Niobium K‑Edge X‑ray Absorption Spectroscopy of Doped TiO2
Produced from Ilmenite Digested in Hydrochloric Acid
Richard G. Haverkamp,* Peter Kappen, Katie H. Sizeland, and Kia S. Wallwork

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ABSTRACT: Niobium doping of TiO2 creates a conductive material with many new energy applications. When TiO2 is
precipitated from HCl solutions containing minor Nb, the Nb in solution is quantitatively deposited with the TiO2. Here, we
investigate the structure of Nb doped in anatase and rutile produced from ilmenite digested in hydrochloric acid. Nb K-edge X-ray
absorption near edge structure (XANES) and extended X-ray absorption fine structure (EXAFS) are used to characterize the
environment of 0.08 atom % Nb doped in TiO2. XANES shows clear structural differences between Nb-doped anatase and rutile.
EXAFS for Nb demonstrates that Nb occupies a Ti site in TiO2 with no near neighbors of Nb. Hydrolysis of Ti and Nb from acid
solution, followed by calcination, leads to a well dispersed doped material, with no segregation of Nb. Production of Nb-doped TiO2
by this method may be able to supply future demand for large quantities of the material and in energy applications where a low cost
of production, from readily available natural resources, would be highly desirable.

1. INTRODUCTION
The dominant use of TiO2 is for the white pigment.

1 However,
many non-pigment applications of TiO2 have been developed
or are being considered. One important feature of TiO2 that
enables new applications is the ability to modify this normally
insulating material to become an electrical conductor. One way
to produce this conductivity is by doping of TiO2 with
niobium. If these applications are to become economical, a
means to produce Nb-doped TiO2 on an industrial scale is
required.
A proposed production method for TiO2 from the relatively

abundant titanium containing ore, ilmenite, is the digestion in
HCl2 and subsequent precipitation of a titanium oxide hydrate
directly from solution,3 followed by calcination. In this process,
Nb naturally present in the ilmenite dissolves and is
quantitatively precipitated with the TiO2. Either rutile or
anatase forms can be made directly or anatase formed and
converted into rutile by calcination.
Nb-doped TiO2 can act similar to a transparent metal which

could be used in the place of indium-tin-oxide (Sn-doped
In2O3), which is widely used in flat panel displays, touch
panels, and light-emitting devices.4 Other uses for Nb-doped
TiO2 include photovoltaics

5 and dye-sensitized solar cells.6−8

It also finds application in photocatalysis9 including for the
production of H2,

10 photoelectrochemical water splitting,11

and photocatalytic CO2 reduction.
12 Other catalytic applica-

tions are for catalyst supports,13 for example, for the oxygen
reduction reaction14 and H2 production,

15 or for dimensionally
stable anodes for the chlorine evolution reaction,16 and
electrochemical destruction of “forever chemicals”.17 Nb-
doped TiO2 has potential application in batteries and
supercapacitors18 including lithium-ion batteries19,20 and Na-
ion batteries.21 Other potentials uses are for CO sensing22 and
thermoelectric power production.23

These many possible uses of Nb-doped TiO2, many of which
are in new energy developments, suggest that there could be
future demand for large quantities of the material and in
applications where a low cost of production would be required.

Received: April 29, 2022
Accepted: July 22, 2022
Published: August 4, 2022

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Therefore, a process to produce the doped material on a large
scale from readily available natural resources is highly desirable.
For doping of TiO2 to display modified electronic

properties, it may be necessary for the dopant to be distributed
throughout the material. Therefore, any method for the bulk
production of doped TiO2 should also evaluate the nature of
the incorporation of the dopant.
In this work, the nature of the incorporation of Nb into

TiO2 produced from hydrolysis of HCl solutions of Barrytown,
New Zealand ilmenite is investigated. X-ray absorption near
edge structure (XANES) and extended X-ray absorption fine
structure (EXAFS), supported by X-ray diffraction (XRD), are
used to characterize the local environment of Nb in both
anatase and rutile phases of TiO2. The purpose is to determine
whether Nb occupies Ti sites without significantly modifying
the TiO2 structure or if Nb is intermixed as a distinct oxide
phase.

2. EXPERIMENTAL SECTION
2.1. Preparation of TiO2. Placer ilmenite from Barrytown,

New Zealand, containing 0.05% Nb2O5, was digested in 35 wt
% hydrochloric acid and precipitated as TiO2 hydrate with
either rutile or anatase structure, as described in more detail
elsewhere.3 Rutile hydrate is the natural product from the
hydrolysis of HCl solutions, while the anatase hydrate was
obtained by the addition of H3PO4 in the seeding stage of the
hydrolysis (equivalent to 0.35% P2O5 in the final TiO2
product). The hydrate, after the addition of KCl to the level
of 0.3% K2O, was calcined at 925 °C for 1 h, thus yielding the
TiO2 material used in this study.
2.2. Elemental Analysis. The elemental composition of

the titanium dioxide hydrates,3 prior to calcination, were
determined by Spectrachem Analytical Services, Lower Hutt,
New Zealand. The analyses were performed on a Siemens
SRS303AS wavelength-dispersive X-ray fluorescence spectrom-
eter. Pressed powder samples were used with the Siemens
“Spectraplus” semi-quantitative multi-element analysis.
2.3. X-ray Diffraction. XRD was performed at the bending

magnet powder diffraction beamline at the Australian
Synchrotron. This beamline uses a Mythen II silicon microstrip
detector with an intrinsic angular resolution of 0.004°.24 For
the experiments, the wavelength was set at λ = 0.58959 (1) Å
(E = 21 keV), and the vertical beam size was 0.4−0.5 mm at
the sample. Samples were packed in 0.3 mm quartz capillaries,
with 0.01 mm wall thickness (W. Müller, Schönwalde). The
wavelength was determined accurately through Rietveld
analysis of the diffraction pattern from LaB6.
2.4. X-ray Absorption Spectroscopy. X-ray absorption

spectroscopy (XAS) was performed at the wiggler XAS
beamline at the Australian Synchrotron. Samples were finely
ground with a mortar and pestle and pressed into pellets.
Spectra across the Nb K-edge (E0 = 18,985.6 eV25) were
recorded in the fluorescence mode with a 100-element
detector (Canberra). The samples were held in a He-cooled
cryostat (T < 20 K). Energy steps of 10 eV pre-edge and 0.35
eV across the edge (1 s/step) were used. In the EXAFS range,
k-steps of 0.035 Å−1 (up to 5 s/step) were used. The energy
scale was calibrated by simultaneously measuring a Nb foil
placed between the two downstream ion chambers. The
photon flux at the sample was around 1010 photons s−1. No
signs of radiation damage were detected from repeat scans,
permitting multiple scans to be summed in order to improve
signal-to-noise. Reference standards were Nb foil as well as

0.02% NbO2 (Aldrich) and Nb2O5 (Aldrich) both diluted to
0.02% in boric acid and loaded into 1 mm thick sample
holders. The beam size at the sample was about 1.5 × 0.4 mm
(H × V).
XANES and EXAFS data were processed using the freeware

package Athena/Artemis,26 with scattering paths provided
through FEFF6.27

3. RESULTS AND DISCUSSION
3.1. X-ray Diffraction. The two samples of TiO2 prepared

for this study are shown to be highly crystalline anatase or
rutile (Figure 1) with no detectable admixture of the two
phases in either sample and no other phases present.

3.2. Elemental Composition. The TiO2 hydrates that
were the basis for the anatase and rutile materials used in this
work both contained Nb at the level of 0.13% “Nb2O5” (see
Table 1, taken from ref 3); this oxide represents the
conventional method to convey elemental concentrations and
does not mean the oxide must be in this form. The reported
concentration corresponds to 0.08 atom % Nb of the Ti + Nb.
These also contain phosphorous, a portion of which comes
from inclusions in the ilmenite ore28 and with additional
phosphorous added to produce anatase instead of the naturally

Figure 1. X-ray diffraction patterns of Nb-doped anatase (blue) and
Nb-doped rutile (red) with the peaks identified by their Miller indices
(and indicated A for an anatase diffraction, R for a rutile diffraction).

Table 1. Elemental Composition of the Two TiO2 Materials
Prior to Calcinationa

element as oxide anatase wt % rutile wt %

TiO2 94.3 95.8
P2O5 0.82 0.49
Nb2O5 0.13 0.13
Ta2O5 0.009 0.009
Cl 4.4 3.1
FeO 0.015 0.044
SiO2 0.19 0.28
Al2O3 0.07 0.03
CaO 0.01 0.02
K2O 0.006 0.01

aReprinted with permission from Haverkamp, R. G.; Wallwork, K.;
Waterland, M.; Gu, Q.; Kimpton, J. A. Ind. Eng. Chem. Res. 2022, 61
(19), 6333−634. Copyright 2022 American Chemical Society.3

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formed rutile. Si, Al, Ca, and K (except the K added later as a
calcination flux) are likely to be present as discrete finely
divided gangue material, from inclusions in the ilmenite ore,
that passed through filtration steps and became incorporated in
the hydrolysis product and subsequently in the calcined
material. This work did not specifically ascertain the form of
the Si, Al, Ca, and K, but as the solubility of these materials is
low in hydrochloric acid used for the ore digestion. Small
inclusions of minerals such as garnet and feldspar are typically
present in ilmenite, thus posing a likely route to the inclusion
of these elements in the rutile and anatase precipitates.
3.3. X-ray Absorption Near Edge Structure. The

primary purpose of this work is to determine the nature of
the Nb present in the TiO2 of the two different structures.
XANES can be used to compare materials of interest with
reference materials, primarily providing chemical information.
Here, we compare the Nb-doped anatase with the Nb-doped
rutile and with two niobium oxide reference compounds.
There are clear differences in the Nb K-edge XANES between
the Nb-rutile and the Nb-anatase forms (Figure 2). The edge
energy is the same in both Nb-doped anatase and rutile (at
∼18,988 eV). However, there are significant differences above
the absorption edge, both in the whiteline and further into the
XANES region. The reference compounds (Figure 2) contain
Nb in NbO2 with oxidation state 4+ and Nb2O5 with oxidation
state 5+. The Nb2O5 has an edge energy of 19,000.2 eV which
is similar to that of the Nb-doped TiO2 materials and also
contains a pre-edge feature similar to that present in those
doped materials. The NbO2 has a lower edge energy of
18,997.7 eV than Nb2O5, reflecting the lower oxidation state of
NbO2. The edge for the Nb metal at 18985.6 eV lies at a lower
energy than these oxides.25 From the XANES, it is therefore
apparent that Nb in the doped anatase and rutile has a different
chemical/structural environment in each of the two forms. A
published XANES study of 7 atom % Nb doping in anatase
TiO2 presents a similar spectrum to the 0.08 atom % Nb-
doped TiO2 here for the anatase form.

9 XANES of 1.5% Nb in
anatase TiO2 has also recently been published, where ab initio
finite difference method near edge structure using the density
functional theory simulation was used to model the spectrum
providing good agreement between the data and the model of
Nb in anatase.29

3.4. Extended X-ray Absorption Fine Structure. The
Nb K-edge EXAFS of the doped anatase and rutile produced
markedly different spectra. The Fourier transforms of these
spectra, plotted as radial distance versus the magnitude of the
Fourier transform (Figure 3), show that the structural
environments of Nb in these two materials are quite different.

The EXAFS was therefore analyzed by solving the EXAFS
equation for crystal structures that might be present and
comparing these with the experimental data using the following
procedure.
TiO2 structures were obtained from the Crystallography

Open Database30 for anatase and rutile, as well as a wide range
of niobium oxide structures. For the oxides, structure models
were loaded into Artemis as is. For Nb-doped TiO2 structures,
the Atoms routine in Artemis was run with Nb set as the
absorber in place of Ti; scattering paths and phases were then
calculated using FEFF. Due to the low concentration of Nb, it
was assumed that Nb was isolated on the scale of the
attenuation length of electrons in the structure. This
assumption was tested against experimental data by fitting.
Anatase and rutile structures (with Nb substituted) were
evaluated, as were a wide variety of Nb2O5 and NbO2
structures. None of the niobium oxide structures gave good
fits, and these attempted fits are provided in the Supporting
Information.
Fitting of the structures to the EXAFS data was performed in

real space (R), with the quality of fit parameters shown in
Table 2. Fairly good fits were obtained, with a fit to a Nb
substituted for Ti in anatase and a Nb substituted for Ti in
rutile matching well to the experimental data for the Nb-doped
anatase and rutile samples, respectively (Figure 4). These
unconstrained fits gave S02 close to 1 in both cases (1.05 ±
0.23 and 1.06 ± 0.14) which provides good confidence that the
fitted model in each case is appropriate.
The EXAFS fit reveals that Nb is substituted into Ti sites of

the crystal structure adopted in each case, anatase or rutile, as
represented in the structures shown in Figure 5. The Nb is well
dispersed and does not appreciably interact with other Nb

Figure 2. Nb K-edge XANES of Nb-doped TiO2 (anatase and rutile) and Nb oxide standards (offset by +0.3).

Figure 3. Comparison of recorded spectra for Nb K-edge EXAFS of
Nb-doped anatase (blue) and Nb-doped rutile (red) TiO2. (a) k3, (b)
Fourier transform.

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atoms in the structure, and it is not present as niobium oxide
clusters within TiO2 of clusters separate to TiO2, consistent
with the XANES data discussed above. The EXAFS results also
indicate that it is unlikely that Nb forms dimers as has been
postulated in some Nb-doped TiO2.

33 In other studies, anatase
thin films were prepared from a composite TiNb target by
reactive magnetron sputtering giving a level of 1.5% Nb doped
in TiO2 anatase.29 The authors conclude that “the local
environment of Nb atoms in the film is close to that of Ti
atoms in the anatase phase [...]. This suggests that the

substitution of Ti by Nb ions occurs in the film without a
strong influence on the TiO2 matrix”. Here, we come to a
similar conclusion for these bulk materials formed by
hydrolysis and with both anatase and rutile where the Nb
takes Ti sites in either structure without appreciably modifying
the structure. However, it is noted that in other work with
TiO2 doped to 2.9 atom % Nb or higher, there was a phase
change from anatase to rutile on calcination the Nb was
segregated, leading to the formation of NbO nanoclusters on
the surface of the TiO2 rutile nanoparticles even before the
phase change took place.34 In the work described here, we see
no segregation of Nb in the calcined material, with the data
supporting highly dispersed Nb substituting for Ti in TiO2
structures. This is a positive consideration for energy
applications of this material, as discussed further below
(Section 3.7).
3.5. Industrial Production of Nb-Doped TiO2. If the

many potential applications of Nb-doped TiO2 are to be
realized, a low-cost and scalable method to produce such a
material is highly desirable. Ilmenite is the main primary source
of TiO2 and sometimes also contains niobium. The Barrytown,
New Zealand ilmenite used here is from a large placer deposit
and has an average concentration of 0.05% Nb2O5 (350 ppm
Nb) in the bulk ilmenite with 200−800 ppm Nb in individual
ilmenite grains.28 It is one potential source for the large-scale
production of Nb-doped TiO2 materials and has been shown
to be amenable to this hydrothermal method both being highly
soluble in HCl2 and readily precipitated3 and calcined to form
either anatase or rutile. This process is similar to the “sulfate

Table 2. EXAFS Structure Fitting Conditions and Quality of Fit Parameters

sample
structure fitted (CIF

file)
data range used,

k (Å−1)
fitting range in

R (Å)
reduced

χ2 R factor S02 (error) ΔR
σ2

(Debye−Waller)
Nb-doped anatase 1010942 anatase31 2−11.5 1.01−6 99 0.19 1.05 (0.22) 0.068 0.0058
Nb-doped rutile 9004141 rutile32 2−14 1.01−6 35 0.16 1.06 (0.14) 0.048 0.0067

Figure 4. Nb K-edge EXAFS structure fits to data for Nb substituted in a Ti site in k-space and R-space; (a,b) anatase; and (c,d) rutile. Recorded
spectra are shown in blue and Artemis fits to the data for the best fit crystal structures are shown in red.

Figure 5. Crystal structures with Nb in anatase (left) and Nb in rutile
(right). Green, Nb; blue, Ti; red, O. Generated using CrystalDiffract,
CrystalMaker Software Ltd, Oxford, England (www.crystalmaker.
com).

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process” for the large-scale production of pigment grade TiO2
which uses sulfuric acid rather than hydrochloric acid.
Other ilmenite deposits have also been reported to contain

Nb at useful levels. A West Australian sand deposit contains
1000 ppm Nb on average with up to 3500 ppm Nb in some
mineral grains.35 Ilmenite from Richard’s Bay, South Africa,
contains 460 ppm Nb.36 An ilmenite sand deposit, at Walikale,
North Kivu, Democratic Republic of Congo, contains 157 ppm
Nb on average with 40−341 ppm Nb in individual grains.37 An
ilmenite deposit in Kuru town, Jos South, Plateau State,
Nigeria may have up to 4.4% Nb.38 Some of these deposits
may be suitable for this process; however, only some ilmenites
are readily soluble in hydrochloric acid and other elements
present in these ores may also contribute either favorably or
unfavorably to the product formed.
The Nb doping level could be increased by the addition of

extra Nb, either by concentration from the digestion solutions,
adding a soluble Nb ore to the digestion process, or by the
addition of a suitable Nb salt to the digestion liquor prior to
hydrolysis. The XANES and EXAFS presented here have
demonstrated that the Nb is incorporated into the lattice of
TiO2 by this preparation method.
Production of Nb-doped TiO2 has been proposed by sol−

gel synthesis, for example, for anatase beads with 0.1−10 atom
% Nb from starting material of titanium(IV) isopropoxide with
1-hexadecylamine as a structure-directing agent for use in Li-
ion batteries.20 A similar sol−gel synthesis of TiO2 doped with
about 10% Nb from titanium tetrabutyl titanate with 1-
hexadecylamine and polydimethylsiloxane as structure deter-
mining agents was prepared for use in electrorheological
fluids.39 A sol−gel preparation followed by spark plasma
sintering with repeated oxidation and reduction is another
method proposed for the preparation of Nb-doped TiO2
powders.33 Another possible synthesis route is by grinding
precursors of TiNb2O7 and TiO2 in a mechanochemical
synthesis15 which results in a mixture with a gradient in Nb
concentration with more Nb on the surface. However, for
large-scale production, preparation by the hydrolysis of
aqueous acid solutions to form TiO2 is well established on
an industrial scale and therefore readily adaptable to produce
Nb-doped material.
3.6. Other Effects of Nb on TiO2 Production. The

action of doping TiO2 with Nb at hydrolysis to produce a
material suitable for the many proposed applications may lead
to changes to other properties of the material. The anatase to
rutile phase change may be retarded by increased Nb
doping34,40−44 which may be desirable in some circumstances
(when anatase in the desired end product) but undesirable in
other circumstances (e.g., more flux may be required to
produce rutile from anatase at a suitably small particle size).
Final product color may be influenced by the presence of Nb,
giving a blue tint. However, this color change is often desirable
for pigments and can counteract a yellow tint produced by iron
or some other impurities.9,43,45,46

3.7. Why the Placement of Nb is Important for the
Electrical and Optical Properties. It has been shown here
that Nb doped into TiO2 substitutes in the Ti site for either
anatase or rutile and can therefore be fully dispersed within the
TiO2 material. It is important that Nb does this, rather than
forming discrete clusters, in order for the electronic, optical,
and photocatalytic properties to be realized.
The electrical conductivity of Nb-doped TiO2 depends on

both the level of Nb doping with the conductivity increasing

approximately linearly with Nb content over the range 0.003−
0.03 atom % Nb.47 The electrical conductivity is also
dependent on the number of oxygen vacancies. The Nb(V)
substituting for Ti(IV) requires a charge balance which is met
by oxygen deficiency. However, oxygen deficiency can also
result from the reducing conditions in the treatment of the Nb-
doped TiO2. Under mildly reducing conditions (in a study of
0.65 atom % Nb in TiO2), an n-type semiconductor is formed,
whereas under strongly reducing conditions metallic charge
transport is developed.48 A density functional theory
calculation (screened exchange hybrid functional method)
showed that shallow conduction bands should be present in
Nb-doped anatase TiO2, but deep conduction bands in rutile
TiO2.

49 The calculations suggested that Nb donors are
compensated by interstitial oxygen anions except at low
oxygen partial pressures and low O pressures prevent O
interstitials being formed rather than create extra O vacancies.
If too much O is removed, the material is no longer transparent
as a thin film.49

We therefore might expect the Nb-doped rutile form
produced in the work presented here to be less electrically
conductive than the Nb-doped anatase form, when compared
in similar oxygen partial pressure environments. We have
shown that the Nb is placed in the Ti sites rather than as
dimers or a discrete phase and this enables the electronic
properties of the doped material to be realized.

4. CONCLUSIONS
Niobium-doped TiO2, at a level of 0.08 atom % Nb, was
produced by the hydrolysis of liquor from the digestion of
ilmenite in hydrochloric acid as either anatase or rutile and
calcined to form Nb-doped anatase or rutile. XANES of these
doped materials showed that Nb in the doped anatase and
rutile has a different chemical/structural environment in each
of the two forms and the Nb is not in the form of a previously
known Nb oxide structure. An EXAFS analysis revealed that
Nb is substituted into Ti sites of the crystal structure adopted
in each case, anatase or rutile, and that Nb is well dispersed
and does not appreciably interact with other Nb atoms in the
structure and is not present as niobium oxide clusters within
TiO2. Segregation of Nb did not occur on calcination. This
placement of Nb in Ti sites, well dispersed, is important in
order for the electronic, optical, and photocatalytic properties
to be realized. Because this method produces Nb substituted
into Ti sites and because it is analogous to current industrial
scale TiO2 production methods, it may be a suitable low cost
method of producing Nb-doped material to realize the many
potential applications of Nb-doped TiO2, especially if Nb levels
are boosted by the addition of Nb to the hydrolysis solution..

■ ASSOCIATED CONTENT
*sı Supporting Information
The Supporting Information is available free of charge at
https://pubs.acs.org/doi/10.1021/acsomega.2c02676.

EXAFS attempted fits of structures that are not a good
match to the data (PDF)

■ AUTHOR INFORMATION
Corresponding Author

Richard G. Haverkamp − School of Engineering and
Advanced Technology, Massey University, Palmerston North

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4442, New Zealand; orcid.org/0000-0002-3890-7105;
Email: r.haverkamp@massey.ac.nz

Authors
Peter Kappen − Australian Synchrotron, ANSTO, Clayton
3168 Victoria, Australia

Katie H. Sizeland − Australian Synchrotron, ANSTO, Clayton
3168 Victoria, Australia

Kia S. Wallwork − Australian Synchrotron, ANSTO, Clayton
3168 Victoria, Australia

Complete contact information is available at:
https://pubs.acs.org/10.1021/acsomega.2c02676

Notes
The authors declare no competing financial interest.

■ ACKNOWLEDGMENTS
This research was undertaken on the XAS and PD Beamlines
at the Australian Synchrotron, part of the ANSTO; grant
numbers M4871 and M2870. Travel funding was provided by
the New Zealand Synchrotron Group.

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