Improved performance of titanate nanostructures for manganese adsorption and posterior pollutants photocatalytic degradation

Article abstract of DOI:10.1016/j.jphotochem.2019.04.009

In this work, the synthesis of novel manganese-modified titanate nanowires, with enhanced photocatalytic activity for pollutants degradation, is described. Distinct modified titanate nanowires samples were produced by manganese doping (Mn-TNT) and by Mn ion-exchange (TNW/Mn). The characterization of the samples was performed by XRD, DRS, TEM, PL and XPS. The structural characterisation indicates that Mn can be incorporated in the crystalline structure in two distinct positions: replacing some Ti⁴⁺ in the TiO₆ octahedra and/or replacing Na⁺ in the interlayers. For the TNW/Mn sample, the Mn was found only in the interlayers. However, for Mn-TNW the metal was present in both possible positions. The produced materials demonstrate to be photo-active in a wider range of radiation then pristine TNW, even starting to absorb in the visible range. The PL and XPS results indicate a decrease on the photogenerated charge carriers recombination for the metal-containing samples. The photocatalytic performance of the modified samples was tested for the hydroxyl radical production. Both Mn modified samples (Mn-TNW and TNW/Mn) demonstrated to be catalytic for this reaction. The photocatalytic degradation of the preservative methyl-paraben, considered an emergent pollutant was also studied. For this process, the results indicate that the mechanism of degradation is dependent on the photocatalyst surface ionic character. The best photocatalytic performance was achieved by the doped Mn-TNW sample, with 62% of removal of a 10 ppm methyl-paraben aqueous solution (0.13 g catalyst/L solution), after 90 min of irradiation.

Full text of DOI:10.1016/j.jphotochem.2019.04.009

Accepted Manuscript
Title: Improved performance of titanate nanostructures for
manganese adsorption and posterior pollutants photocatalytic
degradation
Authors: Sara C.A. Sousa, J.C. Cardoso, O.C. Monteiro
PII:
S1010-6030(18)31711-8
DOI:
Reference:
https://doi.org/10.1016/j.jphotochem.2019.04.009
JPC 11798
To appear in:
Journal of Photochemistry and Photobiology A: Chemistry
Received date:
Revised date:
Accepted date:
25 November 2018
8 March 2019
8 April 2019
Please cite this article as: Sousa SCA, Cardoso JC, Monteiro OC, Improved performance
of titanate nanostructures for manganese adsorption and posterior pollutants
photocatalytic degradation, Journal of Photochemistry and amp; Photobiology, A:
Chemistry (2019), https://doi.org/10.1016/j.jphotochem.2019.04.009
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Improved performance of titanate nanostructures for manganese adsorption and
posterior pollutants photocatalytic degradation
Sara C.A. Sousa, J.C. Cardosoa and O.C. Monteiro*1
Centro de Química e Bioquímica and Centro de Química Estrutural, Faculdade de Ciências,
Universidade de Lisboa, 1749-016 Lisboa, Portugal
*
Corresponding author
a
present adress: Faculdade de Engenharia da Universidade do Porto - Departamento de
Engenharia Química, 4200-465 Porto, Portugal.
Graphical abstract
Research highlights



Novel Mn-modified titanate nanowires were prepared using distinct approaches;
If Mn is adsorbed after TNW synthesis, Mn replaces Na in the interlayers;
In Mn doped TNW, Mn is in two positions: replacing Ti4+ and in the interlayers;



Modified samples were tested for the hydroxyl radical photocatalytic production;
Mn-TNW was the best photocatalyst for methyl-paraben degradation.
In this work, the synthesis of novel manganese-modified titanate nanowires, with enhanced
photocatalytic activity for pollutants degradation, is described. Distinct modified titanate
nanowires samples were produced by manganese doping (Mn-TNT) and by Mn ion-exchange
(TNW/Mn). The characterization of the samples was performed by XRD, DRS, TEM, PL and
XPS. The structural characterisation indicates that Mn can be incorporated in the crystalline
structure in two distinct positions: replacing some Ti4+ in the TiO6 ocathedra and/or replacing
Na+ in the interlayers. For the TNW/Mn sample, the Mn was found only in the interlayers.
However, for Mn-TNW the metal was present in the both possible positions. The produced
materials demonstrate to be photo-active in a wider range of radiation then pristine TNW, even
starting to absorb in the visible range. The PL and XPS results indicate a decrease on the
photogenerated charge carriers recombination for the metal-containing samples. The
photocatalytic performance of the modified samples was tested for the hydroxyl radical
production. Both Mn modified samples (Mn-TNW and TNW/Mn) demonstrated to be catalytic
for this reaction. The photocatalytic degradation of the preservative methyl-paraben, considered
an emergent pollutant was also studied. For this process, the results indicate that the mechanism
of degradation is dependent on the photocatalyst surface ionic character. The best photocatalytic
performance was achieved by the doped Mn-TNW sample, with 62% of removal of a 10 ppm
methyl-paraben aqueous solution (0.13 g catalyst/L solution), after 90 min of irradiation.
Keywords: titanate nanowires; Mn doping; metal intercalation; photocatalytic pollutants degradation,
methyl-paraben
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1. Introduction
Nowadays, pharmaceutic and personal care products (PPCPs) and heavy metals are two classes
of problematic pollutants. Several methodologies have been proposed to remove them from the
environment, but clear improvements are needed. The majority of the proposed methods for
metals removal, mainly by adsorption, are not effective, since they only influence their
distribution between aqueous and other phase and don’t really eliminate them [1-6].
At the same time, photocatalytic degradation has been seen as an excellent way to effectively
oxidize persistent organic pollutants, including PPCPs. In this context, nanocrystalline TiO2 has
been thoroughly investigated. However, its high charge recombination rate and high bandgap
energy, limiting the electrons and holes photo-generation under visible irradiation, are
drawbacks that need to be overcome before its widespread use in environmental applications.
Titanate elongated nanostructured materials, including titanate nanowires (TNW) and nanotubes
(TNT) have attracted increasing attention in recent years [7-18], as an interesting alternative to
nanocrystalline TiO2. These titanate nanomaterials possess a layered structure composed by TiO6
octahedra layers intercalated by Na+ ions. These ions can be easily exchanged by other cationic
species, including metallic ones. Titanate elongated nanostructures possess similar structure and
bandgap when comparing to TiO2, but they possess larger surface area. Usually, TNW presents
higher surface area than TNT [19]. Because of their elongated morphology, they also exhibit
improved photogenerated charge carrier separation [9,20,21]. However, some of the optical and
photocatalytic TNW/TNT properties can still and need to be improved, before to propose them
for new and more challenging applications.
Numerous experimental strategies have been proposed to tailor TNW optical and photocatalytic
properties, including the control over the Na+ amount in the interlayers [10,22] or the
combination with other semiconductor nanoparticles, like Bi2S3, ZnS and Ag2S [23-25]. The
simultaneous control over the adsorption and photocatalytic titanate nanostructures properties
can also be achieved using multiple semiconductor layout arrangements, like ZnS-Bi2S3 or ZnS-
Ag2S [20,21]. An enhancement on their photocatalytic performance has also been described for
titanate nanomaterials modified with amines or other organic entities [19,26-29]. Recently, and
with the same purpose, the incorporation of metal ions in the TNW structure was successfully
reported [30-33]. In these cases, the improvement on the photocatalytic activity was attributed to
the creation of additional levels on the forbidden zone.
However, the incorporation of a metal in the catalyst structure is not always successful and the
additional formation of metallic or metal oxide nanoparticles over the matrix is sometimes
described [34]. For instance, after treat TNW with aqueous Ag+, the formation of nanocrystalline
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silver particles has been described over the semiconductor surface, during phenol photocatalytic
degradation [35]. On the other hand, no metallic or oxide formation happens, when the
modification of TNW with Co2+ ions is performed. In this case, only the metal intercalation
between the TiO6 layers was confirmed [36]. Studies concerning these types of modification
mechanisms and their relation with the exhibited materials properties are still scarce, but
essential.
Several experimental and theoretical studies have been shown that 3d transition metals are
potential dopants to modify TiO2 based materials [37]. From those metals, manganese is an
excellent candidate for doping because it can contribute for a reduction on the bandgap energy. It
can also induce the introduction of intermediate bands (IBs) in the forbidden zone, allowing
multi-band optical absorption [38].
Since their first use in the beginning of the 20th century, as preservatives in drugs, parabens are
considered the most abundant organic specie present in several PPCPs formulations, like facial
and body cosmetics, skin and hair care products. Methyl-paraben (MPB) is a widely used
preservative, exhibiting endocrine-disrupting properties with estrogenic activity. Even in very
low concentrations parabens may induce carcinogenic reactions and male infertility [39]. In the
limit, these compounds can be degraded in the environment; however, with extremely slow
reaction rates. Several studies have been proposed to face this problem, including innovative
photocatalytic degradation processes [40]. For instance, the use of a UV/persulfate process was
reported as an effective approach to remove MPB from aqueous solutions [41]. The combination
of TiO2/H2O2 was also reported to successfully remove MPB [42]. Also, the use of
nanocrystalline β-Bi2O3 as an alternative to TiO2 for methyl-paraben photocatalytic removal
under visible-light irradiation was recently reported [43].
This work proposes a new and green methodology to produce novel Mn-modified TNW
materials that can posteriorly be used for organic pollutants photocatalytic degradation. Due to
its worldwide relevance, methyl-paraben was chosen as model pollutant for this study.
Experimentally, and to better evaluate the oxidation state and relative position of Mn, within the
TNW crystalline structure, two distinct synthesis methodologies were used: a Mn doped sample
(Mn-TNW) was obtained using a doped precursor, and a Mn- ion exchange sample (TNW/Mn)
produced by submitting the TNW powder to an Mn adsorption process. The highest level of Mn
incorporation was attained by the TNW/Mn sample. This can open the possibility of using TNW
materials to remove toxic heavy metallic species from wastewaters. On the other hand, and in
what concerns the methyl-paraben photocatalytic removal, all the samples were catalytic for this
degradation process, with the best results being obtained using the doped Mn-TNW sample.
4/23

2. Materials and Methods
2.1. Materials
All the chemicals were used without previous purification. Titanium trichloride (TiCl3, Acros),
ammonia hydroxide (NH4OH, Fluka) and sodium hydroxide (NaOH, Sigma-Aldrich) were used
in the synthesis of titanate nanowires. Manganese dichloride (MnCl2.4H2O, Merck) was used as
Mn source. Terephthalic acid (C6H4(CO2H)2, Sigma-Aldrich) and methyl-paraben
(CH3(C6H4(OH)COO), Acros) were used for photocatalytic activity tests. The solutions were
prepared using distillate water (pH = 6.5).
2.2. Synthesis procedure
2.2.1. undoped and Mn-doped TNW precursors
TNW precursor was prepared based on a previously reported procedure [10,44]. A titanium
trichloride solution (20 w/v in 2M HCl) diluted in a ratio of 1:2 in deionized water was used as
titanium source. A 4 M NH4OH aqueous solution was added drop wise to the Ti solution under
vigorous magnetic stirring, until complete precipitation of a white solid. This suspension was
kept overnight at room temperature. After, it was filtered and vigorously rinsed several times
with deionized water. The obtained solid (precursor) was stored to posterior use for TNW
synthesis.
An identical experimental procedure was carried out to prepare the Mn-doped TNW precursor
by adding the required amount of manganese (nominal percentage of 5%) to the titanium
trichloride solution. After precipitation, the Mn-doped precursor, a greyish solid, was used to
produce the Mn-TNW sample.
2.2.2. Synthesis of TNW and Mn-TNW powders
TNW and Mn-TNW powders were obtained using a hydrothermal method previously described
[45]: 5 g of each precursor (Mn-doped and undoped) in ca. 50 mL of a NaOH 10 M aqueous
solution were maintained at 160ºC during 24 hours, using an autoclave system. After natural
cooling, until room temperature, the solids were isolated and washed several times with
deionized water until pH 7 on the filtrate solution and then were dried and stored.
2.2.3. TNW/Mn particles
The Mn ion-exchanged titanate nanowires sample (TNW/Mn) was obtained by treated 100 mg of
TNW previously prepared with 150 mL of MnCl2 aqueous solution (10 ppm). The suspension
was kept for 24 h under stirring at room temperature. After, the solid was isolated by
5/23

centrifugation and vigorously rinsed with deionized water to remove the remaining manganese
ions. The supernatant solution was stored in a plastic container to be posteriorly analysed by
Atomic Absorption Spectroscopy. The light brown/grey solid was dried and stored.
2.3. Photocatalytic experiments
The photodegradation experiments were conducted using a 250 mL refrigerated photo-reactor
[46]. A 450 W Hanovia medium-pressure mercury-vapour lamp was used as a radiation source.
The distribution of the total irradiated energy was 40-48% in the ultraviolet range and 40-43 %
in the visible region of the electromagnetic spectrum.
2.3.1. Hydroxyl radical (•OH) production
The photocatalytic activity of TNW, Mn-TNW and TNW/Mn samples was investigated for the
catalytic production of hydroxyl radical (•OH). Therephthalic acid (TA) was used here as probe
molecule. The •OH radical can converts TA into 2-hydroxyterephthalic acid (HTA), a
fluorescent product that can be easily detected and quantified by fluorescence spectroscopy [47].
Experimentally, 10 mg of each solid was added to 150 mL of 3 mM TA in 0.01 M NaOH
solution. The suspension was stirred in darkness for 30 min to establish adsorption equilibrium,
and after submitted to 30 min of irradiation. Several aliquots were collected during this period,
centrifuged and analysed by fluorescence spectroscopy.
2.3.2. Methyl-paraben (MPB) degradation
The catalytic photodegradation tests were performed by adding 20 mg of each catalyst to 150
mL of methyl-paraben 10 ppm aqueous solution. Prior to irradiation, the suspension was stirred
in darkness for 60 min to ensure adsorption equilibrium. The suspension was then irradiated
during 90 min. During this period, several aliquots were collected, centrifuged and analysed by
UV-vis spectrophotometry.
2.4. Characterization
The structural characterization of the samples was performed by X-ray powder diffraction
(XRD) using a Philips Analytical X-ray diffractometer (PW 3050/60) with automatic data
acquisition (X’ Pert Data Collector (v2.0b) software), employing Cu Kα radiation ( = 0.15406
nm) working at 40 kV/30 mA. The diffraction patterns were collected in the 2θ range of 5° to
70° with a 0.02° step size and an acquisition time of 200 s/step. Transmission electron
microscopy (TEM) images were acquired using a JEOL 200CX electron microscope operating at
300 kV. Specific surface areas were obtained by the Brunauer–Emmett–Teller (B.E.T.) method,
6/23

from nitrogen (Air Liquide, 99.999%) adsorption data at -195 °C, using a volumetric apparatus
from Quantachrome mod NOVA 2200e. The samples were degassed under vacuum at 150 °C for
2.5 h prior to measurement.
The XPS analyses were performed at CEMUP – Centro de Materiais da Universidade do Porto,
using a Kratos AXIS Ultra HSA, with VISION software for data acquisition and CASAXPS
software for data analysis. The analysis was performed with a monochromatic Al Kα X-ray
source (1486.7 eV) operating at 15 kV (90 W) in FAT mode, with a pass energy of 40 eV for
regions of interest (ROI) and 80 eV for survey. Data acquisition was performed with a pressure
lower than 1 × 10−6 Pa, and it was used as a charge neutralization system. The effect of electric
charge was corrected with reference to the carbon peak (285 eV). The fluorescence spectra of 2-
hydroxyterephthalic acid were measured using a Spex Flourolog 3-22/Tau 3 (excitation and
emission wavelengths were 315 and 425 nm, respectively).
UV–vis diffuse reflectance spectra (DRS) were performed using an Shimadzu UV-2600PC
spectrometer equipped with an ISR 2600plus integrating sphere, in the wavelength range of 200-
1400 nm. BaSO4 was used as reference. The same equipment was used for monitoring the
absorption of the methyl-paraben solutions. A SpexFlourolog 3-22/Tau 3 equipment was used to
quantify 2-hydroxyterephthalic acid, with excitation and emission wavelength at 315 and 425
nm, respectively. An atomic absorption Analytik Jena AAS novAA® 350 Flame AA (230 V,
50/60 Hz), Flame Compact spectrometer, operating in absorption mode, was used do quantify
aqueous Mn. A manganese hollow cathode lamp was employed. Operational parameters of the
equipment were: wavelength of 279.5 nm, slit width of 0.2 nm, current of 9 mA air as oxidizer
and acetylene as fuel (air/acetylene) and a stoichiometric flame.
3. Results and discussion
3.1. Identification and morphological characterization
The XRD patterns of the TNW powder before and after Mn incorporation, by doping (sample
Mn-TNW) and by ion exchange (sample TNW/Mn) are shown in Figure 1. The peaks observed
at 9.97°, 24.39°, 28.55° and 48.44°, for TNW, corresponds to the (100), (102), (111) and (303)
planes respectively, and are in agreement with a Na2Ti3O7 layered titanate crystalline structure
(CAS nº 12034-36-5). The broad peaks at 2θ = 9.1° and 28.6° are related to the interlayer
spacing in the layered structures and the ones at 2θ = 24.3° and 48.4° are characteristic of
sodium tri-titanate materials [48-50].
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TNW
TNW/Mn
Mn-TNW
(100)
5
10
15
2θ (°)
(111)
(303)
(102)
5
15
25
35
45
55
65
2θ (°)
Figure 1. XRD patterns of the TNW, Mn-TNW and TNW/Mn prepared samples (inset: detail of
the XRD patterns)
The XRD patterns of the Mn containing samples, Mn-TNW and TNW/Mn, are identical to that
of pristine TNW. However, a closer inspection of Figure 1 shows, for these two samples, a
positive shift (higher 2θ values) in the 10° peak (Figure 1 inset). This indicates a reduction on
the interlayers distance, and it is usually associated with a partial replacement of Na+ by a
cationic specie [51]. Considering the ionic radius of Na+ (1.02 Å) and of Mn2+ (0.67 Å), the
decrease observed on the interlayers distance agrees with the Na+
→
Mn2+ replacement.
The morphology of the samples was analysed by TEM (Figure 2). Very uniform and thin
elongated particles were visualized for all the three samples. This observation allows to conclude
that Mn incorporation, by doping or by adsorption/ion exchange, had no influence on the TNW
native morphology.
The B.E.T. method was used to evaluate the surface area of the synthesized samples (Table 1).
Pristine TNW presents a specific surface of 288.49 m2/g and a decrease of 18%, to 235.98 m2/g,
was observed for the TNW/Mn sample. The powder with lower surface area was the Mn-TNW,
with 211.15 m2/g.
8/23

a)
c)
b)
50 nm
Figure 2. TEM images of the pristine TNW (a) and of the modified TNW/Mn (b) and Mn-TNW
(c) samples.
This decrease is in agreement with previous works [36]. The surface area decrease observed for
the TNW/Mn sample (with Mn in the interlayers) should be attributed to the Na → Mn large
scale replacement. In fact, this was confirmed by the Na/Ti ratio, obtained by XPS, which was
0.67 and 0.057 for TNW and TNW/Mn respectively. In this case, the decrease of the BET
surface area is a direct consequence of the increasing weight of the sample after Mn
incorporation, since the atomic mass of Mn is more than twice that of Na. For the Mn-TNW
sample, with Mn in two distinct positions (interlayers and replacing Ti4+), the combination of
two factors should be considered to justify the surface area decrease: replacement of Na by Mn
as described above for the TNW/Mn sample, and the decrease of the atomic radius due to the
electronic configuration of Ti ([Ar]3d24s2) and Mn ([Ar]4s23d5).
Table 1. Surface area, bandgap energy, Mn/Ti and Ti 2p binding energies for the prepared
samples
surface area Eg
Mn/Ti
(at. %)
---
Ti 2p3/2
(eV)
Ti 2p1/2
(eV)
sample
(m2/g)
(eV)
TNW
288.49
3.21
2.27
2.41
458.7
458.5
458.8
464.5
464.4
464.4
Mn-TNW 211.15
TNW/Mn 235.98
9.1
19.3
3.2. Optical characterization
3.2.1. Diffuse Reflectance spectroscopy
9/23

The optical properties of the prepared samples were evaluated by diffuse reflectance
spectroscopy, in the UV-A/vis/NIR range.
The FKM function, represented in Figure 3, is proportional to the absorption coefficient and was
obtained from the diffuse reflectance spectra data [52]. Mn-TNW was the powder with the
highest absorption capability in the UV-A but also in the vis light range. The analysis of Figure 3
shows that the absorption edge is red shifted for Mn-TNW and TNW/Mn samples, when
compared to pristine TNW. This shift is more evident for the TNT/Mn sample.
The red shift observed for both Mn-containing samples, should be attributed to the insertion of
Mn 3d orbitals within the forbidden band and subsequent charge-transfer transition between the
d-d electrons of the dopant and the conduction band of the semiconductor.
The higher shift observed for the doped sample (Mn-TNW), in comparison to that of TNW/Mn, is
explained by the additional contribution of structural defects associated with oxygen vacancies in this
sample. These vacancies are generated for charge balancing, when ionic Mn replaces Ti4+ in the TiO6
octahedra.
Figure 3. Absorption spectra of the TNW, Mn-TNW and TNW/Mn samples (inset: Tauc plot
]1/2 versus h
[FKM h )
The absorption bands at 475–660 nm, perceptible for Mn-TNW, but also for TNW/Mn even in a
lower scale, are typical features related with transition metal doped semiconductors and are
consequence of the charge transfer and d–d transitions of the metal [33].
1/2
The Tauc plot, [FKM
h

]
versus h, was used to calculate the bandgap energy of the samples
(Table 1) by extrapolating the linear region of the plot to intersect the photon energy axis (Figure
10/23

3 inset). The bandgap energy obtained for TNW was 3.21 eV. After Mn modification, this
material bandgap shifted to 2.27 eV, for the doped Mn-TNW, and to 2.41 eV for the TNW/Mn
sample. By introducing new intra bandgap levels, the presence of Mn provides to TNW the
capacity to absorb lower energetic radiation, thus, promoting the activity of these materials on
the visible region of electromagnetic spectrum [35].
3.2.2. X-ray photoelectron spectroscopy
The electronic structure of TNW and modified Mn-TNW and TNW-Mn samples were analysed
by XPS. The TNW and Mn-TNW survey spectra are shown in Figure 4. The TNW/Mn spectrum
(not shown) is similar to that of Mn-TNW.
8
6
4
2
0
10
8
a)
b)
6
4
2
0
1000
500
0
1200
1000
800
600
400
200
0
binding energy (eV)
binding energy (eV)
c)
d)
e)
40
30
20
10
0
60
40
20
0
1.2
1.1
1
1.28
1.18
1.08
0.98
TNW/Mn
Mn-TNW
TNW
TNW/Mn
Mn-TNW
TNW
TNW/Mn
Mn-TNW
0.9
0.8
525
530
535
540
630
640
650
660
454
459
464
469
binding energy (eV)
binding energy (eV)
binding energy (eV)
Figure 4. XPS survey spectra of TNW (a) and Mn-TNW (b). XPS high resolution spectra of Ti 2p
(c), O 1s (d) and Mn 2p (e) regions for the prepared samples.
11/23

The Ti 2p3/2 and Ti 2p1/2 core spectra of the TNW, Mn-TNW, and TNW/Mn are shown in Figure
4c. For all the samples, the Ti 2p peak doublet splitting energy is 5.8 eV, indicating the

existence of titanium only as Ti4+. The high symmetry of the Ti 2p peaks also confirms that no
Ti4+ reduction had occurred in the samples.
For the TNW pristine sample, Ti 2p3/2 and Ti 2p1/2 peaks appear at 458.7 and 464.5 eV
respectively. After Mn incorporation, some shifts in these peaks position were observed. After
treat TNW with ionic Mn (sample TNW/Mn), a small shift was observed for these peaks, to
458.8 and 464.4 eV respectively. For the doped Mn-TNW sample, a more pronounced shift was
observed for the same peaks but for lower values, to 458.5 and 464.4 eV respectively. This
agrees with some alterations in the electron density of the Ti4+ surrounding structural
environment, imposed by the Mn presence. The shift to lower binding energies values, for Mn-
TNW sample should be due to the substitution of Ti4+ by Mn3+, as discussed later, and the
consequent higher level of oxygen vacancies for this sample.
The O 1s core spectra of the samples are shown in Figure 4d, with the main peak at
∼ 530.3 eV
attributed to the lattice oxygen. This peak has some contribution of the sodium Auger peak (Na
KLL). The small peak at higher binding energies (near 532.9 eV) can be attributed to surface
hydroxyl groups, indicating the possible presence of chemisorbed water molecules [24].
The Mn 2p core spectra for the modified samples are shown in Figure 4e. Manganese can exhibit
six oxidation states (0, 2+, 3+, 4+, 6+, and 7+) and according to literature [53], the Mn 2p peak
can be deconvoluted into 2 peaks, with split spin-orbit components and a Δpeak
= ∼ 11 eV.
The literature concerning the use of Mn to dope TiO2 and related materials is not unanimous,
with some works attesting that Mn4+ is the predominant ionic state for substitutional Mn in TiO2
lattice but others indicating Mn3+ as the dominant specie [35].
In this work, the Mn 2p core spectrum of the TNW/Mn sample shows asymmetric peaks, with
this indicating that the metal should co-exist with more than one oxidation state [35]. To better
conclude about the Mn oxidation state, the XPS results were carefully analysed and their
interpretation supported by previously published data [50,54]. For the TNW/Mn sample, the Mn
2p core spectrum agrees with the co-existence of Mn2+ and Mn4+. In fact, together with the Mn4+
peak features, including the recognizable shape at the top of the narrow Mn 2p3/2 peak, the
spectrum of this sample also shows the characteristic Mn2+ satellite peak (~ 647 eV), that doesn’t
exist for Mn3+ and Mn4+ species.
In opposition, the Mn 2p core spectrum of the Mn-TNW powder doesn’t show any evidence of
these features associated with the Mn2+ and/or Mn4+ existence. Despite this, this spectrum also
agrees with the presence of the metal as Mn+3.
12/23

In this kind of materials, the existence of the low oxidations states, Mn2+ and Mn3+, is usually
originated by several factors, including the synthesis methodology. The use of non-equilibrium
conditions during doped materials synthesis, like hydrothermal treatment, usually conducts to
reduced Mn ions and oxygen vacancies formation [35]. This statement is in accordance with the
predominant existence of Mn3+ in the sample produced directly by hydrothermal treatment, (Mn-
TNW).
The Mn quantification, in Mn-TNW and TNW/Mn samples was also performed by XPS since
the particle thickness, in both cases is inferior to the radiation depth penetration. The Mn/Ti
atomic ratio (%) obtained for Mn-TNW and TNW/Mn samples was 9.1 and 19.3 respectively.
These results indicate that the Mn incorporation was much more effective when performed after
TNW synthesis (sample TNW/Mn) if compared to the doping process (Mn-TNW). However, the
two incorporation processes should not be directly compared since the Mn position and
oxidation state is distinct in both materials as demonstrated above. For the Mn-TNW sample, the
Mn is incorporated between the TiO6 interlayers and replacing Ti4+, and for the TNW/Mn
powder the metallic ions are only in the interlayers. This is in agreement with the amount of Na+
found for both samples, through XPS quantification: values of 5.7 and 51.8% were obtained for
the Na/Ti ratio for TNW/Mn and Mn-TNW samples respectively. These conclusions are also in
agreement with the XRD results discussed above.
3.2.3. Photoluminescence (PL) spectroscopy
To evaluate a possible photo-generated charge-carriers’ recombination decrease, due to Mn
incorporation, the photoluminescence (PL) spectra of the prepared samples were recorded
(Figure 5). In these semiconductors, the emission peak at around 350 nm is attributed to TNW
band-to-band transitions [33]. Figure 5 analyses shows a clear decrease on this peak intensity for
Mn-TNW and TNW/Mn samples indicating a reduction in the recombination of the photo-
generated charge carries. The reduction was more pronounced for the doped Mn-TNW sample.
This effect can be a consequence of a more efficient electron-hole separation due to the creation
of new energetic levels in the forbidden zone.
13/23

Figure 5. Photoluminescence spectra of the pristine TNW and of the modified Mn-TNW and
TNW/Mn samples (inset: detail of the photoluminescence spectra of Mn-TNW and TNW/Mn)
For these semiconductor materials, the surface hydroxyl groups and oxygen vacancies are
usually the main sites for electrons and holes trapping. These trapped carriers are the entities
responsible for the fluorescence in the visible range [55]. The light emission in the visible range
(Figure 5 – inset) for the two Mn-containing powders, is in accordance with literature, with the
emission bands at 420455 nm being assigned to shallow-trap states near the absorption band
edge emissions, consistent with the existence of a high level of oxygen vacancies [56]. In fact,
the sample with the higher level of oxygen vacancies, the Mn-TNW, was the one with the lower
PL emission.
3.3. Photocatalytic degradation studies
3.3.1. Hydroxyl radical production
The photocatalytic activity of the TNW based powders was first evaluated for the hydroxyl
radical (•OH) production. The quantification of this oxidant specie, very relevant for
photocatalytic pollutants degradation processes, was accessed by fluorescence spectroscopy. The
photocatalytic conversion of therephthalic acid (TA) into 2-hydroxyterephthalic acid (HTA) was
used here as model reaction, to evaluate the photo-induced hydroxyl radical production [44].
Figure 6 shows the amount of HTA produced during 30 min of irradiation of a TA suspension,
using the prepared samples as catalysts. TNW demonstrated to be not catalytic for this process.
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Despite this, the Mn modified samples demonstrated to be catalytic for this reaction, with the
Mn-TNW presenting the best performance.
8
photolysis
TNW
Mn-TNW
6
TNW/Mn
4
2
0
0
5
10
15
20
25
30
irradiation time (min)
Figure 6. HTA concentration during 30 minutes of irradiation of a TA solution (3 mM, 150 mL)
using 10 mg of each sample as photocatalyst
These results indicate that Mn incorporation leads to a clear improvement on the TNW
photocatalytic performance for hydroxyl radical production. This is a very interesting result for
the Mn-modified materials, anticipating promising photocatalytic results for degradation
processes where this radical plays an active role.
3.3.2. Methylparaben (MPB) photocatalytic degradation
The photocatalytic activity of TNW, Mn-TNW and TNW/Mn samples was also evaluated for the
degradation of a methyl-paraben solution (10 ppm, 150 mL). Methyl-paraben (MPB) is the
methyl ester of the p-hydroxybenzoic acid. It has been used as food preservative and as anti-
fungal agent in cosmetics and personal-care products. Nowadays it is considered an emergent
pollutant due to its worldwide massive use and its high resistance to degradation.
The samples ability to adsorb MPB was studied before to evaluate their photocatalytic
degradation performances. Figure 7 shows the absorption spectra of the MPB solution before
and after to be in contact with 20 mg of each catalyst, during 60 min. Only slightly changes were
observed in the MPB spectrum using the TNW/Mn sample as adsorbent. On the other hand,
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substantial differences were observed when comparing the original MPB spectrum with the ones
recorded after the solution contact with the TNW and Mn-TNW powders. In these cases, the
absorption peak at 254 nm, typical of the MPB, substantially decreases and a new peak near 294
nm appears. This peak can be attributed to the 4-hydroxybenzoic acid existence, a well-known
MPB degradation intermediate. This product is formed through the alkaline hydrolysis of the
paraben ester bond. When in a pH 6
 9 range environment, the 4-hydroxybenzoic acid tends to
be deprotonated once it has a pKa of 2.7. This is in agreement with the decrease on the 254 nm
peak intensity and the concomitant appearance of the 294 nm peak.
1.8
1.6
MPB solution
TNW
Mn-TNW
TNW/Mn
1.4
1.2
1
254 nm
0.8
0.6
0.4
0.2
0
294 nm
200
220
240
260
280
300
320
340
wavelength (nm)
Figure 7. Absorption spectra of a 10 ppm MPB solution before and after 60 min in contact with
the prepared samples.
The similarity of the TNW and Mn-TNW behaviour, when in contact with a MPB aqueous
solution, can be justified since TNW and Mn-TNW powders were produced using the same
experimental procedure. Under these circumstances identical isoelectric points are expected for
both samples. For the TNW/Mn sample, the treatment with Mn2+, after TNW synthesis, is
responsible for the more acidic surface character and consequently for the MPB stability increase
(pKa = 8.4) [57]. For this reason, and in agreement with reported works, an alkaline media would
facilitate the MPB degradation reaction [36].
Figures 8 shows the absorption spectra of the MPB solution recorded during 90 min of
irradiation, using Mn-TNW and TNW/Mn as catalysts. For comparative proposes, the MPB
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spectra obtained during photolysis are also presented. It has been reported that MPB photolysis
is mainly due to UV-C radiation since the paraben maximum absorption is in the same range of a
strong and very energetic peak of the used Hg lamp ( 250 nm) [39,58]. The results concerning
the use of TNW (not shown) are similar to that of Mn-TNW sample.
It can be concluded, from Figure 8 analysis, that MPB degradation occurs during irradiation
independently on the 4-hydroxybenzoic acid existence at t= 0min. The MPB degradation profile
using TNW/Mn as photocatalyst is similar to that of photolysis.
initial
0 min
5 min
10 min
1.2
a)
1
20 min
30 min
45 min
60 min
75 min
90 min
0.8
0.6
0.4
0.2
0
200
225
250
275
300
325
350
wavelenght (nm)
1.2
initial
0 min
b)
1
0.8
0.6
0.4
0.2
0
5 min
10 min
20 min
30 min
45 min
60 min
75 min
90 min
200
225
250
275
300
325
350
wavelenght (nm)
c1).2
initial
0 min
5 min
1
10 min
20 min
30 min
45 min
60 min
75 min
90 min
0.8
0.6
0.4
0.2
0
200
225
250
275
300
325
350
wavelength (nm)
Figure 8. Absorption spectra of a MPB solution (10 ppm), during photolysis (a) and using Mn-
TNW (b) and TNW/Mn (c) as photocatalysts (0.13 g/L).
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For that samples which promote the formation of 4-hydroxybenzoic acid before irradiation,
TNW and Mn-TNW, the MPB degradation profiles are similar. It is interesting to note that under
these conditions, the simultaneous removal of MPB and 4-hydroxybenzoic acid was observed.
Comparing all the MPB spectra obtained after 90 min of irradiation (Figure 9) it is possible to
infer about the photocatalytic activity, for this process, of the prepared samples. All the powders
demonstrated to be catalytic for the MPB removal. The best catalytic performance was attained
by the Mn-TNW sample, with a pollutant concentration decrease of 62%. Values of 40 and 50%
of MPB degradation were obtained for the TNW and TNW/Mn assisted processes, respectively.
These values were estimated by measuring the intensity of the 254 nm peak (Figure 9 inset).
These results indicate that the modification of TNW through Mn incorporation, by ion-exchange
or doping, always contribute to improve the photocatalytic performance of the pristine material.
Figure 9. MPB solution spectra after 90 min of photocatalytic degradation, using the prepared
samples as catalysts (inset: percentage of MPB photodegradation after 90 min of irradiation)
It is also interesting to conclude that the TNW sample is catalytic in this process even if it is not
for the photo-induced hydroxyl radical production, suggesting that this radical is not the
exclusive responsible for MBP degradation. These results are in agreement with literature, which
proposed three main pathways for the MPB photocatalytic degradation: (i) direct hydroxyl
radical oxidation (ii) reaction with the photogenerated holes, and (iii) direct oxidation by
dissolved oxygen [59]. More work, concerning the elucidation of this topic, together with the
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MBP degradation mechanism and by-products identification/quantification is now ongoing in
our labs.
4. Conclusions
In this work, nanocrystalline titanate nanowires were tested to remove two pollutants from
aqueous media. First, TNW were successfully used to remove Mn ions from a aqueous media,
through an adsorption process. The added value product obtained (TNW/Mn) was then used to
photocatalytic degrade an emergent pollutant, the methyl-paraben, a well-known food and
cosmetic preservative and antimicrobial agent. For comparative reasons, a Mn doped sample,
Mn-TNW, was also prepared and analysed.
The results show that the incorporation of Mn leads to significant changes on the crystalline
structure of the TNW and on its physical and optical properties. These modifications are
dependent on the Mn incorporation methodology: doping or adsorption/ion-exchange. For the
Mn-TNW sample, the Mn was incorporated between the TiO6 interlayers and replacing Ti4+. For
the TNW/Mn powder the metallic ions were only incorporated in the interlayers’ space. By the
XRD and XPS combined analysis it was possible to conclude that Mn is present as Mn3+ in the
Mn-TNW sample and as Mn2+/Mn4+ in TNW/Mn powders. Both modified samples start to
absorb more radiation in the visible range and a reduction of the charge carriers’ recombination
rate was also observed. An enhancement of the photocatalytic performance of the TNW due to
Mn incorporation was observed for hydroxyl radical production and methyl-paraben
degradation. The best photocatalytic results were attained by the Mn-TNW sample, with 62% of
removal after 90 min of irradiation (10 ppm, 150 mL, 0.13 g/L).
5. Acknowledgements
This work was supported by Fundação para a Ciência e Tecnologia through
UID/MULTI/00612/2013, PEst-OE/QUI/UI0612/2013 and IF/01210/2014 projects. The XPS
analyses were performed at CEMUP – Centro de Materiais da Universidade do Porto
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