A comparative run for visible-light-driven photocatalytic activity of anionic and cationic S-doped TiO2 photocatalysts: A case study of possible sulfur doping through chemical protocol

Article abstract of DOI:10.1016/j.molcata.2016.05.003

Photocatalytic degradation of pollutant molecules delivered great challenges for sustainable development of green energy in the field of environmental science. For this purpose, growing interests have been made to witness the fast development of new photocatalysts with improved catalytic efficiency and to monitor the reaction mechanism at the atomic and molecular levels. Therefore, doping of semiconductor metal oxide (e.g. TiO₂) with main group elements, especially with sulfur atoms (S), has gained much interest due to the introduction of a localized band between the conduction bands (CB) and valence bands (VB) that increase the absorbance in the visible light region. More interestingly, photocatalytic practices of the S-doped TiO₂ material are of pronounced significance due to photon-to-carrier conversion ability of S-doping beneath the band gap energy region of pure TiO₂. Therefore, visible-light-activated S-doped TiO₂ photocatalysts were prepared via template free and low-temperature oxidant peroxide method (OPM) assisted hydrothermal treatments. Experimental findings have revealed the successful incorporation of sulfur atoms into TiO₂ crystal lattice and, as a result, substitution of Ti⁴⁺ by S⁶⁺ to form Ti-O-S bonds for cationic S-doping was observed. Whereas, in the case of anionic S-doping, substitution of S^2- by O^2- to form O-Ti-S bonds was achieved. More evidence was observed for the presence of chemisorbed sulfate groups on the surface of S-doped TiO₂ samples and inhibition of crystallite size growth by S-doping, and obviously, upsurge the absorbance in the visible light region. The photocatalytic activity of as-prepared 1-D nanorods shaped photocatalysts and the mechanism involved for the photodegradation of organic molecules (methyl orange and phenol) under visible-light irradiation were investigated by adding different scavengers into the system solution to capture active species. It was found that in cationic S-doped TiO₂ photocatalysts, chemisorbed hydroxyls (OH_ads-) and photoinduced holes (h⁺) played a major role in photocatalysis. Whereas, in the case of anionic S-doped TiO₂ photocatalysts, electrons (e^-) and photoinduced holes (h⁺) played the nearly equal role in photocatalysis.

Full text of DOI:10.1016/j.molcata.2016.05.003

Journal of Molecular Catalysis A: Chemical 421 (2016) 1–15
Contents lists available at ScienceDirect
Journal of Molecular Catalysis A: Chemical
journal homepage: www.elsevier.com/locate/molcata
A comparative run for visible-light-driven photocatalytic activity of
anionic and cationic S-doped TiO2 photocatalysts: A case study of
possible sulfur doping through chemical protocol
Shahzad Abu Bakara,b,∗, Caue Ribeirob
a Department of Chemistry − Federal University of São Carlos, Rod. Washington Luiz, km 235, CEP: 13565-905 São Carlos, SP, Brazil
b Embrapa CNPDIA, Rua XV de Novembro, 1452, 13560-970, CP 741, São Carlos, SP, Brazil
a r t i c l e i n f o
a b s t r a c t
Article history:
Photocatalytic degradation of pollutant molecules delivered great challenges for sustainable develop-
ment of green energy in the field of environmental science. For this purpose, growing interests have been
made to witness the fast development of new photocatalysts with improved catalytic efficiency and to
monitor the reaction mechanism at the atomic and molecular levels. Therefore, doping of semiconductor
metal oxide (e.g. TiO2) with main group elements, especially with sulfur atoms (S), has gained much inter-
est due to the introduction of a localized band between the conduction bands (CB) and valence bands
(VB) that increase the absorbance in the visible light region. More interestingly, photocatalytic prac-
tices of the S-doped TiO2 material are of pronounced significance due to photon-to-carrier conversion
ability of S-doping beneath the band gap energy region of pure TiO2. Therefore, visible-light-activated
S-doped TiO2 photocatalysts were prepared via template free and low-temperature oxidant peroxide
method (OPM) assisted hydrothermal treatments. Experimental findings have revealed the successful
incorporation of sulfur atoms into TiO2 crystal lattice and, as a result, substitution of Ti4+ by S6+ to form
Received 30 March 2016
Received in revised form 25 April 2016
Accepted 5 May 2016
Available online 6 May 2016
Ti
O S bonds for cationic S-doping was observed. Whereas, in the case of anionic S-doping, substitu-
tion of S2− by O2− to form O Ti S bonds was achieved. More evidence was observed for the presence
of chemisorbed sulfate groups on the surface of S-doped TiO2 samples and inhibition of crystallite size
growth by S-doping, and obviously, upsurge the absorbance in the visible light region. The photocat-
alytic activity of as-prepared 1-D nanorods shaped photocatalysts and the mechanism involved for the
photodegradation of organic molecules (methyl orange and phenol) under visible-light irradiation were
investigated by adding different scavengers into the system solution to capture active species. It was found
that in cationic S-doped TiO2 photocatalysts, chemisorbed hydroxyls (OHads−) and photoinduced holes
(h+) played a major role in photocatalysis. Whereas, in the case of anionic S-doped TiO2 photocatalysts,
electrons (e−) and photoinduced holes (h+) played the nearly equal role in photocatalysis.
© 2016 Elsevier B.V. All rights reserved.
1. Introduction
cheap, and widespread applications in solar energy cells water
splitting to create hydrogen, and waste water purification etc. [2].
In recent decades, development of one-dimensional (1-D) metal
oxide semiconductor photocatalysts has been one of a promising
subject for environmental remediation due to their unique stabil-
ity, high reactivity, thermodynamic and transport properties [1].
Therefore, efforts have been made to prepare semiconductor mate-
rials specifically TiO2 (titanium dioxide) in nanorods shape due to
its high specific surface area, enhanced properties, easy acceptance,
Another characteristic feature of TiO2 is its ability to hold on the
higher absorption of hydroxyl groups on the surface and facilitate
the destruction of pollutants molecules (organic and inorganic) in
an aqueous medium by interacting with the adsorbed hydroxyl
groups [3]. However, intrinsic flaws of the TiO2 limit the advantages
of these unique properties as a photocatalyst due to its large band
gap needs ultraviolet radiation to excite electron–hole pairs and
a higher rate of recombination for photogenerated electron–hole
pairs [4]. More importantly, the adsorption capacity of TiO2 lies in
the range of 4.8-63.5 mg/g deprived of photocatalytic reduction due
to performed reaction conditions [2]. This may result in the inferior
pollutants destruction efficiencies and feebler reproducibility due
∗
Corresponding author at: Department of Chemistry − Federal University of São
Carlos, Rod. Washington Luiz, km 235, CEP: 13565-905 São Carlos, SP, Brazil.
E-mail address: shazad 158@yahoo.com (S.A. Bakar).
http://dx.doi.org/10.1016/j.molcata.2016.05.003
1381-1169/© 2016 Elsevier B.V. All rights reserved.

2
S.A. Bakar, C. Ribeiro / Journal of Molecular Catalysis A: Chemical 421 (2016) 1–15
to the fast ruination of the active surface area by self-aggregation
[5,6]. Furthermore, the greater adsorption capacity is desired for
TiO2 to add its application range deprived of photocatalytic reduc-
tion [7]. Therefore, efforts have been focused to overcome these
deficiencies by tweaking the band gap of TiO2 to prolong the light
absorption response in the visible-light region and to enhance the
separation efficiency of photoinduced charge carriers. For this pur-
pose, considerable research has been focused on doping TiO2 crystal
structure with either metal or nonmetal elements [8,9], loading
noble metals [10,11] and coupling with narrow band gap semicon-
ductors [12,13].
One of the important aspects of the photocatalytic activity
of non-metal doped TiO2 is to understand the keen relationship
between the origins of absorption shift due to doping on the pho-
tocatalytic chemistry. First time, Asahi et al. assumed that N-doping
creates a rise in valence band due to delocalized coupling between
the orbital of O 2p and N 2p [14]. Whereas, in the recent reports,
it has been proposed that a mid-gap level is generated above the
valence band due to dopant atom orbitals [15]. Therefore, to under-
stand the photocatalytic chemistry, these distinguished views are
very important to elaborate for initiation of catalytic oxidation of
pollutant degradation pathway on the surface of TiO2. As an over-
all lowering of oxidizing power for the generation of photoinduced
holes on the surface of the pristine catalyst due to rising of valence
band energy level, there should be possible interfere with the pho-
tocatalytic degradation reaction. On the other hand, the role of the
chemically active and low energy hole (h+) trap is unclear due to
isolated mid-gap level as compared to the core-valence band holes
[16]. It is expected that absorption-excitation process in the low
energy edge would probably direct the generation of such hole
trapped. Nonetheless, these low energy holes trapped could also
migrate towards the dopant center after UV excitation. Therefore,
it is apparent that expected excitation over pristine TiO2 might have
lower oxidation potential due to rapid trapping and constraint of
high charge mobility. Whereas, we might observe a wavelength
dependence on the photocatalytic chemistry due to adsorbed sub-
strates oxidation of pollutant molecules via competitive charge
migration within the crystal lattice of TiO2 [17].
adverse peculiarity of dyes is that it also absorbs light in the visible
region.
Perhaps, the most scorching research topic continues to be on
S-doped TiO2 photocatalysis. Generally, it is mentioned that non-
metal doping into TiO2 is regarded as difficult and hectic route
especially for large atomic radius S atom [30]. For this purpose,
common routes adopted for the successful synthesis of anionic S-
doped TiO2 materials include the oxidation of TiS substrate [31]
and/or high-temperature heating of the titania film under the flow
H2S gas [32]. Later on, both of these approaches have encountered
serious limitation to control the concentration of doping elements
into TiO2 crystal lattice [30]. Afterward, different forms of Sol-gel
method have been adopted for the synthesis of S-doped TiO2 mate-
rials with controlled doping concentration, but Sol-gel method
lacks serious drawback due to the use of expensive starting materi-
als and worthless of reproducibility [33,25,31]. Nevertheless, to the
best of our knowledge, no report has been cited yet for the synthesis
ofvisible-light-activatedS-dopedTiO2 nanorodsby surfactant-free,
low temperature and facile OPM route, which is expected to be both
practically and scientifically important as compared to previously
reported methods [21,23–25]. Herein, we have developed a sim-
ple OPM route for the synthesis of S-doped TiO2 nanorods followed
by crystallization through hydrothermal treatment. In this work,
carbon disulfide and thiourea (S donating chelating ligands) were
used as sulfur sources and were added into the stabilized Ti-peroxo
complex solution to form a Ti-monometallic complex for the incor-
poration sulfur into TiO2 crystal lattice and crystallized through
hydrothermal treatment to prepare S-doped TiO2 nanorods. The
cited route proofed to be more feasible for the incorporation of S
into TiO2 crystal lattice due to the negative enthalpy of formation
for the chelating agents to form a stable Ti-monometallic complex.
The chelating ligands attained the stable geometry for the most
accessible substitution among departing/incorporating atoms into
TiO2 crystal lattice to occur i.e. Ti4+ substitution by S6+ to form
cationic doped samples and O2− substitution by S2− to form anionic
doped samples. To evaluate the mechanism of photocatalytic activ-
ity of the nanorods under visible-light irradiation, either colored
(MO) or colorless (phenol) dyes were used as model pollutants.
In details, S-doping into TiO2 crystal structure has been studied
widely as it produced alike band gap tapering effect as observed
in case of other non-metals doping [8,9], due to increased width of
the valence band (VB) by mixing 3p states of the sulfur with the
VB to narrowing the bandgap. Recently, it has been recognized that
sulfur sources and preparation routes promote the ionic form of
the S-doping. Ohno and other groups [18,19] reported that use of
thiourea or sulfate as sulfur sources could incorporate sulfur atoms
as cations by substitution of Ti ions in the S-doped TiO2 nanopow-
ders. It was found that the cationic substitution of Ti4+ by S6+ was
more favorable chemically due to their different ionic radius, and
this substitution induced new states into the CB of TiO2 from the S3s
states just exceeding the valence states of O2p [18]. Whereas, Ume-
bayashi et al. [20] have reported that use of TiS2 as the sulfur source
was more favorable for anionic doping of sulfur by replacing oxygen
atoms of TiO2 crystal lattice. In the following years, it was pledged
that use of TiS2 or CS2 as sulfur sources also promote the anionic
doping of sulfur atoms into TiO2 crystal lattice [21]. They suggested
that excess of sulfur oxidation occurred in the case of TiS2 or CS2
precursors and remaining sulfur naturally exists as S2− that favors
the replacement of oxygen atoms in the O–Ti–O framework. More
interestingly, some of the reports later suggest that use of thiourea
as sulfur precursors could promote the formation of ionic form in
the TiO2 crystal lattice [22]. Many other studies have probe the
photocatalytic activity of doped titania by using dyes as chemical
inquiries to follow either easily oxidative or reductive degradation
pathways as compared to other common pertinent pollutants. An
2. Experiment
2.1. Sample preparation
All chemicals used in the synthesis of S-doped TiO2 photocata-
lysts were of analytical grade and were used without any further
purification. The undoped TiO2 and cationic S-doped TiO2 photo-
catalysts were prepared via OPM route as described in detail by
our group [26,27] and were crystallized through the decomposition
of Ti-peroxo complex via hydrothermal treatment. For the prepa-
ration of anionic S-doped TiO2 photocatalysts, the cited method
is modified with the introduction of carbon disulfide as S chelat-
ing ligands into the stable Ti-peroxo complex solution i.e. carbon
disulfide is reported as an anionic S-doping source into TiO2 crys-
tal lattice [20,21]. Briefly described, the synthesis process starts
with the dissolution of the specific amount (250 mg) of metallic Ti
(99.7%, Aldrich) into a mixture of 80 mL (3:1 ratio) H2O2–NH3 (both
29.0%, Synth) solution. The mixture solution was placed on ice-bath
until the complete dissolution of metallic Ti and this results in the
appearance of yellow transparent solution due to the formation
of stable peroxytitanate ion [Ti(OH)3O2]− [28]. Different amount
of thiourea and carbon disulfide was added to different batches of
the peroxytitanate ion precursor solution with different starting
Ti:S molar ratios of 1:0.01, 1:0.02 and 1:0.03. The basin precur-
sor solutions were marked as CTO and ATO correspond to cationic
and anionic S-doped TiO2 samples. General scheme for the prepa-

S.A. Bakar, C. Ribeiro / Journal of Molecular Catalysis A: Chemical 421 (2016) 1–15
3
Fig. 1. Proposed mechanism for the incorporation of S-doping into the TiO2 crystal lattice.
ration and incorporation of S into the TiO2 to form S-doped TiO2
photocatalysts is presented in Fig. 1. A similar procedure was fol-
lowed to prepare the pristine TiO2 sample without the addition of
any S-doping source and was marked by TO sample. To remove
the excess of solvents, the precursor solution was heated to the
boiling point and then placed on ice–water cooling bath for quick
cooling to obtain the precipitate, which was vigorously stirred for
overnight to vaporize any excess of solvents (NH3 and H2O2). In the
next step, a relevant amount (200 mg) of freeze-dried precipitate
was suspended in the water for annealing in a temperature con-
trol hydrothermal cell. The synthesis conditions for all the samples
were set for 4 h heating at 200 ◦C. After the treatment, a crystalline
material was obtained by centrifugation of rapidly quenched sam-
ples in an ice bath. Finally, the samples were washed with doubly
deionized water for several times until to neutral pH and then with
isopropanol, and were dried for 8 h at 60 ◦C.
RTMS detector (PIXcel 1D, PANalytical). Field emission gun scan-
ning electron microscope (FEG-SEM) used for characterization of
surface morphology of samples as STEM FESEM, Zeiss Supra 35, at
4.0 kV. Brunauer–Emmett–Teller (BET) method was used to esti-
mate specific surface area (SA) of the sample using Micromeritics
ASAP 2000 adsorption analyzer operated at 77 K from physical
adsorption of nitrogen. Raman spectra was obtained at room tem-
perature from FT-Raman Bruker RFS100/S spectrometer operated
between 100–1000 cm−1 frequency range to the yttrium aluminum
garnet laser (1064 nm line to 450 nm) at scans rate of 200. Cary
5 G spectrophotometer was used to measure the diffuse reflectance
spectra of samples and Fourier transforms infrared spectra (FTIR)
was obtained from PerkinElmer Spectrum 1000.
Osram radiation source was used to test the photocatalytic
activity of samples for MO dye (Aldrich, 10.0 mg L−1) and phenol
(sigma Aldrich, 10.0 mg L−1) photodegradation under visible-light
irradiation, having a model of ModelL18/10 daylight with the inten-
sity of 18W and at the 440 nm maximum intensity. For this purpose,
10 mg of photocatalyst was dispersed in 20.0 mL of the aqueous
dye solution. Shimadzu-UV-1601 PC spectrophotometer was used
for absorbance spectra of samples to monitor the degradation of
dyes for different light exposure periods. Effect of dye adsorp-
tion on the catalytic activity of photocatalyst was tested before
the photocatalytic test by soaking all samples in the aqueous dye
solution for overnight. Different radical scavengers were added
to the reaction system to monitor the role of active species in
the photocatalytic degradation of MO including AgNO3 (electrons
scavenger), t-BuOH (•OH scavenger), Na2C2O4 (holes scavenger),
and benzoquinone (•O2 scavenger), respectively. An Auto Lab PGSt
(Methrom, Autolab) was used for photocurrent measurements of
S-doped TiO2 photocatalyst using the three-electrode configura-
tion from 302 N potentiostat–galvanostat system. The modified
2.2. Characterization of material
X-ray powder diffraction (XRPD) was used for semi-quantitative
phase analysis of the crystalline phases in the samples by
using Rigaku D-Max 2500 diffractometer having Cu anode
(␭Cu-K␣ = 1.5456 Å). The range of 10◦ and 75◦ was investigated as
the width of scanning routine to define peak position at 1 min at
2␪ between an angular pass of 0.02◦ and having an exposure time
of 1 s. Scherer s equation and Lorentzian approximation were used
to calculate the length of crystallographic coherence and full width
at half maximum (FWHM) from (101) of monocrystalline Silicon
wafer reflection [29]. PANalytical X’Pert Pro (NL) ␪/␪ diffractometer
was used to collect data having 0.5◦ divergence and antiscatter-
ing slits, and a 15 mm copper mask with 0.04 rad Soller slits for
the pathway of the incident beam. Diffracted arm consisted of fast

4
S.A. Bakar, C. Ribeiro / Journal of Molecular Catalysis A: Chemical 421 (2016) 1–15
Fig. 2. (a) Rate of MO photocatalytic degradation over pristine and S-doped TiO2 photocatalysts, (b) phenol photocatalytic degradation kinetics over pristine and S-doped
TiO2 photocatalysts. Effect of scavenger’s addition on the photocatalytic degradation of MO over (c) 2CTO and (d) 2ATO samples.
electrode was dipped into 1 mol L−1 Na2SO4 (pH = 13.6) electrolyte
visible light irradiation (≥420 nm). Prior to the photocatalytic
experiment, a separate experiment has been performed to observe
the adsorption-desorption equilibrium between the photocatalysts
and MO molecules in dark condition as shown in Fig. 1S. The change
in absorbance of MO aqueous solution over the as-prepared sam-
ples under the visible-light irradiation is shown in Fig. 2S. The
photocatalytic efficiency was higher for the ambient amount of sul-
fur dopants samples (2CTO and 2ATO) than the other samples and
showed the photocatalytic efficiency of 89.32% and 85.43% for 2ATO
and 2CTO samples, respectively. The synergistic effect of S-doping
levels into TiO2 crystal lattice is proved beneficial for enhancing
the photocatalytic activity. For the higher concentration of sul-
fur dopant samples, the photocatalytic efficiency is lower. This is
expected due to increased concentration of dopant elements into
the TiO2 crystal lattice started to act as recombination center and
increased the recombination rate of photogenerated electron–hole
pair as observed for higher S-doping concentration samples (3CTO
and 3ATO). Thus, the photocatalytic activity has been increased
for S-doped TiO2 photocatalysts to an ambient dopant concentra-
tion and rests unaffected at upper proportion. Therefore, it was
critical to control the S-doping level into TiO2 crystal lattice to
obtain optimal photocatalytic efficiency. The MO photodegradation
efficiency rate over as-prepared photocatalysts for the different
interval of visible-light irradiation is shown in Fig. 2(a). The con-
centration of blank MO aqueous solution deteriorated dimly after
240 min under visible-light irradiation without adding any pho-
tocatalyst into the MO aqueous solution. This indicates that no
photolysis of MO aqueous solution occurred under visible-light
irradiation. In the case of TO sample, a dark adsorption is obtained
from the photodegradation plot, which indicates that TiO2 is not
and the system consist of working electrode consists of nanorods
(active surface area of 0.5 cm2) covered by titanium foil, a refer-
ence electrode (Ag/AgCl/0.1 M KCl) and a counter electrode of Pt
net. The electrochemical workstation (CHI750B) was used for lin-
ear sweep voltammograms was in simulated illumination source
and the reversible hydrogen electrode (RHE) was used to monitor
the potential of the photoelectrode.
3. Result and discussion
The undoped and S-doped TiO2 photocatalysts were prepared by
OPM route and crystallized through hydrothermal method. Prior
to photocatalytic studies, the as-prepared samples were charac-
terized by XPS, SEM/TEM, XRD, and DRS analysis to have better
comparison and understanding of their expected visible-light-
driven photocatalytic activity for the degradation of the organic
pollutants aqueous solution. It was noted that the color of the
as-prepared samples changes with different S-doping level from
gray-white (for pristine TiO2) to yellowish appearance for high
sulfur concentration samples. Herein, we have reported a facile,
low-temperature, surfactant-free and template-free OPM route
assisted by hydrothermal treatment for the synthesis of anionic and
cationic S-doped TiO2 photocatalysts. More interestingly, the cited
route is more compatible with an industrial process for the incor-
poration of the sulfur source and superior to the sol-gel synthesis
which required post annealing process.
The photocatalytic activities of the as-prepared pristine and
S-doped TiO2 photocatalysts were assessed for the degradation
of organic pollutants (MO and phenol) aqueous solution under

S.A. Bakar, C. Ribeiro / Journal of Molecular Catalysis A: Chemical 421 (2016) 1–15
5
N
N
SO3 Na+
-
N
Methyl Orange
Intermediate
M = 327.33 g/mol
O
N
N
S
O-Na+
N
O
Fragments
M = 115 g/mol
M = 174 g/mol
N
SO3 Na+
-
sodium benzenesulfonate
(i)
N,N-dimethylbenzenamine
(ii)
(i)
M = 87 g/mol
SO3 Na+
-
O-Na+
OH
O
M = 174 g/mol
M = 110 g/mol
M = 88 g/mol
COOH
(ii)
CO2 + H2O
COOH
N+
N
M = 118 g/mol
N
COOH
O
O
O
M = 117 g/mol
M = 101 g/mol
M = 115 g/mol
COOH
Fragmentation
Fig. 3. Proposed pathway for the photodegradation of MO.
active in the visible-light region. Whereas, higher visible-light-
driven photocatalytic efficiency was noted in the case of S-doped
TiO2 photocatalysts with optimum S doping level after 240 min
of visible-light irradiation. The photocatalytic activity of S-doped
TiO2 photocatalysts remains unchanged after 240 min of visible-
light irradiation, which is expected due to the successful/brilliant
partition of photogenerated charge carriers.
Phenol, a typical organic pollutant found in industrial effluent
waste water and having no absorbance in visible-light region, was
tested for photocatalytic activity experiment to avoid the chance
of photosensitization of MO aqueous solution under visible-light
irradiation. The change in absorbance of phenol aqueous solution
under visible-light irradiation over S-doped TiO2 samples is shown
in Fig. 3S. The rate of phenol photodegradation over as-prepared
samples under visible-light irradiation is shown in Fig. 2(b). It was
observed that phenol follows similar photodegradation pathway as
was observed for MO photodegradation under visible light irradi-
ation. This indicates the photocatalytic capability of anionic and
cationic S-doped TiO2 photocatalysts for colorless organic com-
pounds. Thus, indirect dye photosensitization process was not
occurred on the surface of photocatalysts under visible-light irradi-
ation. Therefore, photoinduced charge carriers separation could be
the effective photodegradation mechanism due to band gap excita-
tion in anionic and cationic S-doped TiO2 photocatalysts. The high
TOC (total organic compound) removal rate (50%) indicates that the
mineralization of phenol occurred over the surface of S-doped TiO2
photocatalysts.
A number of scavengers were added into the MO aqueous
solution to detect the role of main active species in the photodegra-
dation process over highly active cationic and anionic S-doped TiO2
photocatalysts (2CTO and 2ATO). The main active species which are
involved in the mineralization of organic molecules includes; the

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S.A. Bakar, C. Ribeiro / Journal of Molecular Catalysis A: Chemical 421 (2016) 1–15
1,0
0,8
0,6
0,4
TO (At/Ao)
(a)
2ATO (TOCt/TOCo)
2ATO (At/Ao)
2CTO (TOCt/TOCo)
2CTO(At/Ao)
0,2
1,0
0,8
0,6
0,4
0,2
TO (At/Ao)
(b)
2ATO (TOCt/TOCo)
2ATO (At/Ao)
2CTO (TOCt/TOCo)
2CTO(At/Ao)
-20
0
40
60
80
100
120
20Irradiation time (min)
Fig. 4. The ratios of residue concentration based on the TOC and absorbance of (a) MO and (b) phenol solution mineralization over as-prepared samples under visible-light
irradiation.
1,0
5th run
3rd run
4th run
2nd run
1st run
0,8
0,6
0,4
(a)
0,2
1,0
0,8
0,6
0,4
0,2
0,0
1st run
2nd run
3rd run
4th run
5th run
(b)
0
60 120 180 240 300 360 420 480 540 600
Time (min)
Fig. 5. Comparison of five consecutive recycling runs for the degradation efficiency of MO over (a) 2ATO and (b) 2CTO samples under visible-light irradiation.

S.A. Bakar, C. Ribeiro / Journal of Molecular Catalysis A: Chemical 421 (2016) 1–15
7
Fig. 6. Schematic representation of mechanism involved in the photocatalytic degradation of MO.
hydroxyl radical (•OH), holes (h+), electrons (e−), and superoxide
radical (•O2−). Therefore, different scavengers were added sepa-
rately on the photodegradation experiment of MO aqueous solution
under visible-light irradiation for 120 min. They were added to
eliminate the specific reactive species and to have clear authen-
tication of the photocatalytic mechanism. For this purpose, sodium
oxalate (Na2C2O4), silver nitrate (AgNO3), tert-butyl alcohol (t-
BuOH) and benzoquinone (BQ) were added as scavengers into the
individual MO aqueous solution for h+, e−, •OH, and •O2−, respec-
tively [34–37]. In particular, the addition of scavenger suppressed
the photocatalytic degradation reaction of MO aqueous solution.
The extent of decrease in photodegradation rate by individual scav-
enger helps to locate the role of corresponding active species taking
part in the photo-oxidation mechanism.
Fig. 2(c) shows the substitution of •OH radical on 2CTO sur-
face by the addition of t-BuOH (5 mmol L−1) into the MO aqueous
solution. This indicates that •OH radicals taking an active part in
the photo-oxidation reaction mechanism over CTO sample. Simi-
larly, an addition of Na2C2O4 (0.5 mmol L−1) significantly reduced
the photodegradation efficiency of MO aqueous solution under
visible light irradiation. This indicates that photogenerated hole
(h+) has also participated in the photo-oxidation mechanism of
MO aqueous solution. Furthermore, photodegradation efficiency
did not vary noticeably with the addition of AgNO3 (1 mmol L−1
and BQ (1 mmol L−1) for the similar interval of time (120 min).
)
0,8
0,6
0,4
TO under visible-light
2CTO under visible-light
2ATO undervisible-light
0,2
0,0
-0,5
0,0
0,5
1,0
1,5
E/V vs SCE
Fig. 7. Photocurrent responses of the undoped and S-doped TiO2 (2ATO and 2CTO) photocatalyst in 1 mol L−1 Na2SO4 (pH = 13.6) solution under visible-light irradiation.

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S.A. Bakar, C. Ribeiro / Journal of Molecular Catalysis A: Chemical 421 (2016) 1–15
Thus, the photogenerated electrons (e− located in the CB) and •O2
in MO molecule to small fragments (CO2 and H2O) and to evade
secondary pollution for industrial applications.
−
played the minor role in the photodegradation of MO aqueous
solution over CTO sample. Interestingly, e− scavenging could limit
its recombination rate during the photogeneration of e−–h+ pairs
and, therefore, a slight increase in the photodegradation rate was
observed.
The stability of the S-doped TiO2 photocatalysts was studied by
performing repeated photodegradation experiments of MO aque-
ous solution under the same conditions. Fig. 5(a) and (b) show the
photocatalytic activity of 2ATO and 2CTO samples for degradation
of MO (10 mg/L) after five consecutive cycles with the minor change
in the photodegradation efficiency. It is noteworthy that photocor-
rosion did not occur and as-prepared photocatalysts are stable to
show enhanced photocatalytic activity. Thus, the recycling ability
of S-doped TiO2 photocatalysts has shown an improved constancy
for higher photocatalytic activity. Fig. 6 shows the schematic repre-
sentation of mechanism involved in the photocatalytic degradation
of organic compounds on the surface of S-doped TiO2 photocata-
lysts. The substitution of Ti4+ by S6+ in CTO samples, where thiourea
was used as a source of sulfur doping, would be a kind of transition-
metal ion doping into the TiO2 crystal lattice [39]. This results in the
creation of an intra-band-gap state between VB and near to the CB
edges that encourage the visible-light absorption. As a result, this
substitution of Ti4+ ions by S6+ ions has created a charge dispar-
ity in the bulk TiO2 crystal structure. The resulted charge disparity
attracts a number of anions (e.g. hydroxyl ions) from the organic
pollutant aqueous solution towards the surface of the photocat-
alyst and assisted in the neutralization of the additional positive
charge [40]. Whereas, the visible-light absorption in the CTO sam-
ple is expected due to the creation of confined sulfur mid-gap levels
from the replacement of Ti4+ by S6+ into the TiO2 crystal lattice
below the Ti 3d CB and likely narrow the band-gap level as com-
pared to pristine TiO2. Another interesting factor is the generation
of photoinduced h+ and it played an important role to generate
•OH radical from the oxidation of hydroxyl ions in the solution. The
similar kind of finding was observed in the h+ scavenger experi-
ment. As we know that hydroxyl radicals are the most active species
to perform the photo-oxidation reaction. The creation of charge
imbalance into the structure of bulk TiO2 crystal lattice enabled the
adsorption of more hydroxyl ion on the surface of the photocata-
lyst and has facilitated them to capture photoinduced h+ to form
hydroxyl radicals. The newly generated •OH radical played a major
part in the mineralization of organic pollutants. In the sideway, par-
tial e− transferred from less electronegative sulfur atoms to more
Fig. 2(d) shows the photodegradation plot of MO aqueous solu-
tion over 2ATO sample. It was observed that addition of t-BuOH
(5 mmol L−1) did not make a significant change to the photodegra-
dation reaction. This indicates that less amount of hydroxyl ions
are present on the surface of ATO samples. Whereas, an addi-
tion of Na2C2O4 (0.5 mmol L−1) into MO aqueous solution reduced
the photodegradation efficiency markedly from 89% to 55%. This
suggests that photoinduced h+ has played an important role for
the visible-light photodegradation reaction of MO aqueous solu-
tion over ATO samples. In particular, the addition of h+ scavenger
promotes the photoinduced e− to play an active role for the
photodegradation of MO aqueous solution. More interestingly,
photodegradation efficiency of MO aqueous solution followed dif-
ferent mechanism over ATO and CTO photocatalysts. The addition
of e− and •O2− scavengers have hindered the rate of recombination
for photoinduced e−–h+ pairs.
One the basis of these findings, it could be inferred that photo-
oxidation reaction of MO aqueous solution followed different
pathway over the anionic and cationic S-doped TiO2 photocata-
lysts. GC–LC analysis was performed on the different batches of
MO aqueous solution taken during the course of photodegradation
experiment for the different interval of visible-light irradiation as
shown in Fig. 4S. A schematically representation for the most proba-
ble pathway for photodegradation of MO molecule over anionic and
cationic S-doped TiO2 photocatalysts under visible light irradiation
is shown in Fig. 3.
To reconfirm the production of •O2 under visible-light irra-
−
diation over S-doped TiO2 photocatalysts, the photodegradation
experiment of MO aqueous solution was performed under aerobic
and anaerobic conditions. For this purpose, MO aqueous solution
was sparked with air and/or nitrogen gas before illumination to
the visible-light irradiation. For CTO samples, the photodegradation
efficiency of MO aqueous solution was marginally changed after
120 min of visible-light irradiation in the aerobic and anaerobic
condition. Whereas, in the case of ATO samples, the photodegrada-
tion efficiency of MO aqueous solution was largely affected due to
the generation of •O2− radicals under visible-light irradiation. Thus,
it was concluded that for CTO samples, •O2− did not take part in the
electronegative oxygen atoms through Ti
O S bonds. As a result,
electron deficient sulfur atoms hold on photoinduced e− for more
time to reduce the recombination rate and facilitated h+ to spend
more time on the surface of the photocatalyst to generate more
•OH radical, and resulted in enhanced quantum efficiency. There-
fore, the higher photocatalytic efficiency of the CTO samples could
be explained due to an involvement of above possible parameters.
These are main parameters to sum-up the enhanced photocatalytic
activity of cationic S-doped TiO2 photocatalysts such as; (1) quick
rate of separation for photoinduced charge carriers, (2) enhanced
absorbance in the visible-light region due to partial substitution of
Ti4+ by S6+ and surface adsorbed hydroxide ions capture the excited
holes to form •OH radicals, (3) partial transfer of electrons from S
photodegradation of MO aqueous solution, and •O2 radicals did
−
not exist in the photodegradation reaction of MO aqueous solution
over cationic S-doped TiO2 photocatalysts. However, in the case
of anionic S-doped TiO2 photocatalysts, the photodegradation effi-
ciency−of MO aqueous solution is dependent on the concentration
of •O2 radicals. Thus, photocatalytic degradation of MO aqueous
solution followed different pathway over anionic and cationic S-
doped TiO2 photocatalysts.
The photodegradation of organic compounds (MO and phenol)
was evaluated by total organic carbon (TOC) mineralization index
during the photocatalysis experiment over anionic and cationic
S-doped TiO2 photocatalysts. Fig. 4(a) and (b) shows the ratio of
residue concentration (TOC) of MO and phenol in solution as a
function of visible-light irradiation time [38]. It was found that
as-prepared S-doped TiO2 photocatalysts showed substantial min-
eralization ability for organic compounds (MO and phenol) under
visible-light irradiation. After 2 h of visible-light irradiation, the
TOC removal rate of MO dye and phenol was reached to 48% and
44%, respectively. Thus, the photodegradation efficiency of S-doped
TiO2 photocatalysts is good enough to degrade not only the bleach-
ing (chromophore groups) but also to mineralize the aromatic rings
to O atoms due to formation of Ti
O S bonds, (4) and inhibition of
photoinduced e−–h+ pairs recombination rate due to capture of e−
by electron deficient S atoms.
For anionic S-doped TiO2 photocatalysts, we employed CS2 as
the sulfur originators. Higher photodegradation efficiency and nar-
row band gap were observed in the case of ATO samples from the
photocatalytic experiments of organic pollutants (MO and phenol)
and DRS spectra as compared to the pristine and CTO samples.
It is expected that anionic S-doping into the bulk of TiO2 lattice
structure has generated more oxygen defects/vacancies and it has
shifted the absorption edge in the visible-light region [41,42]. This
also inhibits the recombination rate of photoinduced e−–h+ pair’s
by capturing the photoinduced e−. Thus, enhanced the quantum

S.A. Bakar, C. Ribeiro / Journal of Molecular Catalysis A: Chemical 421 (2016) 1–15
9
Fig. 8. (a) to (c) SEM images of undoped TiO2 (d) EDX of S-doped TiO2 sample, (e) to (g) HR-TEM analysis of S-doped TiO2 sample, and (h) FFT of S-doped TiO2 sample.
efficiency. As discussed above for anionic S-doped samples, the
transfer of photoinduced e− at the surface of the photocatalyst
prompts the degradation of MO aqueous solution by producing
hydroxyl and superoxide radicals via reacting with adsorbed O2
gap, (4) and formation of stabilized anatase structure by anionic
S-doping enhanced the photocatalytic activity.
Furthermore, the cylindrical-shaped nanorods structures have
many porous channels on the surface which endorse their pho-
tocatalytic activity. The responses of transient photocurrent of
S-doped TiO2 modified electrode confirmed this phenomenon
when irradiated with visible-light (Fig. 7). The S-doped TiO2
modified electrodes showed enhanced photocurrent response as
compared to undoped TiO2 electrode under visible-light irradi-
ation. The enhanced photocurrent response is generated due to
more efficient separation of photoinduced charge carriers and fast
transfer pathway through the nanorods structure. As a result, 1-D
S-doped TiO2 nanorods showed higher photocatalytic activity than
undoped TiO2 under visible-light irradiation.
and H2O. Another important factor is the formation of Ti
O S and
O
Ti S bonds in the TiO2 crystal structure. These bonds tempt
fresh impurity levels among the VB and CB bands that result in
the promotion of more photoinduced h+ to produce active radicals
(hydroxyl and peroxide) due to the transfer of more photoinduced
e− to the CB under visible-light irradiation. Additional character-
istics for enhanced photocatalytic activity of anionic S-doped TiO2
photocatalysts include; (1) presence of (101) facets entertained the
photoinduced electron as reservoir to slow down the recombina-
tion rate of photoinduced electron–hole pairs and to produce •O2
−
by reduction of surface adsorbed O2 atoms, (2) anisotropic shape
of nanorods having larger surface area facilitate the fast transport
of photoinduced charge carriers to pass through different longitu-
dinal directions for successive photocatalytic oxidation reactions,
(3) creation of oxygen vacancies causes red shift for optical energy
Fig. 5S shows the XRD diffraction patterns for anatase TiO2 and
S-doped TiO2 photocatalysts. The presence of well-defined and
broad diffraction peak at (101) crystal plane revealed that diffrac-
tion planes are alignment along the favored (101) direction for
anatase pristine sample. No badge of diffraction peaks was noted

10
S.A. Bakar, C. Ribeiro / Journal of Molecular Catalysis A: Chemical 421 (2016) 1–15
Fig. 9. XPS survey spectra of undoped and S-doped TiO2 nanorods, (b), (c) and (d) XPS spectra for Ti3p, O1s and S2p region, respectively.
which corresponds to the presence of rutile phase in the XRD
diffraction patterns of as-prepared samples. It was evident that
sulfur doping could inhibit the growth of rutile phase. The Debye-
Scherrer equation was used to calculate the average crystallite size
of the pristine and S-doped samples and is given in Table 1. It
was found that S-doping strongly influenced the anatase degree
of crystallization in the photocatalysts. For higher concentration
of S-doping levels, the crystallinity of samples slightly decrease
and is an indication of successful S-doping into TiO2 crystal lattice
as reported for other non-metal doping e.g. for nitrogen doping
[43,44]. Meanwhile, systems with higher crystallinity are estab-
lished to ensure superior photocatalytic activity as compared to
ailing crystalline systems and thus, the higher concentration of dop-
ing element could halt the photocatalytic activity [45]. The lattice
parameters for the pristine sample having tetragonal crystal sys-
tem were slightly varied after S-doping samples and were ascribed
due to an assimilation of the larger sulfur atoms into the crystal
lattice of TiO2.
More FE-SEM images of anionic and cationic S-doped TiO2 samples
are shown in supporting information Fig. 6S.
It was observed that the S-doped TiO2 nanorods consist of hier-
archical core–shell structure and are compactly aligned having an
anatase phase. From the EDX mapping of S-doped TiO2 samples
(see Fig. 7S), it was observed that sulfur atoms are equivalently
disseminated in the TiO2 crystal lattice structure. EDX analysis
was performed to analyze the S-doping concentration in the as-
prepared samples as shown in Fig. 8(d). It was found that sulfur
atoms are almost uniformly distributed along the axial and radial
direction of nanorods. A sharp peak at about 2.45 keV was observed
for the sulfur element in the S-doped TiO2 samples and was ascribed
due to the presence of small amount of S dopant element. This peak
is absent in the EDX spectrum of pristine sample. More interest-
ingly, the edges and surfaces of S-doped TiO2 nanorods started
to develop out of flatness for higher sulfur content (3ATO and
3CTO). It is expected due to the decrease of overall crystallinity to
a certain level. TEM and HR-TEM analysis were used for structural
characterization of pristine and S-doped TiO2 samples. The TEM
and HR-TEM images of S-doped TiO2 photocatalysts confirm the
presence of the anatase phase with hierarchical core–shell nano-
structure. The calculated values for average diameter and length of
nanorods are consistent with the FE-SEM results. Fig. 8(g) shows
the HR-TEM image of the pristine sample which comprises of
the single-crystalline anatase structure with well-defined/resolved
inter-planer spacing and having lattice fringes of about 0.352 nm.
Fig. 8(a–c) shows the FE-SEM images of the undoped TiO2 sam-
ple that consist of slanting or vertically allied uniformly grown
nanorod arrays. The pristine nanorods have the uniform rectan-
gular cross section and diameter in the range of 20–30 nm with
the distinctive length of nanorods are about 50–70 nm. From the
FE-SEM images, no clear noticeable change in the morphology of
nanorods was observed for the pristine and S-doped TiO2 samples.

S.A. Bakar, C. Ribeiro / Journal of Molecular Catalysis A: Chemical 421 (2016) 1–15
11
The dominant facets, expected from the symmetry of crystallo-
graphic anatase TiO2 structure was corresponding to the (101)
planes and, which is considered as the most stable anatase TiO2
facets plane for thermodynamically [46]. Furthermore, the HR-TEM
image demonstrates the high crystalline nature of the as-prepared
samples.
S-doped TiO2 samples were washed vigorously with double deion-
ized water prior to the XPS analysis. The XPS findings suggest that
none of the physisorbed sulfur species such as SO42− are present on
the surface of photocatalysts and SO42− are chemically coordinated
at the surface of S-doped TiO2 nanorods. The probable coordination
binding between TiO2 lattice and SO42−, which could play a vital
role as an efficient electron trapper (see Fig. 1). It was reported
that thiourea is mainly responsible for cationic S-doping due to the
substitution of Ti4+ by S6+, and is in accordance with our results as
shown in Fig. 9(d) [40,19,24]. The insertion of S atoms as cations
into TiO2 crystal lattice generates a charge disparity that results
in to bring the additional positive charge on the surface of the
photocatalyst which is nullified by the free hydroxide ions [40].
As a result, photoinduced holes on the surface of photocatalysts
are captured by adsorbed hydroxide ions to form hydroxyl rad-
ical, an important active species to bring photocatalytic reaction
for the degradation of pollutant molecules. More interestingly, the
relatively larger atomic radius of Ti4+ (0.64 Å) as compared to S6+
(0.29 Å) might cause a decrease in the crystal size due to the replace-
ment of Ti4+ by S6+ in S-doped TiO2 samples (CTO). By comparison
of the XPS finding to the XRD result, we have observed that the
lattice parameters for CTO samples were smaller than the pristine
sample.
The XPS spectrum of 2ATO sample in the S 2p region corresponds
to the binding energy at about 162.7 eV for sulfur atoms and has
been assigned to the replacement of O2− by S2−to form Ti S bonds
[20,23,31]. This peak is regarded to the presence of S2− at lower
valence state for sulfur atoms in the ATO samples. It was found
that use of either CS2 or TiS2 precursors for S-doping are mainly
responsible for anionic form in the S-doped TiO2 samples [21,56].
Therefore, the presence of sharp XPS peak at about 162.7 eV in the S
2p region for 2ATO sample is accredited to the S2− species due to the
exchange of O2− by S2− during the curse of S-doping into the crystal
lattice of TiO2. Umebayashi et al. [23] and Hebenstreit et al. [57]
reported that a binding energy of 162 eV could be assigned to the
substitutional sulfur for oxygen either anatase and/or rutile phase
S-doped TiO2. Thus, sulfur atoms from the CS2 precursor replaced
oxygen atoms to form Ti S bond. As expected, the anionic S-doping
into the TiO2 crystal lattice would result in changing the unit cell
parameters of pristine TiO2 due to a difference in the ionic radius
of S2− (1.7 Å) and O2− (1.22 Å). As a result, the unit cell parameters
of 2ATO sample after successful substitution of O2− by S2− were
slightly increased than the pristine and CTO samples, probably due
to the incorporation of S2− with larger ionic radius into the crystal
lattice of TiO2, which is in accordance with the XPS results.
Fig. 8S shows the FT-IR spectra of pristine and S-doped TiO2
Recently, it was found that the reductive (101) facets of anatase
•
−
nanocrystals facilitate the reduction of O2 to O2 by providing a
highly reactive surface and were likely acting as a reservoir for pho-
togenerated electrons [47,48]. Likewise, the sole structure features
of octahedral anatase crystals having (101) facets assist the oxida-
tive reduction/disintegration of pollutant molecule and enabled
the higher photocatalytic activity [49]. Therefore, we could expect
that S-doped TiO2 photocatalysts with preferential reductive (101)
facets displayed important role to improve the photocatalytic effi-
ciency by lowering the rate of recombination for photogenerated
electron-hole pairs. Conversely, it was noted that particles having
anisotropically shaped including nanorods [50], nanobelts [51], and
nanotubes [52] have a lower rate of electron-hole pairs recombi-
nation due to their structural characteristic of providing a quick
pathway for transport of charge carriers alongside the longitudinal
track. Thus, this results in enhanced mobility for charge carri-
ers. We can infer from these findings that S-doped TiO2 nanorods
with anisotropically long shape are efficient materials for relatively
ambient separation of photogenerated electron-hole pairs and for
subsequent supply of charge carriers towards reactive sites to carry
out photocatalytic reactions.
High-resolution XPS analysis was used to quantify the existence
of sulfur atoms in the TiO2 crystal lattice. Fig. 9(a) shows wide-scan
survey spectra for the pristine and S-doped TiO2 photocatalysts,
which consist of Ti, O, and S elements corresponding to Ti2p, O1s,
and S2p peaks, respectively. The presence of carbon peak is ascribed
due to XPS instrument as adventitious hydrocarbon and no peak
corresponding to nitrogen atoms was observed in the XPS spec-
trum. It was observed from the XPS spectra in the S2p region that
sulfur atoms are presents for the ATO and CTO samples. Whereas,
for the pristine sample, there is no speak for S atom presents in
the S2p region. This finding suggests that sulfur atoms have been
effectively introduced into the crystal lattice of TiO2 photocatalyst.
It can be seen in the XPS spectra of Ti 2p for pristine and S-doped
photocatalysts that comprise of two peaks for Ti 2p3/2 (458.3) and Ti
2p1/2 (464.1 eV) as shown in Fig. 9(b). It was found that the binding
energy of Ti 2p slightly shifted negatively after S-doping as com-
pared to pristine TiO2 photocatalyst and which is possibly due to the
2−
rise of interaction between coordinated SO4 anions to titanium
atoms. This interaction drags more electrons towards Ti atoms. As is
mentioned above, that this interaction is more favorable due to the
samples. It was attributed that the stretching vibration of Ti
O Ti
formation of O Ti S and Ti
O
S bonds by anionic and cationic S-
bonds are mainly responsible for the generation of an intense
broadband at around 900–700 cm−1. Moreover, the characteristic
peaks at around 1080 and 1423 cm−1 for S-doped TiO2 samples are
ascribed due to coordinated sulfate groups [58]. The presence of
SO stretching vibration is attributed at about 1365 cm−1 [59] in the
S-doped TiO2 samples. As discussed in the XPS analysis that these
peaks further support the presence of chemically coordinated sul-
fate groups in the S-doped TiO2 samples. It is noted that distinct
doping into TiO2 lattice and enable the most electronegative atoms
(Ti and O) to drag electrons from the sulfur atoms that cause a
charge disparity. This results in shifting of binding energy nega-
tively due to the formation of new band energy structure [53,54].
Fig. 9(c) shows the O1s XPS spectrum that consists of two peak fit-
tings. The main peak ascribed to lattice oxygen appeared at about
529.6 eV of lower binding energy and peak at higher binding energy
(530.8 eV) is attributed to the oxygen atoms of sulfate groups and
surface adsorbed hydroxyl groups [54].
band was observed for Ti
O
S bonds at 1043 cm−1 for CTO sam-
ple [60]. Furthermore, the presence of Ti S stretching vibration is
confirmed from absorbance band at 1135 cm−1 in the ATO sample
[59]. These findings further confirmed the successful substitution
of O2− by S2− in anionic S-doping and substitution of Ti4+ by S6+ in
the cationic S-doping TiO2 samples. Whereas, in the case of the
pristine sample, no such bands corresponding to either Ti S or
Fig. 9(d) shows the XPS spectra for the 2CTO and 2ATO samples.
For 2CTO sample, thiourea was used as precursor for S-doping, and
it showed two strong broadened peak probably due to spin–orbit
coupling [55], of the split sublevels of 2p3/2 and 2p1/2 states, at
168.3 and 169.5 eV with a separation of 1.4 eV corresponding to
presence/incorporation of S6+ into TiO2 lattice by partial substitu-
S
O/S O was observed, which indicate that opted OPM route is
tion of Ti4+ to form Ti
O
S bonds [40]. In order to eliminate the
superior to number of early reported methods for the synthesis of
S-doped TiO2 and it limits the use of expensive organic compounds
to prevent the involvement/doping of impurities from synthesis
chance of presence/indication of physisorbed sulfur species from
the experimental process on the surface of the photocatalyst, the

12
S.A. Bakar, C. Ribeiro / Journal of Molecular Catalysis A: Chemical 421 (2016) 1–15
Table 1
Structural parameters and photocatalytic performance of TO, ATO and CTO samples.
Lattice constants of cell
unit (Å)
SA
Vp
IEP
Sample
Ti:S molar
ratio
Crystallite
size (nm)
Band gap
(eV)
Photodegradation
efficiency (%)
(cm3/g)
(m2/g)
(a)
(c)
TO
1:0.0
19
20
22
23
17
16
14
3.7852
3.7859
3.7865
3.7916
3.7803
3.7758
3.7737
9.4558
9.4837
9.5182
9.5203
9.4505
9.4489
9.4453
71
77
90
79
80
88
81
0.41
0.43
0.44
0.48
0.42
0.43
0.45
3.09
2.91
2.86
2.82
2.94
2.88
2.85
4.30
4.40
4.70
5.10
4.33
4.35
4.39
5
1ATO
2ATO
3ATO
1CTO
2CTO
3CTO
1:0.01
1:0.02
1:0.03
1:0.01
1:0.02
1:0.03
81.38
89.32
83.57
82.19
85.43
81.21
steps. The reproducibility of the as-prepared material through OPM
route with the same phase composition make this another advan-
tage over other previously reported methods. More interestingly,
intense broad peak at around 1640 cm−1 was observed correspond-
ing to OH bending vibrations in the CTO sample, and the broad band
is more intense as compared to pristine and ATO samples. Thus, we
can conclude that cationic S-doping favored the presence of more
hydroxyl groups on the surface of the photocatalyst as compared to
anionic S-doping and this could enhance the photocatalytic activity
of cationic S-doped TiO2 photocatalysts.
Fig. 10 shows the UV–vis diffuse reflectance spectra of 1-D
undoped and S-doped TiO2 nanorods to investigate the opti-
cal absorptions. The sulfided surface shows stronger solar light
absorbance in visible-light region for the S-doped TiO2 photocat-
alysts as compared to pristine sample. Compared to the pristine
sample, all S-doped TiO2 samples show significantly improved
absorptions in the visible region while highest absorption edge
was observed near 400 nm for higher sulfur level samples and
is ascribed due to S-doping effect. This absorption enrichment is
greatly reliant on the S-doping levels and reaches to the sulfidation
saturation for higher sulfur concentrated samples and prolongs the
light absorption from UV into the visible region and broadens the
solar spectrum absorption due to the successful incorporation of
S dopants. From the comparison of absorption spectra for pristine
and S-doped TiO2 nanorods, it was demonstrated that creation of
oxygen deficiencies could enhance the wide-spectrum absorption
due to the high electron concentration in the surface layer. Inset
of Fig. 10 shows the plot of Kubelka–Munk function versus energy
of light for the pristine and S-doped samples. For the pristine TiO2
sample having a large value of band gap (3.09 eV), no appropri-
ate light absorbance was observed in the visible region. Whereas,
the S-doped TiO2 nanorods (ATO and CTO) showed enhanced light
absorbance in the visible region due to the decrease in band
gap energy by S-doping to 2.82 eV (ATO) and 2.85 eV (CTO). It is
expected that this alternation/reduction in band gap energy is due
to the incorporation of the sulfur atoms into the bulk of TiO2 and,
as a result, the electronic and crystal structure of the doped mate-
rial altered which result in strong absorption edge into the visible
light region. A broad absorption background was observed for both
doped samples (ATO and CTO) as compared to the pristine sample
in the visible region and is ascribed due to the creation of new bonds
(Ti
O S or O Ti S) into TiO2 crystal structure, as discussed in the
XPS results and discussion session. This could facilitate the partial
flow of electron from low electronegative atom (S) towards higher
electronegative atoms (Ti and O). Furthermore, this could result in
the creation of a new low-valence state [61] between the valence
and conduction band of TiO2 and enhanced the light absorption in
the visible region greater than 500 nm.
The photocatalytic ability of nanoparticle for the degradation
of organic compounds depends on the particle size distribution
in solution and aggregation in solution caused the derivation of
measured particle size from the actual powder form. Therefore, the
zeta potential measurements were calculated for undoped and S-
doped TiO2 photocatalysts as shown in Fig. 9S. Table 1 shows the
measured value of the isoelectric point (IEP) for undoped and S-
doped TiO2 photocatalysts. As expected, undoped TiO2 sample has
the lower zeta potential as compared to S-doped TiO2 samples. For
anionic S-doped TiO2 samples, the increase in the IEP value was
more prominent with the increase of S-doping level and shifted
towards negative pH (<6.8). Whereas, for cationic S-doped TiO2 the
increase of IEP value is not so convenience for negative pH [62]. As
a result, the surface of S-doped TiO2 photocatalysts is engaged by
the more net charge as compared to undoped TiO2 and make it
more feasible for organic compound to approach on the surface of
photocatalysts. Therefore, an increase in the S-doping concentra-
tion resulted in the increase concentration of approaching organic
compounds on the surface of photocatalyst due to increasing the
number of net charge on the surface of the photocatalyst. One
can predict from these findings that one factor for the enhanced
photocatalytic degradation efficiency of S-doped TiO2 nanorods in
solution is due to increasing net charge on the surface as compared
to undoped TiO2. The surface adsorbed hydroxyl groups increase
the wettability and contact angles of S-doped samples due to the
photocatalytic oxidation reaction of organic compounds on the
surface. Conversely, the pH of solution played an important part
for the enhanced photocatalytic performance of S-doped TiO2 in
an aqueous medium. Higher zeta potential suggest increased the
number of surfaces adsorbed organic compound for S-doped TiO2
photocatalysts to perform photodegradation reactions [63]. More
interestingly, after an ambient concentration of S-doping level, the
photocatalytic efficiency started to decrease due to higher surface
wettability and higher surface net charge [64].
Fig. 10. DRS absorbance spectra of pristine and S-doped TiO2 samples, Inset shows
the band-graph plot of all samples.

S.A. Bakar, C. Ribeiro / Journal of Molecular Catalysis A: Chemical 421 (2016) 1–15
13
Fig. 11. N2 absorption-desorption plot of pristine and S-doped TiO2 samples, inset shows the pore volume distribution graph of TiO2.
The incorporation of S atoms into the TiO2 crystal structure was
further confirmed with the help of Raman spectroscopy. Fig. 10S
shows the Raman spectra for pristine and S-doped TiO2 samples.
The Raman spectra was executed in the range of 100–800 cm−1
and corresponds to the existence of pure anatase phase. These
findings are consistent with XRD results for all samples and fur-
ther confirmed the absence of any other phase impurity. The six
Raman active modes [3Eg + 2B1g + A1g] are assigned to the anatase
phase at frequencies 143 cm−1, 197 cm−1, 397 cm−1, 514 cm−1, and
638 cm−1 [65]. It was found that the main anatase Raman active
band sharpened with the increase of S-doping concentration at
about 143 cm−1 (Eg mode) and attributed to a considerable blue
shift in the Raman band. The characteristic shift and broadening of
Raman peaks are attributed due to change in the original symme-
try/structure of TiO2 crystal lattice caused by S-doping.
The Raman peak intensities increased with sulfur concentra-
tion and is expected due to Raman resonance effect of TiO2 due to
change in electronic environment [66]. Fig. 11 shows the nitrogen
adsorption–desorption isotherm measurements for pristine and
S-doped TiO2 photocatalysts to characterize the specific surface
areas. It is well-known that high surface area enhanced the pho-
tocatalytic activity due to increased number of active sites per unit
mass for photocatalytic reaction. The BET specific surface area of S-
doped TiO2 photocatalysts (ATO and CTO) was much larger than the
pristine sample. Furthermore, the small diameter of TiO2 photocat-
alyst reduce the detrimental size effects by tumbling the distance
between electron–hole pair which limits the recombination rate of
the free hole at the surface with a trapped electron.
(S and O), and surface adsorbed negatively charged hydroxide neu-
tralized the positive charge at the surface of the photocatalyst.
Some chemisorbed sulfate groups (SO42−) were also present on the
photocatalyst surface. In the case of anionic S-doped TiO2 photo-
catalyst, sulfur atoms substitute the oxygen atoms in the O Ti
O
network to form O Ti S bonds and sulfur atoms exist in the S2−
valence state in the crystal lattice of TiO2.
4. Conclusion
A facile, surfactant-free chemical route has been developed
to synthesize anionic and cationic S-doped TiO2 photocatalysts
via OPM assisted hydrothermal treatment. The as-prepared S-
doped TiO2 photocatalysts demonstrated worthy visible-light
photocatalytic activity and mineralization performance for organic
compounds (MO and phenol) as compared to pristine sample. It
was found that S6+ substituted for Ti4+ in cationic S-doped sample
reduced the crystalline grain size due to a smaller ionic radius of S6+
as compared to Ti4+. Whereas, S2− substitution for O2− increased
the crystalline grain size in anionic S-doped TiO2 samples and
reduced the BE shifting of parent atoms (Ti and O) peaks due to
higher electronegativity difference, which consequently enhanced
the absorbance in the visible-light region. The surface adsorbed
2−
SO4 groups were observed for both samples and intimated due
to either chemisorptions or surface oxidation of sulfur atoms.
The anionic S-doped TiO2 photocatalysts showed higher photocat-
alytic activity due to reduced crystallite size and scavengers test
revealed that holes and electrons played the nearly equal role for
its higher photocatalytic activity. Whereas, in the case of cationic
S-doped TiO2 photocatalysts the creation of new impurity levels
could be mainly responsible for enhanced higher photocatalytic
efficiency due to oxidation of surface chemisorbed hydroxyl ions
by photoinduced holes. Therefore, it was revealed that doping lev-
els and degree of surface chemisorbed hydroxylation are largely
accountable for the enhanced photocatalytic activity of S-doped
TiO2 photocatalysts. Thus, the present report may offer a signifi-
cant suggestion for the construction of visible-light photocatalysts
through doping and morphology designing.
Thus, the proposed pathway for S-doping into the TiO2 crys-
tal lattice and arrangements of impurity dopant atoms (S) into the
TiO2 crystal lattice (see Fig. 1) are confirmed by the XRD, XPS, EDX
and FT-IR findings. In fact, S-doping precursors (thiourea and CS2)
played an important role in the creation of valence states of sulfur
and correspondingly helped in the incorporation of S atoms into the
TiO2 structure to form cationic and anionic S-doped TiO2 nanorods.
The negative enthalpy or formation for the chelating ligand (CS2)
enabled to replace oxygen atoms in the TiO2 crystal lattice. In detail,
the formation of Ti
cation generated charge imbalance between the neighboring atoms
O
S bonds due to substitution of S6+ for Ti4+

14
S.A. Bakar, C. Ribeiro / Journal of Molecular Catalysis A: Chemical 421 (2016) 1–15
[20] T. Umebayashi, T. Yamaki, S. Yamamoto, A. Miyashita, S. Tanaka, T. Sumita,
Acknowledgment
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The authors like to thank, post-graduate program by TWAS-
CNPq for financial support to this project. The authors also like to
thank, Brazilian Nanotechnology National Laboratory LNNano for
XPS analysis and LIEC lab for HR-TEM analysis.
Appendix A. Supplementary data
Supplementary data associated with this article can be found, in
the online version, at http://dx.doi.org/10.1016/j.molcata.2016.05.
003.
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