ defects, for example, were obtained by them.35,36 By combining plasma and electrolysis, Zhang et al. synthesized oxygen-deficient TiO2 microspheres that exhibited optical absorption covering the range from ultraviolet to infra-red.37 In addition, vacuum treatment of TiO2 at elevated temperatures can also produce oxygen-deficient TiO2 materials due to the release of lattice oxygen atoms under vacuum conditions (TiO2 → TiO2−x + O2);39 and the e-beam irradiation method was reported to produce Ti3+–TiO2 because of the surface reduction by electrons.40
(v). Photochemical synthesis. The photocatalysis phenomenon was discovered in 1972.9 However, only in recent years has the photochemical technique been employed to synthesize Ti3+ self-modified TiO2 materials.41–46 This is because (1) in common TiO2 materials the amount of photogenerated Ti3+ species is rather limited, and (2) the photogenerated Ti3+ was usually considered as an intermediate state in a photocatalytic system, and thereby its modification effects on the properties of TiO2 were often ignored. To date, there are three main types of TiO2 materials that can be efficiently modified by photogenerated Ti3+. They are porous amorphous TiO2, TiO2 gels and amorphous TiO2 nanoparticles. Let us take the porous amorphous TiO2 material as an example.41,42 This porous material possesses an ultra-large BET surface area of ∼530 m2 g−1. With UV irradiation, the color of the porous TiO2 turned from white to intense blue under the protection of inert gas (Fig. 4A), which is indicative of the presence of a large number of Ti3+ species in the porous TiO2. The presence of Ti3+ in the porous TiO2 was confirmed by EPR spectroscopy (Fig. 4B).41 No paramagnetic signals were observed for porous amorphous TiO2 before UV irradiation, whereas after UV-irradiation, an intense signal at g = 1.925 was displayed. This EPR signal is ascribed to surface Ti3+. In addition, the amount of photogenerated Ti3+ can be further increased by the introduction of a dopant (e.g. V4+) into porous TiO2.
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Fig. 4 Digital images (A) and EPR spectra (B) of porous amorphous TiO2 before and after UV irradiation; (C) schematic representation of the photochemical synthesis (step 1 and step 2) of Ti3+-containing TiO2, and (step 3) the oxidation of Ti3+ in TiO2 by an oxidant, such as O2. TiOs and ROH denote the surface oxygen atoms of TiO2 and the hole scavenger, respectively. In this scheme, photogenerated electrons are present in the form of Ti3+, and Ti3+ storage and oxidation are proton-coupled processes.41 Reproduced by permission of The Royal Society of Chemistry.

The photogenerated Ti3+ in TiO2 is fundamentally different from those formed through other routes. The differences mainly include: (1) the photogenerated Ti3+ ions only exist on the TiO2 surface; (2) they are sensitive to oxidants such as O2; (3) their electric charges are balanced by the surface protons, rather than the oxygen vacancies; and (4) their generation and consumption are proton-coupled processes. Because of these differences, the photochemically-synthesized Ti3+–TiO2 showed many unique properties and functions (see below).

Fig. 4C shows a schematic representation of the photochemical synthesis (step 1 and step 2) of Ti3+–TiO2; and (step 3) the oxidation of Ti3+ in TiO2 by an oxidant such as O2. When incident light possessing energy greater than the band gap of TiO2 (i.e., UV light) hits the material, electrons in the valence band are excited to the conduction band, concomitantly leaving holes in the valance band. The photo-generated electrons and holes will allow reduction and oxidation reactions, respectively, to occur on the TiO2 surface. In a typical photochemical system for the synthesis of Ti3+–TiO2, a proper hole scavenger (e.g., ethanol) but no electron scavenger is needed in the reaction system. In this case, the photogenerated holes on the valence band react with the hole scavenger, and the electrons are comfortably stored in the form of Ti3+ (step 1 and 2). When the photogenerated Ti3+ contacts with a proper oxidant, such as O2, it will be consumed rapidly (step 3). It should be emphasized that the Ti3+ storage and oxidation are proton-coupled processes.

2.2. Partial oxidation method

Although the +4 oxidation state dominates titanium chemistry, compounds in the +2 and +3 oxidation states are also common. The Ti-based compounds with low oxidation states include TiH2, TiO, Ti2O3 and TiCl3. Considering that it is possible to obtain Ti3+ self-doped TiO2 materials by the partial oxidation of these compounds under appropriate conditions, researchers have been exploring synthetic approaches for translating this idea into reality.47–51

Recently, Grabstanowicz et al. reported the synthesis of Ti3+ self-doped rutile TiO2 using TiH2 as the precursor.47 As shown in Fig. 5, they first used a 30% aqueous solution of H2O2 to oxidize grey TiH2 powder, yielding a yellow or green gel-like product.47 H2O2 was selected as the oxidation agent because (1) TiH2 is stable in air and water, but very reactive with H2O2 at room temperature, and (2) the only byproduct of H2O2 is H2O, avoiding any unnecessary contamination. Finally, the dried gel was thermally treated at 630 °C in Ar to form Ti3+ self-doped rutile TiO2. Furthermore, Liu et al. showed that rice-shaped Ti3+ self-doped anatase nanoparticles were synthesized by the direct hydrothermal treatment of TiH2 in H2O2 aqueous solution (Fig. 6).48 From the above results, it can be concluded that even when using the same precursor (i.e., TiH2) and oxidation agent (H2O2), different thermal treatment methods can lead to different products (e.g., rutile phase obtained by the former method, but anatase phase obtained by the latter method).


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Fig. 5 Scheme of the synthesis of Ti3+–TiO2 from TiH2. Grey TiH2 powder was first converted to a yellowish gel after reaction with H2O2. The gel was further heated to yield the black Ti3+–TiO2 powder at high temperatures.47 Reprinted with permission from ref. 47. Copyright 2013 American Chemical Society.

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Fig. 6 (A) Schematic of the formation mechanisms for the Ti3+ self-doped anatase TiO2 nanoparticles, and (B and C) the interface diffusion–redox diagram. The green arrows indicate ion diffusion.48 Reproduced by permission of The Royal Society of Chemistry.

In addition to TiH2, TiO, Ti2O3 and Ti were also proved to be efficient precursors for the synthesis of self-modified TiO2 materials.49–51 Pei et al. demonstrated the synthesis of a self-doped TiO2 material with Ti3+ and Vo˙ defects by the hydrothermal treatment of TiO in HCl solution.49 They claimed that in the present reaction system, Ti3+ species can be generated via the reaction between TiO and HCl (TiO + HCl → Ti3+ + H2 + H2O). Liu et al. showed that the direct thermal oxidation of a Ti2O3 and TiO2 mixture in air at 900 °C could also produce the Ti3+ self-doped TiO2 material.50 In another study, Zuo et al. synthesized Ti3+-doped rutile TiO2 with {110} active facets by using titanium powder as the precursor in a HCl-containing hydrothermal system.51

3. Properties and functions of self-modified TiO2

Defects in TiO2 have often been considered to have only negative influence on the properties of materials. For example, the structural defects in TiO2 were frequently referred by researchers to function as combination sites of photogenerated changes that lead to a low photocatalytic activity. However, recent studies have shown that proper defects can improve the properties of materials in some cases, and sometimes even result into some new useful functionalities that are not available to the “perfect” material. Herein, we summarize the favorable effects of these defects on materials' properties and applications.

3.1. Conductivity

In 2000, Diebold’s group investigated the influence of Ti3+-related bulk defects on the properties of TiO2 (110) single crystals.52 In their study, TiO2 (110) cubes were cut from the same crystal and were heated in an ultrahigh vacuum to produce Ti3+-related bulk defects in TiO2. As shown in Fig. 7, five rutile crystals with different colors were prepared. The lighter crystal possessed a lower amount of bulk defects, and the amount of bulk defects increased in the order of cube 2 < cube 5 < cube 1 < cube 4 < cube 3. Table 1 shows the resistivity of the five cubes at 300 K. The results show that the darker crystal with a larger amount of defects has a lower resistivity than the lighter crystal with a smaller amount of defects. For example, the resistivity of cube 2 (the lightest cube) is 1835.0 Ω cm−1, whereas the resistivity of cube 3 (the darkest cube) is only 8.94 Ω cm−1. The resistivity of the former is about 205 times as high as that of the latter. In another study, in 2013 Chen’s group reported a porous crystalline titania material with heavily self-doped Ti3+ species.53 In the material obtained, 29% of the titanium species in TiO2 are present in the form of Ti3+, so this material is heavily self-doped by Ti3+. They further measured the room temperature electrical conductivity, and the results showed that the material exhibited a conductivity of 2.7 × 10−3 S cm−1 whereas the conductivity value of the Ti3+-free sample was 9.7 × 10−8 S cm−1. In other words, the conductivity of Ti3+–TiO2 increased by ∼5 orders of magnitude in comparison with that of the Ti3+-free TiO2 sample. The increase in conductivity (or the decrease in resistivity) of TiO2 by Ti3+ self-modification was considered to be because Ti3+ species in TiO2 can function as efficient donors, and the electrons of which could hop to the conduction band (or adjacent Ti4+ sites).53–57
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Fig. 7 Digital images of five rutile crystals with increased amounts of bulk defects in the order of cube 2 < cube 5 < cube 1 < cube 4 < cube 3. The lighter crystal possesses a lower amount of bulk defects.52 Reprinted with permission from ref. 52. Copyright 2001 American Chemical Society.
Table 1 Sample resistivity (Ω cm−1)52
  Cube 2 Cube 5 Cube 1 Cube 4 Cube 3
Resistivity 1835.0 108.24 46.76 24.06 8.94


3.2. Magnetism

Dilute magnetic semiconductors (DMSs), in particular the ferromagnetic oxides, have received great attention because of their potential applications in spintronics.58 The study of DMSs was significantly accelerated by the discovery of room temperature ferromagnetism in Co-doped TiO2 film.58 With the development of DMS studies, researchers surprisingly found that some defect-containing undoped TiO2 materials (without introducing magnetic ions in the materials) also exhibited room temperature ferromagnetism although TiO2 itself is intrinsically non-magnetic.

In 2006, Yoon et al. observed that oxygen-deficient anatase TiO2 film showed high-temperature ferromagnetism with a Curie temperature up to 880 K.59 In this study, the oxygen defects were located at the interface between the anatase TiO2 film and the (100) oriented lanthanum aluminate (LaAlO3) substrates because of the presence of an atomic mismatch at the interface. The authors also showed that the interfacial oxygen defects in the anatase TiO2 film on a (100) LaAlO3 substrate are the origin of the observed magnetism. Furthermore, the authors proposed that the oxygen vacancy-related Ti3+ and Ti2+ species might provide magnetic moments because of their 3d1 and 3d2 electronic configurations respectively, although no evidence was provided to support the presence of Ti3+ and Ti2+ in the material. However, in a theoretical study by Kim et al., the ferromagnetism in undoped TiO2 was thought be the result of the charge redistribution owing to the oxygen vacancy-induced lattice distortion.60

More recently, several different groups experimentally showed that Ti3+ ions with one unpaired 3d electron (3d1) provided the local magnetic moments in the undoped TiO2 materials. Zhou’s group prepared Ti3+ self-doped rutile TiO2 single crystals by the oxygen ion irradiation method, and observed ferromagnetism in the obtained material.61 In addition, Chen’s group prepared porous amorphous TiO2 with a large amount of Ti3+ by a photochemical method.41 The investigation into the magnetic properties of Ti3+–TiO2 revealed the co-existence of ferromagnetism and paramagnetism in the material. When the Ti3+ in TiO2 was completely consumed by air, the as-obtained Ti3+-free TiO2 turned diamagnetic. This observation strongly confirmed that the magnetism of Ti3+–TiO2 was an intrinsic feature associated with the Ti3+ species. Furthermore, Santara and co-workers reported on the oxygen vacancy-induced ferromagnetism above room temperature in undoped TiO2 nanoporous nanoribbons (Fig. 8).62 The observed magnetism was explained by the s–d interaction between the 1s1 electron spin in Vo˙ and the 3d1 electron spin of the Ti3+ ions. However, it should be noted that the exact mechanism of the magnetism for undoped TiO2 is still not very clear at present. Thus, further experimental and theoretical studies on this unusual magnetic phenomenon are strongly encouraged.


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Fig. 8 Temperature-dependent magnetization (MT) curve of oxygen-deficient TiO2 nanobelts showing a ferromagnetic to paramagnetic transition at ∼800 K.62 Reproduced by permission of The Royal Society of Chemistry.

3.3. Sensing application

TiO2, a wide band gap semiconductor, is a well-established oxide material that has been employed as a promising candidate in sensing applications. Previously, surface modification with noble metal particles or other metal oxides was often attempted to improve the sensing properties of TiO2.3 Recently, Chen’s group showed the importance of Ti3+ defects in TiO2-based sensors for the first time, providing a new strategy to optimize the sensing properties of TiO2. In their study, thermally stable, Ti3+ self-modified TiO2 nanomaterials were demonstrated to exhibit enhanced sensitivity and ultrafast response/recovery (<3 s) for the detection of various organic vapors, such as ethanol, methanol and acetone.63 The enhanced gas-sensing performance of Ti3+ self-modified TiO2 materials could be related to the Ti3+-induced enhancement of oxygen absorption on the TiO2 surface. Because the Ti3+ species in these nanomaterials were deeply buried in the bulk of the TiO2 particles, the Ti3+ effect on increasing the surface reaction activity of TiO2 is limited. This thereby led to the requirement for a high operating temperature (300 °C) in sensing applications. In view of this, the same group reported another porous crystalline titania material with heavily self-doped Ti3+ species.53 This Ti3+ self-doped TiO2 material contained a considerable proportion of Ti3+ in the subsurface region of the titania particles, and thus served as an efficient room temperature gas-sensing material for specific CO detection with fast response/recovery. The self-dopant (Ti3+) in the titania material was proved to possess dual functions: (1) decreasing the resistance of TiO2 significantly, and (2) increasing the chemisorbed oxygen species substantially.

As shown in Fig. 9, the sensing mechanism of Ti3+ self-doped TiO2 was discussed by Chen et al.,53 based on the interaction of surface chemisorbed oxygen and CO gas at room temperature. When the TiO2 sensor is exposed to air, the oxygen molecules in air are chemisorbed on the TiO2 surface and then extract electrons from the TiO2. This will result in a potential barrier and thus a high-resistance state. On the other hand, when the sensor is exposed to CO gas, the latter reacts with the surface oxygen species, reducing the amount of surface adsorbed oxygen. This finally leads to a decreased potential barrier and thus a low-resistance state. For such a surface reaction, the increase in the amount of surface-adsorbed oxygen should be beneficial to the reaction activity/kinetics for oxidation of the CO gas on the TiO2 surface. In addition, the decrease of the electrical resistance not only makes it feasible to detect the resistance (and its variation) for a room temperature sensor but also enables rapid electron transport between the Au electrodes of the sensor. Therefore, the simultaneous accomplishment of high oxygen adsorption and low electrical resistance by Ti3+ self-doping played a crucial role in the room temperature sensing performance of the Ti3+–TiO2 material.


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Fig. 9 Schematic representation of the sensing mechanism of Ti3+-containing TiO2. Resistance ↓ means resistance decrease. Percolation path describes the electron transport pathway between the Au electrodes of the sensor.53 Reprinted with permission from ref. 53. Copyright 2013 American Chemical Society.

In another study, Pan et al. showed that Ti3+ self-doped rutile was used for the direct and ultrafast detection of H2O2.64 This detection was based on the fast color change from blue to yellow when Ti3+ self-doped rutile was exposed to H2O2. The proposed chemical mechanism is shown by the equation:

2Ti3+ (blue) + 3H2O2 + 2H2O → 2H2TiO4 (yellow) + 6H+

3.4. Photocatalytic hydrogen evolution and degradation of organic compounds

In 1972, Fujishima and Honda discovered the photoinduced water splitting at TiO2 electrodes.9 Since then, photocatalytic H2 production over TiO2 materials has long been considered as a promising approach for solar energy exploitation, especially because H2 is attractive as a clean fuel. However, the widespread use of TiO2 is limited by its wide band gap energy, which causes the catalyst to exploit only a very small proportion (about 3–5%) of solar radiation. Recently, the self-modification strategy was demonstrated to be one of effective methods to shift the photo-responsive range of TiO2 to the visible spectral region.

The first attempt at visible light-driven photocatalytic H2 evolution over self-modified TiO2 was reported by Feng’s group.31 Through a facile combustion method, Feng et al. prepared Ti3+ self-doped TiO2, which possessed high stability in air and water under irradiation, and visible light activity for H2 evolution from a methanol–water mixture (Fig. 10). Because the combustion process was very harsh, this method only yielded irregularly-shaped products. To overcome this shortcoming of the combustion method, the same group developed a hydrothermal method to grow Ti3+ self-doped rutile TiO2 crystals with highly active facets.51 This material showed enhanced photocatalytic activity relative to the irregular nanoparticles prepared by a combustion method, which was attributed to the exposure of active {110} facets on the material. In another study, this group also reported a dopant-free, visible light-responsive TiO2 photocatalyst with Vo˙ defects. This material exhibited not only satisfactory thermal and photo-stability, but also superior photocatalytic activity for H2 evolution (115 μmol h−1 g−1) from water with methanol as a sacrificial reagent under visible light (λ > 400 nm) irradiation.32 After Feng's pioneering work, there were several reports on new methods to synthesize the self-modified TiO2 materials (e.g., vacuum activity, and selective oxidation by TiH2 or TiO) and their visible light activity for H2 evolution.28,39,48,49,65


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Fig. 10 Time courses of evolved H2 under visible light (>400 nm) illumination over Ti3+ self-doped TiO2 synthesized by a combustion method.31 Reprinted with permission from ref. 31. Copyright 2010 American Chemical Society.

In addition to photocatalytic H2 evolution, the photocatalytic degradation of organic compounds by self-modified TiO2 materials was also reported for potential environmental remediation using visible light.21,22,25,27,28,38,39,47,48,50,65,66 In these studies, a range of organic compounds, including benzoic acid, 2-propanol, methylene blue (MA) and methyl orange (MO), were decomposed into CO2 and H2O under visible light. For example, Liu et al. demonstrated that the Ti3+ self-doped TiO2 becomes an efficient visible light active photocatalyst for the degradation of 2-propanol by the grafting of Cu(II) oxide amorphous nanoclusters.50 The results showed that the presence of Ti3+ did not significantly narrow the band gap, but led to the formation of isolated states between the forbidden gap. These isolated states have various electric levels from 0.3 to 0.8 eV below the conduction band minimum, resulting in the broad visible light absorption of the Ti3+ self-doped TiO2 materials. On the other hand, Cu(II) oxide nanoclusters functioned as co-catalysts to suppress the recombination of photogenerated charges.

3.5. Other photocatalytic reactions

Zhao and co-workers studied the photocatalytic reduction and oxidation of nitrosobenzene over Ti3+ self-doped TiO2 mesocrystals.34 They found that the synergetic effect between Ti3+ and the (101)/(001) facet ratio are responsible for the higher activity of the TiO2 mesocrystals for both the oxidation and reduction of nitrosobenzene. The authors claimed that the (101) facet was intrinsically more active than the (001) facet, and the existence of Ti3+ defects could shift the valence band maximum downwards and facilitate the generation of strongly reductive electrons. Note that in their study, no visible light photocatalytic activity was observed.

However, in another study, Mul et al. showed that Ti3+ self-doped titania with a band gap of 2.93 eV served as a photocatalyst for the liquid phase selective oxidation of methylcyclohexane under both UV and visible light irradiation.67 The activity of Ti3+ self-doped titania surpassed those of the commercial titania catalysts, such as P25 TiO2. In addition, Nakamura et al. demonstrated that self-modified TiO2 with Vo˙ defects possessed photocatalytic activity for NO oxidation in the visible light region up to 600 nm.35,36 The oxidation of NO under visible light proceeded consecutively as NO → NO2 → NO3. The visible light activity of Vo˙-modified TiO2 was attributed to the presence of an oxygen vacancy state between the valence and conduction bands in the TiO2 band structure.

3.6. Photoelectrochemical water splitting

In 2011, Li and co-workers demonstrated for the first time that hydrogen treatment on TiO2 nanowires and nanotubes created a high density of oxygen vacancies, and thus significantly improved the performance of TiO2 materials for photoelectrochemical water splitting (Fig. 11).26 In comparison to pristine TiO2 nanowires, the TiO2 samples after hydrogen treatment showed a significantly enhanced photocurrent in the entire potential window, and low photocurrent saturation potentials of −0.6 V vs. Ag/AgCl (0.4 V vs. RHE). The optimized TiO2 nanowire sample gave a photocurrent density of ∼1.97 mA cm−2 at −0.6 V vs. Ag/AgCl under the illumination of simulated solar light (100 mW cm−2 from a 150 W xenon lamp), and the solar-to-hydrogen efficiency was calculated to be around 1.1%. Furthermore, the incident photon-to-current conversion efficiency (IPCE) spectra showed that the photocurrent enhancement by hydrogen treatment was mainly due to the improved photoactivity of TiO2 in the UV region (Fig. 11).
image file: c3ra47757f-f11.tif
Fig. 11 IPCE spectra of pristine TiO2 and several TiO2 nanowires prepared by hydrogen treatment at 350, 400 and 450 °C.26 Reprinted with permission from ref. 26. Copyright 2011 American Chemical Society.

To enhance the visible light water splitting performance of TiO2 nanowires, C. B. Mullins and coworkers showed hydrogenation and nitridation cotreatment of TiO2 nanowires to be a simple and effective method. This cotreatment led to the codoping of nitrogen and Ti3+ in TiO2 nanowires.23 The synergistic effect between the N-dopant and Ti3+ was believed to be the key to the extension of the active spectrum and the superior visible light water splitting activity. In other studies,30,68 Ti3+ self-doped TiO2 nanotubes were prepared by either chemical reduction or electrochemical reduction, and they all exhibited enhanced water splitting performance, regardless of the method used.

3.7. Proton-coupled electron transfer agents

Although it is well-known that Ti3+ self-modified TiO2 materials (Ti3+–TiO2) can be synthesized by a photochemical method (as mentioned before), it is little-known that the Ti3+–TiO2 materials obtained by such a method can serve as proton-coupled electron transfer agents. In 2010, Zou et al. developed a photochemical approach for the preparation of porous TiO2 with a large number of Ti3+ species.41 When studying the interaction of the Ti3+ in TiO2 with nitrobenzene, they found that the TiO2 with Ti3+ species supplied not only electrons (i.e., Ti3+), but also protons (H+) for the transformation of nitrobenzene into aniline. They realized that when an electron (in Ti3+) was released to an electron acceptor such as nitrobenzene, a proton (H+) on the surface of TiO2 was also removed at the same time. However, in their paper,41 the concept “proton-coupled electron transfer (PCET)” was not proposed. In 2012, Schrauben et al. showed that photochemically reduced (or Ti3+ self-modified) TiO2 nanoparticles in solution transferred an electron and a proton to phenoxyl and nitroxyl radicals, indicating that e and H+ were coupled in this interfacial reaction.44 The concept of PCET was suggested by the authors, and correspondingly the Ti3+–TiO2 materials synthesized by a photochemical method could serve as proton-coupled electron transfer agents. The above observations had important implications for the understanding and development of chemical energy technologies, such as photocatalysis, that were usually previously considered as electron transfer processes, rather than proton-coupled electron transfer processes.

In a recent study, Su et al. reported vanadium-doped porous TiO2 with highly active hydrogen (V–TiO2(H*)) prepared by a photochemical method.42 The vanadium doping could increase the amount of active hydrogen in the obtained TiO2 material. The active hydrogen (H*) in this material is not molecular hydrogen, but a proton with an electron (i.e., Ti3+) in its vicinity. Furthermore, the authors used the as-obtained TiO2 materials as proton-coupled electron transfer agents for the chemoselective hydrogenation of nitroarenes. The results showed that the TiO2 materials can instantly (<10 s) and selectively reduce nitroarenes to aminoarenes under ambient conditions. During the reduction process, the V–TiO2(H*) provided rich H* species (i.e., coupled e and H+), which could be regenerated through UV light irradiation of the reactant in methanol after the consumption of the H* species (Fig. 12).


image file: c3ra47757f-f12.tif
Fig. 12 Schematic diagram for the selective hydrogenation of nitroarenes to aminoarenes by V–TiO2(H*), and the regeneration process of V–TiO2(H*). The active hydrogen (H*) in our material is not molecular hydrogen but a proton with an electron in its vicinity.42 Reproduced by permission of The Royal Society of Chemistry.

4. Conclusions

In this article, we reviewed recent developments in the synthesis of self-modified TiO2 materials with Ti3+ ions and/or oxygen vacancies, and some favorable effects of these defects on the properties and applications of TiO2 materials. Despite the enormous strides and many achievements made in the field of TiO2 defect chemistry, as outlined in this review, there are still a lot of intractable difficulties ahead, such as the identification of the nature and local microstructure of the Ti3+/oxygen defects. In addition, exploring new applications related to Ti3+/oxygen defects and understanding the functions of these defects still remain challenging. This therefore calls for more research efforts in the general defect chemistry of metal oxides, a burgeoning and fascinating area of chemistry.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (91022019, 21331004) and the National Basic Research Program of China (2011CB808703, 2013CB934102).

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