Visible-Light-Active Titania Photocatalysts: The Case of N-Doped TiO₂s—Properties and Some Fundamental Issues

Abstract

This article briefly reviews some factors that have impacted heterogeneous photocatalysis with next generation TiO₂ photocatalysts, along with some issues of current debate in the fundamental understanding of the science that underpins the field. Preparative methods and some characteristics features of N-doped TiO₂ are presented and described briefly. At variance are experimental results and interpretations of X-ray photoelectron spectra (XPS) with regard to assignments of N 1s binding energies in N-doped TiO₂ systems. Relative to pristine nominally clean TiO₂ with absorption edges at 3.2 eV (anatase) and 3.0 eV (rutile), N-doped TiO₂s display red-shifted absorption edges into the visible spectral region. Several workers have surmised that the (intrinsic) band gap of TiO₂ is narrowed by coupling dopant energy states with valence band (VB) states, an inference based on DFT computations. With similar DFT computations, others concluded that red-shifted absorption edges originate from the presence of localized intragap dopant states above the upper level of the VB band. Recent analyses of absorption spectral features in the visible region for a large number of doped TiO₂ specimens, however, have suggested a common origin owing to the strong similarities of the absorption features, and this regardless of the preparative methods and the nature of the dopants. The next generation of (doped) TiO₂ photocatalysts should enhance overall process photoefficiencies (in some cases), since doped TiO₂s absorb a greater quantity of solar radiation. The fundamental science that underpins heterogeneous photocatalysis with the next generation of photocatalysts is a rich playing field ripe for further exploration.

1. Introduction

As a first-generation material, pristine TiO₂ has served well in the photoassisted (often dubiously referred to as photocatalytic) disposition of contaminants in aqueous and atmospheric ecosystems. The science that underlies heterogeneous photocatalysis has shown that the lowest energy level of the bottom of the conduction band (CB) of TiO₂ is a measure of the reduction potential of the photogenerated electrons, whereas the higher energy level of the valence band (VB) is a measure of the oxidation potential of photogenerated holes. pH-dependent flatband potentials, 𝑉_fb, of the CB and VB bands of this metal oxide determine the energy of electrons and holes at the interface. Accordingly, reductive and oxidative processes of couples with redox potentials more positive and more negative than the 𝑉_fb of CB and VB, respectively, can be driven by surface trapped electrons (e⁻) and holes (h⁺) that are poised to engage in various processes, the most important of which are photoreductions and photooxidations. An important issue regards the notion that once photogenerated, e⁻ and h⁺ tend to recombine somewhat efficiently and rapidly relative to an otherwise slow redox chemistry at the TiO₂ surface. An additional, no less important issue is that TiO₂ absorbs a relatively small fraction (ca. 3–5%) of the solar radiation reaching the Earth's surface.

Of the two important polymorphs of TiO₂, anatase begins to absorb UV light around 387 nm (band gap energy, 𝐸_bg∼3.2eV), whereas the absorption onset of rutile occurs around 413 nm (𝐸_bg∼3.0eV) increasing sharply to shorter wavelengths. Accordingly, in the late 1980s studies began to develop the next generation of titanium dioxides [1] that could absorb and make use of both UV (290–400 nm) and visible (400–700 nm) radiation to enhance process efficiencies. To achieve this feat required that the absorption edge of TiO₂ be shifted to longer wavelengths (>400 nm). One way to accomplish this necessitated photosensitizing TiO₂ either with suitable dyes that unfortunately led to their own destruction, or with suitable metal-ion dopants that sometimes act as recombination centers of e⁻ and h⁺; metal-ion dopants are often ineffective in aiding surface redox reactions, as appears to be the case when metal doping is achieved by wet impregnation [1]. However, metal-ion implantation methods have produced metal-doped TiO₂ specimens that enhance photoinduced surface processes even in the visible-light spectral region [2], where wet chemical methods failed.

First reports of anion-doped TiO₂, began to appear in the early 1990s, although Sato [3] had earlier hinted at a N-doped TiO₂. The 2001 study of Asahi and coworkers [4] on doping TiO₂ with various anions to prepare visible-light-active (VLA) N-doped TiO₂s was the catalyst needed to produce second-generation TiO₂ materials that are photoactive over the UV and much of the visible-light region. Subsequent studies reported several other visible-light-active N-doped TiO₂s, together with C-doped TiO₂ and S-doped TiO₂. The reports by Asahi et al. [4, 5] has led to a lively debate on the causes that lead the absorption onset of TiO₂ to be red-shifted to the visible region. They proposed that N-doping of TiO₂ red-shifts the absorption edge of TiO₂ and increases photoactivity by a narrowing of the TiO₂band gap. Carbon- and sulfur-doped TiO₂ displayed similar red-shifts accompanied by increased photoactivity. As we will see later, others have proposed that electronic transitions in N-doped TiO₂ systems activated by visible-light irradiation involve transitions from N 2p localized states to the CB of TiO₂. Clearly, just as first-generation TiO₂ led to lively debates on the fundamental science that underpins TiO₂-assisted photoredox surface processes following photo-activation, so are the second-generation VLA TiO₂s generating enthusiastic discussions on the root cause that shifts the absorption onset to longer wavelengths. Three recent reviews have summarized some of the facets of first (undoped) and second generation (doped) titanias. [6–8].

This review article focuses briefly (a) on some preparative methods of visible-light-active N-doped TiO₂ materials and their XPS spectroscopic features, (b) on their visible absorption spectra that display the red-shift of the absorption edges, to terminate with (c) a brief visit into the lively debate concerning band gap narrowing.

2. Nitrogen-Doped TiO₂s– Syntheses and Characterization

Asahi et al. [4] initially set three requirements to achieve visible-light-activity for TiO₂: (i) doping should produce states in the band gap of TiO₂ that absorb visible light; (ii) the CB minimum and the dopant states of doped TiO₂ should be as high as or higher than the H₂/H₂O level to ensure photoreductive activity; and (iii) the intragap states should overlap sufficiently with the band states of TiO₂ to transfer photoexcited carriers to reactive sites at the TiO₂ surface within their lifetime. Metal dopants were undesirable because they did not meet conditions (ii) and (iii) as they produce localized d states deep in the band gap of TiO₂ and tend to act more as e⁻/h⁺ recombination centers. Calculations of density of states (DOS) of substitutional doping with several nonmetals (C, N, F, P, or S) into O sites in anatase TiO₂ by the full-potential linearized augmented plane-wave (FLAPW) formalism in the framework of the local density approximation (LDA) led Asahi et al. [4, 5] to choose N since the 2p states apparently contribute to band gap narrowing through mixing with O 2p states in the valence band. On the other hand, Yates and coworkers [9] classified the methods of synthesizing N-doped TiO₂s into (i) modification of existing TiO₂ by ion bombardment, (ii) modification of existing TiO₂ in powdered form, film, and single crystal, or else modify TiN by gas phase chemical impregnation, and (iii) grow TiO_2-xN_x (crystals) from liquid or gaseous precursors.

Early on, Sato [3] had noted that calcination of Ti(OH)₄ in the presence of NH₄Cl (or aqueous NH₃) led to photosensitization of TiO₂ when exposed to visible-light radiation. The powdered samples were deduced to be NO_𝑥-doped TiO₂ with the NO_𝑥 impurity acting as the sensitizer. Noda et al. [10] reported a yellow-colored anatase TiO₂ powder obtained from aqueous hydrazine and TiCl₄ solutions, and deduced that visible-light absorption was due to the presence of oxygen vacancies V_Os.

In their 2001 seminal report, Asahi et al. [4] prepared crystalline TiO_2-xN_x films by sputtering a TiO₂ target in a N₂/Ar gas mixture followed by annealing at 550 C in a N₂ atmosphere. The yellowish TiO_2-xN_x films absorbed light below 500 nm. X-ray photoelectron N 1s spectra (XPS) of the N-doped TiO₂ showed bands at 402, 400 and 396 eV; the undoped TiO₂ film showed no 396 eV band. The latter was assigned to atomic 𝛽-N in TiO_2-xN_x, whereas the 402-eV and 400-eV bands were attributed to molecularly chemisorbed dinitrogen 𝛾-N₂ [11]. Powdered samples prepared with NH₃/Ar as the source of N followed by calcination at 550–600 C produced a TiO_2-xN_x systems that showed XPS peaks at 396 eV; these systems were photoactive toward the decomposition of methylene blue (optimal loading, ∼0.25 at.% N). The sites for photoactivity under visible-light irradiation were those when N substitutionally replaced O, that is, sites associated with atomic 𝛽-N at 396 eV [4]. Lee et al. [12] fabricated N-doped TiO₂ anatase films by MOCVD using Ti(i-PrO)₄ and N₂O at 420 C; XPS Ti 2p spectra showed N was incorporated into the TiO₂ lattice to form Ti–N bonds. Hydrolysis of Ti(SO₄)₂ with NH₃ in dry air at 400 C produced a visible-light-active anatase TiO₂ (𝜆 < 550 nm) [13]; however, XPS spectra showed only trace amounts of N, with visible-light response due to an oxygen-deficient stoichiometry.

Pale yellow, yellow, and dark green TiO_2-xN_x (𝑥=0,0.0050,0.011,0.019) powdered samples can be prepared by annealing anatase TiO₂ powder (ST-01) in a flow of NH₃ at 550, 575, and 600 C, respectively [14]. XRD patterns indicated that the samples retain the anatase structure; no TiN phase was present. The XPS peak at 396 eV confirmed substitutional N doping of O sites yielding O–Ti–NO bonds. Noticeable shifts of the absorption edge into the visible spectral region were evident for TiO_2-xN_x. The feature at 𝜆 > 550 nm was attributed to Ti³⁺ since NH₃ decomposes into N₂ and H₂ at ca. 550 C, and H₂ reduces Ti⁴⁺ under these conditions. The band gap energy remained at 3.2 eV. Mineralization of isopropanol to CO₂ with UV-light and visible-light radiations resulted in different quantum yields, suggesting that N-doping forms a narrow N 2p band above the valence band of TiO₂. Note that band gap narrowing in TiO_2-xN_x, as inferred by Asahi et al. [4], would have required identical quantum yields. On irradiating with visible light, the quantum yields decreased with increase in x of the dopant N because of the increase in oxygen vacancies, V_Os, with increase of x in TiO_2-xN_x. In this case, V_Os act as recombination centers for e⁻ and h⁺. Under UV irradiation, the quantum yields also decreased with increase in x, suggesting that the doping sites also act as recombination centers.

Nanocrystalline porous N-doped TiO₂ thin films, prepared by introducing N into anatase TiO₂ by means of DC magnetron sputtering in N₂-containing plasma [15], displays new spectral features in the spectral range 410 < 𝜆 < 535 nm at low N concentrations owing to excitation of e⁻ to unoccupied states from local states located slightly above the VB edge. N-doping had no effect on the conduction band edge. Band gap narrowing was deemed somewhat questionable by these authors [15]. Despite the intense recombination of charge carriers caused by N-doping, the new band gap states created by N-doping improved the visible-light photoresponse at the expense of some losses of the UV response.

Sakthivel and Kisch [16] prepared yellow N-doped anatase TiO₂ with various N loadings by hydrolysis of TiCl₄ with a N-containing base {aqueous NH₃, (NH₄)₂CO₃ or NH₄HCO₃} followed by calcination at 400 C. XPS spectra showed only a broad signal at ∼404eV (but no 396-eV peak) attributed to the hyponitrite (NO⁻) species that was confirmed by infrared spectral techniques. No changes in the valence band edge occurred on N-doping, despite the red-shift of the TiO₂ absorption edge to ∼250nm . Contrary to the inference by Lindgren et al. [15], however, photoelectrochemical results [16] showed a slight change in the electrochemical potentials of the CB of TiO₂ (see Figure 1) for three of the specimens. According to the authors [16], N-doping led only to a “modest band gap narrowing.”

Figure 1 

Electrochemical potentials (versus NHE) of band edges of three N-doped TiO2s: (a) TiO2, (b) TiO2–N/1, (c) TiO2–N/2, (d) TiO2–N/3. Shaded areas denote surface states; the oxygen redox potential at pH 7 is also shown. Reproduced with permission from [16]. Copyright Wiley-VCH Verlag GmbH & Co. KGaA.

High-energy ball milling of P-25 TiO₂ (ca. 80% anatase and ∼20% rutile) with various quantities of hexamethylenetetramine (HMT) at near-ambient temperatures yields yellowish N-doped rutile TiO₂ [17], which subsequent to calcination in air at 400 C gives an N-doped product that displays absorption edges at ∼400 and 550 nm and a good visible-light photoresponse toward oxidation of NO.

Ion-assisted electron-beam evaporation of rutile titania powder and N₂ yields crystalline anatase TiO_2-xN_x films with a considerable amount of substitutional N atoms (1.8 at.%) and chemisorbed molecular N₂ [18]. XPS spectra showed peaks at 402 and 396 eV assigned to molecularly chemisorbed 𝛾-N₂ and atomic 𝛽-N, respectively; Ti 2p XPS spectra revealed Ti⁴⁺2p_3/2 and Ti⁴⁺2p_1/2 in the anatase TiO₂ film indicating that the majority of titanium in the TiO_2-xN_x film consisted of Ti⁴⁺, thus confirming the results of XRD patterns. N-doping caused no changes to the anatase TiO₂ structure as attested by Raman spectroscopy.

In a simplified synthesis, Gole and coworkers [19, 20] produced TiO_2-xN_x samples at room temperature using direct nitridation of anatase TiO₂ with alkylammonium salts. The samples could be tuned to respond to wavelengths up to 𝜆∼550nm. The method first yielded metal-oxide colloids by the controlled hydrolysis of Ti(i-PrO)₄ in aqueous/isopropanol media (pH 2; HNO₃), subsequent to which treatment in excess (C₂H₅)₃N led to TiO_2-xN_x nanocolloids. XRD and HRTEM results demonstrated that the treated TiO_2-xN_x nanoparticles were predominantly anatase. Diffuse reflectance spectra (DRS) of the TiO_2-xN_x crystallites rose sharply at ∼450nm ; the corresponding DRS spectrum of nitrided TiO_2-xN_x from partially agglomerated nanoparticles rose at ∼550nm. XPS analysis with Ar⁺-ion sputtering revealed the presence of N dopants at the surface and in the sub-layers of TiO_2-xN_x agglomerates (N content, 3.6–5.1 at.%). No evidence was found for conversion of the anatase structure into rutile on N-doping for the initial TiO₂ nanocolloids and for the agglomerated gel solutions. Little if any XPS evidence of atomic 𝛽-N binding at 396 eV was found in any of the TiO_2-xN_x samples. Rather, the XPS results were consistent with nonstoichiometric surface-based Ti–O–N bonding.

In a later related detailed XPS study of a series of TiO₂-based nanometer-sized photocatalysts that included nitrogen-doped TiO₂ nanoparticles, Chen and Burda [21] noted a broad N 1s binding energy peak of the nitrogen-doped TiO₂ nanoparticles that was centered at ca. 401.3 eV (see Figure 2) and extended from 397.4 eV to 403.7 eV, a range greater than the typical binding energy of 397.2 eV in titanium nitride, TiN. These findings were attributed to the formation of an O–Ti–N structure that was suggested as the chemical entity formed during the substitutional doping process and responsible for the significant increase in photocatalytic activity of synthesized nitrogen-doped nanoparticles. Moreover, the XPS observations reported by Chen and Burda [21] were not consistent with a Ti-N entity within the TiO₂ nanocolloid lattice [22]. The shift in the N 1s binding energy for the TiO_2-xN_x nanocolloid to higher energies relative to TiN was likely due to a more positive oxidation state of nitrogen in relation to the N 1s binding energy in NaNO₃ (408 eV), in NH₃ (398.8 eV) and adsorbed NO (400–401.5 eV) [23]. Attribution of the N 1s peak at 400 eV to 𝛾-N₂ in N-doped titanias has been questioned by Chen et al. [23] in that N₂ is not chemisorbed on metal oxides such as TiO₂ at ambient temperature, preferring instead to assign the 400 eV peak to NO species consistent with the attendant heat release on the creation of these sites in the TiO₂ lattice.

Figure 2 

X-ray photoelectron spectra of various TiO2 specimens; N-Degussa is nitrided P25 TiO2 and TiO2-xNxNP denotes TiO2-xNx nanoparticles. Reproduced with permission from [21]. Copyright (2004) American Chemical Society.

Gopinath [24] recently questioned the validity of the conclusions reached by Chen and Burda [21] as not being consistent with the reported XPS observations, suggesting (among others) rather that there may have been surface contamination of the analyzed nanocolloidal N-doped TiO₂ material as a result of atmospheric degradation. Based on earlier studies of XPS spectra of NH₃ and primary alkyl/aromatic amines (398–399 eV), anionic N³⁻ in TiN (396–397 eV), adsorbed NO and NO₂ on ZnO (401 and 405 eV, resp.) and Sato and coworkers’ [25] observation of the N 1s core level at 400 eV for N-doped TiO₂ prepared by a wet method and attributed to an impurity sensitization such as NO, Gopinath [24] argued that the 401.3-eV peak in the N–TiO₂ nanocolloid reported by Chen and Burda [21] was more likely due to oxidized nitrogen as in N–O–Ti–O or O–N–Ti-O, and that the high binding energy of 401.3 eV was associated with some partial positive charge on N as noted by György and coworkers [26] in the nitridation of titanium for the 400–401 eV peak attributed to an O–N–Ti structure. Also questioned by Gopinath [24] were the origins of the O 1s core levels at binding energies of 530 and 532 eV ascribed by Chen and Burda [21] to O-Ti-O and to N-Ti-O structural units, respectively. Gopinath [24] insisted that the 532-eV peak was likely due to surface contamination by some carbon oxide, such as CO₂, and further claimed that the large level of N-doping in nanocolloids (ca. 4–8%) was rather unlikely. In an appropriate response, Burda and Gole [27] pointed out that Chen and Burda [21] used the shorthand notation N−Ti−O to mean that N, Ti, and O were bound to next neighboring atoms (Ti or O) in the TiO₂ lattice and not to mean an isolated triatomic molecular species with a nonoxidized nitrogen, and were thus rather surprised by Gopinath's misinterpretation since such notation was chemically obvious even to others [28]—also see below. In fact, as clearly stated by the groups of Gole and Burda [19, 20, 22, 23] the discussion dealt with a minimal structure fragment within a doped TiO₂ lattice (characterized by NO sites), and not some species where the nitrogen was not oxidized. As well, the apparent discrepancy noted by Gopinath [24] in the XPS data reported earlier by others (see refs 13–20 in [27]) did not take into account that the data were taken under vastly different conditions across distinct physical entities that included films and undoped nanoparticles, not to mention the anatase and rutile structures. Burda and Gole [27] further noted that Gopinath’s misinterpretation of their studies originated with the misconception that N, Ti, and O are isolated from their TiO₂ anatase environment, when in fact the XPS results reported by Chen and Burda [21] clearly demonstrated that nitrogen was partially oxidized and that the neighboring Ti was reduced relative to Ti in pure TiO₂. The discussion concerning the valencies of the nitrogen had indeed attributed the 401-eV peak to an NO site [22, 23]; however, the precise location of the NO site within the titania lattice remains a subject yet to clarify. Finally, that the doping levels are as high as 4 to 8% in the N-doped TiO₂ colloids are not unusual based on nucleophilic substitution chemistry, as pointed out previously [23].

The above reports, misinterpretations and misunderstandings, together with other XPS results described below, clearly demonstrate that XPS results do not in themselves provide unequivocal understanding of the exact nature of N-doped titania specimens, not to mention the nature of other anion- and cation-doped TiO₂s.

Nakamura and coworkers [29] produced anatase TiO_2-xN_x materials by two methods: (a) in the dry method anatase ST-01 TiO₂ was heated to 550 C in a dry NH₃ flow and (b) in the wet method the Ti(i-PrO)₄ precursor was hydrolyzed in aqueous NH₃ at 0 C followed by calcination at 400 C. Nitrogen-doped nanocrystalline TiO₂ (yellow) powders have also been synthesized by a procedure developed by Ma and coworkers [30]. Commercial anatase ST-01 TiO₂ heated at 500 C under a dry N₂ gas flow in the presence of a small quantity of carbon also yields N-doped yellow TiO₂ whose XRD patterns reveal the sample to have the anatase structure, even after annealing at 500 C. XPS spectra showed peaks at 396.2, 398.3, and 400.4-eV in the N 1s region, with the first two peaks attributed to chemically bound N-species and to O–Ti–N linkages within the crystalline TiO₂ lattice, respectively, whereas the 400.4-eV peak was ascribed to molecularly chemisorbed N₂ species.

Ion implantation of atomically clean TiO₂ (110) surfaces in single crystals with mixtures of N⁺₂ and Ar⁺ ions, followed by subsequent annealing under ultrahigh-vacuum conditions, led to incorporation of N into the TiO₂ lattice [31]. XPS spectra revealed only the N 1s feature at 396.6 eV attributed to substitutionally bound nitride nitrogen (O²⁻ ions substituted by N²⁻ anions). Contrary to expectations, N-doped crystals containing only nitride ions exhibited a shift in the photothreshold energy of 0.2 eV to higher (shorter wavelengths) rather than lower (longer wavelengths) energy compared to undoped TiO₂ (110). N-doped TiO₂ (110) rutile single crystals previously treated in the presence of an NH₃/Ar gas mixture at ca 600 C exhibited photoactivity at the lower photon energy of 2.4 eV, that is, 0.6 eV below the band gap energy of rutile TiO₂ (3.0 eV) [32]. The active dopant state of the interstitial N responsible for this effect showed a N 1s binding energy at 399.6 eV attributed to a form of nitrogen likely bound to H. This is distinctively different from the substitutional nitride state, which displays a N 1s binding energy at 396.7 eV. Apparently a co-doping effect of N and H probably enhanced the visible-light photoactivity. Doped and undoped TiO₂ (110) samples also showed an impurity XPS feature at 399.6 eV, which on UV treatment in air and/or Ar⁺-ion sputtering led to extensive depletion of the signal indicating that traces of nitrogen may have contaminated the metal-oxide surface. Such inferences by the Yates group are in stark contrast to those of Asahi et al. [4] and those of others who claimed that nitridic nitrogens that substitute O²⁻ ions in the TiO₂ lattice are the necessary dopant species for TiO₂ photoactivity in the visible-light spectral region.

In a later study, Thompson and Yates [33] re-emphasized that the exclusive XPS N 1s signal at 396.7 eV attributed to substitutional 𝛽-N in ion-implanted N-doped TiO₂ does not account for the decrease in the photothreshold of TiO₂ (110), as observed for interstitially located N–H bound species. Rather, they pointed out that the 0.2-eV increase in photothreshold energy of N-doped TiO₂ systems arose from deposition of charge in the low levels of the CB (the band-filling mechanism). Although clear XPS evidence exists for the incorporation of 𝛽-substitutional N in N-doped TiO₂, there is no strong and firm evidence of any appreciable photoactivity when these doped systems are irradiated with visible light according to Yates et al. [9], a point also raised by Frach et al. [34] who further noted no improvement in visible-light activity on N-doping TiO₂, and by Li and coworkers [35] who reported that the nature and level of visible-light activity depended on the nitriding compound employed.

With TiCl₄, ethyl acetate and NH₃ as precursors and N₂ as the carrier gas, Yates and coworkers [9] used an atmospheric pressure thermal CVD coater to grow thin films of N-doped TiO₂ on glass substrates. Only three grown samples displayed the XPS N 1s peak at 396 eV of atomic 𝛽-substituted N. XPS N 1s spectra showed no evidence of the 397-eV signal typically due to the N³⁻ ion (TiN), but did reveal weak signals at 400 and 402 eV probably arising from molecularly chemisorbed N₂, or from NH_𝑥 species located at interstitial sites (399.6 eV), or from NO_𝑥, or NH_𝑥 (400 eV), or from an oxynitride (399.3 eV) of stoichiometry equivalent to TiN_0.5O_0.5. Some of the films displayed visible spectral absorption features, but so did nominally undoped TiO₂ films indicating that N incorporation cannot be assumed on the basis of red-shifts of the absorption edge. Even though the N-doped TiO₂ specimens revealed the presence of 𝛽-N incorporation and absorption spectral features in the visible region, they were photo-inactive under visible light irradiation, while the UV photoactivity was reduced considerably compared to films grown in the absence of NH₃. Clearly, the presence of 𝛽-nitrogen alone cannot be claimed to induce visible-light activity in N-doped TiO₂ films, a point also raised by Mrowetz and coworkers [36] who prepared two different yellow-colored N-doped TiO₂ samples: sample A prepared by the method of Gole et al. [19, 20] and sample H prepared by the high-temperature nitridation of commercial anatase TiO₂ at 550 C in a NH₃/Ar gas flow. XPS spectra of sample A surface revealed intense peaks at 399.6 and 404.5 eV in the N 1s region, whereas the peak at 396 eV in the XPS spectra of sample H powders was weak and diffuse, even after Ar⁺-ion sputtering. Despite these observations, the N-doped TiO₂ materials failed to catalyze the oxidation of HCOO⁻ to CO₂^−•, and NH₃–OH to NO⁻₃ under visible-light illumination.

The solvothermal process using a TiCl₃/HMT/alcohol (methanol or ethanol) mixed solution in an autoclave at 90 C and then at 190 C yields N-doped TiO₂ nanoparticles consisting of pure anatase (pH 9, methanol), rutile (pH 9, ethanol) and brookite (pH 1, methanol) phases, which showed good visible-light absorption and visible-light activity at 𝜆 > 510 nm [37]. The two-step absorption in the reported diffuse reflectance spectra (DRS) became apparent only after calcination at 400 C. The first absorption edge is related to the band structure of nondoped TiO₂, whereas the second absorption edge around 520–535 nm is due to the formation of a N 2p band located above the O 2p valence band in the TiO_2-xN_x specimen.

Electrochemical anodization of titanium in HF/H₂SO₄ electrolyte, followed by calcination at different temperatures (range 300–600 C) in pure NH₃, yields self-organized N-doped TiO₂ nanotubes [38]. The initial amorphous nanotubes were converted to anatase, with some rutile present depending on the heat-treatment temperature. Absorption spectra of the TiO_2-xN_x nanotubes displayed a sub-band gap energy of ∼2.2eV (that may be referred to as an extrinsic band gap ) and the intrinsic band gap of anatase (∼3.2eV). The N 1s XPS spectrum showed two clear peaks, one at 400 eV, ascribed to molecularly chemisorbed dinitrogen (𝛾-N₂ state), and the other at 396 eV, attributed to the atomic 𝛽-N state.

The RF-MS deposition method produces N-substituted TiO₂{N–TiO₂(𝑋)} photoactive thin films using various N₂/Ar mixtures as the sputtering gas (𝑋=2,4,10,40) and a calcined TiO₂ plate as the source material [39]. The absorption edge of the films red-shifted to 550 nm, with the extent of the shift depending on the concentration of N(𝑋) substituted within the TiO₂ lattice (range 2.0–16.5%). The specimen with 6.0% N exhibited the highest visible-light activity in the photooxidation of isopropanol (aqueous media; irradiation 𝜆 ≥ 450 nm), and the photooxidation of H₂O (irradiation 𝜆 ≤ 550 nm). XPS and XRD measurements showed significant substitution of lattice O atoms of TiO₂ by N atoms, which Kitano and coworkers [39] suggested as playing a crucial role in the band gap narrowing of the TiO₂ thin films (range 2.58–2.25 eV relative to 3.2 eV, depending on X) enabling the visible-light photoresponse. In samples with 𝑋>4, the Ti³⁺ species formed in the N–TiO₂(𝑋) samples acted as recombination centers of e⁻ and h⁺ and thus led to a decreased photoactivity.

Joung et al. [40] used the hydrolysis of Ti(i–PrO)₄ in anhydrous ethanol containing HCl, followed by treatment of the resulting colloids in an NH₃ stream at 400 C, 500 C and 600 C at various time intervals (5 to 60 min) to prepare visible-light-active N-doped TiO₂ materials. The highest photoactivities were seen for samples prepared at 400 C and 600 C and calcination times of 5 and 10 min, whereas for samples prepared at 500 C the highest photoactivity was observed for a calcination time of 60 min. XRD patterns of the TiO₂ powders before and after N-doping showed that N-doping caused no changes to the anatase phase. The N 1s XPS spectra of the TiO_2-xN_x samples displayed a peak at 399.95 eV that was tentatively assigned to adsorbed NO or to N in Ti–O–N; however, no peak attributable to Ti–N bonding at 396 eV was observed. Band gap energies of N-doped TiO₂ inferred from absorption spectra ranged from 2.92 to 3.04 eV, for samples prepared at 400 C (5 min) and at 500 C (60 min), respectively. For samples prepared at 400 C (5 and 10 min), the active species were described as being NO, NO₂, NO₂⁻, NH₂, whereas for samples prepared at 500 C (60 min) the active species was identified as doped atomic N; the active species for samples prepared at 600 C (5 and 10 min) were inferred to be doped atomic N along with NO, NO₂, NO₂⁻, and NH₂ species.

At this time, it is important to realize that visible-light photoactivity of N-doped TiO₂ materials appears to be highly sensitive to the preparative routes, because although such materials may absorb visible light, they are nonetheless frequently inactive in photooxidations. To the extent that photogenerated charge carriers in and by themselves do not impart photoactvity and that charge carrier recombination must be muted to allow the carriers to reach the metal-oxide surface, In and coworkers [41] prepared a series of TiO_2-xN_x systems with nominal N loadings from 0.2 to 1.0 wt.% involving the sequential reaction of H₂O with a small known excess of TiCl₄ in toluene (step 1) under dry O₂-free argon, followed by the stoichiometric reaction of the remaining TiCl₄ with a standard solution of NH₃ in dioxane (step 2). The resulting species were heat-treated in air at 400, 500, and 600 C. Tests of the TiO_2-xN_x specimens for visible-light photoactivity revealed (i) that calcination at 400 C yields a solid with pronounced absorption in the visible spectral region but yet no visible-light photoactivity, (ii) that 500 C calcination produces an effective (yellow) visible-light-active sample, and (iii) that the heat treatment at 600 C results in an inactive white material.

In two extensive reports, Belver and coworkers [42, 43] prepared and characterized a series of nanosized N-doped TiO₂-based materials by a reverse micelle microemulsion method using a Ti(i-PrO)₄ precursor and three N sources, for example, 2-methoxyethylamine, N,N,N',N'-tetramethyl-ethylenediamine and 1,2-phenylenediamine, to produce Ti(IV) complexes in dry isopropanol under a N₂ atmosphere. Dropwise addition of the solution to the inverse microemulsion, that contained H₂O dispersed in n-heptane and Triton X-100 as the surfactant with hexanol as the cosurfactant, produced materials that were subsequently calcined at 200 C and then at 450 C. A XANES examination confirmed the anatase structure of TiO_2-xN_x. XANES spectra also showed a lack of correlation between the number of oxygen vacancies (V_Os) and the N content in the samples. Above a certain limit, the association of point defects, such as V_Os and/or the presence of nonpoint extended defects, was detrimental to photoactivity. The distribution of defects and the nature of defects present in the N-doped samples were examined in a joint XANES/EXAFS investigation, which revealed that the distribution of defects was not simply related to the oxygen vacancies V_Os, since strong differences existed in the first cation-cation coordination shell that inferred the possible presence of nonpoint defects. The joint study also confirmed the point defects to be the V_Os; no interstitial defects were seen and the O/Ti atom ratio was <2. Evidently, there exists an optimal O/Ti ratio for maximum photoactivity achieved when oxygen vacancies are located in the bulk lattice that act as electron traps subsequent to visible-light photoactivation of the doped specimens. No apparent effect due to N-doping on the valence band edge was detected. Some localized states were, however, detected at the bottom of the conduction band with broad absorption around 500 nm. Results from DRIFT spectra indicated the presence of several anion-related impurities of a substitutional (Nⁿ⁻) and interstitial (NO⁺) nature. Although these species contributed to the absorption features, the authors [42, 43] found no clear correlation between any of these species and photoactivity. In fact, photoactivity best correlated with an optimal number of oxygen vacancies, above and below which a decrease of steady-state reaction rates occurred.

N-doped titania samples with high visible-light activity have been synthesized using a layered titania/isostearate nanocomposite from a sol-gel technique [44], with N-doping achieved by treating the composite with aqueous NH₃ followed by calcination either in an O₂/N₂ mixture or in pure N₂ at various temperatures (300, 350, 400, 450, and 500 C). The vivid yellow samples absorbed visible light in the 380–500 nm spectral region, and correlated with the extent of doped-N content in the samples. However, the visible-light photoactivity did not correlate with N content. The highest visible-light photoactivity was observed for the 400-C calcined sample. The quantity of N content in the sample decreased on increasing the calcination temperature, which was particularly significant between 300 and 350 C, with the decrease being more important for the sample calcined in O₂/N₂ than for the sample calcined in pure N₂.

Thus far we have witnessed that visible-light-active (VLA) TiO₂ systems doped with nitrogen possess, in most cases, good attributes toward the photooxidation of organic and inorganic (e.g., NO_𝑥) substrates. Of particular interest have been the materials doped with N whose preparative methods have been varied but otherwise simple in a large number of cases. Most important, however, although all the N-doped TiO₂ materials displayed absorption features and absorption edges red-shifted to the visible spectral region (at least to 550 nm), photoactivity of these systems under visible-light irradiation has not always correlated with these absorption features. In a recent study, Tachikawa and coworkers [45] addressed some of these issues and described mechanisms of the photoactivity of VLA TiO₂ specimens. Using solid-state NMR measurements combined with transient diffuse reflectance (TDR) spectroscopy, these workers provided direct evidence of the degradation of ethylene glycol with VLA-active TiO_2-xN_x under visible-light irradiation. It appears that photoassisted oxidations of organic compounds on the surface of TiO_2-xN_x proceed by surface intermediates generated from oxygen reduction (the superoxide radical anion, O₂^−•) or otherwise water oxidation (the •OH radical) and not by direct reaction with h⁺ that may be trapped at the N-induced mid-gap level (see Figure 3). Based on their experimental results, it is rather evident that both an appropriate lower-energy photo-threshold for visible-light absorption and high carrier mobilities are needed for advanced visible light-active TiO₂-based photocatalysts.

(a)

(b)

(c)

Figure 3 

Cartoons illustrating possible photoassisted processes of a substrate adsorbed on the surfaces of pure, N-, S-, and C-doped TiO2 nanoparticles. Reproduced with permission from [45]. Copyright (2007) American Chemical Society.

3. DFT Computations of Band Gap Energies in N-Doped TiO₂

Different preparative methods and strategies of N-doping TiO₂ as described above can lead to anion-doped metal-oxide materials with entirely different properties—these are the next generation TiO₂ photocatalysts. The key question that keeps recurring in the literature is the chemical nature and the location of the species that lead(s) the absorption edge of TiO₂ to be red-shifted and consequently to the visible-light activity of doped TiO₂. Species such as NO_𝑥, NH_𝑥, and N²⁻ have been proposed, not to mention NO⁻, NO₂⁻ and NO₃⁻ species that have been confirmed experimentally. Another key question regards the electronic structure(s) of the (anion)-doped materials and their fate when subjected to UV- and/or visible-light irradiation. Although, these questions have been addressed in several interesting computational studies, a consensual acceptance of the results has yet to be reached. Significant advances can be made in clarifying key questions by a combination of experimental and computational studies within the same laboratory or among collaborating laboratories.

Densities of states in anatase TiO₂ for substitutional doping of oxygen in the lattice by C, N, F, P, and S dopants were first reported by Asahi and coworkers [4, 46] using the full-potential linearized augmented plane-wave (FLAPW) formalism in the framework of the local density approximation (LDA). The calculations were carried out without geometry optimization for the five anion-dopings because the resulting atomic forces were apparently too large to obtain reasonable positions in the unit cell (eight TiO₂ units per cell). Three types of doping for N were considered in the computations: (a) substitutional N doping (N_S), (b) interstitial N doping (N_I), and (c) both types of doping (N_S+I) in the anatase TiO₂ architecture. Optimization of the N positions in the cell inferred molecularly bonding states (NO and N₂) for cases (b) and (c) with bond lengths (improved by the generalized gradient approximation GGA) in fair agreement with accepted values: 1.20 A versus 1.15 A (N–O) and 1.16 A versus 1.10 A (N₂). According to the authors [4], substitutional doping of N was the most effective because its 2p states contribute to band gap narrowing by mixing with O 2p states of the valence band. Calculated imaginary parts of the dielectric functions of TiO_2-xN_x showed a shift of the absorption edge to lower energy by N doping, with dominant transitions from N2p_𝜋→Tid_𝑥𝑦 rather than from O2p_𝜋 as in TiO₂. However, the calculated band gap energies were considerably underestimated relative to the experimental value (e.g., 𝐸_bg = 2.0 eV versus 3.2 eV for anatase) attributed, in part, to the well-known shortcomings of the LDA approach. The underestimated band gap was corrected using a scissors operator (a sort of fudge factor) that displaces the empty and occupied bands relative to each other by a rigid shift of 1.14 eV to bring the minimum band gap in line with experiment for the band gap of anatase TiO₂ (corrected 𝐸_bg = 3.14 eV). Accordingly, the band gaps of N-doped TiO_2-xN_x systems were also adjusted by the factor 1.14 eV [4] on the assumption that the underestimated energy of the band gap in the LDA approach is not affected by N-doping because long-range screening properties in TiO_2-xN_x were likely similar to those in TiO₂.

The picture as to the exact cause of the red-shift of the absorption edge of TiO₂ in various N-doped TiO₂ ( anatase ) powders becomes somewhat confused with the report from Yates group [30] that the absorption edge of a N-doped TiO₂ rutile single crystal shifts to higher energy by 0.20 eV. Spin-polarized density functional theory (DFT) calculations within the GGA approximation by Di Valentin and coworkers [47] showed that whereas in anatase the localized N 2p states, located just above the O 2p states of the valence band, red-shift the absorption edge to lower energy, in rutile the tendency to red-shift the absorption edge is offset by a concomitant contraction of the O 2p valence band resulting in an overall increase in the optical transition energy by ca. 0.08 eV. Compared to anatase, rutile has a wider (W) O 2p band due to both its higher density and its different structure (see Figure 4). In this work [47], N-doping was modeled by replacing 1, 2, or 3 oxygen atoms in a 96-atom anatase supercell and 1 or 2 oxygen atoms in a 72-atom rutile supercell giving a stoichiometry comparable to that used in experiments for TiO_2-xN_x∶0.031<𝑥<0.094 for anatase and 0.042<𝑥<0.084 for rutile. Note that Asahi et al. [4, 46] used a higher level of N-doping, which yielded a stoichiometry of TiO_1.875N_0.125. Inclusion of more N atoms in the same supercell yielded more accurate results than using smaller supercells. Nonetheless, calculated band gaps [47] were still underestimated at 2.19 eV and 1.81 eV (at the Γ position) versus the experimental 3.2 eV and 3.0 eV, respectively, for pure undoped anatase and rutile TiO₂, again because of the shortcomings of the DFT method. Analysis of the electronic energy levels (see Figure 4) shows that N-doping causes no shift of the position of both top and bottom of the O 2p VB band and of the CB band relative to pure undoped anatase TiO₂, in significant contrast with the conclusions of Asahi and coworkers [4, 46] with respect to the undoped material. Structural variations in rutile TiO₂ subsequent to substitution of one O atom with N in the 72-atom supercell appear significant in rutile relative to anatase in which the variations were inconsequential. In any case, the N impurity states can act as deep electron traps in TiO_2-xN_x systems (see Figure 4). Di Valentin et al. [47] also considered the contribution of oxygen vacancies (V_Os), estimated experimentally at 0.75 to 1.18 eV below the conduction band 𝐸_𝐶 (DFT calculations placed them at 0.3 eV below 𝐸_𝐶), to the overall visible-light photoactivity of N-doped systems when V_Os trap electrons to produce 𝐹-type color centers. The simultaneous presence of N dopants and V_Os can also lead to charge transfer states (reaction 1) that can further contribute to the visible-light photoactivity, V_O(𝐹)+N⟶V_O^•(𝐹⁺)+N⁻. (1)

(a)

(b)

Figure 4 

Schematic representation of the energy band structure of pure and N-doped anatase and rutile (energies not to scale). Note the modest shift of 0.03 eV in 𝐸𝐶 of the conduction band to higher energies and the contraction energy 𝐸4 in the N-doped rutile TiO2. 𝐸3 represents the energy of the N dopant levels above the valence band. Higher levels of doping, for example, three N atoms per supercell, cause a small shift of ∼0.05eV to higher energies for 𝐸𝐶 which is overcompensated by the presence of N-derived states just above VB, so that the excitation energy 𝐸2 from these states to the conduction band is reduced by <0.1 eV compared to pure anatase (𝑊 denotes the width of the valence bands). Adapted from results reported in [47].

In a later study that combined experiments (EPR, XPS) and DFT calculations performed using the plane-wave-pseudopotential approach, together with the Perdew-Burke-Ernzerhof (PBE) exchange correlation functional and ultrasoft pseudo-potentials, Di Valentin and coworkers [48] characterized the paramagnetic species present in N-doped anatase TiO₂ powders obtained by sol-gel synthesis, and unraveled some of the mechanistic details of the visible-light activity of N-doped TiO₂ as to whether photoactivity is due to NO_𝑥 or NH_𝑥 species or simply to substitutional N-doping. The TiO_2-xN_x sample was obtained by hydrolysis of Ti(i-PrO)₄ in isopropanol media in the presence of aqueous NH₄Cl as the N source, followed by calcinations of the N-doped specimen at ca. 500 C for 2 hrs. XPS N 1s spectra showed only a peak at ca. 400 eV attributed to interstitial N (without precluding others); no 396 eV peak was seen that might have originated from substitutional N-doping. EPR spectra indicated the presence of two different paramagnetic species that were attributed to substitutional and interstitial N species. These results led them to consider two structurally different locations for the N dopant in their DFT calculations: substitutional N(N_S) and interstitial N(N_I) atoms in the TiO₂ anatase matrix. In the substitutional model, the N that replaces O in the TiO₂ lattice was taken to be bonded to three Ti atoms in the 96-atom supercell (a) so that the paramagnetic species is formally N²⁻, whereas in the interstitial model N is added to the 96-atom supercell bonded to one or more O and thus is in a positive oxidation state as in NO⁻, NO₂⁻ and/or NO₃⁻ (b)(see Figure 5). Figure 6 illustrates the DFT band structure of the N-doped TiO₂ and reports the calculated albeit still underestimated band gap energy of titania. The two bonding 𝜋 states in Figure 6(b) for NO lie deep below the O 2p band but the two 𝜋^∗ still occupied states lie above (0.73 eV) the O 2p band and lie at higher energy than the 2p states of substitutional N (0.14 eV). A more interesting consequence of the picture of Figure 6 is that the two electrons left in the formation of an oxygen vacancy, which typically would form two Ti³⁺ color centers, may also be trapped by substitutional N_S and interstitial N_I yielding the azide species (N³⁻ denoted as N_S⁻) and the hyponitrite species (NO⁻). Another significant result deriving from the DFT calculations of Di Valentin and coworkers [48] is that N-doping leads to a substantively reduced energy of formation of V_Os (4.3 eV to 0.6 eV for anatase) with important consequences in the generation of 𝐹-type and Ti³⁺ color centers. Experimentally, which of the two types of N dopants predominates in the N-doped TiO₂ will depend on the experimental conditions, for example, nitrogen and oxygen concentrations, and calcination temperatures. What Figure 6 also implies is that moving from substitutional N to interstitial N is in fact an oxidative step, which according to DFT estimates is ca. 0.8 eV exothermic. Thus, there is a cost for the reverse, that is, interstitial N-doping is preferred when TiO_2-xN_x systems are prepared in excess nitrogen and oxygen, whereas high-temperature calcination of N-doped systems, commonly done in most experiments, both substitutional N and formation of oxygen vacancies V_Os are likely the preferred occurrences.

(a)

(b)

Figure 20 

(a)

(b)

Figure 5 

Electronic band structure for (a) substitutional and (b) interstitial N-doped anatase TiO2 as given by PBE calculations at a low-symmetry 𝑘-point. In the former, the site contains the paramagnetic N2− species making the site electrically neutral (replaced O2−), whereas in the latter the site is occupied by the radical NO. The estimated band gap energy is also indicated. Reproduced with permission from [48]. Copyright (2005) American Chemical Society.

The question on the blue-shift of the absorption edge of N-doped TiO₂ rutile single crystals contrasting the red-shifts in N-doped TiO₂ powders was also taken up in a DFT study by Yang and coworkers [49] using the plane-wave method. Results confirmed those of Di Valentin et al. [47] that some N 2p states lay above the O 2p valence band when N substitutes O in the TiO₂ lattice and when N is located at interstitial positions. However, no band gap narrowing was predicted by the calculations of Yang et al. [49]. When N substitutes Ti atoms in the rutile lattice, a band gap narrowing in the rutile crystal is apparently possible because the N dopant introduces some energy states (the N 2p states) into the bottom of the conduction band [49]. The authors rationalized this inference by the fact that removal of electrons from the supercell on replacing one Ti with a N atom leads to a reduction of the Coulomb repulsion and thus to a shift of the energy band edges, that is, the band gap energy is reduced by ca. 0.25 eV relative to the undoped rutile supercell whose estimated band gap energy was calculated to be 1.88 eV. This is reminescent of the assertion by Asahi et al. [4, 46] that N-doping TiO₂ causes band gap narrowing because the N 2p states mix with the O 2p states in the valence band, thereby widening the valence band and shrinking the band gap.

Substitutional N-doping can be stabilized by the presence of oxygen vacancies (N_S−O+V_O) under oxygen-poor experimental conditions, whereas under oxygen-rich conditions interstitial N species (N_I) become favored. The N-doped sample prepared by the sol-gel process of stoichiometry near TiO_1.907N_0.062, examined earlier by Di Valentin and coworkers [48], was re-examined more closely by Livraghi et al. [50] in a series of experiments and DFT calculations aimed at determining the fate of the doped specimen when irradiated at different wavelengths in the presence of adsorbates. The experiments asserted that N species were responsible for the absorption of visible-light radiation, and consequently for the visible-light activity, as well as for the photoinduced electron transfer from the solid to surface electron scavengers (adsorbates) such as molecular O₂. The UV-visible diffuse reflectance spectrum of the sample (see Figure 7) is nearly identical to the many reported DRS spectra of N-doped TiO₂ specimens prepared in a variety of ways. However, as we will see below, this spectral behavior is identical to the spectral behavior of so many other doped TiO₂ samples that have been doped with different types of dopants (e.g., transition metal ions, C, S, and others) and synthesized by different methods. Previous EPR work (Di Valentin et al. [48]) had identified two distinct N-related paramagnetic species in N-doped TiO₂, one of which was the molecular NO radical [51] located in closed pores within the crystals and thus had no influence on the electronic structure of the solid. No evidence of NH_𝑥-type paramagnetic species was found as had been reported by Yates’ group [31, 32]. Whatever the nature of the paramagnetic species, it was stable to washing and to calcination in air up to ca. 500 C. This was taken to mean that the nitrogen radical species, identified as N_b^• in Figure 8 interact strongly with the TiO₂ lattice. DFT calculations carried out on the 96-atom supercell involved two interstitial nitrogens (N_I^•) or two substitutional nitrogen (N_S^•) paramagnetic species plus an oxygen vacancy (V_O) located far away from these N-centers to avoid direct defect/impurity interactions. Note that removal of an O atom from the TiO₂ lattice leaves behind two electrons to form the neutral𝐹center (V_O in the Kroger-Vink notation), or they may be trapped by neighboring Ti⁴⁺ species to give two Ti³⁺ color centers, which Henderson et al. [52] positioned at 0.8 eV below the bottom of the conduction band. Other studies indicated otherwise, although there are electron traps around this energy. Ti³⁺ color centers certainly do exist as demonstrated by EPR measurements [53, 54]. One of the consequences of the high number of V_Os under oxygen-poor conditions in N-doped TiO₂ is the partial quenching of N_b^• paramagnetic species, which are transformed [48] into N_b⁻ through reduction by Ti³⁺ color centers (see Figure 8). The energetically favored reduction of N_b^• species may be the cause for the small energy cost in the formation of V_Os in N-doped TiO₂ (see above). The EPR peaks attributed to N_b^• centers disappeared on reduction of the sample (reaction (2)) whether by annealing in vacuo or by other means to then reappear on re-oxidation. Thus, the N-doped TiO₂ specimen (at least the one prepared N_b^•+Ti³⁺⟶N_b⁻+Ti⁴⁺ (2)by the sol-gel method) contained paramagnetic N related species in the bulk lattice (N_b^•) and a number of diamagnetic species (N_b⁻), the presence of which depended on the oxygen content in the metal-oxide sample. The EPR signal due to N_b^• increased on irradiating the doped sample at 437 nm in O₂(p_O2=5kPa). A new EPR line appeared that was attributed to O₂^−• radical anions (reactions (3) and (4)). These anions are apparently stabilized on two different surface Ti⁴⁺ species, which Livraghi et al. [50] claimed could be typical of N-doped TiO₂. Figure 9 illustrates the process embodied in the formation of the superoxide radical anions.

Figure 7 

UV-visible diffuse reflectance spectra of undoped and N-doped TiO2. Reproduced with permission from [50]. Copyright (2006) American Chemical Society.

Figure 8 

Electronic band structure changes from interactions between Nb• (NS• or NI•) and Ti3+ color centers. Reproduced with permission from [50]. Copyright (2006) American Chemical Society.

Figure 9 

Sketch of the proposed mechanism for the processes induced by vis-light irradiation of the N-doped sample in O2 atmosphere. Reproduced with permission from [50]. Copyright (2006) American Chemical Society.

N_b⁻+h𝜈⟶N_b^•+e⁻_surf (3)

e⁻_surf+O₂⟶O₂^−•. (4)

So far all the DFT-based calculations have failed to calculate experimentally commensurate band gap energies for undoped anatase and rutile TiO₂, and consequently for all anion-doped TiO₂ systems unless, as some have done, one resorts to the scissor operator. In a comprehensive theoretical investigation of substitutional anion doping in TiO₂, Wang and Lewis [55] explored the electronic properties of C-, N-, and S-doped TiO₂ materials using an ab initio tight-binding method (FIREBALL) based on density functional theory and a nonlocal pseudopotential scheme. The method uses confined atomic-like orbitals as the basis set, which led to a calculated direct band gap from Γ to Γ of 3.05 eV for rutile that accords with the experimental band gap of 3.06 eV [56]. The local density approximation (LDA) approach generally underestimates the experimental band gap for insulators and semiconductors; ab initio plane-wave calculations for TiO₂ also place the band gap at around 2.0 eV [57]. In their theoretical treatment, Wang and Lewis [55] compensated for the underestimation by the LDA approach. For anatase, the direct band gap from Γ to Γ of 3.26 eV accords with the experimentally observed 3.20 eV [58]—see Table 1. The upper valence bands are composed mainly of O 2p states, whereas the lower conduction bands consist primarily of unoccupied Ti 3d states.

Table 1 

Comparison of calculated band gaps of undoped rutile and anatase TiO₂ with experimental and calculated values reported by others (𝐸_BG is the direct band gap). From Wang and Lewis [55].

In the computations of Wang and Lewis [55], two O atoms were replaced by one N because of the odd number of electrons in the N atom to yield an effective N-doping level of 0.52% in the low concentration case. The CB minimum remained unchanged. However, new states were introduced by N-doping just above the valence band edge of bulk TiO₂, as well as states that penetrated into the upper valence band of the bulk states. No significant energy shift (<0.05 eV) in the VB edge was seen in N-doped rutile TiO₂ at the high N content of 5.2%, contrary to the significant shift observed for the low N-doping level of 0.52% which gave a narrowed band gap of 2.55 eV. Density-of states (DOS) calculations thus inferred that low doping N levels greatly improve the visible-light photoactivity. No significant overlap occurred between the N 2p states and the O 2p states for the 0.52% N-doping level. Moreover, at the low N concentration the valence band edge was more localized compared to the high N-doping level in N-doped rutile TiO₂. Unlike N-doped rutile TiO₂, however, identical shifts of ca. 0.44 eV were obtained in N-doped anatase TiO₂ at both high and low doping levels resulting in a band gap of ca. 2.82 eV (440 nm). DOS calculations indicated significant overlap between the N 2p states and O 2p states for the 5.2% N-doping. By contrast, at the low N-doping level of 0.52% N, the states introduced by N were distinct, highly localized on the single dopant state, and there was no significant overlap. A result of this is that the high N-doping level in N-doped anatase TiO₂ would lead to greater visible-light photoactivity.

The brief discussion above points to a lack of consensus on whether or not there is band gap narrowing in doped TiO₂ materials based on DFT calculations. Some of the theoretical studies have deduced from these calculations that there is a rigid shift of the valence band edge to higher energies, thus narrowing the intrinsic band gap of TiO₂ as a consequence of doping. The discrepancies cannot be attributed to a simple semantic problem. Experimentally, anion-doping and cation-doping of TiO₂ do red-shift the absorption edge of TiO₂ in the doped samples, thus yielding potentially visible-light photoactive materials that might prove useful in several important applications of surface processes occurring on the TiO₂ surface. In the past, we have referred to the longest visible-light wavelength at which photoactivity is seen as the red limit of photocatalysis. We wish to emphasize, however, that what does change is the lowest photo-threshold (i.e., extrinsic absorption edge) of the actinic light that can activate TiO₂ by introducing dopants into the metal-oxide lattice. When a N-doped TiO₂ system is photoactivated by visible light absorption, it also generates electrons and holes, although the latter carriers will have a decreased oxidative power (lower redox potential) vis-a-vis holes photogenerated from pristine TiO₂ (see, e.g., Figure 3). The intrinsic absorption edge of the metal oxide itself is not changed by the doping. In other words, the valence and conduction bands are not affected by the doping, at least at low doping levels and weak interactions. But if they were to be affected through strong coupling interactions between the dopant states and the O 2p states of the VB of TiO₂, as some have surmised by DFT calculations, then we must face the inescapable conclusion that the material is no longer TiO₂, but is some titanium oxynitride material (for N-doped TiO₂) that possesses entirely different properties, not least of which are new electronic structures of their respective valence and conduction bands. In this regard, a recent study by Kim and coworkers [61] has demonstrated that a solid solution prepared from ZnO and ZnS, which could be referred to as S-doped ZnO(ZnO_xS_1-x) displays an absorption edge in the visible spectral region at 2.4 eV vis-à-vis the absorption edges of both ZnO (3.1 eV) and ZnS (3.5 eV). The XRD patterns indicated a new material, namely a zincoxysulfide phase, whose band gap is 2.4 eV, much narrower than either of the band gaps of the two initial substrates owing to strong coupling of the S 2p states with the O 2p states in the valence band of the new material. Note that in all the doped TiO₂ systems, XRD patterns showed that the doped TiO₂ retained the anatase (or rutile) structure; no new phase or new material could be ascertained.

4. Optical Properties of N-Doped TiO₂

An examination of the optical properties of doped TiO₂ provides evidence based on the photobleaching phenomenon that the absorption bands observed in the visible spectral region for (any) doped TiO₂ material can be bleached. That is, the species that give rise to or that are responsible for the absorption bands in the visible spectral region of doped TiO₂ can be destroyed by irradiating with visible-light wavelengths corresponding to the absorption bands in the visible spectral region, or by a heat treatment. Under these conditions, it is important to recognize that neither the intrinsic VB nor the intrinsic CB can be destroyed.

Optical properties of doped TiO₂ specimens can be discussed in terms of difference diffuse reflectance spectra (ΔDRS) calculated from the DRS of various doped TiO₂ samples absorbing in the visible region [𝜌_abs(h𝜈)] and undoped TiO₂ samples that do not absorb in the visible spectral region [𝜌_non-abs(h𝜈)]. The latter is typically the DRS of a nominally clean TiO₂ sample or the DRS of the doped sample prior to any treatment that might induce visible-light absorption [62]. Where the transmittance spectrum of a thick sample is 0, the change in reflectance Δ𝜌(h𝜈), that is, [𝜌_non-abs(h𝜈)−𝜌_abs(h𝜈)] is then identical to the change in absorbance ΔA(h𝜈). Moreover, where optical properties of doped specimens are characterized by absorption spectra A(h𝜈), the difference absorption spectra ΔA(h𝜈) can be calculated in a manner similar to the difference DRS spectra, Δ𝜌(h𝜈). Usage of difference diffuse reflectance and/or difference absorption spectra provides a means for numerical analysis of the optical characteristics of the samples. The analysis typically involves (i) the characterization of each absorption spectrum by the position of the spectral maximum (h𝜈_max), the intensity of this maximum (Δ𝜌_max,ΔA_max), and the spectral bandwidth at half-maximum amplitude, (ii) the comparison of the spectra of different samples after normalization by the Δ𝜌_max/ΔA_max factor, and (iii) the analysis of the shape of the absorption spectra to display the spectra as a sum of individual absorption bands.

Resulting absorption spectra of various nondoped TiO₂ specimens displaying maximal absorption around 3.0 eV and shown in Figure 10 are those of (i) mechanochemically activated N-doped TiO₂ [17], (ii) N-doped oxygen-deficient TiO₂ [13], (iii) N,F-codoped TiO₂ sample prepared by a spray pyrolytic method [63], (iv) N-doped anatase TiO₂ specimen prepared by a solvothermal process [37], (v) N-doped rutile TiO₂ sample also prepared by a solvothermal process [37], (vi) yellow N-doped TiO₂ specimen synthesized in short time at ambient temperatures using a nanoscale exclusive direct nitridation of TiO₂ nanocolloids with alkyl ammonium compounds [19, 20], (vii, viii) N-doped TiO₂ samples prepared by evaporation of the sol-gel with N-doping carried out under a stream of ammonia gas at different temperatures [40], and (ix) N-doped TiO₂ prepared via sol-gel by mixing a solution of titanium (IV) isopropoxide in isopropyl alcohol in the presence of an NH₄Cl solution [50].

Figure 10 

Absorption spectra of various anion-doped TiO2 specimens before averaging (see text) and the diffuse reflectance spectrum (DRS) of Degussa P25 TiO2.

Temperature and time of calcination are frequently reported as factors that affect the shape of the absorption spectra. For instance, an increase of temperature from 247 C to 347 C for 2 hrs [64], or prolonging the time of calcination from 5 min to 30 min at 400 C [40], decreased the absorption in the range h𝜈 < 2.0 eV, such that the absorption spectra then adopted a narrower shape. As an example, the broad absorption spectrum of the orange N-doped TiO₂ specimen, prepared in nearly the same manner as the yellow N-doped TiO₂ sample by Gole et al. [19, 20], showed a shoulder on the high-energy side at ca. 2.5 eV (see Figure 12, curve 3 below). Surprisingly, Figure 10 demonstrates a strong similarity between selected spectra in the visible region at energies h𝜈 < 3.0 eV, and noticeable differences in the range of intrinsic absorption at h𝜈 > 3.0 eV. Such differences are not surprising since the samples differed in phase composition (see, e.g., [37]) and sample thickness. Moreover, some workers often choose any available sample, for example, Degussa P25 TiO₂, as the nonabsorbing specimen in the visible spectral region, rather than a specimen prepared in an otherwise identical fashion as the doped samples. Spectral similarities in Figure 10 afford averaging the spectra to obtain the mean spectrum of visible-light-active N-doped TiO₂ samples illustrated in Figure 11 (curve 1; standard error of the mean spectrum was less than 2.3% at h𝜈 < 3.0 eV). Figure 11 also displays the absorption spectrum of the N-doped TiO₂ rutile crystal prepared by an NH₃ treatment at 597 C (curve 2) [21] and the absorption spectrum of the anatase crystal (curve 2a) reported by Sekiya and coworkers [65, 66].

Figure 11 

Average absorption spectrum of visible-light-active N-doped TiO2 specimens (curve 1); difference absorption spectra of N-doped rutile crystal (curve 2) and of the color centers in the yellow anatase TiO2 crystal (curve 2a). Curve 3 (solid squares) depicts the difference between curves 2 and 2a; curve 3a (line) is the Gaussian fit of curve 3 (see text for more details).

Figure 12 

Average absorption spectra (Δ𝜌) of various titania systems. See text for details of the origins of these spectra. Reproduced with permission from [62]. Copyright (2006) American Chemical Society.

The difference in spectra 2 and 2a in the region h𝜈 > 3.0 eV originates from the difference in the phase composition of the TiO₂, whereas the difference between curves 2a and 2 at h𝜈 < 3.0 eV exhibits a single absorption band with h𝜈_max=2.6eV, half-width of 0.35 eV and a near-Gaussian shape, that is, the band is very similar to the 2.55-eV AB2 band reported earlier by Kuznetsov and Serpone [62]. Sekiya and coworkers [65, 66] attributed the 3.0-eV band in the spectrum of anatase to oxygen vacancies that can trap electrons to yield 𝐹-type centers [66]. It should be emphasized that the AB1 absorption band with h𝜈_max at 3.0 eV can be obtained in a variety of ways: (i) by annealing the as-grown crystals under an oxygen atmosphere at T > 374 C [65, 66], and (ii) by annealing colorless crystals by subjecting them first to a reductive H₂ atmosphere at 647 C and then to an O₂ atmosphere at 497 C [66]. In both cases, only prolonged (ca. 60 hrs) annealing of the crystals at 797 C in an O₂ atmosphere (not inert or nitrogen atmosphere) transformed the yellow crystals displaying a band at 3.0 eV to a colorless state [66]. The origin of oxygen vacancies is associated with an uncontrolled reduction of TiO₂ assisted by impurities introduced into the crystal during its growth [65]. Absorption of N-doped rutile crystals in the visible spectral region is only partially associated with reduction of the TiO₂ bulk lattice [32]. According to Yates and coworkers [31–33] the red-shift in the photochemical threshold from 3.0 eV to 2.4 eV originates from the N dopant located in an interstitial site and probably bonded to hydrogen. This suggestion contrasts the interpretation given by Sekiya and coworkers for the spectral features in the visible region [65, 66], by Di Valentin et al. from their EPR study [48], and with our assignments of absorption bands in the visible spectral region of various titania/polymer compositions [62].

The remarkable similarities in the absorption spectra of doped TiO₂ systems are displayed in Figure 12 as averaged spectra. Curve 1 depicts the average spectrum obtained from the absorption spectra (see [62] for details) of (i) Cr-implanted TiO₂, (ii) Ce-doped TiO₂, (iii) mechanochemically activated N-doped TiO₂, (iv) N-doped oxygen-deficient TiO₂, and (v) from Sr_0.95La_0.05TiO_3+𝛿 treated with HNO₃ acid. Curve 2 represents the average spectrum obtained from the difference DRS’s (absorption spectra) of various anion-doped titania specimens: (i) N,F-codoped TiO₂, (ii) N-doped anatase TiO₂, (iii) N-doped rutile TiO₂, and (iv) yellow nitrided TiO_2-xN_x nanocolloids. Curve 3 illustrates the averaged spectra of cation-doped TiO₂, namely (i) Fe-doped TiO₂ nano-powders prepared by oxidative pyrolysis of organometallic precursors in an induction thermal plasma reactor, (ii) zinc-ferritedoped titania (TiO₂/ZnFe₂O₄) synthesized by sol-gel methods followed by calcinations at various temperatures, and (iii) the orange N-doped TiO₂ sample prepared by a procedure otherwise identical to that of yellow N-doped TiO₂ but with the former consisting of partially agglomerated nanocolloids (i.e., larger TiO_2-xN_x clusters). Note the remarkable overlap of the relatively narrow average spectra 1 and 2 in Figure 12, which illustrates convincingly the independence of the spectra on the method of photocatalyst preparation. Such overlap features can only result from electronic and spectral features of color centers/defects in TiO₂. Comparison of the broader mean spectrum 3 in Figure 12 with the narrower spectra 1 and 2 shows that broadening of the absorption spectrum of TiO₂ photocatalysts originates from the long-wavelength AB2 absorption band at 2.55 eV [62].

The remarkable coincidence of the absorption bands in the visible spectra of reduced TiO₂ with those of visible-light-active TiO₂s infers that processes involved in the preparation of visible-light-active TiO₂ specimens (irrespective of the method) likely implicate a stage of TiO₂ reduction. Indeed, most of the syntheses included a heating stage at various temperatures. For example, the 3.0 eV absorption band of anatase crystals attributed to oxygen vacancies results from the removal of impurities introduced during the crystal growth at ca. 300 C [65]. Related to this, the visible-light absorption of metal-ion-implanted TiO₂s was observed only after the samples had been calcined in the temperature range 450–550 C [2]. Accordingly, the absorption features displayed by TiO₂ specimens in the visible spectral region likely originate from the formation of color centers by reduction of TiO₂ after some form of heat treatment or some photostimulated process. Kuznetsov and Serpone [62] concluded that the visible absorption spectra of anion-doped (or otherwise) TiO₂ originated from color centers, and not from the narrowing of the intrinsic band gap of TiO₂ (𝐸_bg = 3.2 eV; anatase), as originally proposed by Asahi et al. [4, 46] through mixing of oxygen and dopant states. True narrowing of the intrinsic band gap of a metal oxide such as TiO₂ would necessitate heavy anion or cation doping at high concentrations of dopants. In such case, however, one must question whether the metal oxide retains its original integrity. We think not as exemplified from the recent reported case of the zincoxysulfide [61].

The next question regards the nature of these color centers. Loss of an O atom in a metal oxide (reaction (5)), leaves behind an electron pair that is trapped in the V_O cavity (reaction (6)) giving rise to an 𝐹 center; an 𝐹⁺ center is equivalent to a single electron trapped in the oxygen vacancy, V_O^• (reaction (7); Kroger-Vink notation). The electron-pair deficient oxygen vacancy, V_O^••, also known as an anion vacancy, V_A, is a doubly charged 𝐹⁺⁺ center (reaction (5)). Thus, color centers associated with oxygen vacancies imply 𝐹-type centers in TiO₂ and other metal oxides. In addition, electrons can also be trapped by Ti⁴⁺ ions in regular lattice sites (Ti⁴⁺_reg) adjacent to V_O or in interstitial lattice sites (Ti⁴⁺_int) to give Ti³⁺_reg and Ti³⁺_int color centers, respectively (reactions (8a)); 𝐹-type centers can also generate Ti³⁺ color centers through charge transfer (reaction (8b)),

Ti⁴⁺−O²⁻−Ti⁴⁺⟶Ti⁴⁺−□−Ti⁴⁺+V_O^••(𝐹⁺⁺)+2e⁻ (5)

V_O^••+2e⁻⟶V_O(𝐹) (6)

V_O^••+2e⁻⟶V_O^•(𝐹⁺) (7)

e⁻+Ti⁴⁺_reg{Ti⁴⁺_int}⟶Ti³⁺_reg{Ti³⁺_int} (8a)

𝐹(or𝐹⁺)+Ti⁴⁺_reg{Ti⁴⁺_int}⟶Ti³⁺_reg{Ti³⁺_int}+𝐹⁺(or𝐹⁺⁺). (8b)

Spectrum 3 of Figure 12 consists of three overlapping bands depicted in Figure 13 [67], one of which is centered at 2.1 eV (590 nm; band 1), another at ca. 2.40 eV (517 nm; band 2), and band 3 occurs around 2.93 eV (413 nm) in accord with band positions reported earlier by Kuznetsov and Serpone [62] for reduction of TiO₂ in polymeric media. The congruence of the bands in the absorption spectra of such disparate TiO₂ systems is remarkable when considering the large variations in the experimental conditions. This lends credence to the notion that the absorption bands likely share the same origins, namely electron transitions involving F-type centers and/or d-d transitions in Ti³⁺ color centers. Evidence for both has appeared in the literature (see references in [67]).

Figure 13 

Deconvolution of spectrum 3 of Figure 12 (curve 5 herein). Band 4 represents the convolution sum of curves 1, 2, and 3. Reproduced with permission from [67]. Copyright (2006) American Chemical Society.

Using the embedded-cluster numerical discrete variational method, Chen et al. [68] estimated the band gap energy of rutile TiO₂ as 3.05 eV in good agreement with the experimental 3.0 eV for this polymorph. Calculations of energy levels of 𝐹-type centers gave energies for the F, 𝐹⁺ and 𝐹²⁺ centers, respectively, of 0.87 eV, 1.78 eV and 0.20 eV below the bottom level of the CB band (see Figure 14). The band at 760 nm (1.61 eV) was ascribed to the electron transition 𝐹⁺→CB of TiO₂.

Figure 14 

Photoinduced detrapping of electrons from the F center to the conduction band followed by retrapping by the shallow 𝐹²⁺ centers can increase the number of 𝐹⁺ centers. The deconvoluted bands 3 and 2 of Figure 13 at 2.9–3.0 eV (428–413 nm) and 2.4–2.6 eV (ca. 517–477 nm), respectively, have been attributed [67] to Jahn-Teller split ²T₂→²E transitions of Ti³⁺ centers. Existence of these centers has been confirmed by EPR examination of N-doped TiO₂ specimens calcined at various temperatures [40, 69]. Band 1 at 1.7–2.1 eV (729–590 nm) is likely due to the transition 𝐹⁺→𝐹^+∗, though the transition 𝐹⁺→CB is not precluded. The transition S_o(𝐹)→S₁(𝐹^∗) or S_o(𝐹)→CB of the F center likely occur at much lower energies (in the infrared). Rigorous systematic studies are needed to ascertain such assignments in anion- and cation-doped TiO₂ specimens examined by diffuse reflectance spectroscopy; additional EPR and photoconductivity studies should aid in such task.

5. Is The (intrinsic) Band Gap of TiO₂ Narrowed in N-Doped TiO₂?

Taken literally, band gap narrowing in doped TiO₂ materials means that the intrinsic band gap energy of TiO₂ decreases in the presence of dopants. We re-emphasize that what does change is the energy photothreshold for activating doped titania specimens to carry out surface photoinduced redox processes. A better term to refer to the red-shift of the absorption edge might be (i) the red-limit of TiO₂ photocatalysis, used in the past to refer to redox processes occurring in the visible spectral region or (ii) the extrinsic band gap(s) of doped TiO₂versus the term intrinsic band gap which for anatase is 3.2 eV and for rutile is 3.0 eV; the latter refer to pristine undoped titania. The term band gap narrowing used by Asahi et al. [4, 46] and others meant a rigid upward shift of the valence band edge toward the conduction band of TiO₂. If the red-shift of the absorption edge were truly due to this rigid shift, then irradiation into the visible spectral bands of Figure 13 should cause no bleaching of the absorption bands. However, if the absorption features are due to the existence of color centers (𝐹-type and/or Ti³⁺), then bleaching of the spectral features should occur as observed in our recent studies on TiO₂/polymer compositions [70] and on a N-doped TiO₂ system [71] that we now examine briefly.

The photocoloration of TiO₂/polymers compositions and the photobleaching of color centers (see Figure 15) at various irradiation wavelengths (UV to near-IR region) were examined to probe the photoactivation of color centers on irradiating into the absorption bands at 2.90 eV (427 nm; AB1), 2.55 eV (486 nm; AB2) and 2.05 eV (604 nm; AB3). Such exercise should lead to two principal types of photostimulated absorbance changes: (i) increase in absorbance or (ii) decrease in absorbance. The decrease in absorbance (ii) is a direct experimental manifestation of the photobleaching phenomenon of colored TiO₂/polymer compositions and would clearly demonstrate the presence and photoinduced disappearance/destruction of color centers in such TiO₂/polymers systems. The average spectrum of the bleaching of the colored TiO₂/[P(VDF-HFP)] composition {P(VDF-HFP) is poly(vinylidene fluoride/co-hexafluoropropylene polymer)} is depicted as curve 3 in Figure 15 and is compared to the average heat-induced absorption spectrum (curve 1) and photoinduced absorption spectrum (curve 2) of several other TiO₂/polymer compositions [70]. These observations confirm an earlier proposal [62] that absorption of light by various TiO₂ systems in the visible region originates only from color centers and not from a narrowing of the band gap of pristine TiO₂. Results also indicate that photobleaching of colored TiO₂/polymer compositions originates both from intrinsic absorption of light (h𝜈 > 3.2 eV, anatase) by TiO₂ and from (extrinsic) absorption of light by the color centers at wavelengths corresponding to their absorption spectral bands in the visible region. These bands are also active in the photodestruction of the color centers. Spectrum 3 in Figure 15 corresponds to the nearly complete discoloration of the compositions under irradiation mostly in the visible region. Hence, the total overlap of the absorption and bleaching spectra of Figure 15 demonstrate unambiguously that the same color centers are formed during the treatment that induced the absorption, and that they are subsequently destroyed on irradiation during the photobleaching process. These data conclusively negate any inference of broadening of the valence band of TiO₂ to account for the red-shifts of the absorption edges in doped visible-light-active TiO₂ systems. The VB and CB bands can neither be photodestroyed nor phototransformed, contrary to the color centers. An additional study [71] examined the effect of molecular oxygen and hydrogen on the photostimulated formation of defects (color centers) on irradiation of TiO_2-xN_x with visible light (546 nm). Results are displayed in Figure 16 as ΔR versus 𝜆; note that ΔR and Δ𝜌 have the same meaning, namely absorbance. A similar behavior was observed on irradiating at 436 nm and 578 nm. The influence of hydrogen on photocoloration on irradiation at 546 nm is nearly the same as on UV irradiation, in that the number of photoinduced defects increased, contrary to when O₂ is present for which the photoadsorption of O₂ is the exact opposite to that seen under UV irradiation. The ultimate level of photocoloration (increase in absorption) in the presence of O₂ is considerably greater under 546-nm irradiation relative to the level in vacuum and relative to what is observed under UV irradiation. This inferred the mechanism of photoexcitation and surface photoreaction occurring under visible-light excitation of TiO_2-xN_x in the presence of O₂ is different from the mechanism of UV-induced processes. Photobleaching of photoinduced color centers by red light at 𝜆 > 610 nm in vacuum and in the presence of oxygen and hydrogen is illustrated in Figure 17. No significant changes in absorption of photoinduced color centers occur during photoexcitation in vacuum and in the presence of H₂. However, the presence of O₂ causes significant photo-bleaching (negative absorbance) of the UV-induced defects, a typical behavior of electron-type color centers (i.e., 𝐹-type and Ti³⁺ centers).

Figure 15 

Averaged absorption spectra (1, 2) of various TiO2/polymer compositions normalized by Δρmax and averaged bleaching spectrum (3) of the TiO2/[P(VDF-HFP)] composition irradiated at different wavelengths. Reproduced with permission from [70]. Copyright (2007) American Chemical Society.

Figure 16 

Absorption spectra of photoinduced color centers in N-doped TiO2 obtained after pre-irradiation at 546 nm in vacuum, in the presence of O2 and H2. Reproduced with permission from [71]. Copyright (2007) American Chemical Society.

Figure 17 

Kinetics of photobleaching (recorded at 𝜆 = 595 nm) of photoinduced color centers on irradiation of TiO2-xNx at 𝜆 > 610 nm in vacuum, in the presence of oxygen and in the presence of hydrogen. Reproduced with permission from [71]. Copyright (2007) American Chemical Society.

6. Concluding Remarks

In this survey, we have attempted to expose and explore some of the root causes that have had such impact and so changed the field of Heterogeneous Photocatalysis involving the next generation of TiO₂ photocatalysts. Preparative methods and some characteristic features of N-doped TiO₂s have been described briefly. At variance are the experimental results and interpretations of X-ray photoelectron spectra with regard to assignments of binding energy peaks. Relative to pristine nominally clean TiO₂, whose absorption edges are 3.2 eV (anatase) and 3.0 eV (rutile), N-doped TiO₂ specimens display red-shifted absorption edges into the visible spectral region. Several workers have surmised that the (intrinsic) band gap of TiO₂ is narrowed by the coupling of dopant energy states with the O 2p states in the VB band, an inference based on DFT computations of band gap energies, which are severely underestimated owing to the inherent faulty local density approximation (LDA). Using similar DFT calculations, others proposed that the red-shifted absorption edges originate from the presence of intragap dopant states above the upper level of the VB band. Analyses of spectral features in the visible region, however, inferred a common origin for the doped TiO₂ as deduced from the strong similarities of absorption features of a large number of TiO₂ specimens, regardless of the preparative methods employed and the nature of the dopants. This next generation of TiO₂ photocatalysts should enhance process engineering photoefficiencies, in some cases, since doped titania absorbs a greater quantity of sunlight radiation. The fundamental science that underscores Heterogeneous Photocatalysis with this new generation of photocatalysts is a rich playing field that is ripe for further exploration, limited only by one’s imagination, creativity and resourcefulness.