Rapid solid-state metathesis route to transition-metal doped titanias

Article abstract of DOI:10.1016/j.jssc.2015.09.028

Rapid solid-state metathesis (SSM) reactions are often short-lived highly exothermic reactions that yield a molten alkali halide salt that aids in product growth and crystallization. SSM reactions may also produce kinetically stabilized structures due to the short (seconds) reaction times. This report describes the investigation of rapid SSM reactions in the synthesis of transition-metal doped titanias (M-TiO₂). The dopant targeted compositions were ten mol percent and based on elemental analysis, many of the M-TiO₂ samples were close to this targeted level. Based on surface analysis, some samples showed large enrichment in surface dopant content, particularly chromium and manganese doped samples. Due to the highly exothermic nature of these reactions, rutile structured TiO₂ was observed in all cases. The M-TiO₂ samples are visible colored and show magnetic and optical properties consistent with the dopant in an oxide environment. UV and visible photocatalytic experiments with these visibly colored rutile M-TiO₂ powders showed that many of them are strongly absorbent for methylene blue dye and degrade the dye under both UV and visible light illumination. This work may open up SSM reactions as an alternate non-thermodynamic reaction strategy for dopant incorporation into a wide range of oxide and non-oxides.

Full text of DOI:10.1016/j.jssc.2015.09.028

Journal of Solid State Chemistry 232 (2015) 241–248
Contents lists available at ScienceDirect
Journal of Solid State Chemistry
journal homepage: www.elsevier.com/locate/jssc
Rapid solid-state metathesis route to transition-metal doped titanias
n
Nathaniel Coleman Jr., Sujith Perera, Edward G. Gillan
Department of Chemistry, University of Iowa, Iowa City, IA 52242, United States
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 12 August 2015
Accepted 24 September 2015
Rapid solid-state metathesis (SSM) reactions are often short-lived highly exothermic reactions that yield
a molten alkali halide salt that aids in product growth and crystallization. SSM reactions may also pro-
duce kinetically stabilized structures due to the short (seconds) reaction times. This report describes the
investigation of rapid SSM reactions in the synthesis of transition-metal doped titanias (M–TiO2). The
dopant targeted compositions were ten mol percent and based on elemental analysis, many of the
M–TiO2 samples were close to this targeted level. Based on surface analysis, some samples showed large
enrichment in surface dopant content, particularly chromium and manganese doped samples. Due to the
highly exothermic nature of these reactions, rutile structured TiO2 was observed in all cases. The M–TiO2
samples are visible colored and show magnetic and optical properties consistent with the dopant in an
oxide environment. UV and visible photocatalytic experiments with these visibly colored rutile M–TiO2
powders showed that many of them are strongly absorbent for methylene blue dye and degrade the dye
under both UV and visible light illumination. This work may open up SSM reactions as an alternate non-
thermodynamic reaction strategy for dopant incorporation into a wide range of oxide and non-oxides.
& 2015 Elsevier Inc. All rights reserved.
Keywords:
Metathesis
Metal doping
Titania
Photocatalysis
1. Introduction
to be synergistic electronic effects between anatase and rutile
particles [5].
Metal oxide coatings and particles have been utilized in a wide
range of materials applications including energy storage and cat-
alysis for energy or fuel production. In the photocatalytic arena,
titanium dioxide (titania or TiO2) is an extensively studied UV
absorbing oxide with utility in both organic dye photocatalytic
oxidation and water splitting reactions. TiO2 photocatalysis has
been utilized for self-cleaning window coatings [1,2]. Titania exists
in primarily two synthetically accessible forms with UV band gap
(Eg) energies: anatase (Eg¼3.2 eV) and rutile (Eg¼3.0 eV). Typical
solution precursor condensation routes to TiO2 involve heating
amorphous titanium hydroxide precipitates near ꢀ500 °C to
produce the more catalytically active anatase form. Anatase irre-
versibly transforms to the thermodynamically stable rutile form
upon heating to higher temperatures near 600 °C, with the tran-
sition temperature influenced by precursor used and presence of
trace impurities or dopants in the amorphous structure [3]. While
bulk crystalline rutile TiO2 is less UV photocatalytically active than
anatase, likely due to recombination of photogenerated electrons
and holes, rutile nanoparticles have shown significant photo-as-
sisted oxidation properties [4]. Commercial TiO2 that is a mixture
of anatase: rutile in ꢀ4:1 ratio (e.g., Degussa P25) shows notably
high photocatalytic activity. The nature of this activity is appears
In addition to being composed of earth-abundant low toxicity
elements, TiO2 shows high UV photocatalytic activity and chemical
stability in acidic and basic environments. Renewed interest in
TiO2 photoactivity stems from its ability to produce hydrogen gas
fuel from renewable resources, particularly by providing photo-
generated electrons to a platinum co-catalyst that produces H2
from water under UV illumination [6]. Both gold and platinum
metal particles deposited on titania show photocatalytic utility
[7,8]. Synthetic efforts in recent years have focused on modifying
titania light absorption properties into the desirable visible light
solar spectrum. For example, organic dyes placed on its surface
allow anatase titania nanoparticles to act as photovoltaic light to
energy conversion (solar cell) materials [9].
Alternate methods to improve titania visible light absorption
take advantage of metal and non-metal dopant incorporation into
the titania structure. The dopant species can either substitute for
Ti or O ions in the lattice or to occupy interstitial sites in the
structure or form oxyanions embedded in the lattice. Anion doping
by main-group elements (e.g., C, N, S, halogens) [10] onto the
oxygen site or into interstitials can potentially raise valence band
or lower the conduction band energy levels or introduce new
dopant levels within the band gap for visible light absorption [11–
16]. Ionic radii of many common oxidation states of transition
metal of ꢀ70 to 90 pm are similar to the Ti4þ ionic radius or
75 pm [17]. Cation doping of titania with transition-metal and
main-group metals includes Fe [18,19], Co [20], Cr [21], Ni [22],
n
Corresponding author.
E-mail address: edward-gillan@uiowa.edu (E.G. Gillan).
http://dx.doi.org/10.1016/j.jssc.2015.09.028
0022-4596/& 2015 Elsevier Inc. All rights reserved.

242
N. Coleman Jr. et al. / Journal of Solid State Chemistry 232 (2015) 241–248
and Mn, often leading to colored titanias [23,24]. Most titania
doping methods begin with solution reaction/precipitation or ion-
implantation followed by thermal annealing or crystallization.
Typically, dopant additions to TiO2 are at low ꢀ1% to 5% levels to
limit thermodynamically preferred second phases that will form
during high temperature annealing. While transition-metal do-
pants may impart visible light absorption properties to TiO2, such
dopants may also act as recombination centers for photogenerated
electron and holes [25,26]. Apart from photocatalytic activity,
transition-metal doped TiO2 can also exhibit interesting dilute
magnetic semiconductor (DMS) properties when doped with
magnetic ions [27–29]. Metal doped titanias have also found ap-
plication in hydrogen evolution catalysis and lithium ion batteries
[30–32].
This paper describes a solvent-free single-step reaction to ra-
pidly produce metal-doped titanias using a solid state metathesis
(SSM) reaction strategy that takes advantage of precursor ther-
mochemical exothermicity to produce crystalline products in
seconds in self-propagating reactions that often require little or no
external energy input. Rapid SSM reactions are an alternate
strategy to solution phase reactions and can rapidly produce
crystalline metal oxide and non-oxide materials using highly
exothermic ion exchange reactions without the need for sub-
sequent annealing. The SSM reactions extend back to the early
1900’s, but were actively developed as a rapid materials growth
strategy over the past couple of decades. Work in the early 1990’s
by Kaner and Parkin demonstrated the utility of rapid self-pro-
pagating SSM reactions to produce a wide range of inorganic
materials in seconds using initiation methods including hot wires,
ampoules placed in heated furnaces, and by external flames [33–
35]. Crystalline products rapidly formed by SSM reactions include
layered MX2 (M¼Mo, W, X¼S, Se) [36], GaE (E¼N, P, As) [37,38],
transition-metal and lanthanide nitrides, phosphides, and borides
(e.g, ZrN, GdN, ZrP, and TiB2) [39–42].
deionized water (18 M ) and 1 M HCl (Fisher Scientific, diluted)
Ω
was used for wash processes. P25-TiO2 (Degussa Corp.) and me-
thylene blue (high purity, Alfa Aesar) or methyl orange (85% dye
content, Sigma Aldrich) dyes were used in photocatalysis studies.
2.2. Synthesis of transition-metal doped TiO2
First-row 3d transition metals were incorporated into the TiO2
structure using multiple metal halides precursors in a solid state
metathesis (SSM) reaction based on our prior work with SSM TiO2
synthesis [47]. All reagent manipulations were performed in a
Vacuum Atmospheres argon filled glove box. Each anhydrous
metal chloride (MCl2 or MCl3) was used in an amount to produce a
1 M to 9 Ti molar ratio for products (e.g., M0.1Ti0.9O2 target) or
10 at% M with respect to total metal amount. Typically, 2.00 g
(13.0 mmol) of ground TiCl3 was mixed and ground in a mortar
and pestle with the dopant metal halide. Specifically, dopant
amounts used were: CrCl3 (0.228 g, 1.44 mmol), MnCl2 (0.182 g,
1.45 mmol), FeCl3 (0.236 g, 1.45 mmol), CoCl2 (0.188 g, 1.45), NiCl2
(0.184 g, 1.42 mmol), and CuCl2 (0.193 g, 1.44 mmol). The mixed
metal halide powder was then mixed with ground Na2O2 powder.
For the MCl2 and MCl3 reactions, respectively, 1.63 g (20.9 mmol)
or 1.69 g (21.7 mmol) of Na2O2 was used to properly balance NaCl
salt elimination from the SSM reaction. The intimately ground
powders were placed inside a stainless steel crucible that was then
placed inside a custom-made thick wall steel SSM ignition reactor
that resembles a non-sealed bomb calorimeter (Fig. 1). Ceramic
and quartz crucibles were initially used for these SSM reactions,
however, due to the violent nature or rapid thermal changes that
occur in these reactions, such crucibles frequently fractured after
only a few uses so a stainless steel crucible was generally used.
After placing the steel crucible in the reactor, a 0.64 mm diameter
nichrome wire was attached to two electrical posts on the reactor
lid and was buried in the precursor powder. The closed reactor
was removed from the glove box and the wire was resistively
heated to a red hot level (ꢀ500 °C) using a setting of ꢀ10 V on a
Variac, which initiated the SSM reaction. Reaction initiation was
Relevant to the current work, solid-state oxygen source re-
agents (e.g., Na2O, Na2O2, Li2O) have been used for SSM growth of
transition-metal oxides, including complex AMxOy structures
[33,43–45]. A rapid SSM reaction was also utilized to produce
cubic stabilized ZrO2 by doping zirconia with Ca, Y, or Ce (ꢀ5% to
10%) during the SSM process [46].
In previous work, we produced crystalline rutile TiO2 micro-
particles using a rapid and exothermic SSM reaction between TiCl3
and Na2O2 (Eq. (1)) [47].
(1)
TiCl3 + 1.5Na2O2 → TiO2 + 3NaCl + 0.5 O2
Rutile TiO2 is synthesized in seconds using this exothermic
exchange reaction that transiently reaches temperatures as high as
the NaCl boiling point of ꢀ1400 °C. We also synthesized anatase
TiO2 nanoparticles using similar exchange reactions under sol-
vothermally heated conditions [48]. In the current study, we ex-
amine the ability of rapid SSM reactions to introduce moderate
(ꢀ10 at%) levels of transition-metals into rutile TiO2. The struc-
tural and physical properties of these SSM-synthesized metal-
doped titanias and their utility in UV and visible organic dye ad-
sorption and oxidation photocatalysis are described.
2. Experimental section
2.1. Reagents
All starting materials were used as received: TiCl3 (Aldrich,
99%), CrCl3 (Alfa Aesar, 98%), MnCl2 (Specialty Inorganics, 99.5%),
FeCl3 (Alfa Aesar, 98%), CoCl2 (Alfa Aesar, 99.7%), NiCl2 (Alfa Aesar,
99%), CuCl2 (Alfa Aesar, 98%), Na2O2 (Sigma Aldrich, 97%). Distilled
Fig. 1. Exploded view image of a home-built steel SSM reactor.

N. Coleman Jr. et al. / Journal of Solid State Chemistry 232 (2015) 241–248
243
observed by wisps of fine powder or smoke exiting the non-sealed
edges of the reactor lid and reactions were usually complete
within a few seconds leaving the reactor exterior walls warm to
the touch. The inside of the reactor was usually covered with a
thin coating of yellowish, off-white material, and a large amount of
a darker glassy product was in the crucible. In order to purify the
crude products and remove sodium containing impurities, metal
doped titania samples were washed with 100 ml 1 M HCl for
ꢀ30 min at room temperature under constant stirring. The acid-
washed samples were then rinsed several times with distilled
water until the pH of the rinse was neutral. All solid products were
dried in air at room temperature. Selected samples were also an-
nealed for one day at 1000 °C in air in a box furnace at a heating at
a rate of ꢀ100 °C/hr. The samples were cooled naturally to room
temperature.
quantitative surface compositions and peak deconvolutions were
performed using CasaXPS software package (www.casaxps.com)
with a KratosAxis specific element library (KratosAxis-F1s.lib) and
the following RSF factors: Ti2p (2.0), Cr 2p (2.43), Mn2p (2.66),
Fe2p (2.96), Co2p (3.59), Ni (4.04), Cu (5.32). Relative atomic sur-
face compositions were measured from survey scans.
2.4. Photocatalytic oxidative degradation of dyes
Photocatalytic oxidative degradation of methylene blue (MB)
and methyl orange (MO) in air was performed using an Ace-Ha-
novia medium pressure 450 W mercury lamp in a water cooled
Pyrex jacket. Approximately 10 mg of doped and undoped titanias
were loaded into 20 ml pre-cleaned glass scintillation vials with
10 ml of a 3.00 ꢁ 10ꢂ5 M MB solution or 6.11 ꢁ 10ꢂ5 M MO solu-
tion and a stir bar. A dye sample with no powder was run as a
blank. SSM synthesized undoped rutile TiO2 and Degussa P25 TiO2
(ꢀ80% anatase) samples were also used for comparison. The
sample vials were placed on a large stir plate about 25 cm away
from the mercury lamp, all of which were contained in a closed
photochemical reactor cabinet. The samples were allowed to stir in
the dark for 30 min to allow surface equilibration or adsorption of
the MB or MO dye. UV irradiation was in regular intervals of 5 min
or longer. Between each interval, the samples were centrifuged
and UV–vis measurements were taken on the solution. The ana-
lyzed solutions were returned to the original vial and the irra-
diation was repeated. A similar set of experiments were performed
using 420 nm cut off filters (Edmund Optics) to limit the UV lamp
output to visible light wavelengths. According to our testing, these
cut off filters have a stop band limit or 0.001% T point at 430 nm.
Linear regression of ꢂln (C/Co) versus time data for 4 data points
in the first 20 min of UV irradiation (initial 40 min for visible) was
used to estimate initial rate constants for MB dye degradation.
2.3. Product characterization
Phase identification was conducted using Siemens D5000 or
Bruker D8 DaVinci powder X-ray diffraction (XRD) systems that
analyzed ground powders affixed to glass slides with either va-
cuum grease or using an acetone slurry. Morphologies and semi-
quantitative elemental analysis was obtained by scanning electron
microscopy (SEM) and energy dispersive spectroscopy (EDS) using
a Hitachi S4800 or S3400 system. Samples were ground to fine
powders and some were lightly pressed into thin pellets for EDS
with an IR pellet hand press then affixed onto aluminum stubs
with carbon tape. Samples were carbon coated to minimize char-
ging. Quantitative analysis by ICP–OE spectrometry (Varian 720-
ES) was performed on acid dissolved samples. Approximately 5 mg
of each sample was dissolved in an acid mixture of 5 ml of con-
centrated H2SO4 and 1 ml of concentrated HNO3, which was he-
ated to ꢀ385 °C for 1 h. The cooled solutions were diluted to the
1–100 ppm range depending on the metal concentration. Cali-
bration standard curves were used to determine weight percent
content for dopant metals and titanium. Magnetic susceptibility
measurements were performed on solid powders at room tem-
perature using a Johnson-Matthey MSB (Evans) magnetic sus-
ceptibility balance. Strongly magnetic samples were diluted
in NaCl prior to analysis. All molar susceptibility results were
corrected for sample core diamagnetism for TiO2 of
2.9 ꢁ 10ꢂ5 cm3/mol or M–TiO2 of 2.98 ꢁ 10ꢂ5 cm3/mol. Spin-only
magnetic moments were calculated from molar susceptibility per
mole of dopant metal using mB¼2.83(χm ꢁ T)1/2. Scaled molar
susceptibility was calculated by subtracting χm from undoped
SSM–TiO2 and dividing by the relative molar amount of M de-
termined by ICP MxTi1ꢂxO2 structure. Solid diffuse reflectance UV–
vis measurements were made with a LabSphere RSA accessory on
an HP 8453 UV–vis spectrometer. The powders were physically
embedded onto filter paper supports sandwiched between glass
microscope slides. Each sample's diffuse reflectance (R) data was
converted to Kubelka–Munk (K–M) units and plots of F(R)¼(1ꢂ
R)2/2R versus energy used to estimate absorption energy onsets or
band gaps. The absorption onset data is derived from extrapolation
of linear region of absorption rise down to baseline spectral region.
Single baselines were used except for the Co– and Mn–TiO2 data
where different baselines better represented onset starting points
of different regions. FT-IR spectra were obtained for each sample
using KBr pellets in a Nicolet Nexus 760 spectrometer. X-ray
photoelectron spectroscopy (XPS) data were obtained on a Kratos
3. Results and discussion
3.1. Synthesis of metal-doped titania via rapid SSM reactions
Rapid and exothermic self-propagating SSM methods for the
synthesis of inorganic metal oxide and non-oxide materials have
broad flexibility to produce binary solids and more complex
structures containing intimately mixed multiple metal and non-
metal components. In the current study, we take advantage of the
rapid and non-equilibrium exothermic processes of SSM reactions
to incorporate moderate amounts of a second transition-metal
into the rutile TiO2 structure. Typical thermodynamically driven
methods for doping transition-metals into titania use sol–gel or
solution precipitation methods followed by thermal processing,
which leads to low metal dopant levels (ꢀ1%) in titania. Given the
rapid heating/crystallization afforded by SSM reactions, there is
potential for kinetically stabilized higher dopant level incorpora-
tion, thus we “overloaded” the reaction system with 10 at% levels
of dopant metals. Such additions, should impart visible optical
absorption properties to the UV absorbing titania. The ideal SSM
reactions for MCl2 dopants (Mn, Co, Ni, Cu,) and MCl3 (Cr, Fe) are
shown below in Eqs. (2) and (3).
0.9 TiCl3þ0.1 MCl2þ1.45 Na2O2-M0.1Ti0.9O2þ0.45 O2þ2.9 NaCl (2)
Axis Ultra Imaging spectrometer using monochromatic Al K ra-
α
0.9 TiCl3þ0.1 MCl3þ1.5 Na2O2-M0.1Ti0.9O2þ0.5 O2þ3 NaCl (3)
diation. Powders were embedded in indium foil for analysis. Sur-
vey spectra were obtained for acid washed samples (nominally
10 at% M doped TiO2) and regional area spectra were obtained for
Ti2p, O1s, as well as M2p (M¼Cr, Fe, Mn, Co, Ni, Cu). Peak posi-
tions are reported relative to the C1s peak at 284.5 eV. Semi-
In all cases, the SSM reactions were easily initiated in a self-
propagating mode using a heated filament in the steel SSM re-
actor. The doped titania products (generally referred to as M–TiO2)
after water and acid workup were visibly colored, with colors

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N. Coleman Jr. et al. / Journal of Solid State Chemistry 232 (2015) 241–248
Table 1
Experimental conditions and results for acid-washed transition-metal doped tita-
nias from rapid exothermic SSM reactions between MClx/TiCl3 and Na2O2 targeting
M0.1Ti0.9O2.
M–TiO2
MClx
XRDa
Product color,
percent yield
ICP (EDS) analysis
Ti:M molar ratio
Cr–TiO2
Mn–TiO2
Fe–TiO2
Co–TiO2
Ni–TiO2
Cu–TiO2
CrCl3
MnCl2
FeCl3
CoCl2
NiCl2
CuCl2
Rutile TiO2
Rutile TiO2
Rutile TiO2
Rutile TiO2
Rutile TiO2
Rutile TiO2
Yellow, 71%
Brown, 86%
lt brown, 73%
Green, 68%
Gray, 67%
1.0:0.037 (1.0:0.046)
1.0:0.104 (1.0:0.174)
1.0:0.063 (1.0:0.061)
1.0:0.102 (1.0:0.115)
1.0:0.042 (1.0:0.090)
1.0:0.069 (1.0:0.115)
lt brown, 58%
a
Minor crystalline secondary phases were sometimes observed (see text for
details).
generally consistent with the dopant metal in an oxide environ-
ment. Table 1 lists several qualitative and quantitative results ob-
tained for these SSM M–TiO2 products with some of these results
highlighted below. The isolated yields for the acid washed mate-
rials (assuming ideal M0.1Ti0.9O2 compositions) were generally in
the ꢀ60% to 80% range. These yields likely reflect some TiCl3 loss
via its decomposition to volatile TiCl4 during the highly exother-
mic SSM reaction.
XRD data showed the presence of Na–Ti–O phases (e.g.,
Na2Ti6O13), consistent with our earlier SSM TiO2 work [47]. An acid
wash step removed most detectable NaTixOy impurities. EDS data
from water versus acid washed samples show that the acid wash
significantly decreases sodium content along with removing some
dopant metal. Powder XRD data in Fig. 2 demonstrates that acid-
washed metal-doped titanias are primarily crystalline rutile TiO2
(PDF #21-1276). The rutile TiO2 phase is expected as these SSM
reactions can reach transient temperatures over 1300 °C and so it
would be difficult to produce the low-temperature anatase TiO2
phase under such conditions. Some acid washed samples still
show evidence of Na2Ti9O19 (PDF #78-1590) and Na2Ti6O13 (PDF
#37-0951) impurities. The Ni–TiO2 sample shows two small peaks
consistent with NiO and the Co–TiO2 sample has small peaks that
may be Co2TiO4 (PDF #39-1410), though some of these small peaks
overlap with Na–Ti–O phases.
Fig. 2. Powder XRD data for acid-washed M–TiO2 samples produced by SSM re-
actions where M¼(a) Cr, (b) Mn, (c) Fe, (d) Co, (e) Ni, (f) Cu. The standard peak
positions for crystalline rutile TiO2 are shown in (g). The * marks Na2Ti9O19
,
ꢃ
marks Co2TiO4, þ marks Na2Ti6O13, and ■ marks NiO.
Table 2
Selected acid-washed samples were annealed in air at 1000 °C
to examine whether metal dopant segregation was observed. Gi-
ven the high dopant levels in these M–TiO2 materials, it is not
surprising that crystalline metal titanate or Na–M–Ti–O phases are
observed after prolonged high temperature annealing. In most
annealed products, the major phase remains rutile TiO2 (except for
Co where Na2CoTi7O16 is dominant) with new minor phases seen
for Cr (Na2Cr2Ti6O16), Mn (Na2Mn2Ti6O16), Fe (Na2Fe3Ti6O16), Ni
(NiTiO3, Na0.23TiO2, NiO), and Cu (Na0.86Cu0.43Ti3.57O8,
NaCu2.5Ti4.5O12). This provides further support that the rapid SSM
reactions trap dopants in non-thermodynamic compositions and
structures that will convert to thermodynamically preferred
structures upon annealing.
Summary of XPS analysis on M–TiO2 powders.
Compound Surface Ti:M (rel.
molar ratio)
Ti2p3/2 transition M2p3/2 transitions
(eV)
(eV)a
Cr–TiO2
Mn–TiO2
Fe–TiO2
Co–TiO2
1:0.384
1:0.599
1:0.064
1:0.109
458.3
458.0
458.4
458.2
576.8, 579.4
641.3, 642.7
710.7, 712.7
780.1, 782.0,
784.6, 786.5
855.3, 861.4
931.4, 932.8, 933.7,
939.8, 942.3
Ni–TiO2
Cu–TiO2
1:0.100
1:0.089
458.2
457.2
a
Major transition(s) from peak deconvolution are bolded.
3.2. Compositional analysis of SSM synthesized metal-doped titanias
concentrations on the particle surface.
The acid-washed M–TiO2 powders were analyzed by XPS to
examine relative composition of metals on the surface and che-
mical states of the surface species. Table 2 summarizes survey and
regional scan data for the metal doped titania powders. In most
cases, the semiquantitative surface compositions (Ti:M ratios) for
the powders is comparable to the bulk analysis results described
earlier, with a few notable exceptions. While the bulk and most of
the surface analysis results support that the achieved dopant le-
vels are generally less than the targeted M0.1Ti0.9O2 product, both
the Cr and Mn doped samples show very high surface metal do-
pant amounts relative to their bulk compositions. This result was
In contrast to typically low ꢀ1% metal dopant levels found in
solution methods to metal doped titanias, the ICP results show the
SSM synthesized M–TiO2 materials have a much higher bulk do-
pant metal content (Table 1). The transition-metal dopant levels
range near the targeted Ti:M ratio for M0.1Ti0.9O2 of 1:0.11 down
ꢀ1:0.05. Both ICP and EDS data for these acid-washed SSM reac-
tion products show that they retain significant amounts of dopant
metal. Some of the semiquantitative EDS results show higher do-
pant content than the more quantitative standardized ICP mea-
surements, which may indicate some higher dopant

N. Coleman Jr. et al. / Journal of Solid State Chemistry 232 (2015) 241–248
245
verified on several different products. The high dopant surface
3.3. Morphologies of SSM synthesized metal-doped titanias
content may impact molecular dye adsorption discussed later in
this paper. In comparing ICP with EDS or XPS data in Tables 1 and
2, it appears that the high dopant contents targeted in these short-
lived SSM reactions lead to dopant rich near surface regions even if
not detected by XRD analysis, specifically observed for Cr, Mn, and
Ni doped titanias.
SEM analysis of M–TiO2 samples shows a range of particle sizes
and shapes, consistent with that expected from a rapid exothermic
reaction where powder products reside in a molten NaCl salt flux
for a very short time. Fig. 3 shows representative images for the
M–TiO2 powders that were lightly pressed into pellets (additional
loose powder images are in Supporting information Fig. S1). The
samples are fairly heterogeneous in size and shape, with distinct
facets visible on some crystallites faces. Particles with roughly
spherical shapes and aggregates are visible as are larger faceted
and elongated rod-like structures. The particles roughly range
Degussa P25-TiO2 yields a Ti 2p3/2 XPS peak at 458.3 eV and
undoped SSM–TiO2 has a peak at 458.2 eV, which are very near
literature titania values [49]. The Ti 2p3/2 binding energies for
metal-doped TiO2 in Table 2 are generally close to the pure TiO2
values, but several are shifted to lower energies that may indicate
some reduced Ti3þ on the surface. The commercial P25-TiO2 and
SSM–TiO2 have one major oxygen O1s chemical environment at
ꢀ529.5 eV, which is also the major oxygen surface environment
found in the M–TiO2 samples, in addition smaller peak intensity in
the ꢀ530 to 532 eV region for oxide environments due to the
metal dopants. The metal dopant’s 2p3/2 peak positions listed in
Table 2 are consistent with binary oxide metal oxidation states, for
example (lit. value, eV) Cr3þ (576.9), Mn4þ (642.1), Fe3þ (710.8),
Co2þ (780.6) and Ni2þ (855.6), and Cu2þ (933.6) [50–55]. There
may be some Cuþ present in the Cu–TiO2.
from ꢀ500 nm to 5
μ
m, though much larger 10–30 m crystallites
μ
are also visible. The well-defined crystallite structures and relative
large particles observed from this SSM reaction may be a direct
result of the initial powder forming in a byproduct molten salt flux
that would aid in product crystallization and growth.
3.4. Magnetic and optical properties of metal-doped titanias
The room-temperature magnetic susceptibilities of the M–TiO2
samples are listed in Table 3. Undoped SSM–TiO2 shows a small
paramagnetic response likely due to either some low level Ti3þ or
Fig. 3. Representative SEM images for M–TiO2 materials with M¼Cr (A), Mn (B), Fe (C), Co (D), Ni (E), and Cu (F).

246
N. Coleman Jr. et al. / Journal of Solid State Chemistry 232 (2015) 241–248
Table 3
Room-temperature magnetic and optical results on M–TiO2 powders from SSM
reactions.
M–TiO2
Mass (χg, cm3/g) and Magnetic UV–vis ab-
molar (χm, cm3/mol) moment
sorbance in energy on-
Absorption
magnetic
susceptibility
per mol M nm (type, E sets (eV)
(BM)a
in eV)b
from K–M
analysis
TiO2 (P25) χg¼ ꢂ0.0098 ꢁ 10ꢂ6
χm¼ þ0.298 ꢁ 10ꢂ4
0.27
0.43
5.7
408 (o, 3.04) 3.05
415 (o, 2.99) 3.00
TiO2 (SSM) χg¼ þ0.609 ꢁ 10ꢂ6
χm¼ þ0.775 ꢁ 10ꢂ4
Cr–TiO2
Mn–TiO2
Fe–TiO2
Co–TiO2
χg¼ þ6.34 ꢁ 10ꢂ6
χm¼ þ5.39 ꢁ 10ꢂ4
570 (o, 2.18), 2.15, 1.50
700 (p, 1.77),
800 (o, 1.55)
670 (o, 1.85), 1.80, 1.35
710 (p, 1.75),
920 (o, 1.35)
480 (o, 2.59), 2.45, 2.10
490 (p, 2.53),
580 (o, 2.53)
510 (o, 2.43), 2.40, 1.60
580 (p, 2.14),
660 (p, 1.88),
χg¼ þ7.55 ꢁ 10ꢂ6
χm¼ þ6.38 ꢁ 10ꢂ4
3.8
7.5
4.7
χg¼ þ17.1 ꢁ 10ꢂ6
χm¼ þ14.1 ꢁ 10ꢂ4
χg¼ þ11.2 ꢁ 10ꢂ6
χm¼ þ9.35 ꢁ 10ꢂ4
800 (o, 1.55)
500 (o, 2.48), 2.55, 1.55
740 (p, 1.68),
810 (o, 1.53)
490 (o, 2.53), 2.55
740 (p, 1.68)
Ni–TiO2
χg¼ þ7.54 ꢁ 10ꢂ6
χm¼ þ6.40 ꢁ 10ꢂ4
6.0
3.4
Cu–TiO2
χg¼ þ4.20 ꢁ 10ꢂ6
χm¼ þ3.72 ꢁ 10ꢂ4
a
Estimated spin-only magnetic moment at 298 K for M–TiO2 samples calcu-
lated from χm due to metal dopant (total χm – χm for undoped SSM TiO2) and scaled
per mol M–TiO2 based on ICP composition. Degussa P25-TiO2 and SSM–TiO2 mo-
ments are per mole of TiO2.
b
p¼Broad absorption peak, o¼onset of absorption event.
other magnetic metal content from either steel reactor or ni-
chrome wire. The paramagnetic responses from metal-doped ti-
tanias are all much larger than that for undoped TiO2 and should
be due to the dopant metal. The paramagnetic response from acid-
washed M–TiO2 samples is generally larger for more heavily doped
materials. A estimate of magnetic moment per mole of dopant
metal is listed in Table 3 (ꢀ3.4 to 7.5 BM) and most values fall in a
range expected for spin-only paramagnetic metal ions with several
unpaired d electrons (n¼1–5 unpaired electrons with 1.7–5.9 BM).
If one assumes that dopant ions are ideal spin-only paramagnets,
Fig. 4. Absorption and UV photodegradation of methylene blue (MB) by M–TiO2
powders. The first half hour is equilibration of dye solution in the dark (surface
adsorption) and light was turned on at time¼0 h.
to smaller band gaps, however uncertainty and low intensity of
the data, makes this a qualitative observation.
3.5. Adsorption and photooxidative degradation of organic dyes on
SSM M–TiO2 powders
then the values in Table 3 roughly correspond to Cr2þ, Mn4þ
,
Fe3þ, Co3þ, with the Ni and Cu values exceeding that expected for
Ni2þ or Cu2þ. The magnetic measurements along with XPS show
that metal ions are in oxidized states and possibly clustered to-
gether leading to enhanced magnetic behavior.
Finely ground M–TiO2 powders suspended in aqueous methy-
lene blue (MB) dye solutions were irradiated with broad spectrum
UV illumination with periodic analysis of dye remaining in solu-
tion using UV–vis spectroscopy. The MB dye contains a hetero-
cyclic ring with N/S atoms that is oxidatively degradable by oxygen
on a catalyst surface. For comparison purposes, undoped rutile
SSM–TiO2 (SSM–TiO2) and a commercial anatase standard powder
(ꢀ50 nm particles Degussa P25-TiO2) were analyzed along with
the M–TiO2 samples. The results are shown in Fig. 4. On notable
difference for many M–TiO2 samples versus P25-TiO2 was ob-
served moderate or large degree (ꢀ40% to 90%) of dye surface
adsorption that occurred during the 30 min dark equilibration
step. This is a fairly reversible process as the Mn–TiO2 after dark
dye equilibration will easily release its adsorbed dye when placed
in methanol. After several hours of UV irradiation, SSM–TiO2 and
the M–TiO2 samples (except the chromium and cobalt samples)
retain a bluish color consistent with some adsorbed intact MB dye
and leaving very pale colored blue solutions.
The solid-state optical absorption properties of the SSM-syn-
thesized M–TiO2 materials were examined by diffuse reflectance
UV–vis spectroscopy. In contrast to white commercial P25 TiO2,
the metal-doped titanias are all visibly colored powders (see Ta-
ble 1) and show a range of visible light absorption. Undoped SSM
rutile TiO2 has a visible off-white beige color even after acid
washing, possibly due to some Ti3þ content, but shows little de-
tectable visible absorption versus the M–TiO2 materials. Table 3
lists approximate absorption wavelengths and energies for the
M–TiO2 products. The extrapolated onset absorption energy for
commercial P25 TiO2 and SSM–TiO2 samples is near 3 eV, while
the M–TiO2 samples show red-shifted lower energy absorptions
(ꢀ1.8 to 2.6 eV) and several show with additional absorption
further in the visible region around 500–750 nm (ꢀ1.7 to 2.5 eV).
Optical absorption properties are consistent with transition-metal
ions present in the titania. An examination of low energy valence
band region of the XPS spectra for M–TiO2 samples indicates that
dopants may raise the valence band energy level that would lead
For comparison, highly active commercial anatase-rich P25
TiO2 is more photoactive than these rutile samples and clears dye

N. Coleman Jr. et al. / Journal of Solid State Chemistry 232 (2015) 241–248
247
Table 4
adsorption, Mn, Fe, and Cu doped samples show respectable dye
degradation comparable to the undoped TiO2 sample.
Summary of methylene blue dye absorption and photodegradation on M–TiO2.
Similar UV experiments were performed with a methyl orange
(MO) dye, which has an azo nitrogen double bond linking two
heterocyclic rings. The rutile M–TiO2 samples absorb little MO dye
during dark equilibration and were less photocatalytically active
leaving 80–95% MO dye in solution after 4 h of UV irradiation. For
comparison, anatase P25-TiO2 UV photodegrades ꢀ100% of the
MO dye after 2.5 h leaving a clear, colorless solution and a white
solid. Thus either the rutile structure or interference from metal
dopant makes them less effective for MO dye degradation.
Given the visible absorption properties of M–TiO2 powders, the
visible light photocatalytic degradation of MB dye was investigated
with filtered UV (4420 nm). Dark equilibrium dye absorption is
again significant for most M–TiO2 samples, though magnitudes
vary from earlier UV studies, suggesting surface charge of M–TiO2
samples may impact degree of MB absorption. Upon visible light
irradiation, several M–TiO2 samples showed photodegradation
activity towards MB, but at reduced rates versus the UV experi-
ments (Fig. 5). Due to UV filtering, both P25-TiO2 and annealed
P25-TiO2 (rutile phase) samples showed significantly lower dye
degradation. Table 4 shows comparison of MB photodegradation
after 4 h visible light versus 2 h of UV illumination. While sig-
nificant visible light assisted MB removal is seen for most samples,
only Mn and Fe samples cause dye elimination of 90% or greater.
Compound Total MB % Rate con-
Total MB %
loss after 4 h
vis (% loss
from dark
absorb)
Rate constant k
(hꢂ1) for vis MB
degrad.
loss after
2 h UV (%
loss from
dark
stant k (hꢂ1
for UV MB
degrad.
)
absorb)
MB alone
P25-TiO2
23 (n/a)
98 [7] (after 13.7(5)
0.5 h)
0.14(1)
23 (n/a)
79 [22] an-
nealed: 45 [5] 0.26(3)
79 [43]
67 [50]
97 [62]
90 [81]
78 [46]
69 [45]
86 [48]
0.11(2)
0.42(5) annealed:
SSM–TiO2 99 [55]
2.2(3)
0.8(4)
3.2(3)
2.2(8)
0.54(4)
0.5(1)
3.3(4)
0.29(7)
no MB loss
1.7(4)
Cr–TiO2
Mn–TiO2
Fe–TiO2
Co–TiO2
Ni–TiO2
Cu–TiO2
63 [46]
99 [95]
88 [63]
68 [36]
91 [63]
97 [53]
1.9(8)
0.35(7)
0.12(9)
0.8(4)
4. Conclusions
Rapid SSM reactions are capable of incorporating relatively
large amounts of dopant metals into titania during the short-lived
exothermic reaction. The targeted Ti:M bulk molar ratio was 1:0.11
with actual analyzed values being generally at or below the target
value. The nature of the rapid SSM reaction that involves rapid
cooling from a molten salt may impact the degree of metal dopant
incorporation and observed sodium titanate components. The
washed doped rutile TiO2 materials show optical, magnetic, and
structural properties consistent with dopant primarily residing in
the oxide structure, though some samples show surface enrich-
ment of dopant content. Several of the M–TiO2 products show
both UV and visible photodegradation activity for a methylene
blue dye. In the case of visible light experiments, several M–TiO2
samples more effectively used the incoming light for photo-
degradation than the P25 anatase titania standard. All M–TiO2
samples demonstrated significant dark dye adsorption that may
aid in bringing the dye near oxygen and photogenerated electron/
hole pairs that perform the dye oxidation.
Acknowledgments
The University of Iowa’s GAANN fellowship program (N. C.) and
National Science Foundation (E. G., Grant #CHE-0957555) are
gratefully thanked for partial support for this research. The NSF
REU summer program (Grant # CHE-1062575) supported Joey
Squires, Liam Taylor, and Tyler Van Heest who are thanked for
their early contributions to this work. Jonas Baltrusaitus and Sylvia
Lee are thanked for XPS data acquisition assistance.
Fig. 5. Visible light methylene blue photodegradation by M–TiO2 powders. The first
half hour is equilibration of dye solution in the dark (surface adsorption) and light
was turned on at time¼0 h.
from the solution after ꢀ30 min of UV exposure. (Supporting in-
formation Fig. S2). Table 4 shows dark absorption and total dye
degradation amounts after subsequent 2 h UV irradiation where
the most photoactive samples reach high dye degradation values.
The initial degradation data was used to calculate approximate
first order rate constants from the data (Supporting information
Fig. S3). As shown in Table 4, in addition to high dark dye
Appendix A. Supplementary material
Supplementary data associated with this article can be found in
the online version at http://dx.doi.org/10.1016/j.jssc.2015.09.028.

248
N. Coleman Jr. et al. / Journal of Solid State Chemistry 232 (2015) 241–248
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