# Visible-Light-Active Titania Photocatalysts: The Case of N-DopedTiO2s—Properties and Some Fundamental Issues

Authors: Alexei V. Emeline; Vyacheslav N. Kuznetsov; Vladimir K. Rybchuk; Nick Serpone
Journal: International Journal of Photoenergy
 (2008)

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## Abstract

This article briefly reviews some factors that have impacted heterogeneous photocatalysis with next generationTiO2 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.


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## Body

## 1. Introduction

As a  first-generation material, pristineTiO2 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, Vfb, 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 Vfb 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 ofTiO2, anatase begins to absorb UV light around 387 nm (band gap energy, Ebg∼3.2eV), whereas the absorption onset of rutile occurs around 413 nm (Ebg∼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-dopedTiO2, 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 theTiO2band 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-dopedTiO2 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 TiO2-xNx (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 NOx-doped TiO2 with the NOx 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 VOs.In their 2001 seminal report, Asahi et al. [4] prepared crystalline TiO2-xNx films by sputtering a TiO2 target in a N2/Ar gas mixture followed by annealing at 550 C in a N2 atmosphere. The yellowish TiO2-xNx 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 TiO2-xNx, 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
TiO2-xNx 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 TiO2-xNx (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 TiO2-xNx.
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 TiO2-xNx, 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, VOs, with increase of x in TiO2-xNx. In this case, VOs 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
porousN-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 threeN-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-25TiO2 (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 andN2 yields
crystalline anatase TiO2-xNx 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+2p3/2 and Ti4+2p1/2 in the anatase TiO2 film indicating that the
majority of titanium in the TiO2-xNx 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 TiO2-xNx 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 TiO2-xNx nanocolloids. XRD and HRTEM
results demonstrated that the treated TiO2-xNx nanoparticles were predominantly anatase. Diffuse reflectance spectra (DRS)
of the TiO2-xNx crystallites rose sharply at ∼450nm
; the corresponding DRS spectrum of nitrided TiO2-xNx 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 TiO2-xNx 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 TiO2-xNx 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 ofTiO2-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 TiO2-xNx 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 variousTiO2 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 ofN-doped titania specimens, not to mention
the nature of other anion- and cation-doped TiO2s.Nakamura and coworkers [29] produced anatase TiO2-xNx 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 cleanTiO2 (110) surfaces in single crystals with mixtures of N2+ 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.WithTiCl4, 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 NHx species located at interstitial sites (399.6 eV), or from NOx, or NHx (400 eV), or from an
oxynitride (399.3 eV) of stoichiometry equivalent to TiN0.5O0.5. 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 NO3− under visible-light illumination.The solvothermal process
using aTiCl3/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 TiO2-xNx 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 TiO2-xNx 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 producesN-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 TiO2-xNx 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 ofN-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 TiO2-xNx 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
TiO2-xNx 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 TiO2-xNx. XANES spectra also showed a
lack of correlation between the number of oxygen vacancies (VOs) and the N content in the
samples. Above a certain limit, the association of point defects, such as VOs 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 VOs, 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 VOs; 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., NOx) 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 TiO2-xNx under visible-light
irradiation. It appears that photoassisted oxidations of organic compounds on
the surface of TiO2-xNx 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.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.
(a)(b)(c)


## 3. DFT Computations of Band Gap Energies inN-Doped TiO2

Different preparative methods and strategies ofN-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 NOx, NHx, 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 anataseTiO2 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 (NS), (b) interstitial N doping (NI), and (c) both types of
doping (NS+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 TiO2-xNx showed a shift of the absorption edge to lower energy by N doping, with
dominant transitions from N2pπ→Tidxy rather than from O2pπ as in TiO2.
However, the calculated band gap energies were considerably underestimated
relative to the experimental value (e.g., Ebg = 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 Ebg = 3.14 eV). Accordingly, the
band gaps of N-doped TiO2-xNx 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 TiO2-xNx were likely similar to those
in TiO2.The picture as to the exact cause of the red-shift of the absorption edge ofTiO2 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 TiO2rutile 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 TiO2-xNx: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 TiO1.875N0.125. 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 TiO2-xNx systems
(see Figure 4). Di
Valentin et al. [47] also considered the contribution of oxygen
vacancies (VOs), estimated experimentally at
0.75 to 1.18 eV below the conduction band EC (DFT calculations placed them at 0.3 eV below EC), to the overall
visible-light photoactivity of N-doped systems when VOs trap electrons to produce F-type
color centers. The simultaneous presence of N dopants and VOs can also lead to charge
transfer states (reaction 1) that can further
contribute to the visible-light photoactivity,
(1)VO(F)+N⟶VO•(F+)+N−.Schematic representation of the energy band structure of
pure andN-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].
(a)(b)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 NOx or NHx species or simply to
substitutional N-doping. The TiO2-xNx 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(NS) and interstitial N(NI) 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 NS and interstitial NI yielding the azide species (N3− denoted as NS−) 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 VOs (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 TiO2-xNx 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 VOs are likely the preferred occurrences.(a)(b)Electronic band structure for (a) substitutional and (b) interstitialN-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.
(a)(b)The question on the blue-shift of the absorption edge ofN-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.SubstitutionalN-doping can be stabilized by the presence of oxygen vacancies (NS−O+VO) under oxygen-poor
experimental conditions, whereas under oxygen-rich conditions interstitial N
species (NI) become favored. The N-doped
sample prepared by the sol-gel process of stoichiometry near TiO1.907N0.062, 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 NHx-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 Nb• in Figure 8 interact strongly with the TiO2 lattice. DFT calculations
carried out on the 96-atom supercell involved two interstitial nitrogens (NI•) or two substitutional
nitrogen (NS•) paramagnetic species plus
an oxygen vacancy (VO) 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 (VO 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 VOs under oxygen-poor
conditions in N-doped TiO2 is the partial quenching of Nb• paramagnetic species, which
are transformed [48] into Nb− through reduction by Ti3+ color centers (see Figure 8). The energetically favored
reduction of Nb• species may be the cause for
the small energy cost in the formation of VOs in N-doped TiO2 (see above). The EPR peaks attributed to Nb• 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 (2)Nb•+Ti3+⟶Nb−+Ti4+by the sol-gel method) contained paramagnetic N related
species in the bulk lattice (Nb•) and a number of diamagnetic
species (Nb−), the presence of which
depended on the oxygen content in the metal-oxide sample. The EPR signal due to
Nb• increased on irradiating the
doped sample at 437 nm in O2(pO2=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 andN-doped TiO2. Reproduced with permission
from [50].
Copyright (2006) American Chemical Society.Figure 8
Electronic band structure changes from interactions betweenNb• (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 theN-doped sample in O2 atmosphere. Reproduced with permission from [50]. Copyright (2006) American
Chemical Society.(3)Nb−+hν⟶Nb•+e−surf(4)e−surf+O2⟶O2−•.So far all the DFT-based calculationshave 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
anataseTiO2 with experimental and calculated values reported by others
(EBG is the direct band gap). From Wang and Lewis [55].
SourceEBG (rutile), eVEBG (anatase), eVWang and Lewis3.053.26Experiments3.06 [56]3.20 [58], 3.42 [59]Other calcns2.00 [57], 1.78 [60]2.22 [60], 2.00 [46]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 rutileTiO2 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 rutileTiO2. Unlike N-doped
rutileTiO2, however, identical shifts
of ca. 0.44 eV were obtained in N-doped anataseTiO2 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 anataseTiO2 would lead to greater visible-light photoactivity.The brief
discussion above points to a lack of consensus on whether or not there isband 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(ZnOxS1-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 dopedTiO2 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 dopedTiO2 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 [ρabs(hν)]
and undoped TiO2 samples that do not absorb in
the visible spectral region [ρnon-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, [ρnon-abs(hν)−ρabs(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 (hνmax⁡), the intensity of this
maximum (Δρmax⁡,ΔAmax⁡), and the spectral bandwidth
at half-maximum amplitude, (ii) the comparison of the spectra of different
samples after normalization by the Δρmax⁡/ΔAmax⁡ 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 nondopedTiO2 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 anataseTiO2 specimen prepared by a
solvothermal process [37], (v) N-doped rutileTiO2 sample also prepared by a
solvothermal process [37], (vi) yellowN-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-dopedTiO2 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 orangeN-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-activeN-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 hνmax⁡=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 hνmax⁡ 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 dopedTiO2 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 Sr0.95La0.05TiO3+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
TiO2-xNx 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 TiO2-xNx 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 reducedTiO2 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 (Ebg = 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 anO atom in a
metal oxide (reaction (5)), leaves behind an electron pair that is trapped in the
VO cavity (reaction (6)) giving
rise to an F center; an F+ center is equivalent to a single electron trapped in the oxygen vacancy,
VO• (reaction (7); Kroger-Vink
notation). The electron-pair deficient oxygen vacancy, VO••, also known as an anion
vacancy, VA, 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 VO 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)),(5)Ti4+−O2−−Ti4+⟶Ti4+−□−Ti4++VO••(F++)+2e−(6)VO••+2e−⟶VO(F)(7)VO••+2e−⟶VO•(F+)(8a)e−+Ti4+reg{Ti4+int}⟶Ti3+reg{Ti3+int}(8b)F(orF+)+Ti4+reg{Ti4+int}⟶Ti3+reg{Ti3+int}+F+(orF++).Spectrum 3 of Figure12 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 Figure12 (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 14Photoinduced detrapping of electrons from theF 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 So(F)→S1(F*)  or So(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 dopedTiO2 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 ofTiO2/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 15demonstrate 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 TiO2-xNx 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 TiO2-xNx 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 inN-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 atλ = 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 ofTiO2 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.



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*Source: 258394-2007-12-24.xml*