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Academic research paper on topic " Function of TiO 2 Lattice Defects toward Photocatalytic Processes: View of Electronic Driven Force "

Hindawi Publishing Corporation International Journal of Photoenergy Volume 2013, Article ID 364802,16 pages http://dx.doi.org/10.1155/2013/364802

Review Article

Function of TiO2 Lattice Defects toward Photocatalytic Processes: View of Electronic Driven Force

Huanan Cui,1 Hong Liu,1 Jianying Shi,1 and Chuan Wang2

1 Key Laboratory of Environment and Energy Chemistry of Guangdong Higher Education Institutes, School of Chemistry and Chemical Engineering, Sun Yat-Sen University, Guangzhou 510275, China

2 Chongqing Institute of Green and Intelligent Technology, Chinese Academy of Sciences, Chongqing 401122, China

Correspondence should be addressed to Hong Liu; liuhong@cigit.ac.cn and Jianying Shi; shijying@mail.sysu.edu.cn Received 18 July 2013; Revised 6 October 2013; Accepted 14 October 2013 Academic Editor: M. Muruganandham

Copyright © 2013 Huanan Cui et al. This is an open access article distributed under the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.

Oxygen vacancies and Ti-related defects (OTDs) are the main lattice defects of TiO2, which have great influence on its photocatalytic activity. To understand the relationship between the defects and photocatalytic activities, detailed discussions based on the electronic driven force provided by these defects are carried out during the three commonly accepted processes in photocatalytic reactions. It is found that these defects inevitably (i) influence the energy structure of the pristine TiO2 as the isolate acceptor/donor level or hybrid with the original orbital, (ii) provide a disordered short-range force that confuses the charge carriers transferring to surface active sites, (iii) act not only as the surface active sites for trapping the charge carriers but also as the main chemisorption sites for O2, H2O, and organic species. These effects of the defects make them one of the key factors that determine the efficiency of heterogeneous photocatalysis. Clarifying the role of the defects will further facilitate the exploration and the construction of high-performance photocatalysts for practical applications.

1. Introduction

The construction of photocatalysis system provides a promising strategy to solve energy and environmental issues by converting solar energy to hydrogen/electric energy and oxidizing the organic compounds to reduce the chemical oxygen demand (COD) in the environment. Titanium dioxide (TiO2) has been studied extensively due to its fundamental properties and wide range applications [1-4]. As light can go as deep as about 1 ^m down to the surface [5], photocatalytic reaction automatically occurs on the surface/subsurface. Attentions were paid intensively to surface science of TiO2 and other oxide photocatalysts, and an expectation for the surface properties insights into the molecular level is in urgent need. The surface properties are largely influenced by the defects, and the dominant defects in TiO2 surfaces are oxygen vacancies and Ti-related defects (OTDs) [6-11].

OTDs can be created during the diverse workable synthesis strategies (e.g., doping [12-14], loading [15, 16], and constructing Z-scheme photocatalytic system [17]) and can also be found in many kinds of efficient photocatalysts (e.g.,

solid solution [18], heterostructure composites [17, 19], and multilayer films [20]). During these systems, the enhanced photocatalytic activities are always attributed to the structure [21], composition, particle size [22], or surface area [23], which do not seem to have direct relationship with defects. However, during the deep discussion of the intrinsic factor for photocatalysis, the importance of the defects in reactions is gradually recognized and commonly accepted as the dominant limitation of photocatalytic efficiency nowadays.

It is the energy structure that makes TiO2 a semiconductor photocatalyst. And OTDs have been widely concerned to the so-called "self-doping" effect [24]. Pristine TiO2 is traditionally thought to be inert under visible light for its broad band gap (Eg ~ 3.2 eV). To narrow the band gap, foreign anion elements (e.g., N [12, 25], C [13], F [26], S [27]) and cation elements (e.g., Fe3+ [28], Cr3+ [29], and Ce4+ [16]) are frequently introduced into TiO2 lattice, forming new energy state in solids. Defect state caused by OTDs was also found to influence the electronic structure of pristine TiO2, and visible light response was clearly observed in reduced TiO2_x specimens [11, 30]. OTDs were found to

be the active response sites from the scanning tunneling microscopy (STM), which came out to be one of the powerful techniques in detecting surface configuration in materials [31-33]. The formation of OTDs is called "self-doping." The effect of narrowing the band gap is proved to exist not only in TiO2 but also in other metal oxides [34]. Avoiding introducing excess foreign elements, OTDs self-doping is recognized as a green and promising strategy for exploring environmentally friendly photocatalysts.

When an electron in the ground state absorbs a photon which possesses sufficient energy larger than Eg of the semiconductor, it can be excited from the valence band to the conduction band, leaving a hole behind. Once the electron-hole (e-h) pair is generated, the charge carriers may immediately start the journey to the surface active sites, mainly to the OTDs. Reduced TiO2 OTDs surface is found to have enhanced photocatalytic efficiency than defect-free surface. The efficiency may rise when increasing the concentration of OTDs in a moderate range on surface [35, 36]. Density functional theory (DFT) calculation, photoemission spectroscopy (PL), atomic force microscopy (AFM) [37, 38], and STM reveal that lattice distortion induced electronic density variation at OTDs accumulates the spontaneous charge migration to the surface, where the OTDs act as the trap center. They serve to promote the separation of e-h pair and prolong the life time of charge carriers in TiO2 [39]. OTDs at subsurface are recognized more as the e-h recombination center, also caused by the trapping effect. Kong et al. [40] found from STM and positron annihilation lifetime spectroscopy (PALS) [41, 42] that the larger the proportion of surface defects was in the whole defects of TiO2, the higher the photocatalytic activity was. This competitive relationship between surface OTDs and subsurface OTDs on trapping charge carriers should be attributed to their electronic properties.

Owning to the feature electronic density, OTDs can not only gather charge carriers but also function as the absorbing sites for external species (e.g., O2, H2O, CO2, and plenty of organic substances) [43, 44]. The adsorption of these species toward OTDs may occur in the form of dissociative adsorption (chemical adsorption). This is ofvital importance for mediating the transfer of charges, undergoing from the surface ^ OTDs ^ dissociated species ^ further redox reaction [45]. For instance, H2O can dissociate at bridging oxygen vacancy, forming two neighbouring hydroxyls (—OH). This hydroxyl facilitates the photocatalytic reactions by lowering the charge transfer barrier, assisting the adsorption of organic substances, and further catalyzing the decomposition of them. Besides, the -OH was deduced to have an effect on the redistribution of subsurface OTDs. The dissociating mechanisms of species at OTDs active sites remain very rough, and the function of these dissociated species in photocatalysis still needs further investigation.

This review focuses on the understanding of the function of OTDs in photocatalytic reactions, from the view of the electronic driven force toward neutralizing. The generation of OTDs is always followed by the redistribution of electronic density. This variation has compact relationship with the photocatalytic efficiency. Upon the three basic photocatalytic processes, further discussion was carried out and the effects

of OTDs were provided. The findings of this work would facilitating the design and exploration of high-performance green photocatalysts in the molecule level.

2. General Issues of Photocatalysis

The general mechanism of heterogenous photocatalysis is always described as Figure 1. When the photon energy is sufficient to promote the electrons in the valence band to jump to the conduction band, three main steps can happen successively: (i) photon absorption and electron-hole pair generation, (ii) charge separation and migration to surface reaction sites or to recombination sites, and (iii) surface chemical reaction at active sites containing donor oxidation at valence-band hole and acceptor reduction at electron center. Upon these three photocatalytic processes, several defects-related photocatalytic issues should be clarified.

2.1. Photoexcited Location. The process of heterogenous pho-tocatalysis starts by irradiation. That is, photoexcitation of electron at ground state is the prerequisite. It should be noticed that the excitation step of electron under irradiation may not only occur in the semiconductor but also occasionally happen in the substance adsorbed on its surface, like the reaction happening in dye-sensitized solar cell [22]. The charge carriers may experience different pathways in these two situations. Hence in this review, we focus on the cases that initial excitation happens in the semiconductor itself.

2.2. Point Defects and Standard Specimen. The concept of defects initially lies in the solid state physics description of lattice distortion. Such distortion can be in the form of point defect, liner defect, two-dimensional flaw or interface and three-dimensional valley or heteroimpurity. Among these, point defect is the most investigated case and provides the primary realization of properties of lattice defects, including energetic, thermal, electrical, optical, and magnetic features in solids [46-49]. OTDs are generally recognized as point defects. The analysis of OTDs and OTDs-related clusters in photocatalytic reactions is usually carried out by performing the point trap model.

Lattice defects are inevitably generated during synthesis procedure and are thermal- and preparation-dependent products [50]. Intrinsic point defects exist automatically in lattice as vacancies, interstitials, and atomic impurities which are frequently observed in doping materials. It was observed by STM images that these defects distributed scatteredly on the flat surface or centralized violently at the terrace boundaries which are proved to be responsible for the increased photocatalytic efficiency. However, the existence of the incoordinate and disordered defects in different samples makes it hardly possible to compare the photocatalytic properties precisely even in the same photocatalytic system. The well-defined particles hardly exist at room temperature under ambient conditions. As a result, to simplify the analysis in molecule scale, a hypothesis about well-defined samples is made in the vast majority of researches, and point defects are well-distributed ignoring the boundary defects.

Thus, preparation of comparable standard specimen is in urgent need for better understanding the nature of photocat-alytic reaction.

2.3. Energy Structure. As is known, ground state energy structure of a pure semiconductor is composed by valence band (VB), conduction band (CB), and the band gap between them. Light absorption for photocatalytic reaction is determined by the range of band gap (Eg). When the photon energy is sufficient to excite an electron in valence band to overcome Eg to the conduction band, photocatalytic reactions may occur.

The point defects as impurities are recognized to have a "self-energy" in short range, creating a variation in the host electronic structure. There are two general identified manners on the influence of defect energy level to the host energy structure: (i) to introduce an isolated mid gap as the acceptor/donor level, leaving the primary structures unchanged, and (ii) to hybrid with the host VB or CB, narrowing or broadening the band gap. These two phenomena were proposed in N-doped TiO2 samples by Irie et al. [51] and Asahi et al. [52]. OTDs also have been found to undergo these two manners favorably in oxide semiconductors. It is crucial to study the energy structure of defects, for the long-range force of charge transfer is provided by the position of valence band/conduction band versus the redox potential of the adsorbed substance on the external surface of photocatalyst, which determines whether the photocatalytic reaction with special substances would happen or not (see Figure 1).

2.4. n/p-Type. TiO2 is one of the most extensively studied photocatalysts. Pure TiO2 samples synthesized from conventional preparation methods are mostly oxygen-deficient nonstoichiometric compound or a solid solution of oxygen into TiO2_x lattice [5]. In consequence, oxygen vacancies as well as Ti interstitials in TiO2 make it an n-type property of dominant materials, precisely written as TiO2_x [13]. However, the formation of metal-deficient TiO2 can be obtained under strong oxidation at elevated temperatures [9, 53, 54] and relatively a p-type behavior is found.

It should be clear that, in the nonstoichiometric n-type TiO2, the charge carriers not only are the electrons but also may be the holes during the photocatalytic procedure. As the transfer rate of the holes is several orders of magnitudes slower than electrons, the dominating charge carriers are electrons and thus the TiO2 is called an n-type semiconductor. When the proportion of O atom in TiO2 rises, the TiO2 may be identified as the p-type semiconductor taking holes as the main charge carriers. The coexistence of n- and p-type in TiO2 is shown in Figure 2 [8], as a result of distribution of OTDs.

To simplify the model of photocatalytic reactions in particles, it is hypothesized that n-/_p-type can counteract with each other during the long routes to the surface; thus a conception of "surplus n-type" or "surplus p-type" is usually introduced as there are only photoelectrons in n-type TiO2 and vice versa. When TiO2 is used as an electrode, the electrons and holes move efficiently toward the opposite

direction and the relationship between them seems more to be the coworker than to be the competitor.

The ability of intrinsic defects to influence the transfer of charge carriers can be judged by electrical conductivity [55], considering the cooperation of both the n-type and the p-type aspects. The resistance of TiO2 can be influenced by the trapping of the charge carriers in ionic defects and overcome the barriers in the distorted bond. An increase of the electrical conductivity with the increase of oxygen vacancies was observed by Nowotny et al. when changing the oxygen partial pressure [48]. Accordingly, it is revealed by the enhanced electrical conductivity that the surface investigated is an n-type dominant surface and oxygen vacancies facilitate the charge transfer.

2.5. Main Characterization Methods of Defects. Based on the first principles computation, the modified local density approximation (LDA) and the related generalized method as well as some other computational modelings are used to calculate the ground state energy structure of the host semiconductor, despite of the error [56, 57]. As to the energy level of the defects and the excited states, however, it becomes more complicated to choose the most suitable calculation method/model and confirm the initial settings of the parameters. Estimations are always handled by empirical adjustment without precise specifications. Among these modeling principles, density functional theory (DFT) provides the relatively acceptable data and is widely used in the calculation of electronic structure of TiO2 [58-62].

Several experimental methods for OTDs are listed in Table 1 and they are always used together to get the defects information.

2.6. Transient Local Heat. As has been reported, the kinetic rate of photocatalytic reactions can be varied from different temperature and light intensity [63], and heat is one problem. For the sake of practical application of photocatalysts in ambient environment, a set of coolers (mostly condensate water) is usually equipped to provide moderate temperature for laboratory-scale tests. However, ambient temperature could not prevent the transient local heat that comes from (i) the released nonradiative thermal energy form recombination and (ii) the slow diffusion of the adsorbed radiative infrared lightofthe lightsource(solarorspeciallytheXelampwiththe power of 300 W or 500 W) in some lattice distortions. Here a hypothesis is made that such heat do not or has little influence on the separation and migration of charge carriers.

3. Generation of OTDs in TiO2

3.1. Removal of Oxygen in TiO2. Ideal structure model of bridging oxygen vacancy in TiO2 (110) surface lattice is shown in Figure 3(a). Figure 3(b) gives a direct picture for the position of oxygen vacancies by STM, and it can be found that the bridging oxygen vacancies (Ob-vacs) are the main surface defects. Figure 3(c) represents the electronic density scattering around a single oxygen vacancy [64] and it could be seen that the potential field of the neighbouring Ti is

Table 1: Common characterization techniques for defects.

Technique Characterization Evidence References

Colour of the materials Different from pristine TiO2 The defects maybe exist [123]

High-resolution transmission electron microscopy (HR-TEM) The atomic lattice is blurred Maybe exists [72,123]

Ultraviolet-visible spectroscopy (UV-vis) An optical absorption band above 400 nm Maybe exists [30]

Photoemission spectroscopy (PL) The emission position and intensity Type, relative concentration of defects [124]

Raman spectroscopy Variation in vibration of O and Ti-related region Type of defects [30, 38, 72]

X-ray photoelectron spectroscopy (XPS) Valence state variation Type of defects mainly Ti3+ [30, 37, 72, 89]

Electron paramagnetic resonance (EPR) g factor calculated from the position of the sharp signal Type of paramagnetic defects [35,123,125,126]

Positron annihilation lifetime spectroscopy (PALS) The lifetime of the positrons Size, type, and relative concentration of defects [68]

Scanning tunneling microscopy (STM) Light dot in pictures The type, position of defects [31,113,120]

Atomic force microscopy (AFM) Comparison of pictures The type, position of defects [38]

Temperature programmed deoxidation (TPD) A narrow peak related to partial oxygen loss according to temperature. Rough concentration of oxygen defects [112,123,125]

Electron energy loss spectroscopy (EELS) Energy loss Electronic change in defects [112]

Synchrotron radiation X-ray absorption fine structure spectroscopy (XAFS) Peak position Geometrical structure of active sites [127]

Acœptor^- Potential/eV versus NHE

" (iii) Reduction (pH= 7) Acceptor

H2/H2O (-0.41 eV) O2/O2'- (-0.28 eV)

O2/H2O (+0.82 eV) •OH/OH- (+1.99 eV) •OH/H2O (+2.77 eV)

Donor (iii) Oxidation

Donor"+ +

Figure 1: Main steps occurring in the photoelectrochemical mechanism. ((i) Photon absorption and electron-hole pair generation. (ii) Charge separation and migration (a) to surface reaction sites or (b) to recombination sites. (iii) Surface chemical reactions at surface active sites, donor oxidation at valence-band hole, or acceptor reduction at electron center.)

the most influenced site whereas the variations die largely in distance. It reveals that OTDs can work as short-range traps in capture charge carriers, which is crucial for charge migration in photocatalytic reactions. The formation processes of these defects assist in the understanding of Figure 3(c).

In an ideal defect-free lattice of TiO2, the removal of an oxygen atom in lattice is usually accompanied with the exposure of the neighbouring metal atoms and the material would tend to maintain electrostatic balance according to the following reactions:

2Ti4+ + O2- <—> 2 [Ti4+]+2e/VO" + Oa (1) [Ti4+] + e <—> [Ti3+] (2)

+ e •

[Ti4+ ] + Vo' — [ Ti3+ ] + V,

where Oa represents the oxygen atom that is taken away from the lattice, VO" represents the corresponding empty position (1), and VO' also represents the empty position which originated from the removal of oxygen but with a localized single electron (3). [Ti +] represents the exposed neighbouring Ti + at oxygen vacancy (1) and [Ti3+] represents the exposed Ti reduced by the excess electron (2) from O removal.

It can be seen from (1) that the removal of oxygen from the lattice generates VO" and further causes the formation of VO' (3) and the reduction of the neighbour metals (4) [6,65]. Liu et al. [7] showed the relationship between the formation

of VO' and the corresponding existence of Ti3+ during

1 1 1 1 1 1 1 1 1 1 i i i i I i i i i I i i i i I

- V " VO

- S'\Ti4+ Ti3+ N \ \ \ \ \ \ yd \ / \ /

\ N \ ✓ /

TiÖ2-% 1073 K A = 1.0-10-4 -

- , -

-15 -10 -5 0 5

log p(O2) [p(O2) (Pa)]

Figure 2: Oxygen vacancies and Ti-related defects (OTDs) related n/p-type TiO2 at 1073 K [8].

H2 treatment up to 700°C (Figure 4). As the temperature increased, more Oa were removed by H2. Magnetic Ti3+ defects generation followed the VO' formation, indicating the reaction of the trapped localized single electron of VO' with the near Ti4+ (4). This process was mainly controlled by temperature [66]. Besides, temperature has great influence on the transition of the crystalline structures of TiO2, and the formation energy order of oxygen vacancies on surfaces is brookite (5.52 eV) > anatase (5.58 eV) > rutile (5.82 eV), which may result in the different concentration of OTDs [67].

The control of ambient O2 concentration/pressure can also adjust the concentration and the distribution of defects in a feasible range [8]. The occurrence of VO" may suffer the reverse reaction under a wide range of oxygen activities and Ti vacancies can be obtained from the prolonged oxidation of TiO2 at elevated temperatures [9]. Under ambient condition at room temperature, the VO" in TiO2 dies out gradually, and this usually causes weakened photocatalytic efficiency in TiO2.

Considering the oxygen activity and temperature, surface treatment methods (e.g., annealing in vacuum condition [68], thermal treatment under reducing atmosphere (H2, CO, NO), and bombardment using electron beam [69, 70], neutron, or y-ray) are introduced to obtain defective surfaces. The bulk ODTs can be directly obtained from sputtering method without further modifications on the TiO2 samples.

3.2. Light-Induced Defects in TiO2. Another way generating OTDs came out of the application of TiO2 in photocatalytic

reactions under light irradiation. Once photoinduced e-h pair is generated, the subsequent reactions could happen:

k+vB + O2

e CB + k VB

Ti trapped electron ^ O- trapped hole

4 h+VK + O2

Oa + Vo'

The photoinduced electrons and holes should also be identified as defects [71]. Accordingly, the recombination of electrons and holes may be also called the reaction of "electron defects" with "hole defects" in photocatalysts.

The excited electrons could react with lattice Ti + and then Ti3+ is generated with a trapped electron (6). At the same time, a hole could oxidize a nearby lattice O leaving an O-in lattice (7). Further, oxygen vacancy would be created under strong oxidizing conditions, generating atom O (Oa) in lattice (8). Thus, the photoinduced defective surface/subsurface is performed under irradiation, and light energy can be stored as the form of electronic energy during this process.

3.3. Doping-Induced Defects in TiO2. Foreign elements are usually introduced to pristine TiO2 to make full use of solar light. The formation of dopant defects is frequently accompanied by the generation of OTDs [72].

Anion Doping. Di Valentin et al. [73] found from DFT calculation that N doping was likely to be accompanied by the formation of oxygen vacancy, because the energy consumed by oxygen vacancy was substantially reduced by N doping. Chen et al. [74] obtained an N-doped TiO2 samples in NH3, and OTDs were found to be coexisted on the surface with N impurities. Recent work of Di Valentin and the coworkers [75] revealed that oxygen vacancies generated when doping F in TiO2. The formation of Ti3+ occurred when doping B due to the charge compensation, while C and N did not donate excess electrons to lattice oxygen.

Cation Doping. It is found by Jing et al. [76] that doping of Zn increased the concentrations of oxygen vacancies and oxygen vacancies also served to assist the formation of Zn-doped TiO2 samples. Following the static equilibrium, the Zn atoms would

ZnTi'' + Oa + VO''

where ZnTi" represents Ti substituted by Zn in lattice, Oa is also theOatom removed, andVO" represents the Zn-doping-induced generation of oxygen vacancy, or according to the charge neutrality,

Ti4+ + O2-

T 2+ TT ''

Zn + Vn

The introduction of Cr [77] and Fe [78] as the acceptor-type defects in TiO2 practically undergoes the similar way in generating oxygen vacancy:

2 CrTi' + 3Oa + VO''

Ob -vac

Ob -vac

Figure 3: (a) Pseudospace-filling model of ideal bulk-terminated TiO2 (110) with a bridging oxygen vacancy. Blue (red) spheres are oxygen (titanium) atoms. (b) STM image of (1 x 1) TiO2 (110) surface. The bright rows correspond to surface Ti5c sites (red ball in (a)). Black row is bridging oxygen (Ob) and Ob-vacs are marked as the bright protrusions in the black row [43]. (c) Redistribution of the surface electronic density caused by single oxygen vacancy [64].

0 100 200 300 400 500 600 700 H2 treatment temperature (° C)

— Vo'

Figure 4: EPR intensity of oxygen vacancies and Ti3+ generation under thermal treatment with H2 [7].

When it comes to the donor-type Nb-doped TiO2, the reactions are [79-81]

2Nb205

2 NbTi ' + 5Oa + 2e

+ VTi + Oa

VTi is Ti vacancy in (14). The occurrence of (13) and (14) is also controlled by the doping condition, commonly the oxygen activity. It can be seen in (13) that electron can be released by Nb doping under the reduced conditions [81]. The excess electrons result in the remarkably enhanced conductivity, and this metallic-type property may be helpful to promote the migration of the charge carriers in photocatalytic reactions.

It could be found that chemical valence states of the dopants play an important role on the formation of defects under preparation conditions. Besides, it was also reported that the doping of anions and cations in pristine TiO2 was all accompanied by the formation of oxygen vacancies [82]. At the same time, the formation of the corresponding color centers (e.g., F, F+, F++ [83], and Ti3+) revealed the probable effects of defects in photocatalytic reactions [84].

Cr2O3 + 6Tii'

2CrTi' + 6Tii3+ + 2 O2 (

The Ti/ in (12) presents the interstitial Ti. It could be seen from (11) and (12) that Cr atoms would undergo different reaction pathways. Besides the oxygen activity, the chromium concentration was found to be another factor to influence these two reactions [77]. When the chromium concentration was lower than 3 atom %, it mainly underwent (12), producing interstitial Ti3+ in lattice. When the chromium concentration was in the range of 4-5 atom%, oxygen vacancies were mainly created by (11), balancing the charge variation during Cr-doping as the acceptor defect center.

4. The Function of Defects in Photocatalysis

The function of OTDs in photocatalysis can mainly be

(1) to modify the band energy structure of the pristine TiO2 as the defect states,

(2) to trap charge carriers in the migration pathways as the electron pool or recombination center,

(3) to influence the adsorption of reactants (e.g., H2O, O2, CO2, and organic pollutants) as the active sites.

4.1. Function of OTDs on Energy Structure. The host energy structure of pristine TiO2 is constructed by valence band (O 2p orbitals) and conduction band (Ti +3d orbitals). The energy level of TiO2 as well as other outstanding photocatalysts is shown in Figure 5 [85].

-2.0 -,

O ■>vp>

*.....

O2/H2O

Figure 5: Band structures of TiO2 and other popular semiconductors [85].

The energy level of OTDs is recognized to aid band gap narrowing and the formation of the main active sites in favor of visible light adsorption [34, 46, 66, 86]. Figure 6 gives the STM images of defective TiO2 surface before and after visible light irradiation. These images provide a clear evidence of oxygen vacancies function as the visible light response sites [87], which can be attributed to the electronic structure of OTDs. Accurate calculation of the defect state of OTDs in TiO2 energy structure is in urgent need, because energy structure of photocatalyst will influence light absorption and charge carriers migration.

According to (1)-(4), once oxygen atom is removed, and VO", VO', and Ti3+ are left behind. The defect states of these defects are different in the band gap, as has been reported by Janotti et al. [88] and Zou et al. [89]. Janotti and the coworkers [88] found that oxygen vacancies were shallow donor and VO" defect state presented lower energy than VO' for all Fermi-level positions in the band gap. Zou et al. [89] introduced VO' in TiO2 by calcining TiO2 precursor with imidazole and hydrochloric acid at the elevated temperature. The paramagnetic oxygen vacancies VO' were proven to form mid gap electronic state within the band gap of TiO2, and thus visible light photocatalytic activity was performed by the electron transition from the VO' mid gap to the conductor band of TiO2. Here the VO' acts as the donor. This conclusion is the same with the results that were previously reported by Serpone [90] and Chen et al. [83]. As to oxygen vacancies VO", Zou and the coworkers [89] believed that they could serve as an acceptor as well as Ti3+, forming an unoccupied state below the bottom of the conduction band. The energy levels of OTDs in TiO2 were summarized by Nowotny and his group as in Figure 7 [5].

However, the isolate electronic band fails to explain the contradiction of the strongly localized small polarons versus the delocalized free polarons in experiments. Hence, hybrid function is introduced appropriately and serves as a workable theory. Janotti et al. [91] proposed that there exist two kinds of hybrid functions in the electronic band: (i) between the electrons and the conduction band in the presence of delocalized free electrons and (ii) between the electrons and the oxygen vacancies as the form of oxygen vacancies complexes and the ionized shallow-donor impurities. This reveals the influence of the defect states on shifting the position of the lowest unoccupied molecular orbital (LUMO)

and the highest occupied molecular orbital (HOMO), from the crystal field theory point of view.

More discussion of the relationship between OTDs states and the crystal field arguments was carried out by Morgan and Watson [65]. They used an on-site correction DFT calculation to study the oxygen vacancies in rutile (110), (100), (101), and (001) reduced surfaces, and it was found that the oxygen vacancy of the reduced (110) surface introduced an occupied defect state of 0.7 eV below the bottom of the conduction band. The defect states were also shown in the other three reduced surfaces and varied from each other. However, the defect states seem more important than the exposed surface or crystal form in photocatalytic reactions. Liu et al. [92] compared the oxygen vacancies in anatase, rutile, and brookite obtained from helium pretreatment in moderate temperature. The characterization results revealed that the oxygen vacancies were created in anatase and brookite, which led to a remarkable increase in photocatalytic CO2 reduction ability. On the contrary, the treated defect-free rutile and the untreated TiO2 samples did not have photocatalytic activity in this reaction. Liu et al. [92] also examined the intermediates/radical and the corresponding final products and found that the reduction of CO2 may undergo different pathways. It is deduced that OTDs are crucial to such difference, for the reactants (CO2, H2O, CO2-, and CO) are all tend to adsorb on these surface active sites, which are about to discuss later in this review.

4.2. Function of OTDs on Charge Transfer. When irradiated with light, an excited single electron moves rapidly in response to an applied electric field (i.e., voltage supplied by power source or difference of potential between energy structure of TiO2 and the redox potential of the adsorbed species) by HOMO-LUMO promotion. Franck-Condon factors of this process are usually very small as a result of little lattice distortion when creating an electron.

The transfer of charge carriers follows the band model and the hopping model [93] and is limited by the vanishing reorganization of energy according to Marcus-Hush electron transfer theory. The energy initially provided by a photon to an electron can be consumed by the lattice distortion, and if the remaining energy is sufficient to overcome the surface barrier, the charge could be utilized by the adsorbed species. The annihilation at the recombination center is another quick vanishing approach for the charge carriers. Yu et al. [94] proposed three recombination mechanisms in semiconductors: (i) band-to-band recombination, which happens between the excited electron and the hole lying in the empty VB, and this reaction is limited by the production of available electrons and holes and is a second order to the concentration of charge carrier (ii) trap-assisted recombination, which directly happens between the excited electrons and holes in the VB under the aid of "trap" state, and this reaction is also limited by the concentration of charge carriers described as Shockley-Read-Hall Model (SRH model); (iii) Auger recombination, which happens when the excited electron and hole recombine, releasing the energy to enhance the energy of another electron or hole. It is discovered by Zhang et al.

Figure 7: The energy levels of OTDs in TiO2 [5].

[95] that charge transfer follows the first order kinetic model on surface under UV irradiation, because of the abundant OTDs serving as trap-assisted recombination centers. In the subsurface, charge transfer mainly follows the second order kinetic model for the OTDs.

The exciting sites are widely distributed among the solid. Under light irradiation, the excitation mainly occurs around the OTDs as shown in Figure 6, and the recombination would be in the form of SRH model. However, exact dynamic behavior of a single charge carrier remains unclear. The scope into the molecular level and the study on defect-related characterization techniques are urgently needed.

From the molecular point of view, short-range electronic driven forces provided by bulk OTDs can work efficiently only in the distance of several angstroms in bulk. This force is weaker than that provided by surface OTDs as a result of the broken symmetry in lattice. Despite working in short range, the effect of OTDs cannot be ignored but taken into account seriously.

As calculated by Janotti et al. [91], VO" primly acts as the acceptor but when it receives one electron, then VO' was formed and it acts more as a donor than an acceptor, in the photocatalytic reaction. If the prime VO" is located in

2(gas)

2 (ads)

K2O2-)(latt)

Ti4+— O2-— Ti3+- VO—Ti4— O2-^ Ti3+- O2- Ti4+- O2-— Ti4+

I /I I /I I I

O2- e/ O2- O2- <У O2- O2- O2-

I // I I / I I I

Ti4+ O2- Ti4+ O2- Ti4+ O2- Ti4+ O2- Ti4+ O2- Ti4+

O2- O2- O2- O2- O2- O2

Figure 8: The Ti vacancy functions as the active site [96].

the subsurface of an и-type TiO2, it can be deduced that this one excess electron in VO' would subsequently

(i) meet the h+ and recombine,

(ii) reduce one Ti4+ to Ti3+ hindering the entrance of other electrons into this Ti-site,

(iii) transfer along the surrounding Ti4+ to the surface active sites, maybe as the form of neutralized state,

(iv) enter into another VO".

By this mean, the original VO" is regenerated and this process repeats until photocatalytic reaction ends. This reiteration behavior not only prolongs the migration routes for the electron to the surface but also highly increases the recombination opportunity of e- with h+. If the prime VO" is located on the surface, the entered electrons can be given to the dissociative adsorbed species here. Besides, if the arrival of electrons on the surface occurs along Ti atoms, the finally formed Ti3+ canalsoserve as theactivesites toward adsorbing O2 scavenger. The transfer of the charge carriers to the active sites are shown in Figure 8.

Except for these defects, Ti vacancy reported by Nowotny et al. [96] also assisted the transfer of electrons to the adsorbed species, as shown in Figure 9. OTDs on the surface can enhance the separation of photogenerated electrons

>r->2H,

Electrolyte

>2H2°(ads)

Ti4+—O2

Ti4+—O2

^ - i - - O2 Cathode

/ Active \ -f ^

/ complex v

L2(H2O2+)-(ad^2° Vs) + 4H+ (aq) ^

O2- O2- O2-

—Ti4+— O2-— Ti4— O2-—Ti4+

4+ r«2- T;4+ r,2-_

T;4+ r>2-_rr;4+

Figure 9: The Ti vacancy functions as the active site [96].

and holes by acting as the electron pools on surface and thus prolong the lifetime of both the electrons and holes.

In a typical reaction procedure, the photogenerated charge carriers experience different procedures between the photocatalytic reactions and the photogeneration of electricity when TiO2 is used as the electrode. Photocatalytic excitation mainly happens in surface and subsurface, and the charge carriers must conquer these blockings as

(i) localization or trapping in the recombination centers,

(ii) consuming of migration energy in the distorted Ti-O bond caused by lattice defects in the subsurface,

(iii) localization or trapping in the surface ionic defects,

(iv) consuming of migration energy in the distorted Ti-O bond caused by lattice defects on the surface,

(v) surface barriers caused by the binding of the dissociative adsorbed molecules with surface active sites such as oxygen vacancies and Ti-related defects.

Photocatalytic reaction (i.e., degradation of organic compounds and water splitting) processes mainly suffer the surface/subsurface OTDs, whereas the photovoltaic reaction must bear the OTDs in bulk. Here the movement of charge carriers can be delayed [97] by trapping or localizing in the lattice of bulk OTDs and the migration energy can is reduced by the bulk lattice distortion. Furthermore, in the interface with large amount of lattice defects (e.g., the connected interface of the layered electrode/film be constructed by multilayered materials), the bond distortion may cause a large problem [98, 99] because of the chemical tension. However, the defects, which function as donor or acceptor in the interfaces, could promote the charge transfer by chemical adsorption, which is of vital importance in the photovoltaic cells.

4.3. Function of OTDs on Adsorption. Oxygen, water, or organic compounds with electron-rich functional groups can adsorb at OTDs by the electronic driven force toward electrostatic equilibrium. The adsorption behavior of OTDs promotes the charge transfer efficiency from solids to external reactants, and thus makes OTDs flexible active sites on the surface.

4.3.1. Adsorption of Oxygen. The introduction of O2 is of significant importance in photocatalytic reactions, such as photoinduced refractory organics degradation systems and water splitting. Experimental results have proven that the existence of O2 can significantly enhance the degradation efficiency, and the addition of O2 with different dosages is widely investigated in water treatment processes. During these processes, the adsorbed O2 at the active sites can serve as the electron scavenger [100]. These scavengers can facilitate the charge separation, prevent the electron-hole recombination, and generate the ^O2- for deep oxidation of the organic specials. Figure 10 shows the subsequent reactions

It can be seen that the intermediate products are hydroxyl radical (•OH) and other oxidation species, which further promote the mineralization of organic pollutants. The half reaction led by photogenerated electrons is equally important to the reaction led by photogenerated holes, for these two pathways have a synergistic effect. However, during water splitting, the existence of O2 in water tends to assist the generation of O2 gas but not the H2. By consequence, the experimental tests or comparisons of the H2 evolution activity of photocatalysts are always performed in vacuum or inert gases (i.e., N2, Ar). Besides, the adsorption of oxygen by the active sites can also be capable of causing upward band bending, which is of great importance in many of the applications of TiO2 (i.e., as the film electrodes). Thus it is necessary to study the oxygen adsorption behavior and its distribution feature on the TiO2 surfaces.

It is mostly accepted that adsorbed O2 species occur at oxygen vacancies in an idealized model, and the bridge-bonded oxygen vacancies are believed to be the most preferred sites for oxygen chemisorption on the surface [101]. Xu et al. [102] investigated the interaction between O2 and reduced TiO2 (110) surface by DFT calculations, and the results showed oxygen interaction with oxygen vacancies as the dissociative configuration form of O-O complex between in-plane oxygen and Ti atoms in room temperature. Other reports proposed that surface Ti-related defects (mainly interstitial Ti and Ti +) were also the active adsorption sites for oxygen [34].

The OTDs on the surface can act as the charge donor for the transfer of charge carriers from TiO2 to oxygen atoms. Except for the surface OTDs, bulk OTDs like Ti3+ ion can also provide excess electrons to the adsorbed O2 at ~410 K and the desorption of O2 occurs when the surface interstitial Ti and Ti3+ act as the electron acceptor in the same condition. These two models are shown in Figure 11. Aschauer and the coworkers [103] further proposed that O2 was more favorable to adsorb at shallow subsurface interstitial Ti. The bulk defects were also estimated to have more pronounced effect than lower-lying interstitials at providing excess electrons and contributing to O2 adsorption. Zhang and Yates [95] proposed that desorption of oxygen could happen when photoinduced holes reacted with adsorbed O2-. However, the molecular-scale mechanism of desorption of the oxygen and its subsequent reaction with other adsorbed substances is still unclear.

> o2 + h2o2 e-H+ ho2 " 2 —j—- •oh + OH-HO2 -

e c.b.

TiO2 + hr -

o2-1h2o

^^ O2 + HO2- + OH-

► H2O2

► •OH + OH- + O2

h+vb (1) >TiOH )R_ O--, HOO^, HOOH, HOO-, HO^, OH-, H2O Oxidized . . (2) R •ROH Activated oxygen species products

Thermal oxidation

CO2 mineralization

Figure 10: The photocatalytic reactions with electrons and holes [121].

O2O2 O2 O2O2 O2O2O2

/ Model A

O2 Oad O2 O2 O2 Oad O2

• ■ ■ • - *

■ • . Ti3+ . ■ • .

■ Model B .-■■" fg ~ M JHHHfaj ■ata n - -jw, ¡(ffiti) irf/ir' ■rif

Figure 11: The schematic diagram for the influences of OTDs on O2 adsorption. Model A: O2 adsorption on surface OTDs and Model B: surface oxygen distribution influenced by surface and bulk OTDs [103].

4.3.2. Adsorption of Water. The efficiency of photocatalytic degradation of gas phase organic compounds can be improved by moderating the dosage of water [104]. In aqueous solution, the hydrophilic surfaces tend to possess higher photocatalytic activity than hydrophobic surfaces. It is proposed that water serves as a vital media to promote the fast diffusion of OH radicals from TiO2 surface to the near-surface region and thus remarkably improve the photocatalytic efficiency. Another reason for this improvement is attributed to the tendency of organic contaminant adsorption toward the OTDs (the adsorption of organic compounds by surface defects is illustrated in the next section). The importance of the application of water in water splitting as the reactant and in photovoltaic cell as the electrolyte solution is very clear and needs no further illustration.

The occurrence of H2O adsorption on TiO2 surface can be in the forms of molecular absorption, dissociative adsorption, and thetransitionstate betweenthem(Figure 12). Molecular adsorption as physical absorption occurs mostly at surface radical groups as -OH and surface defects, and this kind of H2O serves more as solution or media than as reactant. The vast majority of dissociative adsorption

Ob-vac

Figure 12: The dissociation of water in bridging oxygen vacancy. Green spheres denote O atoms with two H pink spheres [122].

happens at OTDs [105-108], mainly at the bridge-bonded oxygen vacancies, where H2O dissociates leaving one pair of neighbouring -OH groups (Figure 12). The newly formed -OH can assist the redistribution of the defect electrons which are originally trapped at subsurface sites to its neighboring surface Ti4+ sites [109,110], and thus the excess electrons (e.g., the photogenerated electrons) can be forced onto exposed surface and undergo further reactions. Further, Aschauer et al. [111] proposed that subsurface defects could promote the binding between water and surface defects by lowering the desorption energy of adsorbed water at OTDs. The water adsorption energy to the defect-free stoichiometric surface is higher than defective surfaces [111] (shown in Figure 13), implying a less effective photocatalytic activity on the stoichiometric TiO2.

Reaction coordinate

Ti interstitial

(3) Dissociative adsorption

Figure 13: The water adsorption state on O vacancy, Ti interstitial and stoichiometric surface [111].

Table 2: Possible reaction pathways for defective TiO2.

CO2 photoreduction with H2O vapor

Defective TiO2 anatase and brookite

H2O + h+ ^ H+ + OH (1) OH + CO2+ ^ HCO3- (3)

CO2 + Ti3+ ^ Ti4+ + CO2- (2) CO2- + H+ + e- ^ CO + OH- (4)

(5) CO2- + [Ti3+ - Vo - Ti4+] ^ CO + [Ti4+ - O2- - Ti4+] (6)

CO, HCO3 -, HCOOH C(ads) CH3

CH4 (7)

Defective TiO2 brookite

CO2 + 2H+ + 2e- ^ HCOOH (8) HCOOH ^ CO + H2O(10)

CO2- + 2H+ + e- ^ HCOOH (9)

CO, HCOOH CH4 (11)

CO2~ + CO2~ ^ CO + CO3

It is interesting that the electronic structure of surface

oxygen vacancies can hardly be affected via water dissociation at these sites [112], not influencing the subsequent physical adsorption of O2 toward these oxygen vacancies [113] and the bridging hydroxyls [114]. Henderson and the coworkers [112] also explained the negative effect of excess water in gas-phase photocatalysis. It is proposed that the second-layer H2 O is sufficient to inhibit O2 adsorption towards the active sites. As the effect of O2 scavenger was weakened, the photocatalytic efficiency would be lowered.

4.3.3. Adsorption of Organic Species. Degradation of organic compounds by heterogeneous photocatalysts starts by adsorbing organic specials, the rate of which is commonly recognized to determine the overall photocatalytic efficiency. The adsorption behavior of the organic compounds follows the single molecule adsorption to the surface active sites and can be described by Langmuir adsorption isotherm. The adsorption mass of the substrate by TiO2 particles is usually very small due to the limitation of surface active sites. TiO2 nanostructures with high surface area have been widely studied in the adsorption of organic species. However, the surface area can hardly essentially change the adsorption behavior of the organic substrates, moving from the near-surface region to the surface active sites. It can be supposed by the deduction from O2 and H2O adsorption that surface defects as OTDs are the main sites for the dissociative adsorption/chemical adsorption whereas the subsurface disordering defects tend to assist the physical absorption on the surface. On the other hand, the chemical adsorption of organic compounds and their by-products in

different defects may cause different degradation pathways [45,115,116]. This variation would result in the change of the reaction kinetic, adsorption-desorption completion toward surface active sites with O2 and H2O [117], mineralization degree, photocatalytic quantum yield, and catalyst poisoning degree as well [118]. Therefore, it is still necessary to discuss this problem in the view of microcoscopic view. Zhang et al. [119] studied the adsorption behavior of methanol on TiO2 and found that the methanol molecules are mainly distributed on bridging oxygen vacancies (Figure 14).

The adsorption of organic species to TiO2 surface defects is widely studied by modeling of the process and theoretical calculation. However, it is hard to build the adsorption model because of the complexity of the various organic compounds. There exist no generally acceptable results even for a single alcohol molecule. Zhang et al. [120] introduced an in situ STM to study the methanol adsorption on TiO2 surface. O-H bond scission on oxygen vacancies was found to be the dominant manner for methanol dissociation, prior to C-O bond scission. This result is of great importance for the exploration of the mechanisms in the methanol reforming and organic species degradation. Farfan-Arribas et al. [69] compared the adsorption behavior of ethanol, n-propanol, and 2-propanol. It is found that the coverage of these compounds increased and they tended to undergo decomposition with the increased concentration of oxygen vacancies. Table 2 also provides an evidence for altering the reaction pathways by surface defects as OTDs in photocatalytic reactions. This reveals a surface defects-related change in reaction pathways which is occurring, and it is important to understand the mechanism of photocatalytic reactions.

Figure 14: STM images of the methanol molecules and oxygen vacancies distribution on TiO2 surface. (a) Before methanol exposure, BBOv represents the bonding bridging oxygen in the shallow bright color. (b) After methanol exposure, the strong bright dots reveal the methanol adsorption sites [119].

5. Conclusion Remarks

Oxygen vacancies and Ti-related defects are the main lattice defects in TiO2. The formation of oxygen vacancies, VO" and VO', and the related Ti3+ defects are described. It provides an internal relationship between the defects, which is vital for understanding the behavior of the charge carriers in photocatalysis. Once the defects are introduced, HOMO-LUMO orbital can be reconstructed and the electronic cloud density of pristine TiO2 can be redistributed. Such electric properties directly result in the narrowing of band gap and the trapping of photoinduced charge carriers in surface/subsurface. OTDs in the subsurface mainly serve as the recombination center, and the concentrated lattice distortion would largely consume the motion energy of excited charge carriers. Both of them would deadly lower the life time of the photogenerated charge carriers. OTDs in surface tend to function as the electron pool favoring the e-h separation and serve to adsorb active species as a result of the electronic force toward electrostatic neutralization. Surface OTDs would also mediate the charge transfer between the solid and the external reactants. Moreover, the selective dissociated adsorption of substance onto different kinds of OTDs is probably decisive to the exploration of the reaction mechanism. To study the behavior of defects caused by electronic driven forces is vitally necessary for photocatalysis, and it will promote the construction of environmentally friendly high-performance photocatalysts for diverse specific applications.

Acknowledgments

The research was financially supported by the Natural Science Foundations of China (nos. 21103235, 21067004, 51208539), the Natural Scientific Foundation of Guangdong Province (no. S2012010010775), Science and Technology Plan Projects of Guangdong Province (no. 2010B010900033), the Science and Technology Programme of Guangzhou City (no.

2013J4100110), the Key Laboratory of Fuel Cell Technology

of Guangdong Province, and the Key Laboratory of Environmental Pollution Control and Remediation Technology of

Guangdong Province (no. 2011K0011).

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