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

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.


INTRODUCTION
As a first-generation material, pristine TiO 2 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 TiO 2 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 TiO 2 surface.An additional, no less important issue is that TiO 2 absorbs a relatively small fraction (ca.3-5%) of the solar radiation reaching the Earth's surface.
Of the two important polymorphs of TiO 2 , anatase begins to absorb UV light around 387 nm (band gap energy, E bg ∼ 3.2 eV), whereas the absorption onset of rutile occurs around 413 nm (E bg ∼ 3.0 eV) 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 International Journal of Photoenergy of TiO 2 be shifted to longer wavelengths (>400 nm).One way to accomplish this necessitated photosensitizing TiO 2 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 TiO 2 specimens that enhance photoinduced surface processes even in the visiblelight spectral region [2], where wet chemical methods failed.
First reports of anion-doped TiO 2 , began to appear in the early 1990s, although Sato [3] had earlier hinted at a N-doped TiO 2 .The 2001 study of Asahi and coworkers [4] on doping TiO 2 with various anions to prepare visible-lightactive (VLA) N-doped TiO 2 s was the catalyst needed to produce second-generation TiO 2 materials that are photoactive over the UV and much of the visible-light region.Subsequent studies reported several other visible-light-active N-doped TiO 2 s, together with C-doped TiO 2 and S-doped TiO 2 .The reports by Asahi et al. [4,5] has led to a lively debate on the causes that lead the absorption onset of TiO 2 to be redshifted to the visible region.They proposed that N-doping of TiO 2 red-shifts the absorption edge of TiO 2 and increases photoactivity by a narrowing of the TiO 2 band gap.Carbonand sulfur-doped TiO 2 displayed similar red-shifts accompanied by increased photoactivity.As we will see later, others have proposed that electronic transitions in N-doped TiO 2 systems activated by visible-light irradiation involve transitions from N 2p localized states to the CB of TiO 2 .Clearly, just as first-generation TiO 2 led to lively debates on the fundamental science that underpins TiO 2 -assisted photoredox surface processes following photo-activation, so are the second-generation VLA TiO 2 s 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][7][8].
This review article focuses briefly (a) on some preparative methods of visible-light-active N-doped TiO 2 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.

NITROGEN-DOPED TiO 2 s-SYNTHESES AND CHARACTERIZATION
Asahi et al. [4] initially set three requirements to achieve visible-light-activity for TiO 2 : (i) doping should produce states in the band gap of TiO 2 that absorb visible light; (ii) the CB minimum and the dopant states of doped TiO 2 should be as high as or higher than the H 2 /H 2 O level to ensure photoreductive activity; and (iii) the intragap states should overlap sufficiently with the band states of TiO 2 to transfer photoexcited carriers to reactive sites at the TiO 2 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 TiO 2 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 TiO 2 by the fullpotential 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 TiO 2 s into (i) modification of existing TiO 2 by ion bombardment, (ii) modification of existing TiO 2 in powdered form, film, and single crystal, or else modify TiN by gas phase chemical impregnation, and (iii) grow TiO 2-x N x (crystals) from liquid or gaseous precursors.
Early on, Sato [3] had noted that calcination of Ti(OH) 4 in the presence of NH 4 Cl (or aqueous NH 3 ) led to photosensitization of TiO 2 when exposed to visible-light radiation.The powdered samples were deduced to be NO x -doped TiO 2 with the NO x impurity acting as the sensitizer.Noda et al. [10] reported a yellow-colored anatase TiO 2 powder obtained from aqueous hydrazine and TiCl 4 solutions, and deduced that visible-light absorption was due to the presence of oxygen vacancies V O s.
In their 2001 seminal report, Asahi et al. [4] prepared crystalline TiO 2-x N x films by sputtering a TiO 2 target in a N 2 /Ar gas mixture followed by annealing at 550 C in a N 2 atmosphere.The yellowish TiO 2-x N x films absorbed light below 500 nm.X-ray photoelectron N 1s spectra (XPS) of the N-doped TiO 2 showed bands at 402, 400 and 396 eV; the undoped TiO 2 film showed no 396 eV band.The latter was assigned to atomic β-N in TiO 2-x N x , whereas the 402-eV and 400-eV bands were attributed to molecularly chemisorbed dinitrogen γ-N 2 [11].Powdered samples prepared with NH 3 /Ar as the source of N followed by calcination at 550-600 C produced a TiO 2-x N 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 TiO 2 anatase films by MOCVD using Ti(i-PrO) 4 and N 2 O at 420 C; XPS Ti 2p spectra showed N was incorporated into the TiO 2 lattice to form Ti-N bonds.Hydrolysis of Ti(SO 4 ) 2 with NH 3 in dry air at 400 C produced a visible-light-active anatase TiO 2 (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-x N x (x = 0, 0.0050, 0.011, 0.019) powdered samples can be prepared by annealing anatase TiO 2 powder (ST-01) in a flow of NH 3 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-x N x .The feature at λ > 550 nm was attributed to Ti 3+ since NH 3 decomposes into N 2 and H 2 at ca. 550 C, and H 2 reduces Ti 4+ under these conditions.Alexei V. Emeline et al.The band gap energy remained at 3.2 eV.Mineralization of isopropanol to CO 2 with UV-light and visible-light radiations resulted in different quantum yields, suggesting that Ndoping forms a narrow N 2p band above the valence band of TiO 2 .Note that band gap narrowing in TiO 2-x N 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 O s, with increase of x in TiO 2-x N x .In this case, V O s 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 TiO 2 thin films, prepared by introducing N into anatase TiO 2 by means of DC magnetron sputtering in N 2 -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 TiO 2 with various N loadings by hydrolysis of TiCl 4 with a N-containing base {aqueous NH 3 , (NH 4 ) 2 CO 3 or NH 4 HCO 3 } followed by calcination at 400 C. XPS spectra showed only a broad signal at ∼404 eV (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 TiO 2 absorption edge to ∼250 nm .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 TiO 2 (see Figure 1) for three of the specimens.According to the authors [16], N-doping led only to a "modest band gap narrowing."High-energy ball milling of P-25 TiO 2 (ca.80% anatase and ∼20% rutile) with various quantities of hexamethylenetetramine (HMT) at near-ambient temperatures yields yellowish N-doped rutile TiO 2 [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 N 2 yields crystalline anatase TiO 2-x N x films with a considerable amount of substitutional N atoms (1.8 at.%) and chemisorbed molecular N 2 [18].XPS spectra showed peaks at 402 and 396 eV assigned to molecularly chemisorbed γ-N 2 and atomic β-N, respectively; Ti 2p XPS spectra revealed Ti 4+ 2p 3/2 and Ti 4+ 2p 1/2 in the anatase TiO 2 film indicating that the majority of titanium in the TiO 2-x N x film consisted of Ti 4+ , thus confirming the results of XRD patterns.N-doping caused no changes to the anatase TiO 2 structure as attested by Raman spectroscopy.
In a simplified synthesis, Gole and coworkers [19,20] produced TiO 2-x N x samples at room temperature using direct nitridation of anatase TiO 2 with alkylammonium salts.The samples could be tuned to respond to wavelengths up to λ∼550 nm.The method first yielded metal-oxide colloids by the controlled hydrolysis of Ti(i-PrO) 4 in aqueous/isopropanol media (pH 2; HNO 3 ), subsequent to which treatment in excess (C 2 H 5 ) 3 N led to TiO 2-x N x nanocolloids.XRD and HRTEM results demonstrated that the treated TiO 2-x N x nanoparticles were predominantly anatase.Diffuse reflectance spectra (DRS) of the TiO 2-x N x crystallites rose sharply at ∼450 nm ; the corresponding DRS spectrum of nitrided TiO 2-x N x from partially agglomerated nanoparticles rose at ∼550 nm.XPS analysis with Ar + -ion sputtering revealed the presence of N dopants at the surface and in the sub-layers of TiO 2-x N 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 TiO 2 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-x N 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 TiO 2 -based nanometer-sized photocatalysts that included nitrogen-doped TiO 2 nanoparticles, Chen and Burda [21] noted a broad N 1s binding energy peak of the nitrogendoped TiO 2 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 TiO 2 nanocolloid lattice [22].The shift in the N 1s binding energy for the TiO 2-x N 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 NaNO 3 (408 eV), in NH 3 (398.8eV) and adsorbed NO (400-401.5eV) [23].Attribution of the N 1s peak at 400 eV to γ-N 2 in N-doped titanias has been questioned by Chen et al. [23] in that N 2 is not chemisorbed on metal oxides such as TiO 2 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 TiO 2 lattice.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 TiO 2 material as a result of atmospheric degradation.Based on earlier studies of XPS spectra of NH 3 and primary alkyl/aromatic amines (398-399 eV), anionic N 3− in TiN (396-397 eV), adsorbed NO and NO 2 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 TiO 2 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-TiO 2 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 CO 2 , 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 TiO 2 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 TiO 2 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 TiO 2 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 TiO 2 .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 TiO 2 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 Ndoped titania specimens, not to mention the nature of other anion-and cation-doped TiO 2 s.
Nakamura and coworkers [29] produced anatase TiO 2-x N x materials by two methods: (a) in the dry method anatase ST-01 TiO 2 was heated to 550 C in a dry NH 3 flow and (b) in the wet method the Ti(i-PrO) 4 precursor was hydrolyzed in aqueous NH 3 at 0 C followed by calcination at 400 C. Nitrogen-doped nanocrystalline TiO 2 (yellow) powders have also been synthesized by a procedure developed by Ma and coworkers [30].Commercial anatase ST-01 TiO 2 heated at 500 C under a dry N 2 gas flow in the presence of a small quantity of carbon also yields N-doped yellow TiO 2 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 TiO 2 lattice, respectively, whereas the 400.4-eV peak was ascribed to molecularly chemisorbed N 2 species.
Ion implantation of atomically clean TiO 2 (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 TiO 2 lattice [31].XPS spectra revealed only the N 1s feature at 396.6 eV attributed to substitutionally bound nitride nitrogen (O 2− ions substituted by N 2− 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 TiO 2 (110).N-doped TiO 2 (110) rutile single crystals previously treated in the presence of an NH 3 /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 TiO 2 (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 TiO 2 (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 O 2− ions in the TiO 2 lattice are the necessary dopant species for TiO 2 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 TiO 2 does not account for the decrease in the photothreshold of TiO 2 (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 TiO 2 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 TiO 2 , 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 TiO 2 , and by Li and coworkers [35] who reported that the nature and level of visible-light activity depended on the nitriding compound employed.
With TiCl 4 , ethyl acetate and NH 3 as precursors and N 2 as the carrier gas, Yates and coworkers [9] used an atmospheric pressure thermal CVD coater to grow thin films of N-doped TiO 2 on glass substrates.Only three grown samples displayed the XPS N 1s peak at 396 eV of atomic βsubstituted N. XPS N 1s spectra showed no evidence of the 397-eV signal typically due to the N 3− ion (TiN), but did reveal weak signals at 400 and 402 eV probably arising from molecularly chemisorbed N 2 , 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 0.5 O 0.5 .Some of the films displayed visible spectral absorption features, but so did nominally undoped TiO 2 films indicating that N incorporation cannot be assumed on the basis of red-shifts of the absorption edge.Even though the N-doped TiO 2 specimens revealed the presence of β-N incorporation and absorption spectral features in the visible region, they were photo-inactive under visible light irradia-tion, while the UV photoactivity was reduced considerably compared to films grown in the absence of NH 3 .Clearly, the presence of β-nitrogen alone cannot be claimed to induce visible-light activity in N-doped TiO 2 films, a point also raised by Mrowetz and coworkers [36] who prepared two different yellow-colored N-doped TiO 2 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 TiO 2 at 550 C in a NH 3 /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 TiO 2 materials failed to catalyze the oxidation of HCOO − to CO 2 −• , and NH 3 -OH to NO − 3 under visible-light illumination.
The solvothermal process using a TiCl 3 /HMT/alcohol (methanol or ethanol) mixed solution in an autoclave at 90 C and then at 190 C yields N-doped TiO 2 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 TiO 2 , 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-x N x specimen.
Electrochemical anodization of titanium in HF/H 2 SO 4 electrolyte, followed by calcination at different temperatures (range 300-600 C) in pure NH 3 , yields self-organized Ndoped TiO 2 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-x N x nanotubes displayed a sub-band gap energy of ∼2.2 eV (that may be referred to as an extrinsic band gap) and the intrinsic band gap of anatase (∼3.2 eV).The N 1s XPS spectrum showed two clear peaks, one at 400 eV, ascribed to molecularly chemisorbed dinitrogen (γ-N 2 state), and the other at 396 eV, attributed to the atomic β-N state.
The RF-MS deposition method produces N-substituted TiO 2 {N-TiO 2 (X)} photoactive thin films using various N 2 /Ar mixtures as the sputtering gas (X = 2, 4, 10, 40) and a calcined TiO 2 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 TiO 2 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 H 2 O (irradiation λ ≤ 550 nm).XPS and XRD measurements showed significant substitution of lattice O atoms of TiO 2 by N atoms, which Kitano and coworkers [39] suggested as playing a crucial role in the band gap narrowing of the TiO 2 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 Ti 3+ species formed in the N-TiO 2 (X) samples International Journal of Photoenergy 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 NH 3 stream at 400 C, 500 C and 600 C at various time intervals (5 to 60 min) to prepare visible-light-active N-doped TiO 2 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 TiO 2 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-x N 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 TiO 2 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, NO 2 , NO 2 − , NH 2 , 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, NO 2 , NO 2 − , and NH 2 species.
At this time, it is important to realize that visible-light photoactivity of N-doped TiO 2 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-x N x systems with nominal N loadings from 0.2 to 1.0 wt.% involving the sequential reaction of H 2 O with a small known excess of TiCl 4 in toluene (step 1) under dry O 2 -free argon, followed by the stoichiometric reaction of the remaining TiCl 4 with a standard solution of NH 3 in dioxane (step 2).The resulting species were heat-treated in air at 400, 500, and 600 C. Tests of the TiO 2-x N 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 TiO 2 -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'-tetramethylethylenediamine and 1,2-phenylenediamine, to produce Ti(IV) complexes in dry isopropanol under a N 2 atmosphere.Dropwise addition of the solution to the inverse microemulsion, that contained H 2 O 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-x N x .XANES spectra also showed a lack of correlation between the number of oxygen vacancies (V O s) and the N content in the samples.Above a certain limit, the association of point defects, such as V O s 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 O s, since strong differences existed in the first cationcation coordination shell that inferred the possible presence of nonpoint defects.The joint study also confirmed the point defects to be the V O s; 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 (N n− ) 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 NH 3 followed by calcination either in an O 2 /N 2 mixture or in pure N 2 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 O 2 /N 2 than for the sample calcined in pure N 2 .
Thus far we have witnessed that visible-light-active (VLA) TiO 2 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 TiO 2 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 TiO 2 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-x N x under visible-light irradiation.It appears that photoassisted oxidations of organic compounds on the surface of TiO 2-x N x proceed by surface intermediates generated from oxygen reduction (the superoxide radical anion, O 2 −• ) 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 TiO 2 -based photocatalysts.

DFT COMPUTATIONS OF BAND GAP ENERGIES IN N-DOPED TiO 2
Different preparative methods and strategies of N-doping TiO 2 as described above can lead to anion-doped metaloxide materials with entirely different properties-these are the next generation TiO 2 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 TiO 2 to be red-shifted and consequently to the visible-light activity of doped TiO 2 .Species such as NO x , NH x , and N 2− have been proposed, not to mention NO − , NO 2 − and NO 3 − 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 TiO 2 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) for-malism 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 TiO 2 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 TiO 2 architecture.Optimization of the N positions in the cell inferred molecularly bonding states (NO and N 2 ) 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 (N 2 ).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-x N x showed a shift of the absorption edge to lower energy by N doping, with dominant transitions from N 2p π →Ti d xy rather than from O 2p π as in TiO 2 .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 TiO 2 (corrected E bg = 3.14 eV).Accordingly, the band gaps of N-doped TiO 2-x N 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-x N x were likely similar to those in TiO 2 .
The picture as to the exact cause of the red-shift of the absorption edge of TiO 2 in various N-doped TiO 2 (anatase) powders becomes somewhat confused with the report from Yates group [30] that the absorption edge of a N-doped TiO 2 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 International Journal of Photoenergy 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 E C of the conduction band to higher energies and the contraction energy E 4 in the N-doped rutile TiO 2 .E 3 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 ∼0.05 eV to higher energies for E C which is overcompensated by the presence of N-derived states just above VB, so that the excitation energy E 2 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].
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-x N 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 1.875 N 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 TiO 2 , 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 TiO 2 , in significant contrast with the conclusions of Asahi and coworkers [4,46] with respect to the undoped material.Structural variations in rutile TiO 2 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-x N x systems (see Figure 4).Di Valentin et al. [47] also considered the contribution of oxygen vacancies (V O s), 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 O s trap electrons to produce F-type color centers.The simultaneous presence of N dopants and V O s can also lead to charge transfer states (reaction 1) that can further contribute to the visible-light photoactivity, In a later study that combined experiments (EPR, XPS) and DFT calculations performed using the plane-wavepseudopotential 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 TiO 2 powders obtained by sol-gel synthesis, and unraveled some of the mechanistic details of the visible-light activity of N-doped TiO 2 as to whether photoactivity is due to NO x or NH x species or simply to substitutional Ndoping.The TiO 2-x N x sample was obtained by hydrolysis of Ti(i-PrO) 4 in isopropanol media in the presence of aqueous NH 4 Cl 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 Alexei V. Emeline et al. the TiO 2 anatase matrix.In the substitutional model, the N that replaces O in the TiO 2 lattice was taken to be bonded to three Ti atoms in the 96-atom supercell (a) so that the paramagnetic species is formally N 2− , 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 − , NO 2 − and/or NO 3 − (b)(see Figure 5).Figure 6 illustrates the DFT band structure of the N-doped TiO 2 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 Ti 3+ color centers, may also be trapped by substitutional N S and interstitial N I yielding the azide species (N 3− 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 O s (4.3 eV to 0.6 eV for anatase) with important consequences in the generation of F-type and Ti 3+ color centers.Experimentally, which of the two types of N dopants predominates in the N-doped TiO 2 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-x N 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 O s are likely the preferred occurrences.
The question on the blue-shift of the absorption edge of N-doped TiO 2 rutile single crystals contrasting the red-shifts in N-doped TiO 2 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 TiO 2 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 TiO 2 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 1.907 N 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 O 2 .The UV-visible diffuse reflectance spectrum of the sample (see Figure 7) is nearly identical to the many reported DRS spectra of N-doped TiO 2 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 TiO 2 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 Ndoped TiO 2 , 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 TiO 2 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 TiO 2 lattice leaves behind two electrons to form the neutral Fcenter (V O in the Kroger-Vink notation), or they may be trapped by neighboring Ti 4+ species to give two Ti 3+ 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.Ti 3+ color centers certainly do exist as demonstrated by EPR measurements [53,54] Figure 6: Electronic band structure for (a) substitutional and (b) interstitial N-doped anatase TiO 2 as given by PBE calculations at a lowsymmetry k-point.In the former, the site contains the paramagnetic N 2− species making the site electrically neutral (replaced O 2− ), 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.TiO 2 is the partial quenching of N b • paramagnetic species, which are transformed [48] into N b − through reduction by Ti 3+ 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 O s in N-doped TiO 2 (see above).vacuo or by other means to then reappear on re-oxidation.Thus, the N-doped TiO 2 specimen (at least the one prepared 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 O 2 (p O2 = 5 kPa).A new EPR line appeared that was attributed to O 2 −• radical anions (reactions ( 3) and ( 4)).These anions are apparently stabilized on two different surface Ti 4+ species, which Livraghi et al. [50] claimed could be typical of N-doped TiO 2 .Figure 9 illustrates the process embodied in the formation of the superoxide radical So far all the DFT-based calculations have failed to calculate experimentally commensurate band gap energies for undoped anatase and rutile TiO 2 , and consequently for all anion-doped TiO 2 systems unless, as some have done, one resorts to the scissor operator.In a comprehensive theoretical investigation of substitutional anion doping in TiO 2 , Wang and Lewis [55] explored the electronic properties of C-, N-, and S-doped TiO 2 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 TiO 2 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.
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 TiO 2 , 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 TiO 2 at the high N content of 5.2%, contrary to the significant shift observed for  [60], 2.00 [46] 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 visiblelight 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 TiO 2 .Unlike N-doped rutile TiO 2 , however, identical shifts of ca.0.44 eV were obtained in N-doped anatase TiO 2 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 TiO 2 would lead to greater visiblelight photoactivity.The brief discussion above points to a lack of consensus on whether or not there is band gap narrowing in doped TiO 2 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 TiO 2 as a consequence of doping.The discrepancies cannot be attributed to a simple semantic problem.Experimentally, anion-doping and cation-doping of TiO 2 do red-shift the absorption edge of TiO 2 in the doped samples, thus yielding potentially visiblelight photoactive materials that might prove useful in several important applications of surface processes occurring on the TiO 2 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 TiO 2 by introducing dopants into the metal-oxide lattice.When a N-doped TiO 2 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 TiO 2 (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 TiO 2 , as some have surmised by DFT calculations, then we must face the inescapable conclusion that the material is no longer TiO 2 , but is some titanium oxynitride material (for N-doped TiO 2 ) 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 x S 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 TiO 2 systems, XRD patterns showed that the doped TiO 2 retained the anatase (or rutile) structure; no new phase or new material could be ascertained.

OPTICAL PROPERTIES OF N-DOPED TiO 2
An examination of the optical properties of doped TiO 2 provides evidence based on the photobleaching phenomenon that the absorption bands observed in the visible spectral region for (any) doped TiO 2 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 TiO 2 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 TiO 2 specimens can be discussed in terms of difference diffuse reflectance spectra (ΔDRS) calculated from the DRS of various doped TiO 2 samples absorbing in the visible region [ρ abs (hν)] and undoped TiO 2 samples that do not absorb in the visible spectral region [ρ non-abs (hν)].The latter is typically the DRS of a nominally clean TiO 2 sample or the DRS of the doped sample prior to any treatment that might induce visiblelight 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 TiO 2 specimens displaying maximal absorption around 3.0 eV and shown in Figure 10 are those of (i) mechanochemically activated N-doped TiO 2 [17], (ii) N-doped oxygen-deficient TiO 2 [13], (iii) N,F-codoped TiO 2 sample prepared by a spray pyrolytic method [63], (iv) N-doped anatase TiO 2 specimen prepared by a solvothermal process [37], (v) Ndoped rutile TiO 2 sample also prepared by a solvothermal process [37], (vi) yellow N-doped TiO 2 specimen synthesized in short time at ambient temperatures using a nanoscale exclusive direct nitridation of TiO 2 nanocolloids with alkyl ammonium compounds [19,20], (vii, viii) N-doped TiO 2 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 TiO 2 prepared via sol-gel by mixing a solution of titanium (IV) isopropoxide in isopropyl alcohol in the presence of an NH 4 Cl solution [50].
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 TiO 2 specimen, prepared in nearly the same manner as the yellow N-doped TiO 2 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 TiO 2 , 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 TiO 2 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 TiO 2 rutile crystal prepared by an NH 3 treatment at 597 C (curve 2) [21] and the absorption spectrum of the anatase crystal (curve 2a) reported by Sekiya and coworkers [65,66].The difference in spectra 2 and 2a in the region hν > 3.0 eV originates from the difference in the phase composition of the TiO 2 , whereas the difference between curves 2a and 2 at hν < 3.0 eV exhibits a single absorption band with hν max = 2.6 eV, 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 H 2 atmosphere at 647 C and then to an O 2 atmosphere at 497 C [66].In both cases, only prolonged (ca.60 hrs) annealing of the crystals at 797 C in an O 2 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 TiO 2 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 TiO 2 bulk lattice [32].According to Yates and coworkers [31][32][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 TiO 2 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) Crimplanted TiO 2 , (ii) Ce-doped TiO 2 , (iii) mechanochemically activated N-doped TiO 2 , (iv) N-doped oxygen-deficient TiO 2 , and (v) from Sr 0.95 La 0.05 TiO 3+δ treated with HNO 3 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 TiO 2 , (ii) N-doped anatase TiO 2 , (iii) N-doped rutile TiO 2 , and (iv) yellow nitrided TiO 2-x N x nanocolloids.Curve 3 illustrates the averaged spectra of cation-doped TiO 2 , namely (i) Fedoped TiO 2 nano-powders prepared by oxidative pyrolysis of organometallic precursors in an induction thermal plasma reactor, (ii) zinc-ferritedoped titania (TiO 2 /ZnFe 2 O 4 ) synthesized by sol-gel methods followed by calcinations at various temperatures, and (iii) the orange N-doped TiO 2 sample prepared by a procedure otherwise identical to that of yellow N-doped TiO 2 but with the former consisting of partially agglomerated nanocolloids (i.e., larger TiO 2-x N 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 TiO 2 .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 TiO 2 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 TiO 2 with those of visiblelight-active TiO 2 s infers that processes involved in the preparation of visible-light-active TiO 2 specimens (irrespective of the method) likely implicate a stage of TiO 2 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 visiblelight absorption of metal-ion-implanted TiO 2 s was observed only after the samples had been calcined in the temperature range 450-550 C [2].Accordingly, the absorption features displayed by TiO 2 specimens in the visible spectral region likely originate from the formation of color centers by reduction of TiO 2 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) TiO 2 originated from color centers, and not from the narrowing of the intrinsic band gap of TiO 2 (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 TiO 2 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 TiO 2 and other metal oxides.In addition, electrons can also be trapped by Ti 4+ ions in regular lattice sites (Ti 4+ reg ) adjacent to V O or in interstitial lattice sites (Ti 4+ int ) to give Ti 3+  reg and Ti 3+ int color centers, respectively (reactions (8a)); F-type centers can also generate Ti 3+ color centers through charge transfer (reaction (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 TiO 2 in polymeric media.The congruence of the bands in the absorption spectra of such disparate TiO 2 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 Ftype centers and/or d-d transitions in Ti 3+ color centers.Evidence for both has appeared in the literature (see references in [67]).
Using the embedded-cluster numerical discrete variational method, Chen et al. [68] estimated the band gap energy of rutile TiO 2 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 F 2+ 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 TiO 2 .
Photoinduced detrapping of electrons from the F center to the conduction band followed by retrapping by the shallow F 2+ 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 2 T 2 → 2 E transitions of Ti 3+ centers.Existence of these centers has been confirmed by EPR examination of N-doped TiO 2 specimens calcined at various temperatures [40,69].Band 1 at 1. though the transition F + →CB is not precluded.The transition S o (F)→S 1 (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 anionand cation-doped TiO 2 specimens examined by diffuse reflectance spectroscopy; additional EPR and photoconductivity studies should aid in such task.

IS THE (INTRINSIC) BAND GAP OF TiO 2 NARROWED IN N-DOPED TiO 2 ?
Taken literally, band gap narrowing in doped TiO 2 materials means that the intrinsic band gap energy of TiO 2 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 TiO 2 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 TiO 2 versus 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 TiO 2 .
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 Ti 3+ ), then bleaching of the spectral features should occur as observed in our recent studies on TiO 2 /polymer compositions [70] and on a N-doped TiO 2 system [71] that we now examine briefly.The photocoloration of TiO 2 /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 TiO 2 /polymer 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-x N 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 O 2 is present for which the photoadsorption of O 2 is the exact opposite to that seen under UV irradiation.The ultimate level of photocoloration (increase in absorption) in the presence of O 2 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-x N x in the presence of O 2 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 H 2 .However, the presence of O 2 causes significant photo-bleaching (negative absorbance) of the UV-induced defects, a typical behavior of electron-type color centers (i.e., F-type and Ti 3+ centers).

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 TiO 2 photocatalysts.Preparative methods and some characteristic features of N-doped TiO 2 s 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 TiO 2 , whose absorption edges are 3.2 eV (anatase) and 3.0 eV (rutile), N-doped TiO 2 specimens display red-shifted absorption edges into the visible spectral region.Several workers have surmised that the (intrinsic) band gap of TiO 2 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 TiO 2 as deduced from the strong similarities of absorption features of a large number of TiO 2 specimens, regardless of the preparative methods employed and the nature of the dopants.This next generation of TiO 2 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.

Figure 3 :
Figure 3: Cartoons illustrating possible photoassisted processes of a substrate adsorbed on the surfaces of pure, N-, S-, and C-doped TiO 2 nanoparticles.Reproduced with permission from [45].Copyright (2007) American Chemical Society.

Figure 5
Figure 5 . One of the consequences of the high number of V O s under oxygen-poor conditions in N

Figure 8 :
Figure 8: Electronic band structure changes from interactions between N b • (N S • or N I •) and Ti 3+ color centers.Reproduced with permission from[50].Copyright (2006) American Chemical Society.

Figure 9 :
Figure 9: Sketch of the proposed mechanism for the processes induced by vis-light irradiation of the N-doped sample in O 2 atmosphere.Reproduced with permission from [50].Copyright (2006) American Chemical Society.

Figure 11 :
Figure 11: Average absorption spectrum of visible-light-active Ndoped TiO 2 specimens (curve 1); difference absorption spectra of N-doped rutile crystal (curve 2) and of the color centers in the yellow anatase TiO 2 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 :
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.

Figure 17 :
Figure 17: Kinetics of photobleaching (recorded at λ = 595 nm) of photoinduced color centers on irradiation of TiO 2-x N x 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.

Table 1 :
[55]arison of calculated band gaps of undoped rutile and anatase TiO 2 with experimental and calculated values reported by others (E BG is the direct band gap).From Wang and Lewis[55].