Low-temperature synthesis of anatase TiO2 nanoparticles with tunable surface charges for enhancing photocatalytic activity

Article abstract of DOI:10.1371/journal.pone.0114638

In this work, the positively or negatively charged anatase TiO₂ nanoparticles were synthesized via a low temperature precipitation-peptization process (LTPPP) in the presence of poly(ethyleneimine) (PEI) and poly(sodium4- styrenesulfonate) (PSS). X-ray diffraction (XRD) pattern and high-resolution transmission electron microscope (HRTEM) confirmed the anatase crystalline phase. The charges of the prepared TiO₂, PEI-TiO₂ and PSS-TiO₂ nanoparticles were investigated by zeta potentials. The results showed that the zeta potentials of PEI-TiO₂ nanoparticles can be tuned from +39.47 mV to +95.46 mV, and that of PSS-TiO₂ nanoparticles can be adjusted from -56.63 mV to -119.32 mV. In comparison with TiO₂, PSS-TiO₂ exhibited dramatic adsorption and degradation of dye molecules, while the PEI modified TiO₂ nanoparticles showed lower photocatalytic activity. The photocatalytic performances of these charged nanoparticles were elucidated by the results of UV-vis diffuse reflectance spectra (DRS) and the photoluminescence (PL) spectra, which indicated that the PSS-TiO₂ nanoparticles showed a lower recombination rate of electron-hole pairs than TiO₂ and PEI-TiO₂.

Full text of DOI:10.1371/journal.pone.0114638

RESEARCH ARTICLE
Low-Temperature Synthesis of Anatase
TiO2 Nanoparticles with Tunable Surface
Charges for Enhancing Photocatalytic
Activity
Ye Li1,3, Zhenping Qin1, Hongxia Guo2,3*, Hanxiao Yang1, Guojun Zhang1,
Shulan Ji1,3, Tingying Zeng4
1. College of Environmental and Energy Engineering, Beijing University of Technology, Beijing, China, 2.
College of Materials Science and Engineering, Beijing University of Technology, Beijing, China, 3. Beijing Key
Laboratory for Green Catalysis and Separation, Beijing, PR China, 4. MIT, Elect. Res. Lab, Cambridge,
Massachusetts, United States of America
*hxguo@bjut.edu.cn
Abstract
OPEN ACCESS
In this work, the positively or negatively charged anatase TiO2 nanoparticles were
synthesized via a low temperature precipitation-peptization process (LTPPP) in the
presence of poly(ethyleneimine) (PEI) and poly(sodium4- styrenesulfonate) (PSS).
X-ray diffraction (XRD) pattern and high-resolution transmission electron
microscope (HRTEM) confirmed the anatase crystalline phase. The charges of the
prepared TiO2, PEI-TiO2 and PSS-TiO2 nanoparticles were investigated by zeta
potentials. The results showed that the zeta potentials of PEI-TiO2 nanoparticles
can be tuned from +39.47 mV to +95.46 mV, and that of PSS-TiO2 nanoparticles
can be adjusted from 256.63 mV to 2119.32 mV. In comparison with TiO2, PSS-
TiO2 exhibited dramatic adsorption and degradation of dye molecules, while the PEI
modified TiO2 nanoparticles showed lower photocatalytic activity. The
Citation: Li Y, Qin Z, Guo H, Yang H, Zhang G, et
al. (2014) Low-Temperature Synthesis of Anatase
TiO2 Nanoparticles with Tunable Surface Charges
for Enhancing Photocatalytic Activity. PLoS
ONE 9(12): e114638. doi:10.1371/journal.pone.
0114638
Editor: Vipul Bansal, RMIT University, Australia
Received: August 1, 2014
Accepted: November 12, 2014
Published: December 15, 2014
This is an open-access article, free of all copyright,
and may be freely reproduced, distributed,
transmitted, modified, built upon, or otherwise used
by anyone for any lawful purpose. The work is
made available under the Creative Commons CC0
public domain dedication.
photocatalytic performances of these charged nanoparticles were elucidated by the
results of UV-vis diffuse reflectance spectra (DRS) and the photoluminescence (PL)
spectra, which indicated that the PSS-TiO2 nanoparticles showed a lower
recombination rate of electron-hole pairs than TiO2 and PEI-TiO2.
Data Availability: The authors confirm that all data
underlying the findings are fully available without
restriction. All relevant data are within the paper.
Funding: This work was financially supported by
National Natural Science Foundation of China
(21176005), National technology support program
of China (Project No., 2011BAE29B00). The
funders had no role in study design, data collection
and analysis, decision to publish, or preparation of
the manuscript.
Introduction
Photocatalytic degradation is an efficient and economical method to totally
decompose organic contaminants into benign substances [1–2]. Titanium dioxide
Competing Interests: The authors have declared
that no competing interests exist.
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Anatase TiO2 Nanoparticles with Tunable Surface Charges
(TiO2) nanocrystal has attracted much attention as a photocatalyst due to its
photostability, nontoxicity, high activity and relatively low cost [3]. TiO2 has three
crystalline forms: rutile, anatase and brookite [4]. Anatase titania usually exhibits
higher photocatalytic activity than rutile and brookite, owing to its higher density
of localized states, the surface-adsorbed hydroxyl radicals and slower charge
carrier recombination [5–7]. Much work has been reported for preparation of
anatase TiO2, such as thermolysis [8–9], sol-gel [10–11], ultrasonic technique
[12], solvothermal [13] and hydrothermal method [14–15], by which the resulted
particles have to be annealed at suitable temperature to obtain crystalline TiO2
with anatase phases. The annealing process at relatively high temperature would
inevitably lead to particles agglomeration and hence reduction of their specific
surface area [16–17]. Therefore, the low temperature methods for preparation of
anatase TiO2 particles and films attract much attention, due to the fact that
calcinations step can be eliminated. Mesoporous aggregates of anatase TiO2 were
obtained by controlled hydrolysis reaction of titanium tetrabutoxide in non-
aqueous media, isopropanol, and then aging at ambient conditions for more than
120 days [18]. Gao et al. have synthesized the superfine TiO2 nanocrystals with
tunable anatase/rutile ratios in aqueous ethanol solution by ‘‘low temperature
dissolution–reprecipitation process’’ (LTDRP) and solvethermal treatment [19].
Burunkaya et al. have prepared directly the anatase TiO2 particles by hydrolyzing
titanium ethoxide in alcohols solvent, such as ethanol, n-butanol and n-hexanol,
followed by reflux at low temperature of 93 C, without calcination step [20]. An
˚
anatase hydrosol has been obtained by acidic hydrolysis of titanium isopropoxide
at room temperature [21–22], and the nanocrystalline TiO2 films were prepared
after adding surfactant into the hydrosol. However, all of these anatase TiO2
particles or films have been prepared using titanium alkoxide or chloride in non-
aqueous alcohol media.
Moreover, the TiO2 suspension with a high degree of dispersion stability is
considered one of the important factors during the photocatalytic processes [23–
27]. Thus, modification of TiO2 particles by polymers or surfactants, which
combines unique properties of dissimilar components, would favor dispersing the
suspension. Two ways have been used to modify TiO2 particles: one is to
introduce a surfactant or polymer to a preformed nanoparticle (post-synthesis)
[23–24]; and the other is to incorporate surfactants or polymers simultaneously
during nanoparticle synthesis (in situ synthesis) [25–27]. The anionic surfactant,
such as sodium dodecylsulphate (SDS) and sodium dodecyl benzene sulfonate
(SDBS), have been introduced into TiO2 particles for promoting the dimerization
of rhodamine B (RhB) and subsequent adsorption on the TiO2 surface [26–29].
The nanosized titanium dioxide particles and charged polyelectrolytes (PEs), such
as poly(allyl aminehydrochloride) (PAH) and poly(styrene sulfonate sodium)
(PSS), have been alternatively assembled to form multilayer composite film for
photodegradation of RhB under ultraviolet (UV) irradiation [30]. However, the
charge controllability and the photocatalytic performance of the positive or
negative TiO2 particles modified by polyelectrolyte need further investigation.
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Anatase TiO2 Nanoparticles with Tunable Surface Charges
In this work, the charge-tuned TiO2 nanoparticles with pure anatase phase were
synthesized via a low temperature precipitation-peptization process (LTPPP)
under ambient condition. The water soluble titanium sulfate salt (Ti(SO4)2) was
used as precursor to fabricate either positive or negative anatase TiO2
nanoparticles in the presence of cationic or anionic polyelectrolytes (PEs). The
charges of as-prepared nanoparticles, such as pure TiO2, poly(ethyleneimine)
(PEI)-TiO2 or poly(sodium 4-styrenesulfonate) (PSS)-TiO2, manifested by zeta
potentials, were tuned by the variations of pH and the amount of the
polyelectrolytes, respectively. Then, the photocatalytic activities of as-prepared
TiO2 nanoparticles were investigated by degradation of organic methylene blue
(MB) and rhodamine B (RhB) dyes. The possible mechanism of the photocatalytic
performance of the charged TiO2 nanoparticles is proposed based on
photoluminscence (PL) emission and UV-vis diffuse reflectance spectra (DRS).
This work may provide an additive strategy to charge controllable TiO2
nanoparticles, which would find new application in other fields, such as
electrophoretic particles and the charged films or membrane [31–33], in addition
to photocatalysis.
Experiments
2.1 Materials
Titanium sulfate (Ti(SO4)2), methenamine(C6H12N4), nitric acid, ethanol
(analytically pure), methylene blue (MB) and rhodamine B (RhB) dyes were
provided by Beijing Chemical Factory (Beijing, China) and used without further
purification. Poly(ethyleneimine) (PEI, Mw5750,000) and poly(sodium4-styr-
enesulfonate) (PSS, Mw570,000) were purchased from Sigma- Aldrich Co.,(St.
Louis, MO, USA). Deionized water was used throughout this research.
2.2 Preparation of PEs-modified TiO2 nanoparticles
The PEs-modified TiO2 nanoparticles were obtained at low temperature
precipitation-peptization process (LTPPP). The negatively charged TiO2 nano-
particles modified by PSS were prepared as an example. Specifically, 2.4 g
titanium sulfate, used as titanium precursor, was dissolved in 50 ml of deionized
water; then PSS was added (titanium sulfate: PSS51:0.15,1:0.05 mole ratio)
under stirring at ambient condition until it thoroughly dissolved. Subsequently,
the mixture containing titanium precursor and PSS was added into 50 mL of 2.7
wt% C6H12N4 aqueous solution in dropwise of 0.5 mL/min under stirring, and
meanwhile, the precipitation produced. After stirring for 1 h, the precipitates were
collected and washed with ethanol and deionized water thoroughly. Then the
precipitates slurry was dispersed into 0.3 mol/L HNO3 aqueous solution, and
refluxed for 3 h at 50 C. Thus, the PSS modified TiO (PSS-TiO ) was obtained,
˚
2
2
after thoroughly washed by water and dried. The process for preparing PEI
modified TiO2 (PEI-TiO2) was similar as that of PSS-TiO2, using PEI instead of
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Anatase TiO2 Nanoparticles with Tunable Surface Charges
PSS polyelectrolyte. As a comparison, pure TiO2 nanoparticles were prepared in
the same procedure without adding any additives.
2.3 Characterization
The crystallographic structures of the as-prepared nanoparticles were character-
ized by X-ray diffraction method on a D8 advance installation, (Bruker/AXS,
˚
Germany) using Cu2Ka1 (l51.5406 A) radiation source. Scanning election
microscopy (SEM) (SU8020, Hitachi, Japan) was employed to observe the
morphologies of the samples, and the EDX spectra of the corresponding region
were obtained by the equipped Oxford EDX instruments and AZtec software. The
microstructures of nanoparticles were explored by a high-resolution transmission
electron microscope (HRTEM; JEM-2100, JEOL, Japan) equipped with an
electron diffraction, operated at 200 KeV with LaB6 emission source of electrons.
Zeta potentials and dimensions of the TiO2 nanoparticles in aqueous water
suspension were measured using Zeta Potential/Particle Sizer (NICOMPTM
380ZLS, USA), after ultrasonic for 60 min. The pH of the suspensions was
adjusted using diluted HCl or NaOH solution.
The UV-vis diffuse reflectance spectra (DRS) were performed at room
temperature on a UV-2450 instrument (SHIMADZU, Japan) in the range of
200 nm to 600 nm, with BaSO4 as the standard reflectance sample. The
characteristic adsorption of the particles was obtained by the intercept of the
linear extrapolation to the l axis and the band gap energy (Eg), given as a function
of band edge (lg) as formula (1), according to reference [34–35].
1240
Eg~
ð1Þ
lg
The photoluminescence (PL) spectra excited at 320 nm were recorded by a RF-
5301PC fluorescence spectrophotometer (SHIMADZU, Japan) under ambient
condition.
2.4 The adsorption performance and photocatalytic activity of the
nanoparticles
The adsorption performances of the as-prepared nanoparticles were investigated
by UV-vis intensity of 10 mg/L of methylene blue (MB) solution [36]. Specifically,
100 mg of photocatalyst, pure TiO2, PEI-TiO2, or PSS-TiO2, was added into
100 mL of Methylene blue (MB) solution of 10 mg/L of concentration,
respectively. The resultant slurries were fully stirred in darkness for a given time.
Then 5 mL of the suspension was taken out and centrifuged to remove the
suspended solids within time intervals. The supernatant was subjected into the
colorimetric ware for UV-vis analysis on a UV-vis spectrophotometer (UV-3200
MAPADA, Shanghai Mapada Instruments Co., Ltd., China).
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Anatase TiO2 Nanoparticles with Tunable Surface Charges
The photocatalytic activity of the nanoparticles were evaluated by their photo
degradation of 10 mg/L of methylene blue (MB) or rhodamine B (RhB) solution.
A 500 W UV lamp (365 nm) was used as the light source, and its distance from
the liquid level was 15 cm. The photo degradation on dyes begin after their
sufficient adsorption on photocatalyst in darkness. In a typical procedure, 100 mg
of photocatalytic particles was added into 100 mL of MB or RhB solution with
10 mg/L concentration, respectively. The resultant slurry was fully stirred in
darkness until it reached adsorption/desorption equilibrium. Then, the suspen-
sion was placed under the irradiation of UV light. Within a given time, 5 mL of
suspension were taken out and centrifuged to remove the suspended particles, and
the supernatant was transferred into a colorimetric ware for UV-vis analysis on a
UV-vis spectrophotometer (UV-3200 MAPADA, Shanghai Mapada Instruments
Co., Ltd., China). The dye concentration was determined at a wavelength of
664 nm (for MB) and 554 nm (for RhB) and quantified by a standard calibration
based on Beer-lambert’s law. The photocatalytic activity of the samples was
calculated according to g5((C02Ct)/C0)6100%, where g represents the
degradation efficiency of dyes, C0 (mg/L) and Ct (mg/L) represents the
concentration of dye in the aqueous solution at time t50 and t respectively.
Results and Discussions
3.1 Structure analysis on TiO2 nanoparticles
The anatase TiO2 nanoparticles were prepared by LTPPP method, including
precipitation and peptization steps. First, the amorphous Ti(OH)4 precipitation
was obtained by hydrolysis of titanium sulfate in the presence of alkaline
methenamine. Then, the Ti(OH)4 precipitation was peptized by dehydration in
HNO3 solution media, due to that the strong acidic environment favored
formation of the single-crystalline anatase crystal [37]. The crystalline phases of
pure TiO nanoparticles without any additives, peptized at temperature of 30 C,
˚
2
40 C, 50 C, respectively, were first investigated by XRD patterns (Fig. 1). The
˚
˚
XRD pattern of Fig. 1(a) showed an obvious wide peak at 2h of 15.3–35.6 and the
˚
weak wide peak at 2h of 42.4–52.3 , indicating the formation of amorphous crystal
˚
at relatively low temperature of 30 C. These two wide peaks become slightly
˚
narrowed in Fig. 1(b), indicating the formation of some primary crystallite grain,
when the peptization was performed at 40 C. Then, the separate and sharp peaks
˚
at 2h of 25.24 ,37.7 , 48.1 , 53.8 , 55.0 , 62.6 , 68.7 , 70.2 and 75.0 were observed
˚
˚
˚
˚
˚
˚
˚
˚
˚
in Fig. 1(c), corresponding to (101), (004), (200), (105), (211), (213), (116), (220)
and (215) planes of anatase TiO2 (JCPDS card No. 21-1272) [38], respectively.
This indicated that the crystalline anatase nanoparticles were formed at
temperature of 50 C. The size of pure TiO crystalline was calculated about
˚
2
4,6 nm by Scherrer equation [39]. The XRD patterns of the PEs modified
nanoparticles at 50 C shown in Fig. 1(d)) and Fig. 1(e) indicated the similar peaks
˚
at 2h position as that of the pure anatase TiO2. The average sizes of the PEI-TiO2
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Anatase TiO2 Nanoparticles with Tunable Surface Charges
Fig. 1. XRD patterns of pure TiO peptized at (a) 30 C, (b) 40 C and (c) 50 C; as well as the modified TiO
of (d) PSS-TiO2 and (e) PEI-TiO2.
˚
˚
˚
2
2
doi:10.1371/journal.pone.0114638.g001
and PSS-TiO2 crystallines were estimated as about 9.0,11.5 nm according to
Scherrer equation.
Fig. 2 showed the high-resolution transmission electron microscope (HRTEM)
images of pure TiO2, PEI-TiO2 and PSS-TiO2 nanoparticles. The enlarged TEM
images in Fig. 2 showed that all three nanocrystallites have lattice fringes of
0.356 nm, corresponding to (101) faces of anatase crystalline, indicating that the
introduction of PEI and PSS polyelectrolytes into the precursor had little effect on
lattice structure of TiO2 crystal. Moreover, there were some amorphous regions in
HRTEM images of Fig. 2(b) and Fig. 2(c), indicating that some immature
crystalline in the presence of PEs. And the d spacing calculated from the electron
diffraction pattern in the inset of Fig. 2 matched the values of the three as-
synthesized TiO2 nanoparticles, which is in accordance with the consequence of
XRD patterns of Fig. 1.
The SEM morphologies of the particles shown in the inset on the right bottom
of Fig. 2 further manifested the overall morphology of nanoparticles. The SEM
image of in Fig. 2(a) indicated that the pure TiO2 nanoparticles clumped and
aggregated together, with diameter ranged from ,200 to ,500 nm. In contrast,
the PEs modified nanoparticles in Fig. 2(b) and Fig. 2(c) were relatively uniform
spherical with diameter about ,150 nm to ,200 nm; interestingly, the PSS-TiO2
nanoparticles were formed into some spherical bunched network. This indicated
that the polyelectrolyte can reduce aggregation of the TiO2 nanoparticle,
effectively.
EDX measurements further demonstrated the PEs modification on TiO2
particles. The existent elements on TiO2, PEI-TiO2 and PSS-TiO2 nanoparticles
were shown in the spectra of Fig. 3, and the amounts of the elements were
collected as in Table 1. Besides of peaks of the sprayed Au (2.1 keV) and Si
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Anatase TiO2 Nanoparticles with Tunable Surface Charges
Fig. 2. High-resolution transmission electron microscope (HRTEM) images, SEM images, and the selected-area electron diffraction (SAED)
patterns of (a) pure TiO2, (b) PEI-TiO2 and (c) PSS-TiO2 nanoparticles.
doi:10.1371/journal.pone.0114638.g002
(1.7 keV) of the silicon substrate, Fig. 3(a) indicated two peaks of Ti elements
(0.4 keV and 4.5 keV) and O elements (0.5 keV), indicating the characteristic
constituent of TiO2. In contrast, the new peaks of C (0.2 keV) and N (0.3 keV)
elements in Fig. 3(b), and the peaks of C (0.2 keV), S (2.3 keV) and Na (1.0 keV)
in Fig. 3(c) indicated obviously the existence of PEI and PSS on TiO2 particles.
The amounts of these elements shown in Table 1 indicated that 29.22% of Ti and
70.78% of O were found on the pure TiO2 nanoparticles; while there were 33.73%,
66.27%, 34.62% and 13.18% of Ti, O, C and N elements, respectively, existing on
PEI-TiO2 nanoparticle. Simultaneously, on PSS-TiO2 nanoparticle, the amount of
Ti, O, C, S and Na elements were 22.86%, 28.11%, 26.15%, 12.15% and 10.73%,
respectively. This clearly indicated that the modification and existence of PEI or
PSS on TiO2 nanoparticles.
3.2 Zeta potential, particle size and dispersion stability of the
nanoparticles suspension
Fig. 4(a) showed the variation of zeta potential of the nanoparticles with pH
increasing from 3 to 11. The zeta potential of the PEI-TiO2 nanparticles showed
gradual decrease, except for the slight increase of PEI-TiO2 at pH of 5. The zeta
potential of pure TiO2 nanopariticles varied from +49.16 mV to 244 mV with
iso-electric point at pH of 7, similar as that of pure anatase TiO2 in Song’ s work
[40]. And zeta potential of the PSS-TiO2 showed minus increase from 216 mV to
289.48 mV. This was partly ascribed to the enhanced adsorption of hydroxyl ions
on surfaces of the nanoparticles, yielding significant increase on the negative
2
charge, similar as the change of zeta potential with the adsorption of Cl2, NO3
22
and SO4 [41]. Moreover, the minus increase of zeta potential of TiO2
nanoparticles with pH is attributed to the process shown in formula 2, the change
+
of TiOH2 into TiO2 with pH increasing [28]:
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Anatase TiO2 Nanoparticles with Tunable Surface Charges
Fig. 3. EDX spectra of (a) TiO2, (b) PEI-TiO2 (10%) and (c) PSS-TiO2 (10%).
doi:10.1371/journal.pone.0114638.g003
z
z
-H
-H
TiOHz2
TiOH
TiO{
ð2Þ
z
z
zH
zH
For the PEI-modified TiO2 nanoparticles, the zeta potential showed a highest
value of 69.85 mV at pH of 5.0. This was in accordance with the charge behavior
of PEI, a weak polyelectrolyte, of which the amine groups were protonated in
different level with pH [42], making it varied charges. As shown in the result of
Fig. 4(b), the pure PEI showed a highest zeta potential at pH of 5, due to that its
amine groups can be protonated enough at this pH value, resulting in the highest
zeta potential of PEI-TiO2. The zeta potential of PSS, a strong polyelectrolyte, was
little affected by pH of the solution [43]. Thus, the zeta potential of PSS-TiO2 is
mainly dependent on the adsorptive abilities of hydroxyl ions on surfaces of
nanoparticles with increase of pH, as well as the process shown in formula (2)
[28], resulting in the highest zeta potential of 289.48 mV at pH of 11. Moreover,
the variation of pH can also result in the change of the average dimension of the
TiO2, PEI-TiO2 and PSS-TiO2 nanoparticles. As shown in Fig. 4(c), the TiO2
nanoparticles displayed largest average size at pH of 7, at which the zeta potential
of the particle was zero (Fig. 4(a)). Thus, the almost no charged surface of the
particles resulted in the aggregation more easily and with the largest aggregate size.
In contrast, the PEI-TiO2 particle displayed smallest size at pH of 5. This was
probably due to that PEI-TiO2 particle showed a highest zeta potential at pH of 5
(Fig. 4(a)), and the relatively strong electrostatic repulsion made the particles less
Table 1. The elemental amounts of TiO2, PEI-TiO2 (10%) and PSS-TiO2 (10%) nanoparticles.
nanoparticles
Elements content (wt%)
Ti
O
C
N
S
Na
0
TiO2
29.22
33.73
22.86
70.78
66.27
28.11
0
0
0
PEI-TiO2
PSS-TiO2
34.62
26.15
13.18
0
0
0
12.15
10.73
doi:10.1371/journal.pone.0114638.t001
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Anatase TiO2 Nanoparticles with Tunable Surface Charges
Fig. 4. Zeta potential of pure TiO2, PEI-TiO2 (10%) and PSS-TiO2 (10%) as a function of pH (a); zeta potential of PEI (1.0 mg/mL) solution as a function of pH
(b); Particle size of the TiO2, PEI-TiO2 and PSS-TiO2 nanoparticles at different pH (c); Transmittance changes of TiO2 (pH 3), PEI-TiO2 (pH 5) and PSS-TiO2
(pH 11) suspensions over time (d).
doi:10.1371/journal.pone.0114638.g004
aggregate and with smallest particle size. When pH value was greater than 5, the
decreased zeta potential of PEI-TiO2 (Fig. 4(a) increased the chance of
electrostatic attraction, and made the particles aggregate more easily and show the
increased size. Similarly, the increased minus zeta potential of PSS-TiO2 particles
with pH resulted in the gradually decreased size. This indicated that the variation
of the average size of the particles with pH was much in accordance with that of
zeta potential.
The dispersion stability of TiO2, PEI-TiO2 and PSS-TiO2 suspension were
investigated by the variation of the suspension transmittance with time. The pH
values of the three suspensions were 3, 5 and 11, respectively, in order to get a
relatively higher charges and smaller aggregate size nanoparticles. As shown in
Fig. 4(d), the transmittance of PEI-TiO2 and PSS-TiO2 suspensions displayed
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Anatase TiO2 Nanoparticles with Tunable Surface Charges
slower increase with time than that of pure TiO2 suspension, indicating that the
relatively higher charges of nanoparticles can help to prompt the dispersion
stability of the nanoparticles suspension.
In order to further achieve the tunable charge of nanoparticles, the variations of
zeta potential with PEs concentration were investigated at pH value of the highest
zeta potential of Fig. 4 (a). The results in Fig. 5 showed that the zeta potential of
as-prepared PEI-TiO2 nanoparticles at pH of 5 significantly increased from
+39.47 mV to +95.46 mV with concentration of PEI increased from 1.0 wt% to
15.0 wt%. Meanwhile, the zeta potential of the PSS modified TiO2 at pH of 11
showed obviously negative increase from 256.63 mV to 2119.32 mV, with PSS
concentration increased from 1.0 wt% to 15.0 wt%. The lower zeta potential of
TiO2 in the PSS solution is in accordance with that in SDS solution [27]. This can
be explained by the hemimicelle model [44]. At high pH of 11, the negative PSS
was adsorbed on the TiO2 nanoparticle surface through physical adhesive
interaction, causing the hemimicelle. When, PSS concentration increased, PSS
micelles would be formed and adsorb on the TiO2 nanoparticle surface, and the
increased adsorption volumes resulted in more negative zeta potential [28]. The
similar elucidation can also be illustrated for the change of zeta potential of PEI-
TiO2 with PEI concentration. The increases of zeta potential with the
concentration of PEs indicated that the modification of polyelectrolyte of PEI or
PSS on TiO2 nanoparticles resulted in the tunable surface charge of the particles,
effectively.
3.3 Adsorption and photocatalytic activity of nanoparticles
3.3.1 Adsorption of MB on the surface of nanoparticles
The adsorption of the organic substance on photocatalytic nanoparticles is
considered as an important factor in their photocatalytic degradation. The
adsorption performances of methylene blue (MB) on pure TiO2, PEI-TiO2, PSS-
TiO2 were investigated by the absorbance changes of UV-vis spectra of MB at
given time in darkness. The slightly decreased absorbance of UV-vis spectra in
Fig. 6 (a) indicated that there was weak adsorption of MB on TiO2, which had
little charges at pH of 7. And the absorbance the PEI-TiO2 suspension in Fig. 6 (b)
showed almost no change, indicating the negligible adsorption of MB on
positively charged TiO2. This was probably due to that the electrostatic repulsion
between the positively charged PEI-TiO2 nanoparticles and the positively charged
MB. In contrast, the absorbance of MB in Fig. 6 (c) showed obvious decrease
indicating that there was more MB adsorbed on negatively charged PSS-TiO2
nanoparticles. This was due to that the positively charged MB was prone to adsorb
on the negatively charged PSS-TiO2 through electrostatic attraction, which may
favor the next photocatalytic process.
3.3.2 Photocatalytic degradation of MB and RhB
Methylene blue (MB) is often used as a model contaminant of water for
photocatalytic reaction test. And rhodamine B (RhB) is a typically industrial dye.
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Anatase TiO2 Nanoparticles with Tunable Surface Charges
Fig. 5. The variations of zeta potentials of PEI-TiO2 and PSS-TiO2 nanoparticles with concentration of
polyelectrolytes (pH of PEI-TiO2 and PSS-TiO2 was 5 and 11, respectively).
doi:10.1371/journal.pone.0114638.g005
The photodegradation was started after the thorough adsorption of dyes onto the
photocatalyst in darkness. And the decolorization of dyes occurred with
ultraviolet irradiation time. As shown in Fig. 7, the degradation efficiency on MB
and RhB by photocatalysts, such as PEI-TiO2, PSS-TiO2 and TiO2, were much
similar, that is, the degradation efficiency of PSS-TiO2 higher than that of TiO2.
And the degradation efficiency of PEI-TiO2 is relatively lower than that of TiO2.
Fig. 7 (a) showed that 0.96 degradation efficiency of PSS-TiO2 was observed on
MB dye, a little higher than 0.95 and 0.84 of the pure TiO2 and PEI-TiO2,
respectively, within 260 min of UV-light irradiation. Moreover, the PSS-TiO2
photocatalyst exhibited more efficient photo-degradation on MB in the initial
stage. For example, 0.67 of MB was degradated by PSS-TiO2 within 20 min. In
contrast, only 0.30 and 0.09 of MB were degraded by pure TiO2 and PEI-TiO2
photocatalysts within the same time. Fig. 7 (b) showed the similar results of
Fig. 6. UV-vis spectra of MB by the adsorption on (a)TiO2, (b) PEI-TiO2 and (c) PSS-TiO2.
doi:10.1371/journal.pone.0114638.g006
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Anatase TiO2 Nanoparticles with Tunable Surface Charges
Fig. 7. Photocatalytic degradation with time under UV light in the presence of TiO2, PEI-TiO2 and PSS-TiO2 photocatalysts of: (a) Methylene blue;
(b) Rhodamine B.
doi:10.1371/journal.pone.0114638.g007
degradation of RhB. The degradation efficiency of RhB was about 0.90, 0.95 and
0.99 with PEI-TiO2, TiO2 and PSS-TiO2, respectively, within 360 mins. This
indicated that PSS-TiO2 nanoparticles showed relatively higher photocatalytic
activity. The possible reason is that the negatively charged PSS-TiO2 easily
adsorbed the positively charged MB by electrostatic attraction, as shown in
Fig. 6(c), and made it effectively degradated. The relative lower degradation
activity of MB over PEI-TiO2 than pure TiO2 nanoparticles is mainly ascribed to
the electrostatic repulsion for MB shown in Fig. 6(b). For degradation of the RhB
molecule, which contains both positively charged diethylamine groups and
negatively charged carboxylate group, the carboxylate group is easily to adsorb
onto Ti sites of TiO2 surface [45]. While, the positively charged diethylamine
groups are prone to interact electrostatically with negative species, such as PSS
anion occupied on TiO2 surface. Therefore, PSS-TiO2 exhibited notably
accelerated degradation rate of RhB.
3.3.3 Analysis of the photocatalytic performance of the nanoparticles
Generally, the electrons of TiO2 would transfer from the valence band (VB) to
conduction band (CB), when irradiated by photoelectrons with energy equal to/or
larger than that of the band gap (Eg). Then, the photogenerated oxidative valence
holes (h+) and reductive conduction electrons (e2) were produced [46]. In order
to further expound the different photocatalytic performance of the above three
samples, UV-vis diffuse reflectance spectra (DRS) were measured over the
wavelength range from 200 nm to 600 nm. From the characteristic adsorption
bands shown in Fig. 7, the band edge (lg) and band gap energy (Eg) of TiO2, PEI-
TiO2 and PSS-TiO2 photocatalysts were shown in Table 2. Generally, TiO2 has a
relatively high energy band gap (Eg<3.2 eV) that can be excited under UV
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Anatase TiO2 Nanoparticles with Tunable Surface Charges
Table 2. Band edge (lg) and band gap energy (Eg) of TiO2, PEI-TiO2 and PSS-TiO2 photocatalysts.
Sample
TiO2
lg/nm
499.0
464.5
621.36
Eg/eV
2.88
2.67
2.44
PEI-TiO2
PSS-TiO2
doi:10.1371/journal.pone.0114638.t002
irradiation [47]. Here, the pure TiO2 sample showed the typical adsorption of
anatase with an intense transition in the UV region of the spectrum, which was
due to the promotion of the electron from the valence band to the conduction
band in Fig. 8. The energy band gap was calculated about 2.88 eV as in Table 2.
This Eg was further lowered to 2.44 eV by PSS modification of TiO2, in which the
spectra band of the PSS-TiO2 nanoparticles exhibited red shift in the UV-vis range
of 300–500 nm. This red shift indicated that the PSS with p-conjugated structure
of aromatic ring has high electron mobility [48]. The high photodegrdation of dye
molecules in Fig. 7 may be attributed to the efficient charge separation of the
electrons (e2) and hole (h+) pairs at the interfaces of PSS and TiO2 which is
related to the red shift. However, the spectra band of PEI modified TiO2 exhibited
a blue shift with larger Eg of 2.67 eV. This blue shift is possibly due to n-s*
transition of imine (-NH) in PEI macromolecules [49]. Consequently, the narrow
band gap of PSS-TiO2 in Fig. 7 made it an enhanced photocatalytic efficiency of
TiO2 under UV-light.
The photoluminscence (PL) emission, resulted from the recombination of
excited electrons and holes further indicated the different photocatalytic efficiency
as shown in Fig. 9. The broad-band emission spectra at around 375–410 nm are
attributed to the luminescence signal, which indicated that the polyelectrolyte
modification just affected the response range and intensity of the PL spectra, not
causing new luminescence phenomenon in the PEI-TiO2 and PSS-TiO2
Fig. 8. UV-vis diffuses reflectance spectra of TiO2, PEI-TiO2 and PSS-TiO2 photocatalysts.
doi:10.1371/journal.pone.0114638.g008
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Anatase TiO2 Nanoparticles with Tunable Surface Charges
Fig. 9. PL emission spectra (excited at 320 nm) of TiO2, PEI-TiO2 and PSS-TiO2 photocatalysts.
doi:10.1371/journal.pone.0114638.g009
nanoparticles. In comparison with that of pure TiO2, the peak at about 396 nm
for pure TiO2 was attributed to the excitation luminescence of free electrons from
the CB bottom. After PSS modification, the PL intensity of TiO2 nanoparticles is
significantly decreased, indicating that the PSS modification lowered recombi-
nation rate of electron/hole of TiO2 samples. However, the PEI-TiO2
nanoparticles displayed the higher PL intensity, indicating the higher recombi-
nation rate and Eg than PSS-TiO2. This difference was also probably due to the Eg
differences between p-p* transition of PSS and the n-s* transition of imine
(-NH) group of PEI polyelectrolyte, in accordance with that in Table 2.
For better understanding of the photocatalytic activity differences between PEI-
TiO2 and PSS-TiO2 photocatalysts, we used schemes of Fig. 10 to illustrate the dye
degradation over the photocatalysts surfaces. Fig. 10 (a) indicated that PSS with
aromatic groups showed higher electron mobility, while the VB position of TiO2
was lower than the highest occupied molecular orbital (HOMO) of PSS.
Therefore, the hole (h+) was easily formed by e2 electron transference from VB of
TiO2 to HOMO of PSS for e2 regeneration when TiO2 absorbed UV light. The
generated e2 from PSS can be excitated from HOMO to the lowest unoccupied
molecular orbital (LUMO) of PSS, and then the excitated e2 transferred to the CB
of TiO2. Thus, more and more photogenerated e2 and h+ formed on TiO2
nanoparticles. The photogenerated e2 was so active that it can react with O2 to
2
generate ?O2 . Meanwhile, the generated h+ can react with OH– or H2O to
generate ?OH. These radicals would react with dye molecules, and produce
organic radicals or other intermediates. Finally, the radicals and intermediates
were oxidized into CO2 and H2O. PSS of the TiO2 nanoparticles participated in
charger transferring and favored enhancing the photocatalytic activities: e2
moved to the opposite direction from h+, reducing the recombination of
photogenerated e2-h+ and making charge separation more efficient. The
photogenerated e2-h+ pairs would favor the charge separation and the formation
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Anatase TiO2 Nanoparticles with Tunable Surface Charges
Fig. 10. Schematic illustration of photocatalytic activity of (a) PSS-TiO2, (b) PEI-TiO2 nanoparticles for
dye degradation under UV-light.
doi:10.1371/journal.pone.0114638.g010
of oxyradicals (O2?, OH?). Compared to PSS-TiO2, the transition energy of n-s*
of PEI was much higher than that of p-p* of PSS, as shown in Fig. 10 (b). Hence,
the negatively charged PSS-TiO2 exhibited better photocatalytic activity than pure
TiO2 and positively charged PEI-TiO2.
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Anatase TiO2 Nanoparticles with Tunable Surface Charges
Conclusions
In conclusion, the anatase TiO2 nanoparticles were prepared by simple method of
LTPPP, under a room temperature and atmospheric pressure. The modification
of PEI and PSS has little effect on the anatase crystallite phase of TiO2. The zeta
potential of nanoparticles can be effectively tuned by the variation of pH and the
amount of polyelectrolytes during the synthesis process. Furthermore, PSS
modified TiO2 nanoparticles exhibited the highest degradation efficiency of
Methylene blue and Rhodamine B, in comparison with pure TiO2 and PEI-TiO2
nanoparticles. The higher photocatalytic activity of the PSS-TiO2 was attributed to
the well adsorption of dye molecules, the reduced band gaps and lower
recombination rate of electron/hole. This work would provide a simple and facile
strategy to obtain various TiO2-based functional materials, for which is of
significance to extending the practical applications.
Author Contributions
Conceived and designed the experiments: HG. Performed the experiments: YL
HY. Analyzed the data: YL ZQ GZ. Wrote the paper: YL HG TZ. Contributed all
the characterization condition: SJ.
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