Nitrogen-Doped Titanium Dioxide as Visible-Light ... - ACS Publications

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Nitrogen-Doped Titanium Dioxide as Visible-Light-Sensitive Photocatalyst: Designs, Developments, and Prospects Ryoji Asahi,*,† Takeshi Morikawa,† Hiroshi Irie,‡ and Takeshi Ohwaki† †

Toyota Central Research and Development Laboratories, Inc., Nagakute, Aichi 480-1192, Japan Clean Energy Research Center, University of Yamanashi, Kofu, Yamanashi 400-8511, Japan

‡

6.2.2. Photocatalytic Activity over N-Doped TiO2 Loaded with Cu, Fe, V, or Pt for VOC Oxidation 6.2.3. Charge Carrier Dynamics in N-Doped TiO2 Loaded with Metallic Ions 7. Practical Applications 7.1. Textile 7.2. Interior 7.3. Tooth Bleaching 7.4. Air Purification System 7.5. Water Treatment System 8. Summary and Prospects Author Information Corresponding Author Notes Biographies References

CONTENTS 1. Introduction 2. Materials Design for Visible-Light-Sensitive Photocatalyst 2.1. Electronic Structure of TiO2 2.2. Photoelectrochemistry of TiO2 and Conditions for Visible-Light Sensitization 2.3. Metal Doping of TiO2 2.4. Design of Nonmetal Doping for VisibleSensitization 3. Synthesis and Properties of N-Doped TiO2 3.1. Synthesis 3.1.1. Powder 3.1.2. Thin Film 3.2. Properties 3.2.1. Photooxidation of Volatile Organic Compounds 3.2.2. Water Splitting 3.2.3. Antibacterial, Antiviral, and Antiallergen Properties 3.2.4. Hydrophilic Conversion 3.2.5. Which Is More Applicable to Practical Use, Conventional TiO2 or N-Doped TiO2? 4. Detailed Analysis of N-Doped TiO2 4.1. N States in N-Doped TiO2 4.2. Charge-Carrier Dynamics in N-Doped TiO2 5. The Other Nonmetal-Doped TiO2 5.1. C Doping 5.2. F Doping 5.3. S Doping 5.4. B Doping 5.5. Other Nonmetal Doping 6. Improvement of N-Doped TiO2 6.1. Codoping into N-Doped TiO2 6.2. Cocatalysts Loading to N-Doped TiO2 6.2.1. Overview of Cocatalysts Loading to NDoped TiO2

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1. INTRODUCTION Titanium dioxide (TiO2) is well-known to be the most practical and prevalent photocatalyst,1−17 for it is chemically stable, abundant, nontoxic, and cost-effective. Photogenerated holes on TiO2 have strong oxidizing power, enabling almost all toxic organic compounds to be completely oxidized to carbon dioxide (CO2), while photogenerated electrons are consumed by reducing oxygen (O2) when TiO2 is utilized in the presence of O2. Thus, using TiO2 photocatalysts, extensive research has been performed for the purposes of air, water, and soil purification. In addition to these applications of conventional TiO2 photocatalysts, the surface of TiO2 exhibits high hydrophilicity under ultraviolet (UV) light irradiation with a water contact angle (CA) of 0°.18−35 This phenomenon has already been employed in various industrial applications such as self-cleaning tiles and antifogging mirrors. However, the photocatalytic oxidation process and photoinduced hydrophilic conversion of TiO2 are only activated by irradiation with UV light. Although solar light contains only about 2−3% UV light, the above processes of TiO2 can be induced upon irradiation with solar light to obtain rather strong UV light with an intensity of a few mW/cm2. In contrast, TiO2 alone cannot function effectively when irradiated with room light, such as fluorescent light, which contains a very small amount of UV light with intensity at most on the order of μW/cm2. Therefore,

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and its concept for visible-light sensitization also impact and stimulate solar energy applications such as water splitting and artificial photosynthesis, which attract much attention these days. The scope of this work is to review the previous studies on nonmetal-doped TiO2 for visible-light sensitization; however, among the enormous amount of studies and references on this topic, we particularly focus on N-doped TiO2 and the related science and technology that have been intensely studied and significantly developed since 2001. Although the other nonmetal-doped TiO2 are not fully covered in detail, underlying principles discussed for N-doped TiO2 are quite instructive and generalized to understand nonmetal-doped TiO2. We also note that even though we do not particularly emphasize the size of N-doped TiO2, it is embodied in nanoscale materials such as nano powders, thin films, nanotubes, nanowire, etc., as often seen in nano-TiO2. We start to discuss strategy of materials design for visible-lightsensitive photocatalysts in section 2, where N-doped TiO2 is reasonably selected as a candidate. In section 3, we review several ways to synthesize N-doped TiO2, and show its photocatalytic properties. In section 4, we discuss the detailed analysis of N-doped TiO2. Here, we highlight the N states and charge carrier dynamics in N-doped TiO2, which have been often controversial. In section 5, the other nonmetal-doped TiO2 are reviewed. We discuss in section 6 effective ways to improve N-doped TiO2, codoping and cocatalysts techniques. Even if materials show good properties, significant efforts and development are required for practical applications. In section 7, we pick up some of the applications of N-doped TiO2, which successfully come out as commercial products. With these sections, we describe comprehensive science and technology regarding N-doped TiO2. Finally, we summarize in section 8 with prospects for further development of the related science and technology.

TiO2 has not been widely applied to indoor use. Thus, for the more efficient utilization of incoming solar and room light energy, expanding the range of indoor applications of TiO2 has been investigated for many years through attempts to increase the sensitivity of TiO2 to visible light. Since the early 1980s, there have been numerous studies on the introduction of various transition-metal ions (iron (Fe), chromium (Cr), cobalt (Co), manganese (Mn), vanadium(V), molybdenum (Mo), etc.) as a dopant into the TiO2 lattice36−57 and on the introduction of oxygen (O) defects.58−60 However, the metal-doped TiO2 and O-defective TiO2 photocatalysts proposed in these studies have not been developed for practical applications because some of them exhibited poor photocatalytic activity and others exhibited poor reproducibility in improvement of activity. Thus, studies on the development of visible-light-sensitive TiO2 photocatalysts in the last two decades of the 20th century were generally unsuccessful. The other approach to the visible-light sensitization of TiO2 is so-called dye sensitization, which was studied in the late 1990s. For example, it is well-known that TiO2 containing a covalently attached Ru complex exhibits visible-light absorption. In this system, upon the excitation of the Ru complex, electrons in the conduction band (CB) of TiO2 injected from the excited Ru complex can produce hydrogen peroxide (H2O2) with which organic substances are degraded. Another example of dye sensitization is the platinum−chloride (H2[PtCl6] (or PtCl4))-modified TiO2 system reported by Kisch et al.61−66 and Ishibai et al.67 In this system, Pt(IV) chloride absorbs visible light to generate two new redox centers (2Pt4+(Cl−)4 + hν → Pt3+(Cl−)4 + Pt4+Cl0(Cl−)3). The oxidative center can be described as a type of Cl0/Cl− pair weakly coordinated to a metal center, while the reductive center can be described as Pt4+/Pt3+. Pt3+ reverts to Pt4+ when electrons are injected into the CB in TiO2 followed by the reduction of O2, while Cl reverts to Cl− upon the oxidation of organic compounds (upon extracting electrons from organic compounds). While these dye/complex catalysis systems show a large visible-light response and unique selectivity in photocatalysis, they have low stability under light irradiation.68−70 Also, using expensive metal complex makes it difficult to apply to practical use. The visible-light absorption of TiO2 by nitrification processing was first reported by Noda et al.71 and Sato72 in 1986. Sato showed photocatalytic reactions of CO and ethane under visible light, and concluded that visible-light sensitization was attributed to NOx-doping in TiO2. In 2001, Asahi et al. predicted by the first-principles calculations that doping of nitrogen (N) among possible anions should be effective for visible-light sensitization in TiO2, and demonstrated that Ndoped TiO2 has significant photocatalytic activities under visible light in experiment.73,74 Since then, N-doped TiO2 has attracted considerable attention, and its visible-light sensitivity has been independently confirmed by many researchers. Since the realization of N-doped TiO2, various types of TiO2 doped with nonmetal elements, such as sulfur (S),75−79 carbon (C),80,81 and iodine (I),82,83 have been widely studied as visible-light-sensitive photocatalysts. Numerous papers have been published on nonmetal-doped TiO2, including monoanion- and di-anion-doped TiO2 and even tri-anion-doped TiO2, as well as anion-and-cation-codoped TiO2.84−91 Among them, N-doped TiO2 is still leading visible-light-sensitive photocatalysts; it is now widely used in practice for environmental applications even under indoor irradiation. The material

2. MATERIALS DESIGN FOR VISIBLE-LIGHT-SENSITIVE PHOTOCATALYST 2.1. Electronic Structure of TiO2

Most of the investigations of semiconductor-based photocatalysis have focused on anatase TiO2,1,36−38,92,93 which shows relatively high reactivity and chemical stability. Photocatalytic activity on the TiO2 surface is promoted by photogenerated electrons and holes under ultraviolet (UV) light (wavelength (λ) < 387 nm), whose energy exceeds the band gap of 3.2 eV in the anatase crystalline phase (Figure 1). This condition of illumination requires an exposure of outdoor sunshine or black light, which includes a certain amount of UV light, and thus limits the range of applications. The development of photocatalysts that can yield high reactivity under visible light (λ > 380 nm) should allow the main part of the solar spectrum and even poor illumination of interior lighting to be used. The band gap modification for visible-light sensitization is designed on the basis of understanding the electronic structure of TiO2. The electronic structure of TiO2 has been studied by several ab initio calculations.94−99 A fundamental feature of the electronic structure for anatase TiO2 is shown in a molecularorbital bonding diagram presented in Figure 2.94 There exists the nonbonding O pπ orbital (out of the Ti3O cluster plane) at the top of the valence bands (VBM) and the nonbonding dxy states at the bottom of the conduction bands (CBM). A similar feature can be seen in rutile where, however, it is less significant 9825

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Figure 1. Crystal structure of TiO2 in the anatase phase.

Figure 3. Imaginary parts of the dielectric functions for polarization vectors (a) parallel and (b) perpendicular to the c axis. Solid lines and dashed lines are the calculated results and experiment,101 respectively. Reprinted with permission from ref 94. Copyright 2000 American Physical Society.

underestimation. The experimental dielectric functions, measured for the single crystals of anatase using synchrotron orbital radiation,101 are taken as comparison. Good agreement with experiment is obtained for the dielectric functions in both components, which indicate anisotropic optical absorption. Each peak of the dielectric functions corresponds to interband transitions. From the band structure calculations, the strong optical absorption around the band gap energy is assigned to be electron−hole excitations dominantly between the nonbonding O pπ and Ti dxy states.

Figure 2. Molecular-orbital bonding structure for anatase TiO2: (a) atomic levels, (b) crystal-field split levels, and (c) final interaction states. The thin-solid and dashed lines represent large and small contributions, respectively. Reprinted with permission from ref 94. Copyright 2000 American Physical Society.

2.2. Photoelectrochemistry of TiO2 and Conditions for Visible-Light Sensitization

From the viewpoint of photoelectrochemistry, photocatalytic activity of TiO2 is understood in terms of redox potentials for the corresponding chemical reactions with respect to the energies of photogenerated electrons in CBM and holes in VBM. In case of water splitting, photogenerated electrons can flow to reduce protons, resulting in hydrogen (H2) gas evolution if the CB energy is higher (or more negative on the electrochemical scale) than the H2 evolution potential (H2/ H2O), while the photogenerated holes can be utilized for O2 evolution if the valence band (VB) energy is lower than the O2 evolution potential (O2/H2O). Figure 4 shows the energy band diagram for several semicondcutors.102−104 From this diagram, we understand the TiO2 has a very strong oxidization power but a relatively small reduction power for H2O. CdS and SrTiO3 also have suitable band alignment for water splitting; however, the former has less chemical stability, and the latter is much more expensive than TiO2. In a more general chemical

than in anatase.100 In rutile, each octahedron shares corners with eight neighbors, and shares edges with two other neighbors, forming a linear chain. In contrast, in anatase, each octahedron shares corners with four neighbors, and shares edges with four other neighbors, forming a zigzag chain with a screw axis. Thus, anatase is less dense than rutile. Also, anatase has a large metal−metal distance of 5.35 Å. As a consequence, the Ti dxy orbitals at CBM are relatively isolated, while the t2g orbitals at CBM in rutile provide the metal−metal interaction with a smaller distance of 2.96 Å. The bandwidth of anatase is thus smaller than that of rutile, resulting in a slightly wide band gap of 3.2 eV as compared to 3.0 eV for rutile. Figure 3 shows the imaginary part of the dielectric functions for anatase TiO2.94 The calculations were performed for optical dipole transitions within the local density approximation (LDA) including a scissors operator for the band gap 9826

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ensure photoreduction and photooxidation activities for water splitting, respectively; and (ii) the states in the gap should be shallow or mixed with the band states of TiO2 enough to transfer photo- or thermally excited carriers to reactive sites at the catalyst surface within their lifetime. Probability of the recombination increases with a concentration of trapping sites that capture an electron in the CB or a hole in the VB, as described by the Shockley−Read−Hall model.105 Therefore, if the doping produces a localized level located apart from the band edge with an energy larger than the thermal excitation energy kBT, the lifetime of the mobile carriers may become shorter, giving a low photocatalytic activity. The requirement (ii) often makes the scheme (c) inappropriate in practice because such midgap states are often localized in space so that the photogenerated carriers cannot be utilized for photocatalytic reactions on surface.106

Figure 4. Band edge positions for selected semiconductors at pH 14, together with some important redox potentials. It should be realized that the uncertainty in the band edge positions can amount to a few tenths of an eV for most semiconductors shown here. Reprinted with permission from ref 104. Copyright 2012 Springer.

2.3. Metal Doping of TiO2

For the scheme (a) above, because the conduction band minimum of TiO2 consists of Ti d states, one can consider a substitutional doping of other transition metal for Ti to introduce a lower energy level of d states in the band gap. In fact, there were many studies of metallic doping for TiO2, in most cases, with 3d-transtion metals.36−57,107−111 Mizushima et al. experimentally determined impurity states of V, Cr, Mn, and Fe, to be 1.9−3.0 eV below CBM.112 They also suggested these impurity states may be attributed to cation vacancies. Umebayashi et al. theoretically examined changes in density of states (DOS) by the 3d-transition metal doping as shown in Figure 6.44 A systematic change in the impurity levels reasonably corresponds to atomic 3d levels; however, the discrepancy of the impurity levels from the experiment, such as V doping, suggests possible cation vacancies introduced along with the metal doping. Consequently, these metal-doped materials successfully yield a red shift of the light absorption or a color center. On the other hand, localized impurity states and/or concurrent vacancies introduced by metal doping often suffer from thermal instability,108 and an increase of carrier recombination centers as mentioned above for the requirement (ii). An increase in photocatalytic activity by metal doping, often appearing in papers, could be attributed to the metal loaded on the surface of the photocatalyst as shown in Figure 7. The surface metal may enhance charge separation of the photogenerated electrons and holes, making their lifetimes longer. In addition, it may reduce a chemical reaction barrier and/or change selectivity. Such enhancement of the photocatalytic activity typically appeared with noble metals, such as platinum (Pt), silver (Ag), gold (Au), palladium (Pd), and copper (Cu), loaded on TiO2.69,113−118 We discuss these issues in section 6 more in detail. Forming reduced TiOx photocatalysts also show visible-light sensitization.58−60,119 However, because the reduced TiOx is metastable, it often suffers from poor reproducibility and stability. In addition, the reducing TiO2 introduces localized Ovacancy states located at 0.75−1.18 eV below the conduction band minimum (CBM) of TiO2,60 ensuring that the energy levels of the optically excited electrons will be lower than the redox potential of the H2 evolution (H2/H2O) (see Figure 4 and the corresponding requirement (i)), and that the electron mobility in the bulk region will be significantly reduced because of the localization.

reaction, for example, decompositions of organic compounds, the existence of water adsorption on the surface of TiO2 is considered to play an important role; various forms of active species, such as O2−, •OH, HO2•, and O•, produced by the following processes may be responsible for the decomposition reactions:16 e− + O2 → O2−(ad)

(1)

O2−(ad) + H+ → HO2•(ad)

(2)

h+ + H 2O → •OH(ad) + H+

(3)

h+ + O2−(ad) → 2•O(ad)

(4)

In the above four reactions, the first two and the last two reactions involve CBM and VBM, respectively. Therefore, the band alignments of CBM and VBM with respect to the redox potentials for water are important guidelines required to have high photocatalytic activity. Providing that TiO2 has excellent properties as a photocatalyst, for example, activity and stability, one can consider three schemes of the band gap modifications for visible-light sensitization by doping in TiO2, resulting in (a) a lower shift of CBM, (b) a higher shift of VBM, and (c) impurity states in the band gap, as shown in Figure 5. In addition, to realize good photocatalytic activity, one may set the following requirements for the photocatalyst: (i) the CBM should be higher than the H2/H2O level while the VBM lower than the O2/H2O level to

Figure 5. Three schemes of the band gap modifications for visible-light sensitization with a lower shift of CBM (a), a higher shift of VBM (b), and impurity states (c). 9827

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Figure 6. (A) Bonding diagram of TiO2. (B) DOS of the metal-doped TiO2 (Ti1−xAxO2: A = V, Cr, Mn, Fe, Co, or Ni). Gray solid lines: total DOS. Black solid lines: dopant’s DOS. The states are labeled (a)−(j). Reprinted with permission from ref 44. Copyright 2002 Elsevier.

Figure 7. Metal (M) loaded photocatalyst where charge separation effects make the lifetime of the photogenerated electrons and holes longer. The surface metal can also reduce a chemical reaction barrier and/or change selectivity for the photocatalytic activity.

Figure 8. Comparison of atomic p levels among anions. The band gap of TiO2 is formed between O 2pπ and Ti 3d states.

difficult to incorporate it into the O site of TiO2 crystal because of its large ionic radius, as evidenced by a much larger formation energy required for the substitution of S than that for the substitution of N. The states introduced by C and P are too deep in the gap to satisfy the requirement (ii). The calculated imaginary parts of the dielectric functions of TiO2−xNx indeed show a shift of the absorption edge to a visible-light energy by the N doping (Figure 9). Dominant transitions at the absorption edge are identified with those from N 2pπ to Ti dxy, instead of from O 2pπ as in TiO2.94 Considering a variety of nitrification processes, one may introduce several kinds of N complex species including N, nitoric monoxide (NO), and nitoric dioxide (NO2) into a substitutional site for O (denoted as (NO)O, etc.) or an interstitial site (denoted as (NO)i, etc.) in the anatase TiO2 crystal.122,123 The calculated DOS is shown in Figure 10.123 The interstitial N, (N)i, coupled with a lattice O makes molecularly antibonding states deep in the band gap.73,122,123 The NOx complexes such as (NO)O, (NO2)O, and (NO)i also give rather localized impurity states in the band gap. The O vacancy (V)O results in the localized states below the CBM as

2.4. Design of Nonmetal Doping for Visible-Sensitization

Scheme (b) as mentioned in section 2.2, a red shift of the band gap by modifying the VBM, can be realized by substitution of anion species for the doping rather than cationic metals. On the basis of the band structure of TiO2 where the VBM reflects the nonbonding pπ state of O, one can compare atomic p levels among anions as shown in Figure 8. From this figure, one may find that the requirements (i) and (ii) will be satisfied by N or S doping. Asahi et al. calculated densities of states (DOS) of the substitutional doping of C, N, fluorine (F), phosphorus (P), or S for O in the anatase TiO2 crystal,73 by the full-potential linearized augmented plane wave (FLAPW) formalism.120,121 Although the calculations employed nonspin polarized LDA with a small supercell and thus evaluated positions of impurity states and band gaps rather inaccurately, the results clearly demonstrated a trend for the doping species as expected from the atomic p level. The substitutional doping of N was then chosen to be the most promising because its p states contribute to the band gap narrowing by mixing with O 2p. Although doping with S shows a similar band gap narrowing, it would be 9828

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Table 1. Calculated N 1s Binding Energies, BE in eV, and Their Errors in eV Estimated by the Ti 2p3/2 Levels within the Unit Cella dopant

BE

error

(N)O (N)i (NO)O (NO2)O (NO)i

395.70 397.66 398.10 399.81 399.85

0.17 0.11 0.12 0.03 0.10

a

The BE for (N)O is assumed to be an experimental value.223 Reprinted with permission from ref 123. Copyright 2007 Elsevier.

Figure 9. Calculated imaginary part of the dielectric functions (ε2) of TiO2−xNx (thick lines) as compared to TiO2 (thin lines). Reprinted with permission from ref 73. Copyright 2001 American Association for the Advancement of Science.

expected. Thus, all of these modifications of TiO2 may contribute more or less to the visible-light absorption. Considering two requirements (i) and (ii) as discussed in section 2.2, some N species such as (N)O, (N)i, and (NO)O are suggested to be effective to photocatalysis under visible light, although it is rather difficult to conclude the effectiveness only with these DOS analyses. The formation of these N complex species may depend on the synthesis process in experiment. One way to identify the chemical species is the X-ray photoemission spectroscopy (XPS) measurement where elemental binding energies reveal the detailed chemical environment. The calculated binding energies for the N complex species are listed in Table 1.123 The binding energy of the (N)O state shows the lowest binding energy among the N complex species because it is considered to be the most reduced state of N in the TiO2 crystal. Figure 11 shows the calculated formation energies of N species as a function of the O2 chemical potential or the O2 pressure.122 The more Ti-rich or reduced TiO2 stabilizes the N incorporations but along with (V)O due to requirement of charge neutrality. The formation of these chemical species can be also controlled by the chemical potential of N, for example, choosing nitrification gaseous source, N2, nitrous oxide (N2O), or NO as shown in Figure 12. The results also suggest that

Figure 11. Formation energies (Eform, in eV) as a function of the oxygen chemical potential (μO′) or as a function of the oxygen pressure at fixed temperature (T = 700 K) (top x-axis), for different nitrogen species in anatase: NI at interstitial sites and NS−O at substitutional sites for oxygen with/without the oxygen vacancies, VO. Reprinted with permission from ref 232. Copyright 2005 American Chemical Society.

using NO gas rather than N2 gas for nitrification may efficiently introduce the (N)O doping while creating undesirable (V)O defects as small as possible. Such a processing design is the key to optimizing photocatalysts by doping. In general, the substitutional N is stabilized under the reduced condition with the lowest XPS peak of 396 eV, and the interstitial N and then NOx species with higher XPS peaks are

Figure 10. (a) Total densities of states (DOS) of doped TiO2 and (b) the projected DOS into the doped anion sites, calculated by density functional theory. The energy is measured from the top of the valence bands of TiO2, and the DOS for doped TiO2 are shifted so that the peaks of the Ti t2g states (dot−dashed line) are aligned with each other. Reprinted with permission from ref 123. Copyright 2007 Elsevier. 9829

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Figure 12. Defect formation energies as a function of Δμ, formed by (a) N2, (b) N2O, and (c) NO gaseous sources. The parameter Δμ is ranged −ΔH < Δμ < ΔH where the lower and upper limits represent the O-rich and Ti-rich conditions, respectively, and ΔH is heat of formation of the TiO2 anatase crystal. Reprinted with permission from ref 123. Copyright 2007 Elsevier.

changing the annealing temperature in the range of 550−600 °C.136 However, a greater number of O vacancies are introduced into the N-doped TiO2 with increasing annealing temperature. This is because NH3 decomposes into nitrogen (N2) and H2 at approximately 550 °C and H2 is a reducing gas.145 Thus, when annealing TiO2 powder in an NH3 atmosphere, N is introduced into TiO2 while TiO2 is simultaneously reduced.136 Plasma treatment can also be applied to produce N-doped TiO2 powder.146,147 In this case, anatase TiO2 powder is treated by plasma in N2 gas or N2−H2 mixed gases to produce a yellowish powder. A mechanochemical method can also be applied to prepare N-doped TiO2 powder.148,149 Here, TiO2 powder is mixed with hexamethylenetetramine and urea as N sources. The mixture then is ground using a planetary ball mill at a rotation speed of 700 rpm for 1 h. The obtained powder mainly consists of rutile TiO2, indicating that a high mechanical energy accelerates the phase transformation of anatase to rutile. Furthermore, because the obtained powder contains residual organic substances, a calcination process at 400 °C is required to remove them by combustion. 3.1.1.2. Wet Processes. A sol−gel method can also produce N-doped TiO2 powder. 71,72,77,150−157 Typically, an NH3 solution (NH4OH) is added to a solution of either titanium trichloride (TiCl3)150 or titanium sulfate (Ti(SO4)2)151 while stirring in a N2 atmosphere. The precipitate obtained after filtration is purified by washing with distilled water several times, dried at approximately 80 °C in air, and calcined at 400 °C in air to obtain a yellowish anatase TiO2 powder. Instead of using TiCl3 or Ti(SO4)2 as a source of Ti, titanium tetrachloride (TiCl4),71 titanium hydroxide (H2TiO3) obtained from TiCl4 and NH4OH,72,152 and titanium(IV) isopropoxide (TTIP) can also be utilized.153,158 Instead of using NH4OH as a source of N, ammonium chloride (NH4Cl),153 hydrazine,71 urea,77,154,155 guanidine hydrochloride,155 and triethylamine (TEA) are also workable.156,157 To enhance photocatalytic activity under visible light, nanoparticulate N-doped TiO2 has been developed.156,159−161 A TiO2-nanocrystal-containing colloidal solution is prepared by the controlled hydrolysis of TTIP in water under a controlled pH. To introduce N into the TiO2 nanocrystal, TEA then is added to the colloidal solution. Upon vacuum drying, the treated nanoparticle solution forms deep-yellow crystallites.156,159 Nanoparticles160 or mesoporous162 N-doped TiO2 can also be synthesized using a hydrothermal method or a solvothermal method. Tetrabutyl titanate or titanium butoxide

formed under the O-rich condition. For samples prepared via wet processes, such as the sol−gel and hydrolysis processes, the peak at 400 eV is dominantly observed because of their surface oxidization. On the other hand, experimental results show that these samples exhibit good photocatalysis under visible light. Whether the N species having the XPS peak at 400 eV are photocatalytically active or not is still under debate. One of the keys is a depth profile of the N species from the surface of the samples. We will discuss this issue more in section 4.1. As mentioned above, one should keep in mind that the charge compensation mechanism may limit the doping concentration. It originates from the repulsive interaction between impurities followed by atomistic distortion or diffusion.124 Valentin et al. performed first-principles calculations and pointed out that the N doping leads to a substantial reduction of the energy cost to form (V)O, consistent with XPS and electron paramagnetic resonance (EPR) measurements.122 Such concomitant O vacancies and/or their surrounding lattice distortion increase electron−hole recombinations and thus reduce photocatalytic activity.122,125,126 To prevent from generating defects in TiO2, codoping systems, for example, N and F codoping,85,127−133 N and boron (B) codoping,134 and N and V codoping,135 can be applied to compensate the excess charge by the N doping. These codoping systems are also effective for charge separation between photogenerated electrons and holes resulting in improvement of photocatalytic activity.

3. SYNTHESIS AND PROPERTIES OF N-DOPED TiO2 3.1. Synthesis

3.1.1. Powder. Numerous approaches to preparing Ndoped TiO2 powder have been employed, all of which can be classified into two categories: dry processes and wet processes. 3.1.1.1. Dry Processes. N-doped TiO2 powder can be prepared either by the oxidation of titanium nitride (TiN) powder in an O2 gas flow74 or by the nitrification of TiO2 powder in an ammonia (NH3) gas flow.73,136−144 The TiN powder is converted to N-doped TiO2 by annealing at 400 or 550 °C for 90 min in an O2 gas flow. A homogeneous rutile phase is obtained in samples annealed at 550 °C, while a small amount of TiN still remains among the rutile phase of TiO2 in samples annealed at 400 °C.74 Anatase N-doped TiO2 powder is prepared by treating anatase TiO2 powder in an NH3(67%)/ argon (Ar) atmosphere for 3 h at 600 °C.74 The amount of N doped into TiO2 can be controlled when using an NH3 flow by 9830

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N-doped TiO2 thin films can be prepared by the oxidation of TiN thin films, similarly to the powder. Simple thermal oxidation185 and microarc oxidation processes186 have been proposed so far. In the simple thermal oxidation, upon annealing at above 700 °C in air, TiN is converted to Ndoped TiO2 in the rutile phase. In the microarc oxidation, the surface of a pure Ti plate is nitrified in an NH3 atmosphere in a plasma-nitriding furnace at 540 °C. The nitrified surface then is treated by an electrochemical and plasma-chemical process called microarc oxidation. The obtained films mainly contain the anatase phase with a trace amount of the rutile phase.186 Very recently, a wet process, that is, a sol−gel or hydrolysis process, has been reported. Precursor solutions for N-doped TiO2 coatings are prepared using tetrabutylorthotitanate, isopropanol, NH4OH, or triethanolamine. The precursor solutions are dip-coated. To dry the samples, the coated substrates are left at 100 °C in the dipping system, which is followed by drying at 150 °C in air. The samples then are kept at 450 or 510 °C to allow the film to crystallize in the anatase phase.187,188 An aqueous peroxotitanate (PTA) solution containing N can also be used as the precursor for N-doped TiO2. TiCl4 is added dropwise to distilled water in an ice−water bath, and NH4OH is also added to the solution. After being stirred, the white precipitate is filtered and ultrasonically dispersed in distilled water by the addition of H2O2 to form a PTA solution. A substrate is dipped in the PTA solution, which is followed by calcination in air at 500 °C, resulting in a Ndoped TiO2 thin film.189 A viscous N-doped TiO2 gel is formed by stirring nanopowder and cellulose-containing α-terpineol. The gel then is pasted on a substrate and heated at 400 °C, forming a N-doped TiO2 thin film.190 N-doped TiO2 thin films are also prepared by the post treatment of bare TiO2 thin films. For example, anatase films prepared by a sol−gel method are thermally treated in NH3 flow,191 by an ionized N2 gas,192 and by a cathodic magnetron plasma.193 N-doped TiO2 thin films can also be fabricated by an anodic oxidation of a Ti sheet. The Ti sheet as an anode and a copper sheet as a cathode were dipped in an ammonium sulfate ((NH4)2SO4) solution as an electrolyte with NH4OH, and then an external voltage of 245 V is applied, resulting in a N-doped TiO2 thin film.194

dissolved in nitric acid mixed with various organic N compounds, such as TEA, urea, thiourea, and hydrazine hydrate, can be used as the N-doping source. The mixed solution is transferred to a Teflon-lined stainless-steel autoclave and then heated to 120 °C for several hours. The precipitates obtained by centrifugal separation are calcined at 400 °C, resulting in yellowish N-doped TiO2 nanostructures. The other method of preparing nanoparticles of N-doped TiO2 is to use plasma treatment. In this case, the precursors for TiO2 are TTIP and deionized water. They are vaporized and introduced into a plasma reactor via Ar/N2/O2 gases as the carrier. The N2 carrier gas is excited in a nonthermal plasma environment, and N atoms or N-containing molecules are implanted into the vaporized TiO2 precursors or particles.161 3.1.2. Thin Film. N-doped TiO2 thin films are mainly deposited by a dry process, in many cases by a reactive sputtering (SP) process.73,139,163−169 The process can be classified depending on the SP sources, for example, direct current (DC) SP, radio frequency (RF) SP, ion-beam (IB) SP, and electron cyclotron resonance (RCR) SP. Among them, the DC and RF SP processes have been used for preparing Ndoped TiO2 thin films. The DC SP can be utilized only when a target is electrical conductive, whereas the RF SP can be utilized regardless of the electrical conductivity of a target material. To increase the deposition rate and reproducibility of the films, a magnetron SP is often employed because the magnetic field can confine plasma particles close to the surface of the target. A combination of either DC SP or RF SP and magnetron SP, that is, DC magnetron SP and RF magnetron SP then has been widely used for preparing N-doped TiO2 films. A TiO2 target is sputtered in a N2/Ar gas mixture, which is followed by annealing at 550 °C in N2 gas to obtain a crystalline, yellowish, and transparent N-doped TiO2 thin film. Instead of a TiO2 target, a metal Ti target is also applicable; however, in this case, the SP must be performed in an O- and N-containing gas such as a N2O/Ar or N2/O2/Ar gas mixture. The annealing process can be avoided by heating the substrate to above 400 °C during the SP process. Pulsed laser deposition (PLD) can also be used to prepare N-doped TiO2 thin films. A pulsed neodymiumdoped yttrium−aluminum−garnet (Nd:YAG) laser is used to irradiate a Ti, titanium monoxide (TiO), TiO2, or TiN target in a N2/O2/Ar gas mixture while the substrate is heated to 400 °C.170−172 In addition, Krypton−fluorine (KrF) eximer laser is used to irradiate TiO2 and TiN mixed powders followed by calcination at 300−500 °C.173 N-doped TiO2 thin films can also be fabricated by atomic layer deposition (ALD) with TiCl4, NH3, and H2O as the precursors.174,175 Various types of chemical vapor deposition methods can be applied to prepare N-doped TiO2 thin films, for example, metal−organic CVD (MOCVD) using TTIP and N2,176 plasma-activated CVD (PACVD) using TTIP and Ar/N2,177 atmospheric pressure CVD (APCVD) using TiCl4, ethyl acetate, and tert-butylamine,178,179 using TiCl4 and NH3 or N2O,180,181 and using TTIP and hydrazine as source materials182 and plasmaenhanced CVD (PECVD) using a TTIP, NH3, and Ar mixture.183 N-doped TiO2 thin films can also be prepared by the surface treatment of bare TiO2 thin films. For example, anatase films prepared by SP are surface-treated by a N2/H2 mixed gas plasma followed by calcination in N2 gas at 400 °C.184 In addition, the incorporation of N in a TiO2 thin film has been demonstrated by N2+ ion implantation into a PECVD anatase film.176

3.2. Properties

3.2.1. Photooxidation of Volatile Organic Compounds. There are hundreds of reports on the photocatalytic degradation and detoxication of organic and inorganic substances over visible-light-irradiated N-doped TiO2. The photodegradations of acetaldehyde (CH3CHO),73,195−197 carbon monoxide (CO),72 formaldehyde (HCHO),198 acetic acid (CH3COOH),199 formic acid (HCOOH),199 ethane,72 toluene (C6H5CH3),195,200 2-propanol,136 ethylbenzene,201 xylene,201 benzene,202 ethylene,203 trichloroethylene,195,204 ethyl sulfide,205 diethyl sulfide,205 NOx,206,207 and mixtures of some of these198 have been reported to date. The first demonstration of the photocatalytic activity of N-doped TiO2 under visible-light irradiation was reported by Sato in 1986 for reactions of CO and ethane using oxygen isotope equilibration, although the photooxidative degradation of the organic compounds was not reported.72 In this case, he concluded that the photocatalyst was NOx-doped TiO2, in which N was presumably doped interstitially into TiO2 because the material was synthesized using a wet process.208 9831

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(DRIFTS) and gas chromatography (GC) in an airtight flow reactor irradiated with visible light.200 The side-chain methyl (−CH3) was immediately oxidized to yield mainly benzaldehyde and benzylalcohol, which is strongly adsorbed onto the photocatalyst surface, followed by ring-opening to produce a variety of carboxylic acids as preliminary steps toward complete oxidation to CO2 and H2O. Ion chromatography (IC) revealed that the main products on the surface of N-doped TiO2 were oxalic acid, CH3COOH, HCOOH, and pyruvic acid, as shown in Figure 14 and Scheme 1. The oxidation reactions of

In 2001, Asahi et al. reported the first demonstration of visible-light-induced complete photooxidation of gaseous CH3CHO, a well-known volatile organic compound (VOC), to CO2 over N-doped TiO2 synthesized using a dry process.73 Figure 13a shows changes in the concentrations of CO2 evolved

Figure 13. Photodecomposition of gaseous acetaldehyde over Ndoped TiO2 powder. (a) Evolved CO2 concentration (○, ●, N-doped TiO2; □, ■, TiO2), and (b) complete oxidation to CO2 under visible light. Figure 14. Changes in concentrations of four major intermediate products on the N-doped TiO2 surface during toluene oxidation under visible-light irradiation. Intermediate products on the N-doped TiO2 surface were extracted using distilled water and analyzed using IC. Reprinted with permission from ref 200. Copyright 2006 Royal Society of Chemistry.

by the photooxidation of CH3CHO (485 ppm in a closed vessel reactor containing O2 and N2) as a function of irradiation time. N-doped TiO2 and TiO2 exhibit similar CO2 evolution rates under UV irradiation (ca. 5 mW/cm2 at 310−390 nm), while the rate over N-doped TiO2 is more than 5 times greater than that over TiO2 under visible-light irradiation (Figure 1b; >410 nm, ca. 0.9 mW/cm2 at 410−470 nm). 970 ppm of CO2 was evolved, which indicates that all CH3CHO molecules in the reactor were completely oxidized to CO2 by visible photons only. This analysis clarified that N-doped TiO2 possesses sufficient power for the oxidation of CH3CHO in air, despite the speculated lower oxidative power of visible-light induced holes generated by photoexcitation from N 2p to Ti 3d than that produced from O 2p to Ti 3d. This discussion of the stoichiometry is significant and necessary to validate the application of N-doped TiO2 as a photocatalyst for practical use. In the case of N-doping with insufficient amounts or inappropriate chemical states of N, the photocatalytic oxidation reaction was thoroughly deactivated halfway through the reaction, leaving reaction intermediates on the surface of Ndoped TiO2.136 With appropriate photocatalytic conditions, the photocatalytic N-doped TiO2 system is an important tool to improve indoor air quality.198,201 As an example of a realistic environmental atmosphere containing many organic substances, photooxidation of a mixture of C6H5CH3, aromatic compounds, and HCHO was evaluated in a flow reactor under very weak illuminance (150 lx) corresponding to an indoor-lighting environment.198 It was also shown that Ndoped TiO2 can decompose aromatic C6H5CH3 stoichiometrically into CO2 (and H2O), as will be discussed later.200 In the regime of a VOC mixture, the selectivity for decomposition was dependent on the gaseous species in the mixture. For example, the rate of C6H5CH3 degradation was suppressed in the presence of HCHO, which is attributed to the preferential adsorption of HCHO onto the surface of N-doped TiO2.198 It is important to understand the pathways for photooxidation of the aromatic molecule C6H5CH3, which is hydrophobic, over hydrophilic N-doped TiO2. C6H5CH3 degradation under visible light was investigated using in situ diffuse reflectance Fourier transform infrared spectroscopy

Scheme 1. Simplified Pathways for the Photooxidation of Toluene over N-Doped TiO2 under Visible-Light Irradiation Revealed by in Situ DRIFT-GC and IC Analysesa

a

Reprinted with permission from ref 200. Copyright 2006 Royal Society of Chemistry.

HCOOH and CH3COOH adsorbed on the catalyst surface are generally the most important reactions to achieve the complete oxidation of organic substances into CO2 and H2O. While in aqueous solutions, the photooxidation rates for some organic substances such as thiocyanic ion (SCN−),142 bromide ion (Br−),142 formate,157 and ethylene glycol137 were very low, due to the low oxidative power of holes generated in N-doped TiO2 under visible-light irradiation.11 In contrast, HCOOH and other organic substances can be oxidized in air,209 and even a phosphorus-based pesticide such as chlorpyrifos has been decomposed over visible-light-irradiated N-doped TiO2. This is probably due to the differences in the oxidation mechanisms for systems in aqueous media and in air. In air, gaseous O2 is adsorbed on N-doped TiO2, and CB electrons play an important role in the photocatalytic oxidation of organic compounds. The conduction band electrons reduce O2 molecules to superoxide anions (O2−), which facilitates the formation of species such as H2O2 and hydroxyl radicals (•OH), as shown in Figure 15.210,211 It was also elucidated that the activities for the photocatalytic oxidation of organic 9832

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Figure 15. Proposed band structure of N-doped doped TiO2 under visible-light irradiation. Reprinted with permission from ref 210. Copyright 2009 Elsevier.

compounds in air73,136 under visible-light irradiation are strongly dependent on the N concentration doped into TiO2, which is determined by the balance of the amount of doped N ions and O vacancies induced in N-doped TiO2. Controlling this balance is essential to achieve higher photocatalytic activity under visible-light irradiation. A new absorption shoulder is generated in the visible-light region below 520 nm by the doping of N in TiO2.73,74,152 This absorption tail originates from the optical intragap absorption of the N-centers. Figure 16 presents the degradation rate of methylene blue (MB) over a SP-deposited N-doped TiO2 film as a function of

the formation of colorless leuco-MB photosensitization,212,213 resulting in an electron transfer from photoirradiated MB to TiO2, and light-induced self-decoloration of MB.73,212 Significant photocatalytic activity below 520 nm is observed for the N-doped TiO2 films, which corresponds well with the optical absorption spectra. In contrast, both N-doped TiO2 and TiO2 films exhibit similar activity with respect to UV light (represented by the results under black light (BL) irradiation). Lindquist et al. investigated photoelectrochemical water oxidation over a SP-deposited N-doped TiO2 film, where the incident photon to current efficiency (IPCE) of N-doped TiO2 was lower than that for TiO2 in the UV region, while it was much higher in the visible region between 425 and 575 nm, as shown in Figure 17.214

Figure 16. Properties of sputter-deposited N-doped TiO2 films measured as a function of wavelength: photocatalytic methylene blue decomposition (change in the absorbance ratio of embrocated methylene blue during irradiation). Reprinted with permission from ref 73. Copyright 2001 American Association for the Advancement of Science.

the optical high-pass filter cutoff wavelength under fluorescent light irradiation. It should be noted that a more precise photocurrent action spectrum should be discussed as a function of wavelength using monochromatic light. In this evaluation, which utilizes the colored MB dye under visible light, great care should be paid to eliminate the influence of decoloration due to

Figure 17. Spectral incident photon-to-current efficiency (IPCE) and absorbed photon-to-current efficiency (APCE) of sputter-deposited TiO2−xNx and TiO2 films in 0.1 M NaOH at 0.7 V vs Ag/AgCl, as recorded for illumination from the substrate/electrolyte (SE) interface. Reprinted with permission from ref 214. Copyright 2004 American Chemical Society. 9833

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Yin et al. conducted photocatalytic oxidation of gaseous NOx, one of the harmful polluting gases in the atmosphere, in a flow reactor equipped with visible-light emitting diodes (LEDs); Ndoped TiO2 powder exhibited NOx decomposition activity under irradiation with UV (390 nm), blue (445 nm), and even green (530 nm) light.206 More detailed properties will be discussed in section 6.2.2. 3.2.2. Water Splitting. Photocatalytic water splitting to produce H2 and O2 from water under visible-light irradiation is one of the most valuable photoreactions for the creation of renewable energy sources. Nakamura et al. reported that N doping of anatase TiO2 extended the photocurrent action spectrum for water oxidation from the UV region to the visible region.142 However, they also reported that the IPCE for water oxidation is low as compared to those for SCN− and hydroquinone (H2Q), as shown in Figure 18. Raftery and Figure 19. Antibacterial properties of N-doped TiO2 film after irradiation at a luminance of 2000 lx for 24 h. The experiment was conducted using the method provided by the Society of IndustrialTechnology for Antimicrobial Articles. Reprinted with permission from ref 219. Copyright 2006 Toyota Central R&D Laboratories, Inc.

are observed in the case of MRSA (methicillin-resistant Staphylococcus aureus), Staphylococcus aureus, and Escherichia coli O157, because the survival numbers declined by 6 orders of magnitude to an almost undetectable level. It has also been clarified that N-doped TiO2 photocatalyst exhibits antiviral and antiallergen properties, which is applied to practical applications as will be described in section 7. 3.2.4. Hydrophilic Conversion. It is well-known that the TiO2 surface becomes hydrophilic after UV light irradiation. This property has opened various novel applications of TiO2. One is the self-cleaning function; that is, the stains adsorbed on the TiO2 surface can easily be washed by water, because water soaks between stain and the hydrophilic TiO2 surface. Another function blessed with the photoinduced hydrophilicity is the antifogging function. On a highly hydrophilic surface, no water drops are formed. Instead, a uniform thin film of water is formed on the surface. This uniform water film prevents the fogging. We then expect that various glass products, for example, mirrors and eyeglasses, can be imparted with antifogging functions. The number of reports on hydrophilic conversion of Ndoped TiO2 thin films under visible light has, however, been limited.163−165,176 In ref 73, the retention of the hydrophilic property of a SiO2-coated N-doped TiO2 thin film was evaluated under interior lighting after superhydrophilic treatment, which was superior to that of SiO2-coated TiO2. In refs 163−165, 183, a decrease in the water contact angle (CA) on N-doped TiO2 thin films was clearly demonstrated for various N concentrations under visible light. Figure 20 shows typical photoinduced hydrophilic conversion on N-doped TiO2 (TiO2−xNx, x = 0, 0.020, 0.038, 0.058) upon irradiation with visible light (400−530 nm). Prior to irradiation with visible light, all of the thin films exhibited water CAs of approximately 30°, which is considered to be the initial water CA. After irradiation, the water CAs decreased to 22°, 12°, and 20°, which are defined as the critical water CAs, on thin films with x = 0.02, 0.038, and 0.058, respectively, while the water CA on the TiO2 thin film was unchanged. Figure 21 shows the change in the hydrophilic property upon irradiation with UV light (300− 400 nm). After irradiation with UV light, the water CAs of all films decreased to less than 5°, and superhydrophilicity was

Figure 18. Effect of 0.5 mM I−, H2Q, SCN−, and Br− addition as reductants on IPCE vs wavelength in the visible-light region for Ndoped doped TiO2 in 0.1 M HClO4. The electrode potential was 0.5 V vs Ag/AgCl. Reprinted with permission from ref 137. Copyright 2006 American Chemical Society.

colleagues analyzed the detailed photoelectrochemistry of Ndoped TiO2 and confirmed that N-doping increases the visiblelight absorption of anatase TiO2, which leads to higher photocatalytic activity under UV and visible light. They also showed that the photocurrent under visible-light irradiation increased in the presence of methanol (CH3OH), which indicated that the rate for water oxidation was lower than that for CH3OH oxidation.215 Lindquist et al. investigated photoelectrochemical water oxidation over SP-deposited N-doped TiO2 films and concluded that the IPCE for N-doped TiO2 is lower than that for TiO2 in the UV region, while it is much higher in the range from 425 to 575 nm, as shown in Figure 17.214 3.2.3. Antibacterial, Antiviral, and Antiallergen Properties. The N-doped TiO2 photocatalyst exhibits antibacterial properties and has been investigated as an alternative disinfectant for environmental and medical applications under visible or indoor lighting.216−218 Figure 19 presents the antibacterial activities of N-doped TiO2 films that were irradiated with white fluorescent light at a luminance of 2000 lx for 24 h.219 While all of the bacteria increase in abundance after irradiation in the absence of N-doped TiO2, all apart from Bacillus subtilis are diminished on the surface of irradiated Ndoped TiO2 films. In particular, significant antibacterial effects 9834

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property) can be expressed as a product of photon flux, photoabsorption efficiency, and quantum efficiency. As mentioned previously, N-doping into TiO2 is the technique to control the VB of TiO2, by either formation of the narrow N 2p band above VB of TiO2 or band gap narrowing with shifting the VB top upward as a result of mixing N 2p and O 2p. In both cases, the oxidation power weakens because the potential of photoinduced holes decreases. Thus, as compared to conventional TiO2, the disadvantage of N-doped TiO2 is a decrease of quantum efficiency. In contrast, the advantage of Ndoped TiO2 is an increase of photoabsorption efficiency in visible-light region. That is, the contribution of N-doping is the trade-off between photoabsorption efficiency and quantum efficiency. Therefore, whether to use N-doped TiO2 or conventional TiO2 depends on the light source. While Ndoped TiO2 shows photocatalytic reaction under visible-light irradiation below 520 nm, it does not show superior photoactivity to TiO2 under visible illumination containing a certain amount of UV such as sunlight. This feature of N-doped TiO2 may limit practical applications as discussed in section 7.

Figure 20. Changes in water contact angles of TiO2−xNx thin films (x = 0, 0.020, 0.038, 0.058) upon irradiation with visible light (400−530 nm).

4. DETAILED ANALYSIS OF N-DOPED TiO2 4.1. N States in N-Doped TiO2

One of the major points under debate concerns the chemical state and location of the N species in TiO2 that is responsible for the visible-light sensitivity. As discussed in section 2.4, the doped N in TiO2 may exist in a variety of chemical states depending on the synthesis process. In this section, we review experimental analyses and discussions about the N states in TiO2. Different chemical states have been proposed to be responsible for the visible-light sensitivity: substitutional N,73,74,81,138,139,143,147,155,162−166,176,183,184 interstitial N (in the form of NOx72,122,150,152,154,161,185,187,220−222 or NHx140), and O vacancies produced in TiO2 accompanied by the introduction of N.151 Here, the possibility of O vacancies can be excluded because, as mentioned previously, their level in TiO2 is below the lower end of the CB at 0.75−1.18 eV,58 resulting in an extremely low oxidation potential. In almost all of the studies on this issue, samples were analyzed by XPS. XPS reveals a N 1s peak at a binding energy of 396−397 eV, which is attributed to nitride N in Ti−N bonds in TiO2, and interpreted to be substitutional N at the O sites.223 In contrast, it reveals a N 1s peak at 400 eV attributed to NOx and a N 1s peak at 399.6 eV attributed to NHx species. In addition, the detailed XPS analyses indicate the existence of intermediate states between 396 and 400 eV.123 Combined with the DFT calculations, a N 1s peak at 398 eV is assigned to be interstitial N bonding with the lattice O as listed in Table 1. Valentin et al. also calculated the core shifts of the substitutional and interstitial N species, which differ by 1.6 eV,122 consistent with Table 1. In this respect, one could argue that NOx species with a N 1s peak at 400 eV may be different from the interstitial N that also forms NO with a lattice O. The peak at 400 eV is observed even in commercially available TiO2 (e.g., ST01, Ishihara Sangyo), and not necessary related to the visible-light activity. The chemical states and distribution of the N species in TiO2 are strongly process dependent. It appears that the XPS peak at 396 eV is generally observed in N-doped TiO2 prepared via dry processes, such as the nitrification of TiO2 powder and the SP, PLD, and ion-implantation methods, while the peak at 400 eV is observed in samples prepared via wet processes, such as the

Figure 21. Changes in water contact angles of TiO2−xNx thin films (x = 0, 0.020, 0.038, 0.058) upon irradiation with UV light (300−400 nm).

achieved. The critical water CA decreased as the amount of N decreased, as shown in Figure 21. Irradiating the N-doped TiO2 with UV light leads to much stronger hydrophilic conversion than irradiating with visible light. This trend is plausible considering that the incorporated N forms an isolated narrow band consisting of N 2p above VB that consists of O 2p. In this situation, the isolated narrow band is the origin of visible-light sensitivity. Irradiation with UV light excites electrons from both the VB and the narrow band, while irradiation with visible light only excites electrons from the narrow band. The hole mobility and the light absorption in the isolated narrow band should be low. Thus, the hydrophilic properties under visible light are inferior to those under UV light. The optimal value of x to maximize the hydrophilicity of Ndoped TiO2 is approximately 0.038 in the case of irradiation with visible light. When x increases, the density of states (DOS) of N 2p increases, resulting in a broader localized band. The visible-light absorption and the hole mobility then increase, which leads to high hydrophilicity. Conversely, the hole potential decreases, which leads to low hydrophilicity. Therefore, an optimal dopant concentration that maximizes the hydrophilicity under visible-light irradiation is observed. The hydrophilicity of N-doped TiO2 decreases with increasing x in the case of irradiation with UV light. The Ovacancy level in anatase TiO2 as well as dopant N itself may act as a recombination center for holes and electrons. The number of O vacancies increases with increasing x,136 which explains why the hydrophilicity decreases with increasing x under UV light irradiation. 3.2.5. Which Is More Applicable to Practical Use, Conventional TiO2 or N-Doped TiO2? Photocatalytic activities (both oxidative decomposition and hydrophilic 9835

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The EPR measurements have been often attempted to identify the N species present in a N-doped TiO 2 system.153,221,222 According to their results, in the case of Ndoped TiO2 prepared via sol−gel synthesis, either diamagnetic (Nb−) or paramagnetic (Nb•) bulk centers are responsible for the visible-light sensitivity. The relative abundance of these species depends on the oxidation state of the material because, upon reduction, electron transfer from Ti3+ ions to Nb• results in the formation of Ti4+ and Nb−. Combined with firstprinciples density functional theory (DFT) calculation, they concluded that Nb• is probably at both the interstitial and the substitutional positions within the TiO2 lattice. However, Ndoped TiO2 prepared via sol−gel synthesis was concluded to have dominantly interstitial N species in the form of NO.221 The other point under debate concerns the electronic structure of N-doped TiO2. This question has been addressed by performing computational DOS calculations164 and a number of experiments. Some authors claim that the bandgap of the sample narrows as a result of the negative shift of the VB top upon N doping,139,148,152,154,157,159,184,185,220 while others attribute the observed absorption of visible light by N-doped TiO2 to the excitation of electrons from localized impurity states in the bandgap above the VB.122,136,142,153,163−165,176 The electronic structure of N-doped TiO2 depends on the amount of N dopants; the bandgap narrows with increasing N as evidenced by a DOS calculation.164 However, the heavily Ndoped TiO2 does not show high activity in general because the charge neutrality induces the O vacancies that act as recombination centers for photogenerated holes and electrons.

sol−gel and hydrolysis processes. These different procedures probably lead, at least in some cases, to different N species.123 It has been argued that the N 1s peak at 396 eV originates from the N species formed in the initial calcination state, which is subsequently oxidized to an unknown N species, as well as small amounts of nitrite and nitrate.220 Sputtered N-doped TiO2 films, which are active under visible light, only exhibit an XPS peak at 396 eV and not at 400 eV.163 Irie et al. also confirmed using a sputtered N-doped TiO2 film that the 400 eV peak disappeared completely after Ar+ etching, while the 396 eV peak remained unchanged. In the XPS measurements of N-doped TiO2 powder prepared via a wet process, the intensity of the 400 eV peak decreased after Ar+ etching of the sample surface to a depth of several nanometers, whereas the 396 eV peak increased, as shown in Figure 22. In

4.2. Charge-Carrier Dynamics in N-Doped TiO2

The charge-carrier dynamics in photoexcited semiconductors measured by transient spectroscopy provide powerful information to understand the primary processes in photocatalytic reactions. Katoh et al. studied charge separation and trapping processes in N-doped TiO2 photocatalysts containing 0.25 at. % nitrogen that exhibit the highest rate for acetaldehyde oxidation using time-resolved microwave conductivity (TRMC) in nanosecond region. TRMC signals in TiO2 arise from electrons224,225 because holes are trapped within an early time range, as was confirmed by femtosecond transient absorption measurements.126,226 Accordingly, the observed TRMC signal decays reflect conductivity induced by generated electrons, and therefore the recombination and trapping processes of electrons in the particles can be discussed. It was reported that the trapping rate increased with N-doping due to an increase in oxygen vacancies, as demonstrated using deep level optical spectroscopy (DLOS).227 The charge separation efficiency under visible-light excitation was one-third of that under UV excitation. Yamanaka et al. employed femtosecond time-resolved diffuse reflectance (TDR) spectroscopy under weak excitation conditions228 to clarify the charge-separation and trapping dynamics in N-doped TiO2 powder (0.25 at. % N), which exhibits high activity for photocatalytic VOC oxidation under visible light.73 As shown in Figure 23, the TDR spectrum for Ndoped TiO2 after UV (360 nm) light excitation revealed that the surface-trapped electrons and holes were generated immediately after excitation, similar to that for TiO2. The population of surface-trapped electrons for N-doped TiO2 decreased more rapidly than that for TiO2 due to deep trapping by additionally induced oxygen vacancies. The TDR spectrum for N-doped TiO2 after 450 nm light excitation

Figure 22. (a) N 1s XPS spectra of N-doped TiO2 and TiO2 powders before (a) and after (b) etching to a depth of several nanometers. Sample 1, pure TiO2 (ST-01, commercially available from Ishihara Sangyo Kaisha, Ltd.); sample 2, N-doped TiO2 prepared via a wet process using TiCl4 and MH4OH; sample 3, N-doped TiO2 prepared via a wet process using Ti(SO4)2 and MH4OH; and sample 4, Ndoped TiO2 prepared via a dry process, that is, the nitrification of TiO2 under an NH3 flow.

contrast, for N-doped TiO2 powder prepared via a dry process, both 400 and 396 eV peaks were observed before Ar+ etching, and the 396 eV peak disappeared while the 400 eV peak remained after Ar+ etching. Note that this sample was slightly nitrificated by gaseous NH3. In the case of pure TiO2, the intensity of the 400 eV peak decreased while the 396 eV peak did not appear after Ar+ etching. In a wet process, calcination in air at ca. 400 °C is required, suggesting that N-doped TiO2 powder forms a core−shell particle consisting of the Ncontaining core and the NOx-containing or N-free shell. This is because N existing near the surface should be oxidized or substituted by O during the calcination process. In contrast, in dry processes, particularly the nitrification of TiO2 powder and the ion-implantation method, N is incorporated from the outer surface of the particle, suggesting that N-doped TiO2 forms a core−shell particle consisting of the N-free core and the Ncontaining shell. In fact, N-doped TiO2 thin film prepared by SP, which could be considered to contain a homogeneous distribution of N species in terms of depth, was confirmed to exhibit both 400 and 396 eV XPS peaks on the surface before Ar+ etching, whereas after etching, the intensity of the 400 eV peak decreased and that of the 396 eV peak remained unchanged. 9836

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5. THE OTHER NONMETAL-DOPED TiO2 Since a series of nonmetal-doped TiO2 was proposed as an effective way to modify the band structure for visible-light sensitization,73 numerous experimental and theoretical studies regarding nonmetal doping have been reported. While N-doped TiO2 is still in the majority, there are lots of publications for the other nonmetals as well, many of them for C doping80,81,85,133,231−234 and F doping,129,132,133,235−240 also for S doping44,76,78,241−247 and B doping,133,134,248−,255 and some for P, I, chloride (Cl), and bromine (Br) dopings.82,83,256−265 Most of the results show significant improvement of visible-light sensitization and photocatalytic activities. The origin of the improvement is however rather intricate. Such a nonmetal dopant will be located either at a substitutional site in the TiO2 lattice or at an interstitial one that may bind to lattice atoms or form molecular complexes, providing completely different states in the band gap of TiO2.73,123,133 In general, the substitutional doping for O is preferred in O2-poor conditions while interstitial doping is favored in O2-rich conditions; in most cases, these coexisted in materials, and thus the interplay among possible states of dopants and/or defects may be important. Nontrivial effects by doping including charge separations, charge compensations, lattice distortion, or a change in nanostructural morphology also contribute to photocatalytic activity and selectivity. Figure 23. Spatial and energetic distribution of electrons and holes in N-doped TiO2 (0.25 at. %-N) powder after weak femtosecond laser excitation at (a) 360 nm (15 μJ/cm2), and (b) 450 nm (150 μJ/cm2). VO indicates an oxygen vacancy. Reprinted with permission from ref 228. Copyright 2012 American Chemical Society.

5.1. C Doping

Irie et al. obtained the C-doped anatase TiO2 powders by oxidizing titanium carbide (TiC) powder mildly at 350 °C for 36 h in air and then under O2 flow at 600 °C for 5 h.81 Decomposition of gaseous 2-propanol under visible light was observed. The XPS showed the existence of Ti−C bonds, which was assumed to relate to the photocatalytic activity. Sakthivel et al. prepared the C-doped samples by the hydrolysis of TiCl4 with tetrabutylammonium hydroxide followed by calcination at 400−550 °C.80 4-Chlorophenol and the azo dye remazol red are efficiently mineralized by the C-doped sample under visible light. The XPS did not show Ti−C bonding but the presence of carbonate species. They also suggested surface energy states close to the VB edge, which may be responsible for the visible-light absorption. Density functional theory (DFT) calculations showed that the C atom may favor not only a substitutional site for O but also an interstitial site.232 The C doping induces several localized occupied states in the gap, which may account for the experimentally observed red shift of the absorption edge toward the visible-light range. Surface states and/or morphology are often modified by the C doping. One of the prescriptions to escape from the recombination is to utilize a nanostructure that can promote charge transfer to reactant and charge separation. Park et al. reported the preparation of vertically grown C-doped TiO2 nanotube arrays with high aspect ratios.231 The C concentration was controlled by the CO gas flow at 500−800 °C, but the XPS showed the amount of Ti−C bonding was very low. The synthesized C-doped TiO2 nanotube arrays showed much higher photocurrent densities and more efficient water splitting under visible-light illumination (>420 nm) than pure TiO2 nanotube arrays. Ren et al. reported the low-temperature preparation of C-doped TiO2 using glucose as the carbon source under hydrothermal treatment at 160 °C, showing higher visible-light photocatalytic activity than Degussa P25 on the degradation of rhodamine B in water.233 The surface area was increased by 20% in C-doped TiO2 as compared to

clearly indicated the generation of charge carriers. As compared to that for 360 nm excitation, time evolution for 450 nm excitation showed a significant decrease of charge carriers just after excitation because of the deep trapping of electrons within 1 ps (nearly equivalent to time resolution of the detector). As described in section 3.2, some research has indicated the lower oxidative power of visible-light-induced holes excitation in N-doped TiO2 in aqueous media.137,142,157,229 Tachikawa et al. investigated the photocatalytic degradation process of ethylene glycol (C2H6O2) over pure anatase TiO2 and Ndoped TiO2 powders under UV or visible-light irradiation using nanosecond TDR spectroscopy.137 The scavenging of photogenerated holes by C2H6O2 occurred during 355 nm laser photolysis, while no direct oxidation reaction of C2H6O2 occurred during 460 nm laser photolysis in acetonitrile, although sufficient generation of charge carriers occurred upon excitation. Tang et al. studied the dynamics of photogenerated electrons and holes using transient absorption spectroscopy, and explained that the reason for the lack of activity of nanocrystalline N-doped TiO2 film for photocatalytic water oxidation under visible light is due to rapid electron−hole recombination between doping-induced states.230 They concluded that oxygen production on the nanocrystalline N-doped TiO2 film requires photogenerated holes with a longer lifetime of approximately 0.4 s, which can be achieved, for example, by enhancing the charge separation with cocatalysts such as Ag loading. 9837

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supercritical treatment (553 K and 15 MPa with carbon disulfide/ethanol solution), and obtained the high surface anatase samples.244 They demonstrated significant improvement in photocatalytic degradation of MB under visible light. The XPS spectra showed S−Ti−O bonding where partial electrons transfer from S to O in the TiO2, making the S site electron-deficient. The enhancement of the photocatalytic activity may be attributed to the high surface area and defects, which capture the photoinduced electrons and inhibit the recombination.

undoped one. The XPS showed C−O bonds and suggested a form of Ti−O−C structure in contrast to the samples under high-temperature preparations in which C partially substitutes for some of the lattice O atoms.81,266 Recently, Yu et al. synthesized C-doped TiO2 nanoparticles (a diameter of 25 nm) wrapped with nanosized graphene (a thickness of 0.8−2.0 nm and a size of 30−40 nm), showing significant improvement of phenol degradation under visible light.234 The large interfacial contact of the nanosized graphene decreases the number of surface defects on the C-doped TiO2 resulting in the formation of the larger amount of photocatalytic active species such as • OH.

5.4. B Doping

Chen et al. prepared B-doped TiO2 by the sol−gel method using boron solution (H3BO3) as the B source.248 The doped B exists as the form of B3+ in the interstitial TiO2 structure. At high temperature, however, it segregates from TiO2 to form boron trioxide layer on the surface of TiO2 nanoparticles. This can limit grain growth of the nanoparticles and stabilize the anatase phase. Consequently, photocatalytic activity under UV light was improved. In contrast, In et al. suggested a substitutional doping of B for O site forming direct bonding with Ti leading to photocatalytic improvement under visible light, while boric oxide-like states were considered to be inactive.249 The DFT calculations were performed for several possible states of B in anatase TiO2.252,254,255 It was concluded that the substitutional sites for Ti and O are not preferable as compared to interstitial sites, and that the interstitial B forming BO3 or BO4 coordination should be stable at least after high temperature treatment. Such a cationic character of B can be utilized for synergistic effects with anionic doping. Liu et al. demonstrated that the visible-light activity can be substantially enhanced by the formation of a O−Ti−B−N bonding structure on the surface of TiO2.134 It facilitates the separation and transfer of charge carriers, thereby promoting the photocatalytic activity. Recently, N−B−F-tridoped TiO2 was prepared by H2TiO3 via a microwave-assisted route using the ionic liquid [BMIm]BF4 as a dopant.253 The sample exhibited a large specific surface area, a small crystal size, and a mesoporous structure, and showed an excellent visible-light photocatalytic activity in the degradation of organic pollutants.

5.2. F Doping

Yu et al. presented F-doped TiO2 by hydrolysis of TTIP in the ammonium fluoride (NH4F)−H2O mixed solution.235 The F doping improved photocatalytic activity because it stabilized the crystallinity of anatase and suppressed the formation of brookite or rutile phase. Park et al. prepared surface fluorination by using sodium fluoride added to aqueous TiO2 suspensions.236 Although they observed neither red-shift of the band edge absorption nor improvement of the crystallinity, photocatalytic oxidation of Acid Orange 7 and phenol suggests that the free OH radical-mediated oxidation pathways were enhanced on the surface of the F-doped TiO2. The F doping can be also used for compensation of the charge induced by the N doping so as to prevent from oxygen vacancies, as presented in N and F codoped TiO2.85,129,132,133 Huang et al. prepared N and F codoped anatase TiO2 by the sol−gel solvothermal method using tetrabutyl titanate as precursor with triethylamine as a N source and NH4F as a F source.129 They demonstrated a high visible-light photocatalytic activity for p-chlorophenol decomposition, and concluded that the high activity is attributed to a synergetic effect of the codoping where the doped F atoms improve the visible-light absorption and lead to an increase of surface acidity and adsorption of the reactant. Dozzi et al. studied time-resolved photoluminescence for N and F codoped TiO2.240 They found that the F doping favors the formation of long-living luminescent surface trapping sites that are beneficial for photocatalytic activity because these trapping sites decrease the undesired electron−hole recombination.

5.5. Other Nonmetal Doping

5.3. S Doping

There are more studies where nonmetal doping was utilized for visible-light-sensitive photocatalysis. Hong et al. introduced Idoped TiO2 by direct hydrolysis of tetrabutyl titanate with HIO3 as the iodine source.82 The sample showed the anatase phase, a small particle size (ca. 5 nm), and photocatalytic activity under visible light. Liu et al. also observed visible-light sensitization in I-doped TiO2.83 They suggested I5+−Ti3+ defect states. Su et al. proposed a surface IO4− is responsible for the visible-light response.256 XPS measurement and DFT calculations revealed that the surface iodine exists in both I7+ and I− chemical states that introduce some energy bands in the TiO2 band gap. Liu et al. also suggested the surface-dominant I−O−I and I−O−Ti structures, which result in visible-light photocatalytic activity.258 The I-doped TiO2 interestingly showed high efficiency in phenol degradation and disinfection under visible light.259 The P-doped TiO2 was prepared by sol−gel method with hypophosphorous acid as precursor.260 The sample shows visible-light absorption resulting in photocatalytic degradation of 4-chlorophenol under visible-light irradiation. The observed XPS peak of 132.6 eV is different from the one for a pentavalent oxidation state of 133.8 eV,263 suggesting a lower

Umebayashi et al. synthesized S-doped powder samples by oxidation of the titanium disulfide (TiS2) powder, and found the visible-light absorption44 and photocatalytic decomposition of MB,76 2-propanol, and adamantan.78 As discussed in section 2, the S doping is predicted to be rather difficult to incorporate into TiO2 lattice because of the large ionic radius of S as compared to that of O. Instead, the sulfur ion prefers the substitution for Ti as the cationic S4+ or S6+ doping.78,242 Tachikawa et al. investigated the photocatalytic oxidation reactivity of the photogenerated holes during the UV or vis laser flash photolysis for S or C-doped TiO2 powder using timeresolved diffuse reflectance.241 They found that one-electron oxidation reactions were observed in the order of pure TiO2 > S-doped TiO2 > C-doped TiO2 under the 355 nm laser. However, no one-electron oxidation was observed under the 430 nm laser for either S-doped or C-doped TiO2 powder, although the charge carriers were sufficiently generated upon laser excitation. The results suggested the mobility of the photogenerated holes around the doping sites is significantly small under visible light. The states of S in TiO2 strongly depend on processing. Li et al. prepared S-doped TiO2 via 9838

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for further understanding and optimizing the effect of codoping with N.

oxidation state that may be responsible for the visible-light absorption, while P in a form of PO43− results in a band gap larger than that of TiO2.264

6.2. Cocatalysts Loading to N-Doped TiO2

6.2.1. Overview of Cocatalysts Loading to N-Doped TiO2. Loading of transition metals, metallic ions, and metal oxides to the surface of TiO2 has been extensively studied both in gas−solid and liquid−solid regimes to improve the photocatalytic activity.108,322 The enhanced photocatalytic activity is due to improved separation of photogenerated electron−hole pairs as shown in Figure 7, which elongates carrier lifetime,323 together with improved selectivity for the adsorption/desorption of reactants, intermediates, and products. In this section, the effect of cocatalyst loading onto the surface of N-doped TiO2 is explained. The results for the loading of approximately 50 elements have been examined for TiO2 synthesized by both dry and wet processes. Unfortunately, it is difficult to distinguish whether each cocatalyst is metal, metallic ion, or metal oxide because each condition of cocatalyst is ambiguously described in many papers. Among these, a significant effect on VOC oxidation to CO2, disinfection, water oxidation, and so on was reported for loading with Cu,209,324−326 Fe,199,324,327 V,324,328 Pt,199,324,327 Co,329 Au,324,330 Pd,324,331,326 Ag,326 and Sm332 species. While we discuss more in detail the effects of cocatalysts with Cu, Fe, V, and Pt in section 6.2.2, here we review some other cocatalyst systems. Hoang et al. reported photoelectrochemical water oxidation over N-modified TiO2 nanowire arrays treated with Co species.329 IPCE of N-doped TiO2 in the ultraviolet region is restored to equal or higher values than those of unmodified TiO2 electrodes, and it remains as high as 18% at 450 nm. Tian et al. reported N-doped TiO2 nanoparticles loaded with metallic Au prepared by a wet-chemical method.330 The photocatalyst exhibited higher visible-light photocatalytic activity when compared to single N-doped or Au-loaded TiO2. Li et al. demonstrated removal of As3+ in water using Ndoped TiO2 loaded with PdO.331 Photocatalytic activity under visible-light irradiation was enhanced by loading of PdO, which is explained by strong optoelectronic coupling between PdO and N-doped TiO2. Li et al. investigated the effect of additional Sm species to N-doped TiO2 for reaction of oxidative destruction of nitric oxide.332 They found that doping of Sm3+ in the lattice of TiO2 is not effective to improve the photocatalytic activity, while loading of samarium oxides on the surface of N-doped TiO2 results in improvement of the photocatalytic activity. 6.2.2. Photocatalytic Activity over N-Doped TiO2 Loaded with Cu, Fe, V, or Pt for VOC Oxidation. In this section, we focus on Cu, Fe, V, and Pt cocatalysts loading onto N-doped TiO2. At least two research groups performed systematic experimental investigations on the effects of these cocatalysts. Their results are consistent with each other and thus reliable. More importantly, because of the strong VOC oxidation reaction under visible light, these cocatalyst systems are technologically promising for indoor use. Morikawa et al. loaded the cocatalysts by impregnation of Ndoped TiO2 with aqueous solutions of iron nitrate, copper nitrate, or Pt(NH3)2(NO2)2, followed by calcination at 573 and 673 K.199 X-ray diffraction (XRD) patterns of the Fe, Cu, or Pt loaded N-doped TiO2 revealed no diffraction peaks derived from the loaded particles. Transmission electron microscopy (TEM) of a sample loaded with 0.5 wt % Pt revealed nanoscale

6. IMPROVEMENT OF N-DOPED TiO2 6.1. Codoping into N-Doped TiO2

Toward higher photocatalytic activity of N-doped TiO2, codoping of metallic267 or nonmetallic ions such as B,268 C,269 F,127,129,270 Si,271 P,261 S,84 V,272−277 Cr,272,278−281 Fe, 89,271,282−288 Co, 289−292 Ni, 293,294 Cu, 295−297 Y, 298 Zr,299Ag,300301 Ta,165 W,302−309 Pt,310−312 Au,313 La,314 Ce,315 Pr,316−318 Nb,319 and Sm196,320,321 into N-doped TiO2 have been conducted and reported in a significant amount of publications. Some of the codoping systems of nonmetallic ions into N-doped TiO2 are described in section 5. It is rather difficult to judge which codopant is the best for achieving highest photocatalytic activity because synthesis methods, reactor shapes and sizes, light sources, and substances to be decomposed are different among research papers. Instead, here we tried to describe some general aspects based on theoretical calculations and review some selected experimental results to show prospects in codoping of metallic ions into a N-doped TiO2 system. Codoping approach can be categorized into two cases: to make an activation a lower photoenergy region by forming new intraband absorption in the bandgap, and to compensate N3− for its additional charge in TiO2 with the additional codoping to reduce charge recombination centers and increase the amount of dopant. As mentioned in section 3.2, Mizushima et al. experimentally determined impurity states of V, Cr, Mn, and Fe to be 1.9−3.0 eV below CBM.112 Umebayashi et al. theoretically calculated DOS for doped systems with the 3dtransition metals as shown in Figure 6, where a systematic change of the impurity levels in the bandgap of TiO2 is shown reasonably corresponding to the atomic 3d levels.44 These impurity levels induce additional intraband absorption to the TiO2 band absorption spectrum. Kurtoglu et al. reported that Cr and N form strong coupling in the anatase TiO2 lattice codoped with Cr and N, resulting in a 10-fold increase of photocatalytic activity under ultraviolet irradiation as compared to that of Cr or N doping alone.280 Cong et al. reported that N and Fe3+ ion implantations in TiO2 induce formation of new states close to the valence band and the conduction band, respectively.89,271,282 They described that cooperation of the N and Fe3+ ions leads to the band gap narrowing and greatly improves the photocatalytic activity in the visible-light region. The photocatalyst codoped with N and 0.5% Fe3+ showed that degradation efficiencies were improved by 75% and 5% under visible and ultraviolet irradiation, respectively, as compared to the pure TiO2. It is expected that doping of Ta5+, V5+, or Nb5+ can compensate for stoichiometrically unbalanced charges in N3−doped TiO2, which enables an increase in the amount of dopant in TiO2. For instance, Obata et al. reported Ta and N codoped anatase TiO2 thin films fabricated by RF magnetron sputtering method.165 Incident photon-to-current conversion efficiency (IPCE) spectra indicated the suppression of the isolated N 2p band formation but the enhancement hybridization of valence band with N 2p band in Ta and N codoped TiO2, as compared to N-doped TiO2. In the codoped TiO2 system, experimental investigation for various combinations of dopants in a systematic way is expected 9839

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Figure 24. Concentration of (a) gaseous acetaldehyde and (b) CO2 in glass-vessel reactors as a function of visible-light irradiation time (>410 nm). CO2 was generated by the photooxidation of acetaldehyde over the photocatalysts (2CH3CHO + 5O2 → 4CO2 + 74H2O). Reprinted with permission from ref 211. Copyright 2011 Toyota Central R&D Laboratories, Inc.

Pt particles (1−2 nm) on the surface of N-doped TiO2. In contrast, no such particles were observed for the Fe or Cu loaded samples. The yellow colored N-doped TiO2 was changed to light-brown, light-green, or brown after modification with Fe, Cu, or Pt, respectively. X-ray photoelectron spectroscopy (XPS) measurements of 0.5 wt % Cu loaded Ndoped TiO2 indicated the binding energies of Cu 2p3/2 and 2p1/2 were 932.6 and 952.4 eV, respectively, which suggested that Cu, cuprous oxide (Cu2O), or Cu hydroxides were present on the catalyst surface.333 Furthermore, the Cu LMM Auger region was shifted to a higher binding energy (338.5 eV) than that of metallic copper (334.9 eV), which indicated that Cu2O, Cu hydroxides, or clusters containing Cu−O bonds were probably present on the catalyst surface.199,334 Among these, clusters containing Cu−O bonds were most likely when also considering the results of X-ray absorption fine structure (XAFS) analysis.335 Similarly, the XPS spectrum of 0.5 wt % Fe loaded N-doped TiO2 showed the binding energy of the Fe 2p band was 709.5 eV, which suggested that clusters containing Fe−O bonds or Fe2+ hydroxide were present on the catalyst surface under irradiation. The peak at 710.9 eV corresponding to Fe3+ was negligible (less than the XPS resolution of 0.05 at. %), and the electron spin resonance (ESR) signal corresponding to Fe3+ was also very small. XAFS spectra suggested the existence of clusters containing Fe−O and Cu−O bonds on Feand Cu-loaded N-doped TiO2, respectively. As for Pt, the binding energies of the Pt 4f band were 71.9 and 75.4 eV, respectively, which suggested that 1−2 nm Pt nanoparticles comprised of a mixture of metallic Pt (71.2 eV) and PtO (72.4 eV)336 were present on the catalyst surface. Figure 24 shows the photocatalytic oxidation of gaseous CH3CHO over TiO2 (ST01, Ishihara Sangyo), bare N-doped TiO2, and Fe-loaded, Cu-loaded, and Pt-loaded N-doped TiO2 under visible-light irradiation (>410 nm). In Figure 24a, TiO2 (ST01) adsorbed more acetaldehyde molecules than bare Ndoped TiO2, while loading with cocatalysts did not improve adsorption. Fe or Cu loading markedly enhanced the rate of acetaldehyde photodegradation, as indicated by the significant decrease in the CH3CHO concentration. In Figure 24b, no significant CO2 evolution was observed over TiO2, which indicates that acetaldehyde was not appreciably photooxidized to CO2 over TiO2 under visible light. In contrast, the CH3CHO oxidation rate was significantly improved by N-doped doping, and was almost doubled by loading with Cu, Fe, or Pt. These photocatalysts can stoichiometrically decompose acetaldehyde into CO2 and H2O under visible-light irradiation. CO 2

generation reactions were confirmed under irradiation of >510 nm, but were absent under irradiation of >600 nm over N-doped TiO2, Fe/N-doped TiO2, and Pt/N-doped TiO2. Table 2 summarizes the rates of CO2 generation from HCOOH, CH3COOH, CH3CHO, and toluene (C6H5CH3) Table 2. Rates of CO2 Generation from Formic Acid, Acetic Acid, Acetaldehyde, and Toluene over TiO2 (ST01), Bare NDoped TiO2, Fe/N-Doped TiO2, Cu/N-Doped TiO2, and Pt/N-Doped TiO2 under Visible-Light Irradiation (>410 nm)a rate of Co2 generation under visible light (>410 nm) (ppm/h) organic substance HCOOH (darkness) HCOOH CH3COOH (darkness) CH3COOH CH3CHO C7H8

N-TiO2 Pt/N-TiO2 Fe/N-TiO2

Cu/N-TiO2 TiO2

0.0

16.7

0.0

0.0

0.0

5.5 0.0

123.6 0.0

28.5 0.0

34.6 0.0

0.5 0.0

6.0 20.1 9.8

15.0 30.6 14.5

12.4 32.7 10.4

25.0 33.1 9.9

2.7 3.6 0.5

a

The rates of CO2 generation from formic acid in the dark are also indicated.

over various photocatalysts irradiated with visible light. The CH3CHO oxidation rate over bare N-doped TiO2 was approximately 6 times higher than that over TiO2.73 In contrast, the effect of N-doped doping on CH3COOH oxidation was low as compared to that on acetaldehyde oxidation, and the rate was only 2.2 times that for undoped TiO2. However, Fe, Cu, and Pt loading improved the rate of a CH3COOH oxidation over bare N-doped TiO2. Among these, Cu loading caused a significant acceleration of CH3COOH oxidation, enhancing the rate by a factor of 4.2. HCOOH was oxidized to CO2 over bare N-doped TiO2, and the rate was improved to almost the same as that of CH3CHO oxidation by loading with Cu, Fe, or Pt. With respect to long-term stability, more than 2300 μmol of CO2 was produced from CH3CHO under continuous photooxidation for 110 days over 1100 μmol of Fe/N-doped TiO2 and under visible-light irradiation.199 This would correspond to an exposure time of 6.7 years at 150 lx, assuming a linear dependence of the CO2 generation rate on the illuminance (or number of incident photons) between 0.9 mW/cm2 (this experiment) and 44 μW/cm2 (at 150 lx). After 9840

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Figure 25. Dependence of NOx concentration on the irradiation time over (a) TiO2, (b) N-doped TiO2, (c) Fe/N-doped TiO2, and (d) Pt/Ndoped TiO2 photocatalysts. NO gas (1 ppm) was pumped continuously through the reactor (373 cm3). Irradiation was conducted using various LED light sources (fwhm: 35 nm) with a light intensity of 2 mW for 10 min and then turned off to allow the concentration to return to 1 ppm. Data with and without photocatalysts are shown as solid and dotted lines, respectively. Reprinted with permission from ref 206. Copyright 2008 American Chemical Society.

while Fe/N-doped TiO2 and Pt/N-doped TiO2 decompose NOx even at 627 nm. Higashimoto et al. extensively studied the effect of cocatalyst loading for 48 metal ions with N-doped TiO2 using nitrate, sulfate, chloride, acetate, and oxide precursors.324 N-doped TiO2 was synthesized by the hydrolysis of TTIP with an NH4OH solution, followed by calcination at 673 K. Cocatalyst metal species were impregnated at 1.0 at. % and dried at 343 K. The V species interacts with N-doped TiO2 and acts as a redox mediator with oxidation states between +IV and +V for improvement of the photocatalytic activity.324 The activity of N-doped TiO2 was also reported to be significantly enhanced by the addition of metal species such as V (+IV/+V), Fe (+II/ +III), Cu (+I/+II), and Pt (+III/+IV), of which redox potentials are in the regions of ca. +0.6 to +1.0 V vs NHE as shown in Figure 26. Figure 27 shows the dependence of CO2

the long-term stability test, the CO2 generation rates for bare N-doped TiO2 and Fe-loaded N-doped TiO2 were decreased to 51% and 70% of the initial rates, respectively. This suggests that the loading of Fe not only enhanced the photooxidation rate, but also helped prevent the long-term deactivation of N-doped TiO2. The photocatalytic activities for the elimination of gaseous NOx were also measured in a flow reactor using monochromatic light emitting diode (LED) lamps that emit purple (390 nm), blue (445 nm), green (530 nm), and red (627 nm) light.206 The photocatalytic destruction of NO is significant from a practical perspective because NO is one of the most common pollutants found in exhaust emissions from automobiles. Figure 25 shows that N-doped TiO2 can decompose NOx with visible-light irradiation up to 530 nm, 9841

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Figure 26. Yields of CO2 produced by photooxidation of acetic acid under visible-light irradiation over N-doped TiO2 modified with various metal species. The metal species are, from top left to bottom right in the order corresponding to the periodic table, NaCl, MgCl, Al(NO3)3, KCl, CaCl2, VCl3 (in red), K2CrO4 (in red), KMnO4 (in red), FeCl3 (in red), CoCl2 (in red), Ni(NO3)2, CuCl2 (in red), ZnCl2, Ga(NO3)3, RbCl, SrCl2, YCl3, ZrCl2O, H2MoO4 (in red), RuCl2 (in red), RhCl3 (in red), PdCl2 (in red), AgNO3 (in red), Cd(NO3)2, SnCl2, SbCl3, TeCl4, CsCl, BaCl2, H2WO4, H2PtCl6 (in red), HAuCl4 (in red), Pb(NO3)2, BiCl3, La(ac)3, Ce(SO4)2 (in red), Pr(ac)3, Nd(NO3)3, Sm(NO3)3, EuCl3, Gd(NO3)3, TbCl3, DyCl3, HoCl3, ErCl3, Tm(ac)3, YbCl3, and Lu(ac)3. Reprinted with permission from ref 324. Copyright 2008 Elsevier.

does not induce a red-shift in photoabsorption, but interacts with N-doped TiO2 as a redox mediator to improve the photocatalytic activity. With respect to the loading of cocatalysts onto N- and cation-codoped TiO2, Ozaki et al. reported that loading of Fe and V enhances photocatalytic oxidation rates from CH3CHO to CO2 over N, Si-codoped TiO2.271,337 The codoped powder was synthesized by gaseous CH3CHO treatment of Si-doped TiO2, which was synthesized by thermal reaction (glycothermal method) of TTIP and tetraethyl orthosilicate. Loading of a Cu cocatalyst on N-doped TiO2 also induced a pronounced effect on the disinfectant properties. At 0.5 wt % Cu loading, the disinfectant properties were significantly enhanced for the six bacteria shown in Figure 19, except for Bacillus subtilis.211 The bacteria numbers declined by 6 orders of magnitude down to an almost undetectable level after 2000 lx irradiation for 24 h. Li et al. have also demonstrated high disinfection efficiency toward E. coli bacteria over PdO-loaded N-doped TiO2 under visible-light irradiation.338 6.2.3. Charge Carrier Dynamics in N-Doped TiO2 Loaded with Metallic Ions. Transient charge separation and trapping processes in N-doped TiO2 (0.25% nitrogen) photocatalysts loaded with 0.5 wt % Cu were studied using TRMC.126,226 Figure 28a shows time traces of TRMC signals. The trapping rate increased with N-doping in TiO2, which is probably due to an increase in the O vacancies. The trapping rate was markedly increased when Cu was loaded onto Ndoped TiO2, and this increase indicated a rapid electron transfer from N-doped TiO2 to the Cu cocatalyst. Figure 28a shows time traces of TRMC signals, absorption of microwave

Figure 27. Dependence of CO2 yields on photoirradiation through different low cutoff filters over (a) N-doped TiO2 and (b) VCl3/Ndoped TiO2 (1.0 at. % V). Each CO2 yield was plotted at the wavelength that is transmitted (by ca. 50%) through the low cutoff filters. Photoirradiation was performed for 3 h. Reprinted with permission from ref 324. Copyright 2008 Elsevier.

yield produced by acetic acid oxidation under visible-light irradiation over N-doped TiO2 modified with VCl3. It shows that N-doped TiO2 and VCl3/N-doped TiO2 exhibit photoresponsivity up to 540 nm, which indicates that the V species 9842

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Figure 28. (a) TRMC signal traces for TiO2, N-doped TiO2, and Cu/N-doped TiO2 films obtained before and immediately after 355 nm light excitation at Iex = 0.6 mJ/cm2. Reprinted with permission from ref 126. Copyright 2010 American Chemical Society. (b) Proposed mechanism of photocatalysis for N-doped TiO2 loaded with Cu or Fe cocatalysts excited by visible-light photons. Reprinted with permission from ref 226. Copyright 2013 American Chemical Society.

power (−ΔP/P), for TiO2, N-doped TiO2, and Cu-loaded Ndoped TiO2 before and immediately after 355 nm light excitation at Iex = 0.6 mJ/cm2. The density of electrons in each sample just after excitation was assumed to be the same among the three samples, because the strong absorption of TiO2 in the UV range is dominated by band-to-band transitions (between Ti 3d and O 2p), and therefore the chemical species induced by dopants present in the N-doped TiO2 and Cu-loaded N-doped TiO2 samples would not substantially contribute to the observed photoabsorption under UV excitation. It is also assumed that the mobility of electrons just after excitation is not affected by doping, because the crystal structure of TiO2 is preserved after the doping process. The decay rates for Ndoped TiO2 and Cu-loaded N-doped TiO2 were substantially faster than the decay rate of TiO2. This increase in decay rate for the doped samples indicates that trapping of electrons by chemical species induced by doping occurs efficiently in Ndoped TiO2 and Cu-loaded N-doped TiO2 (Figure 28b). For N-doped TiO2, the introduction of O vacancies by doping with N atoms has been argued and confirmed by several experiments and DFT calculation.122,153,227,339 Thus, it is reasonable that O vacancies act as electron trap sites in Ndoped TiO2. For Cu-loaded N-doped TiO2, Cu forms Cu−O− Cu clusters localized on the surface of N-doped TiO2, and these clusters may serve as electron acceptors. For Pt nanoparticles, fast trapping within the picosecond time region has been observed,340 and for Au nanoparticles, electron trapping occurs within 10 ns, as confirmed by TRMC measurements.341 In Cu-loaded N-doped TiO2 studied here, the TRMC signal approached zero within