Synthesis of protonated titanate nanotubes tailored by the washing step: Effect upon acid properties and photocatalytic activity

Article abstract of DOI:10.1016/j.jphotochem.2017.03.012

In this work, the acid and photocatalytic properties of titanate nanotubes (NTs) were surveyed. The surface acidity of the NTs was characterized by FTIR with lutidine and pyridine. The photocatalytic degradation of phenol in aqueous suspension was performed to test the photocatalytic properties of the NTs. The results were compared with those obtained from commercial TiO₂. NTs were prepared by hydrothermal treatment of TiO₂ nanoparticles in a NaOH aqueous solution. During the washing step, three different acid agents (HCl, H₂SO₄, and HNO₃) were used. TiO₂ nanoparticles were synthesized previously by the sol-gel method. The photocatalytic materials were characterized by FTIR, XRD, XPS, S_BET, UV–vis, and HRTEM. It was found that the used acid agent significantly affected the amount and type of acid sites. Br?nsted acid sites were favored by the use of HNO₃. Lewis acid sites were promoted when HCl was employed during the washing step, which is in contrast with the results obtained using the other acids chosen in this work. Besides, the use of HCl promoted the H₂Ti₃O₇ phase. The acid phase H₂Ti₄O₉·H₂O was favored when HNO₃ was used and H₂SO₄ formed the H₂Ti₂O₄·(OH)₂ acid phase. The presence of Cl, S, and N species on the NTs was not found. Furthermore, Na⁺ ions were completely removed from the surface of the NTs, which were exchanged by H⁺ ions. It was found, in general, that the three catalysts presented a relatively high photocatalytic activity to remove phenol. However, NTs washed with HNO₃ (NT-HNO₃) displayed the best photocatalytic activity compared to the other NTs. After 200?min, NT-HNO₃ reached a phenol degradation yield close to 100%. Commercial TiO₂ presented a phenol degradation yield close to 60%. It could be concluded that the acid phase (H₂Ti₄O₉·H₂O) and concentration of Br?nsted acid sites promoted the photocatalytic activity of NT-HNO₃.

Full text of DOI:10.1016/j.jphotochem.2017.03.012

Accepted Manuscript
Title: Synthesis of protonated titanate nanotubes tailored by
the washing step: Effect upon the acid properties and
photocatalytic activity
´
Authors: R. Camposeco, S. Castillo, Isidro Mejıa-Centeno, J.
´
´
Navarrete, N. Nava, V. Rodrıguez-Gonzalez
PII:
S1010-6030(16)30823-1
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.jphotochem.2017.03.012
JPC 10563
To appear in:
Journal of Photochemistry and Photobiology A: Chemistry
Received date:
Revised date:
Accepted date:
27-9-2016
20-1-2017
9-3-2017
´
Please cite this article as: R.Camposeco, S.Castillo, Isidro Mejıa-Centeno,
´
´
J.Navarrete, N.Nava, V.Rodrıguez-Gonzalez, Synthesis of protonated titanate
nanotubes tailored by the washing step: Effect upon the acid properties
and photocatalytic activity, Journal of Photochemistry and Photobiology A:
Chemistryhttp://dx.doi.org/10.1016/j.jphotochem.2017.03.012
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Synthesis of protonated titanate nanotubes tailored by the washing
step: Effect upon the acid properties and photocatalytic activity
R. Camposeco 1*, S. Castillo2, Isidro Mejía-Centeno3, J. Navarrete3, N. Nava3 and V. Rodríguez-González1
1Instituto Potosino de Investigación Científica y Tecnológica, División de Materiales Avanzados,
78216, San Luis Potosí, S.L.P.; México
2Dirección de Tecnología del Producto, Instituto Mexicano del Petróleo, 07730, México, D.F.;
México
3Dirección de Investigación en Transformación de Hidrocarburos, Instituto Mexicano del
Petróleo, 07730, México, D.F.; México
*Corresponding author.
Tel: + (52) 55 9175 8216;
Fax: + (52) 55 9175 9699
E-mail address: roberto.camposeco@ipicyt.edu.mx

Graphical abstract
hν
10 nm
4
nm
[200]
[110]
e-
6CO2
3H2O
CB
VB
7O2
Nanotubes
h+
C6H6O + 7O2 → 6CO2 + 3H2O
H2O
Highlights





Three different acid agents were used to prepare nanotubes of TiO2
HCl, H2SO4 and HNO3 were employed during the washing steep
Acidity, structure, and photocatalytic activity depend of the acid agent
Lewis acid sites are formed with HCl, Brønsted acid sites are promoted by HNO3
HNO3 improves the photocatalytic activity of the nanotubes of TiO2
Abstract
In this work, the acid and photocatalytic properties of titanate nanotubes (NTs) were surveyed. The
surface acidity of the NTs was characterized by FTIR with lutidine and pyridine. The photocatalytic
degradation of phenol in aqueous suspension was performed to test the photocatalytic properties
of the NTs. The results were compared with those obtained from commercial TiO2. NTs were
prepared by hydrothermal treatment of TiO2 nanoparticles in a NaOH aqueous solution. During the

washing step, three different acid agents (HCl, H2SO4, and HNO3) were used. TiO2 nanoparticles
were synthesized previously by the sol-gel method. The photocatalytic materials were
characterized by FTIR, XRD, XPS, SBET, UV-vis, and HRTEM.
It was found that the used acid agent significantly affected the amount and type of acid sites.
Brönsted acid sites were favored by the use of HNO3. Lewis acid sites were promoted when HCl
was employed during the washing step, which is in contrast with the results obtained using the
other acids chosen in this work. Besides, the use of HCl promoted the H2Ti3O7 phase. The acid
phase H2Ti4O9•H2O was favored when HNO3 was used and H2SO4 formed the H2Ti2O4•(OH)2 acid
phase. The presence of Cl, S, and N species on the NTs was not found. Furthermore, Na+ ions were
completely removed from the surface of the NTs, which were exchanged by H+ ions.
It was found, in general, that the three catalysts presented a relatively high photocatalytic activity
to remove phenol. However, NTs washed with HNO3 (NT-HNO3) displayed the best photocatalytic
activity compared to the other NTs. After 200 min, NT-HNO3 reached a phenol degradation yield
close to 100%. Commercial TiO2 presented a phenol degradation yield close to 60%. It could be
concluded that the acid phase (H2Ti4O9•H2O) and concentration of Brønsted acid sites promoted
the photocatalytic activity of NT-HNO3.
Keywords: Nanostructures; Heat treatment; Crystal structure; FTIR; Rietveld analysis; XPS.
1. Introduction
Titanium dioxide (TiO2) is one of the most extensively studied support overcoats in photocatalysis
and heterogeneous catalysts [1,2]. The optical, electronic and redox properties, surface acidity, and

pollutant-decomposition capacity have made possible the application of TiO2 in gas-sensing
elements, electrode materials, as well as in the production and storage of energy [3–7]. Recently,
new TiO2 structures such as nanotubes, nanofibers, nanoribbons, nanorods and nanowires have
emerged. These TiO2 nanostructures feature physicochemical properties that are different from
those displayed by the traditional TiO2 phases (anatase, brookite and rutile) [8,9]. In fact, TiO2
nanotubes have been widely used principally for their great surface area and tubular structure that
increases the number of active sites [10,11].
TiO2 nanotubes can be synthesized by using several methods. The surfactant directed method,
alumina templating synthesis, microwave irradiation, electrochemical synthesis, sol–gel template
method and hydrothermal method are the most employed [11-13]. However, the alkaline
hydrothermal method is probably the most popular. The high reactivity, low energy requirement,
relatively non-polluting set-up and simple aqueous solution control, represent the main advantages
of this method [14].
The methodology involves the thermal treatment of TiO2 with NaOH loaded into an autoclave.
During the formation of TiO2 nanotubes, in 1999, researchers introduced the washing step with an
acid (HCl) with the purpose of removing the Na+ ion to form Ti-OH bonds, followed by a washing
process producing Ti-O--H-O-Ti [15-17]. The main structures formed after the washing step are
the following acid phases H2Ti3O7, H2Ti2O5•H2O, H2Ti4O9, H2Ti4O9•H2O and
HxTi2−x/4□x/4O4•H2O. These kinds of structures are known as titanate nanotubes. However, the
mechanism for the formation of titanate nanotubes is, up to now, controversial [1,18]. Recently,
we reported [19] TiO2 nanostructures such as nanotubes, nanofibers and nanowires featuring both
Brönsted and Lewis acid sites. Hara et al. [20] showed that TiO2 nanosheets exhibited an excellent
catalytic performance for the Friedel−Crafts alkylation of toluene with benzyl chloride. In general,

it has been reported that the acid properties of the titanate nanotubes are different from those
featured by the starting material, but how the surface acidity can be modified is not clear yet.
In this work, the effect of three different acids employed during the washing step upon structure,
surface acidity and photocatalytic activity of the nanotubes is reported. How the surface is modified
and the kind of acid sites that are favored in the NTs were also analyzed. It was found that the
improved photocatalytic activity of the TiO2 nanotubes is attributed to the modification of the
physicochemical properties as well as the surface acidity.
2. Experimental
2.1. Preparation of titanate nanotubes
Titanate nanotubes were prepared by the hydrothermal method as reported elsewhere [21]. TiO2
nanoparticles were prepared by the sol-gel method. 36.7 mL of titanium (IV) isopropoxide (Aldrich
97%) were dissolved in 145 mL of 2-propanol (Baker 99.9%) under constant stirring. To adjust the
reaction medium at pH 1, HNO3 was used. Bidistilled water was employed to accomplish the
hydrolysis. The solution was maintained under stirring and reflux up to the gel formation. The gel
was dried at 70°C for 12 h. Finally, the material was annealed at 350°C for 4 h.
For the synthesis of nanotubes, 3.0 g of TiO2 obtained by the sol-gel method were dissolved in 60
mL of a NaOH aqueous solution (7N). The solution was mixed in a closed cylindrical Teflon-lined
autoclave Parr reactor at 140 °C for 24 h, rotating the autoclave at 200 rpm. Thereafter, the solution
was filtered with HCl, H2SO4 and HNO3 until reaching a pH close to 2. The solution was washed
with abundant deionized water to reach a pH value of 7. Finally, the material was dried under
vacuum at 80 °C for 12 h. The solid was annealed in air at 300 °C for 4 h. The samples were labeled
as NT-HCl, NT-H2SO4 and NT-HNO3, respectively.
2.2. Characterization

X-ray diffraction (XRD) patterns of samples packed in a glass holder were recorded at room
temperature with a Cu Kα1.2 source (1.5418 Å) on a Siemens D-500 diffractometer having a theta–
theta configuration and a graphite secondary-beam monochromator. The data were collected for
scattering angles (2θ) ranging from 4 to 80° with a 0.02° step for 1 s per point. High resolution
transmission electron microscopy (HRTEM) analyses of the samples were performed in a JEOL
2200FS microscope operating at 200 kV and equipped with a Schottky-type field emission gun and
an ultrahigh resolution pole piece (Cs = 0.5 mm, point-to-point resolution, 0.190 nm). The samples
were ground, suspended in isopropanol at room temperature and dispersed by ultrasonic stirring.
Then, an aliquot of the solution was dropped on a 3-mm-diameter-lacey-carbon-copper grid.
Textural properties were determined on an ASAP-2000 analyzer from Micrometrics. The specific
surface area (SBET) was calculated from the Brunauer–Emmett–Teller equation from N2
physisorption at 77 K. The pore size distribution was obtained by the Barrett–Joyner–Halenda
method from the desorption branch. A Thermo Scientific, Evolution 600, UV–vis spectrometer
(UV-vis) was used to record directly the diffuse reflectance spectra between 200 and 800 nm, using
the reflectance spectra as a reference at room temperature.
Pyridine and lutidine (2,6-dimethylpyridine) spectra were recorded on a Nicolet 8700
spectrophotometer with a 4 cm−1 resolution, accumulating 50 scans. In the cell, all the samples
were treated in vacuum at 400 °C for 1 h. Pyridine and lutidine were admitted and adsorbed.
Evacuations were performed from room temperature up to 400 °C. The acidity per surface area for
Lewis and Brönsted acid sites was calculated by following the pyridine method [22]. Rietveld
refinement was performed with the Maud software, using the reported atomic position as a
reference [23-24].
The suspension of titanate nanotubes was monitored with a Microtrac (Zetatrac) to quantify the
zeta potential. The aqueous suspension was used as a function of the pH. Prior to the experiment,

the pH of the aqueous suspension was set to the desired value with the HCl or NaOH (0.01 M)
solutions.
2.3. Photocatalytic activity.
The nanotubes were tested for the photocatalytic degradation of phenol (Fermont) using a Pyrex
reactor at room temperature, which was irradiated directly with a UV-A lamp (365 nm) in the dark.
For the photocatalytic tests, 200 mg of catalyst were used. The aqueous solution contained 20 ppm
of phenol (0.1062 mol/L of phenol). A Pyrex reactor (250 mL), which was irradiated directly by a
UV-A lamp (365 nm) in the dark, was used. The catalysts were exposed for 215 min to the UV-A
light. The pH of the solution was maintained at a value of 7.
To quantify the photocatalytic degradation of phenol, a UV-vis evolution 600 equipment (Thermo
Fisher Scientific) was employed. During the photoreaction, aliquots were collected at selected time
intervals. Samples of 1 mL were extracted with a syringe. The sample was filtered through a 0.45
mm PTFE filter (Millipore). The photocatalytic degradation efficiency was calculated as follows:
(%) = (Ci-Ct)/Ci * 100, where Ci is the initial concentration of phenol (mol/L) and Ct is the
concentration of phenol after irradiation at a given time (mol/L). The filtered titanate nanotube
samples were also analyzed using a Shimadzu Total Organic Carbon Analyzer (VCPN) with Non-
Dispersive Infra-Red detector (NDIR) to determine the amount of Total Organic Carbon (TOC).
2.4. Phenol absorption
Adsorption experiments were conducted at pH 7.0 using 200 mg of titanate nanotubes contained
in a solution (200 mL) of 20 ppm of phenol (0.1062 mol/L of phenol). A Pyrex reactor (250 mL)
was used for the adsorption experiments. The solutions were stirred at 300 rpm and 25°C. Aliquots
were taken and centrifuged and then analyzed by UV–visible spectrophotometer to determine the
phenol initial and equilibrium concentrations in the solutions. The amount of phenol adsorbed onto

the titanate nanotubes at equilibrium was calculated according to the following equation: qe = (Co
- Ce)*V/ M, where qe is the equilibrium adsorption amount (mg/g), C0 is the phenol initial
concentration (mg/L), Ce is the phenol equilibrium concentration (mg/L) from the absorption band
intensity at 270 nm, V is the phenol solution volume (L) and M is the mass of the titanate nanotubes
(g).
Results and discussion
3.1. XRD
The XRD patterns for the NT samples annealed at 300°C are shown in Fig.1. The results confirm
the presence of the different titanic acids (H2Ti3O7, H2Ti4O9•H2O and H2Ti2O4•(OH)2) that were
obtained by washing with HCl, H2SO4 and HNO3, respectively [25-26]. According to JCPDS, the
different diffraction peaks matched the 36-0654, 36-0655 and 57-0123 cards. For the NT-HCl
sample: (100) at 9.79°, (110) at 24.37°, (311) at 28.3° and (020) at 48.5°. For the NT-H2SO4
sample: (200) at 9.77°, (110) at 24.23°, (311) at 28.01° and (020) at 48.5°. For the NT-HNO3
sample: (200) at 9.18°, (110) at 24.3°, (600) at 28.14° and (020) at 48.14° [28], respectively, see
Fig. 1(A).
The XRD patterns of the NT-H2SO4 and NT-HCl samples show a little change in the peaks located
at 2 = 9.79° and 9.77° with respect to the one of NT-HNO3 located at 2 = 9.18°, which indicates
that these titanate nanotubes are sensitive to the acid agent. This result is in good agreement with
those obtained by some researchers [27-29] who have stated that the acid washing step exerts a
significant effect on the titanate nanotubes in terms of elemental composition and annealing
behavior. The phases for the NT-HCl, NT-H2SO4 and NT-HNO3 samples annealed at 300°C were
determined by the Rietveld method, using the atomic position of Na2Ti3O7 (JCPDS 720-148). It
was found that the structure without H2O is very similar to the space group 11, which is in good

agreement with Donk et al. [24]. In Fig. 1(B), a high proportion of the H2Ti2O5•H2O phase with
respect to the anatase phase can be observed for the NT-HCl, NT-H2SO4 and NT-HNO3 samples.
Table 1 also presents the phase composition of the nanotubes for several temperatures. Morgado et
al. [30] reported a refinement at 400°C with a value of 51.8 % for the TiO2(B) phase and 48.2 %
for the anatase phase. However, by using the Rietveld Method, the optimized lattice parameters in
our XRD patterns were as follows: a= 8.11 b= 4.11 and c= 9.77 with an angle of 100.11°.
3.2. HRTEM morphology
The morphologies of NT-HCl, NT-H2SO4 and NT-HNO3, annealed at 300°C, were investigated by
HRTEM. The micrographs are shown in Fig. 2. A high yield of multiwall titanate nanotubes with
inner and outer diameters between 2 and 3 nm and 8 and 10 nm, respectively, can be observed in
Fig. 2. The wall layers of the NT-H2SO4 and NT-HNO3 samples are composed of two and four
layers with an interlayer spacing between 0.75 and 0.85 nm from reflection d200, respectively. The
fast Fourier transform (FFT) for the NT-HCl sample revealed characteristic spots of titanate
nanotubes which represent the (200) and (110) planes. The NT-H2SO4 sample featured an ordered
tubular structure formed with the d200 and d110 planes. The inset in Fig. 2(B) shows a magnification,
where titanate nanotubes are seen mainly, and unreacted particles of the starting material are not
observed; this fact is consistent with what was found in the X-ray patterns.
NT-HNO3, reported in Fig. 2(C), showed a uniform morphology and an open-end-tubular structure
with external and internal diameters between 8 and 2 nm, respectively. After annealing the
materials at 300°C, it could be observed that the interlayer spacing between the nanotube walls was
still conserved; these fringes have a space of 0.8 nm. This phenomenon is related to the dehydration
of interlayer OH groups of the H2Ti2O4•(OH)2 structure [28], which reduced the interlayer distance.
The morphology variations of the titanate nanotubes were very similar to those featured by the NT-
HCl and NT-HNO3 samples. The NT-H2SO4 sample exhibits a different morphology, principally

inside the nanotubes with well aligned fringes of 0.75 nm, which correspond to the d200 plane of
H2Ti4O9•H2O located at 2 = 9.77°, reported in Fig. 2(B).
3.3. Textural properties
The BET surface areas for the NT-HCl, NT-H2SO4 and NT-HNO3 samples annealed at 300 °C are
shown in Table.1. In our case, the NT-H2SO4 sample showed a high specific area (422 m2/g), while
the NT-HCl and NT-HNO3 samples showed a slight change in the SBET values at around 375 to 369
m2/g, respectively. However, it has been reported [1,13,28] that the acid washing step is a critical
parameter that modifies the surface area and pore volume. In fact, the acid washing step removes
the electrostatic repulsion and promotes the formation of titanate nanotubes. Furthermore, the acid
treatments replace Na+ ions by H+ ions. The variation of surface charge caused by ion exchange of
Na+ by H+ ions leads to the scrolling of sheets into nanotubes, improving the physicochemical
properties of the titanate nanotubes with respect to those exhibited by the starting material.
3.4. Surface acidity
The surface acidity of the materials synthesized in this work is presented in Fig. 4. Table 1 also
presents the concentration of Brönsted and Lewis acid sites for several temperatures. The bands
observed in Figs. 3(A), 3(B) and 3(C) at 1450 cm-1 for the NT-HCl, NT-H2SO4 and NT-HNO3
samples are assigned to coordinated pyridine bound to Lewis acid sites [31]. The band located at
1540 cm-1 is assigned to pyridinium ions formed by Brönsted acid sites [31]. This behavior was
not observed for TiO2 in its rutile, brookite and anatase phases; it was only observed for titanate
nanotubes [32-33], which indicates that titanate nanotubes possess both Brönsted and Lewis acid
sites whose concentration can be modified through the different acid washings [19-20].
The amounts of Brönsted and Lewis acid sites in NT-HCl, NT-H2SO4 and NT-HNO3 are quantified
in Table 1. The three samples showed a remarkable behavior regarding the absorption on Brönsted

and Lewis acid sites. In fact, the intensity of the peak for Lewis acid sites was remarkably higher
than that of Brönsted acid sites, principally for the NT-HCl and NT-H2SO4 samples. The NT-HNO3
sample showed a predominance of Brönsted acid sites from room temperature up to 400°C. At high
temperatures, from 200 to 400°C, the intensities of pyridine adsorption decreased on both Brönsted
and Lewis acid sites. In the NT-H2SO4 sample, the bands of the Brönsted and Lewis acid sites
disappear at 400°C. For the NT-HCl sample, the Lewis acid sites remained up to 400°C, as reported
in Figs. 3(A) and 3(B).
In summary, three samples with different behavior patterns and different quantities of Brönsted
and Lewis acid sites in the 100-400°C range were obtained. This effect is particularly important
because titanate nanotubes can also be used in catalytic or photocatalytic reactions and not just,
because of their high surface areas, as supports of dispersed metals. In this sense, the surface acidity
plays an important role. As for the NT-HCl, NT-H2SO4 and NT-HNO3 samples, they can be
employed at low and middle temperatures since they show thermal stability until 420°C. In this
sense, we reported [34] the catalytic activity of NT supports for the NO reduction with NH3,
obtaining a conversion of 52% of NO at 420°C. The NT samples showed different amounts of
Brönsted acid sites owing to the scrolling of the Ti-O-H bonds.
In Fig. 3(D), it can be observed that pyridine is coordinated in different kinds of sites, depending
of the type of structure. In our case, at 200°C, H2Ti3O7 for NT-HCl, H2Ti4O9•H2O for NT-H2SO4
and H2Ti2O4•(OH)2 forNT-HNO3 were deconvoluted between 1660 and 1680 cm-1 and normalized
to sample weight, where the main absorption peak appeared at 1590 cm-1, solely in the NT-HCl
and NT-H2SO4 samples, corresponding to the 8a ring vibration mode of a H-bonded pyridine [35-
36]. Likewise, in the NT-HCl, NT-H2SO4 and NT-HNO3 samples, the main peak appears at 1603
cm-1, suggesting that pyridine is coordinated to Lewis acid sites mainly in NT-HCl, which is in
contrast to NT-H2SO4 and NT-HNO3.

3.5. Lutidine adsorption
Lutidine (2,6-dimethylpyridine) is considered as a more sensitive molecule for characterizing
Brönsted acid sites. This study was carried out to uphold the results obtained by using pyridine,
and, in addition, the O-H groups that can be formed on the surface of the titanate nanotubes by
employing different acid agents such as HCl, H2SO4 and HNO3 in the washing step. It has been
reported [19,37] that titanate nanotubes possess a high number of OH groups when the starting
material featured the anatase phase. Fig. 5(A) shows the FTIR spectra of Lutidine adsorbed on the
NT-HCl, NT-H2SO4 and NT-HNO3 samples. The analysis was carried out at 200°C and normalized
to sample weight. The bands located at 1642 and 1625 cm-1 for the three samples are characteristic
of Lutidine adsorbed on Brönsted acid sites [38].
For the NT-HCl sample, a strong band located at 1580 cm-1 is observed. This band is assigned to
the vibrating 8b mode of an H-bonding with its corresponding 8a vibrating mode of the peak at
1602 cm-1. These bands are less intense in the NT-H2SO4 and NT-HNO3 samples. Another band is
located between 1592 and 1594 cm-1, which is attributed to the 8a mode of physisorbed lutidine
molecules with their corresponding 8b mode overlapping the 8b mode of H-bonding lutidine
located at 1580 cm-1, Fig. 5(B) [39]. This result supports the one obtained by pyridine FTIR,
showing a strong H-bonding band in NT-HCl in comparison to NT-H2SO4 and NT-HNO3. This
fact suggests that the NT-HCl sample tends to interact more with Lewis acid sites than the NT-
HNO3 sample, which interacts with Brönsted acid sites. This statement was confirmed by
absorption of pyridine and lutidine. It can be then conclude that the kind of acid agent can accelerate
the washing process, playing an important role in the atomic arrangement of the Ti-O---H titanate
nanotubes.
3.6. XPS characterization

The XPS scan is shown in Fig. 6. In the inset shown in Fig. 6(A), the 0-450 eV interval can be
observed, which is where the S 2p (176-160 eV), Cl 2p (185-210 eV) and N 1s (394-406 eV) signals
appeared. The Cl, S and N signals are not found due to the fact that these compounds were
eliminated during the washing process. Besides, these species do not affect the type of acid sites;
they only modify the atomic arrangement of the titanate nanotubes during the washing step.
The survey of the NT-HCl, NT-H2SO4 and NT-HNO3 samples displays signals originated from O
1s, Ti 2p1/2 and Ti 2p3/2, which are easily identified at binding energies around 531, 464 and 458
eV, respectively. The features observed at 1224, 1108, 1075 and 979 eV correspond to the Auger
peaks of C, N, Ti and O. It is important to note that signals related to Na, Mg and Ca are not
detected because they were eliminated during the washing process. Figs. 6(B) and (C) show the O
1s and Ti 2p photoelectron peaks.
The deconvolution of O1s displays three peaks [40]. The first peak is assigned to oxygen in the
crystal lattice at 530 eV [40]. The second peak is correlated with surface hydroxyl groups OH− at
531 eV [40]. The third peak is related to physical water adsorbed at 532 eV [40]. The three peaks
are reported in Fig. 6(B). The content of OH- groups was observed in this order: NT-HNO3 > NT-
H2SO4 > NT-HCl. In these sense, OH- groups are very useful because they contribute to the supply
of active oxygen species and provide active sites during the photocatalytic degradation of phenol.
Ti 2p displays two peaks at around 464 and 458 eV, which correspond to Ti4+ 2p1/2 and 2p3/2. This
information supports the fact that titanate nanotubes manly present the oxidation state Ti4+ [41].
The NT-HCl, NT-H2SO4 and NT-HNO3 samples maintain a constant separation of 5.7 eV.
However, the shift shown in Fig. 6(C) is attributed to the interaction between the titanium and
oxygen atoms [42].
3.7. UV-vis

The optical properties of the NT samples annealed at 300°C are generally related to their
nanostructure, where the electronic state, defect state and energy level structure are important
parameters [43]. Fig. 7 shows the UV–vis spectra for P25, NT-HCl, NT-H2SO4 and NT–HNO3.
The calculated band gaps (Eg) for the different NT samples are also shown in Fig.7. The band gap
(Eg) for the samples obtained employing HCl, H2SO4 and HNO3 during the washing step varies
over a narrow interval of 3.18 and 3.34 eV. In comparison with the TiO2 anatase phase that displays
a band gap of 3.2 eV [44], the absorption edge of NT-HNO3 and NT-HCl is shifted to the UV-vis
light region.
The increase in the band gap of the TiO2 nanotubes is attributed to the quantization of electronic
states and the dimensionality reduction from 3D to 2D and/or 1D [45]. Besides, the variations in
the band gaps for NT-HCl, NT-H2SO4 and NT-HNO3 are caused by the acid washing step, which
modifies the absorption due to changes regarding the structure, phase composition and
morphology. The band gap of the NTs prepared with H2SO4 exhibited a value of 3.18 eV; the
nanotubes with HNO3 increased the band gap sharply to 3.34 eV and the nanotubes with HCl
showed a band gap of 3.28 eV.
3.8. Zeta potential
The changes in the titanate nanotubes employing different acid agents during the washing step were
followed by measuring the Z-potential of their aqueous suspension as a function of the pH. The
results are reported in Fig. 8. The NT-HCl, NT-H2SO4 and NT-HNO3 samples showed an
isoelectric point (IEP) at pH 7 for NT-HCl, at pH 7.2 for NT-HNO3 and 6 for NT-H2SO4,
respectively. The Z-potential curve reached negative values below −60 mV at pH values above 7.5,
reaching a maximum value for NT-HCl of -35 mV. The Z-potential values of the three NT-HCl,
NT-H2SO4 and NT-HNO3 samples were significantly different.

The measured Z-potential for the anatase phase has been reported with a value of 5.1 to 5.4 [46-
47]. For Degussa P25, the reported values range from 5.7 to 6.0 [46-47]. In our case, our samples
display slightly higher values than those reported in the literature [46-47]. Besides, it is possible
that the values of Z-potential are sensitive to the kind of structure. For instance, this study showed
the formation of layered titanate phases (H2Ti3O7, H2Ti4O9•H2O and H2Ti2O4•(OH)2). The use of
HNO3 led to the formation of Brönsted acid sites, displaying an isoelectric point (IEP) at pH 7.2.
In the case of HCl, Lewis acid sites were formed and the IEP was reached at pH 7.
3.9. Photocatalytic activity
Fig. 9(A) displays the photocatalytic degradation of phenol as a function of the UV light irradiation
time over the P25, NT-HCl, NT-H2SO4 and NT-HNO3 samples. The photocatalytic degradation of
phenol is an apparent first-order reaction verified by the linear transform -ln Co/C = f(t) illustrated
in Fig. 9(E). Fig. 9(B) displays the UV–vis spectral change of 20 ppm of phenol as a function of
the irradiation time during the photocatalytic activity of the NT-H2SO4 photocatalysts. It was found,
Fig. 9(A), that phenol with an initial concentration of 20 mg/L was removed by 96% after
irradiation and 215 min for NT-H2SO4. The photocatalytic removal rates of phenol by Degussa P-
25, NT-HCl and NT-HNO3 were 65, 92 and 94%, respectively.
The photocatalytic activity of the nanostructured materials was found to be dependent on the
H2Ti3O7 phase, composition, annealing temperature, surface area and acid properties. In fact, the
acid washing step is a critical process that modifies the surface area, pore volume, structure, phase
composition and morphology, and band gap, as well as the acid properties, which improve the
photocatalytic degradation of phenol with respect to the TiO2 anatase and rutile phases.
The surface areas and acid properties are the key factors that influence both the activity of the
photocatalysts and the efficiency of the photocatalytic reactions. The synergetic effect of a larger
surface area and the presence of Brönsted and Lewis acid sites shown by the scrolling structure of

the NT-HCl, NT-H2SO4 and NT-HNO3 samples play a major role in having a higher photocatalytic
activity degradation of phenol.
Properties such as crystalline phase (H2Ti3O7, H2Ti4O9•H2O and H2Ti2O4•(OH)2), morphology,
surface area and acidity type (Brönsted and Lewis) present in these TiO2 nanostructures make them
very profitable supports for potential applications in environmental pollution control. The
equilibrium adsorption of the nanostructured materials used in this work is reported in Fig. 9(C).
The equilibrium absorption was reached after 1 h. Our results reported in Fig. 9(C) show that
phenol was weakly absorbed on the titanate nanotubes. After 60 min, the adsorption of phenol
reached the equilibrium. The adsorption capacity is 2.1 mg of phenol per g of nanotubes for NT-
H2SO4, 1.98 mg/g for HCl, 1.71 mg/g for NT-HNO3, and 1 mg/g for commercial P-25.
The photocatalytic mineralization of phenol was conducted for the NT-HCl, NT-H2SO4 and NT-
HNO3 samples in order to determine and verify the efficiency in removing phenol and total organic
carbon (TOC). The TOC results are reported in Fig. 9(D). It can be observed, in Fig. 9(D), that the
best sample to remove phenol and carry on until complete mineralization was NT-HNO3 followed
by NT-H2SO4 and NT-HCl. Besides, it was found by the total organic carbon analysis that close to
80% of phenol was degraded to CO2 and 20% of phenol formed other intermediate compounds on
NT-HNO3. The CO2 formation by degradation of phenol was close to 70% for NT-H2SO4 and
around 65% for HCl.
It has been reported [48] that the photocatalytic degradation of phenol follows a pseudo first-order
kinetic reaction. In this sense, the rate constant was obtained for our materials. The results are
reported in Fig. 9(E) and Table 2. Eq. (1) was used to obtain the rate constant. However, Eq. (1)
has been developed for a homogeneous reaction based on Eq. (2).
퐶
퐴
푡푘 = −푙푛 퐶
(1)
퐴
표

푑푁
퐴
= 푟 푉
(2)
퐴
푑푡
Eq. (2) does not consider the catalyst mass. The correct form to obtain the rate constant is the
following Eq. (3):
푑푁
퐴
= 푟 푊
(3)
퐴
푑푡
where W is the mass of catalyst employed during the reaction. Separating and integrating Eq. (3)
we obtain:
푡푘푊 푁퐶 = − ln (퐶퐶
)
퐴0
(4)
퐴0
퐴
퐴0
where NA0 represents the amount of phenol (mol), CA0 is the phenol initial concentration (mol/L),
W is the weight of catalyst (gcat), XA is the conversion of phenol (%), t is the time (min), k is the
rate constant (mol/Lgcat-s). Eq. (4) represents the best approach to obtain the rate constant for the
photocatalytic system reported in this work, instead of Eq. (1). In this case, the rate constant can
be compared for each material considering the weight of catalyst. The slope m of the straight line
reported in Fig. 9(E) gives the rate constant. The highest rate constant for photocatalytic
degradation of phenol was obtained for the NT-HNO3.
Conclusions
The effect of using different acid washing agents such as HCl, H2SO4 and HNO3 upon titanate
nanotubes was analyzed. A high concentration of Lewis acid sites in the sample washed with HCl
was found. The sample washed with HNO3 showed a remarkable concentration of Brönsted acid
sites. The sample washed with H2SO4 displayed the generation of both acid sites at low
temperature. The surface acidity of the nanotubes was confirmed by adsorption of pyridine and
lutidine.
In summary, titanate nanotubes can produce effective Lewis and Brönsted sites by only modifying
the acid washing agent, treating them until 300 °C. Besides, the results show that the acid washing

agent modifies the structure of the titanate nanotubes to form H2Ti3O7 (with HCl), H2Ti4O9•H2O
(with H2SO4) and H2Ti2O4•(OH)2 (with HNO3). In this sense, NTs interact differently with the kind
of acid agent, generating Brönsted and Lewis acid sites, which makes NTs very attractive as
supports for a great number of photocatalytic applications.
Acknowledgments
The authors want to thank the financial support provided by the project D.00477 from the Mexican
Institute of Petroleum. R. Camposeco acknowledges the support provided by CONACyT and
LINAN-IPICYT.

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List of Figures
Fig. 1. (A) XRD patterns for NT-HCl, NT-H2SO4 and NT-HNO3 and the standard (JCPDS 36-
0654, 36-0655, 57-0123 and 047–0124) diffraction patterns for H2Ti3O7, H2Ti4O9.H2O,
H2Ti2O4·(OH)2 and H2Ti2O5·H2O. (B) Rietveld refinement plots for NT-HCl, NT-H2SO4 and NT-
HNO3; the upper marks correspond to H2Ti2O5·H2O and the lower marks correspond to the anatase
phase.
Fig. 2. HR-TEM for the titanate nanotubes: (A) NT-HCl, (B) NT-H2SO4 and (C) NT-HNO3
annealed at 300°C with diffraction planes corresponding to H2Ti3O7, H2Ti4O9.H2O and
H2Ti2O4·(OH)2.
Fig. 3. Pyridine adsorption FTIR spectra of titanate nanotubes: (A) NT-HCl, (B) NT-H2SO4 and
(C) NT-HNO3 in the 1400-1700 cm-1 region. Pyridine was evacuated from room temperature (RT)
to 400°C. (D) Pyridine adsorption for the 8a ring vibrating mode in the 1580-1660 cm-1 region for
the NT-HCl, NT-H2SO4 and NT-HNO3 samples, corresponding to bonded-H weakly interacting
with pyridine evacuated at 200°C.
Fig. 4. Concentration of Brönsted (A) and Lewis acid sites (B) at 100, 200, 300 and 400 °C on the
surface of the NT-HCl, NT-H2SO4 and NT-HNO3 samples.
Fig. 5. (A) Lutidine adsorption FTIR spectra for the HCl, H2SO4 and HNO3 samples. (B) Lutidine
adsorption (8a and 8b ring vibrating mode regions) FTIR for the NT-HCl, NT-H2SO4 and NT-
HNO3 samples annealed at 300°C and evacuated at 200°C.
Fig. 6. (A) XPS spectra for the hydrothermally treated NT-HCl, NT-H2SO4 and NT-HNO3 samples,
(B) O1s and (C) Ti 2p cores.
Fig.7. UV-vis for the titanate nanotubes NT-HCl, NT-H2SO4 and NT-HNO3 annealed at 300°C and
Degussa P25.
Fig. 8. Z-potential profiles for NT-HCl, NT-H2SO4 and NT-HNO3 samples (employed different
acid agents) as a function of pH in water.
Figure. 9. (A) Conversion of phenol at pH 7 for NT-HCl, NT-H2SO4 and NT-HNO3 annealed at
300°C. (B) UV-vis spectra of phenol obtained for NT-H2SO4 photocatalysts. (C) Equilibrium
absorption for NT-HCl, NT-H2SO4 and NT-HNO3. (D) TOC removal for NT-HCl, NT-H2SO4 and
NT-HNO3, and (E) rate constant for NT-HCl, NT-H2SO4 and NT-HNO3 photocatalysts.

NT-HCl
9.79°
(100)
A
(110)
(310)
(020)
H
Ti O
3 7
36-0654
2
9.77°
(200)
NT-H SO
2
4
(110)
(020)
(020)
(310)
H
Ti O .H2O
4 9
36-0655
2
NT-HNO
9.18°
(200)
3
(110)
(600)
H
Ti
O .(OH) 2 57-0123
4
2
2
(310)
(110)
(020)
50
47-0124
O .H O
2
2 5
(200)
10
H
Ti
2
20
30
40
60
70
80
2-Theta (degree)
NT-HCl
B
H2Ti2O5·H2O
Anatase
NT-H2SO4
H2Ti2O5·H2O
Anatase
NT-HNO3
H2Ti2O5·H2O
Anatase
10
20
30
40
50
60
70
80
2-Theta (degree)
Fig. 1
R. Camposeco et al.

A
10 nm
4 nm
[200]
[110]
[200]
B
[110]
0.75 nm
200
C
110
9 nm
2 nm
0.85 nm
Fig. 2
R. Camposeco et al.

L
NT-H2SO4
NT-HCl
3.0
3.0
A
B
L
L
L + B
L
B
L + B
B
B
B
R.T
R.T
100°C
100°C
200°C
200°C
300°C
400°C
300°C
400°C
1400 1450 1500 1550 1600 1650 1700
Wavenumber (cm-1)
1400 1450 1500 1550 1600 1650 1700
Wavenumber (cm-1
)
NT-HCl
L
NT-HNO3
1.0
3.0
D
L
L + B
C
B
B
R.T
1560
1580
1600
1620
1640
1660
NT-H2SO4
1.0
100°C
200°C
1560
1580
1600
1620
1640
1660
NT- HNO3
300°C
400°C
1.0
1400 1450 1500 1550 1600 1650 1700
Wavenumber (cm-1
1580
1600
1620
1640
Wavenumber (cm-1
)
)
Fig. 3
R. Camposeco et al.

1.5
1.0
0.5
0.0
0.6
0.5
0.4
0.3
0.2
0.1
0.0
NT-HCl
NT-H2SO4
NT-HCl
NT-H2SO4
A
B
NT-HNO3
NT-HNO3
100
150
200
250
300
350
400
100
150
200
250
300
350
400
Temperature (°C)
Temperature (°C)
Fig. 4
R. Camposeco et al.

NT-HCl
1592
2.0
1580
H+
3.0
A
B
1625
1642
NT-HCl
1560
1580
1600
1620
1640
1660
NT-H2SO4
2.0
H+
1602
NT-HNO3
H+
1602
1560
1580
1600
1620
1640
1660
NT-HNO3
2.0
NT-H2SO4
1500
1550
1600
1650
1700
1560
1580
1600
1620
1640
1660
-1)
Wavenumber (cm-1
)
Wavenumber (cm
Fig. 5
R. Camposeco et al.

Ti 3p
A
Ti 3s
Cl 2p
=
185-210 eV
C1s
O
2
NT-HCl
S
2p =153-173 eV
Ti Auger
NT-H2SO4
NAuger
O
N
1s 394-406
2 Auger
NT-HNO3
3
0
50 100 150 200 250 300 350 400 450
Binding (eV)
NT-HCElnergy
O1s
Ti 2p
NT-H2SO4
NT-HNO3
300
0
600
900
1200
Binding Energy (eV)
NT-HCl
NT-H2SO4
NT-HCl
NT-HNO3
Ti - O
OH
B
NT-H2SO4
Ti - O
OH
NT-HNO3
OH
Ti - O
526 528 530 532 534 536 538 540
Binding Energy (eV)
O 1s
525
530
Binding Energy (eV)
535
NT-HCl
NT-H2SO4
Ti3/2
NT-HNO3
C
Ti 1/2
Ti 2p
456
460
464
468
Binding Energy (eV)
Fig. 6
R. Camposeco et al.