Molecular weight effects of PEG on the crystal structure and photocatalytic activities of PEG-capped TiO2 nanoparticles

Article abstract of DOI:10.1039/c6ra12988a

Polyethylene glycol (PEG) was used as stabilizer to prepare water-soluble anatase titanium dioxide (TiO₂). The molecular weight effects of PEG on the crystal structure and photocatalytic activities of PEG-capped TiO₂ nanoparticles were systematically studied. The results show that the steric hindrance effect of PEG molecular chains will hinder the PEG molecules from being tied to the surface of TiO₂, resulting in the decrease of PEG molecules capped on the surface of TiO₂ with the growth of PEG molecular chains. It is significant that PEG can effectively promote the dispersion of TiO₂ nanoparticles in water, which becomes better and better with the increase of molecular weight of PEG. Moreover, the PEG-capped TiO₂ aqueous solutions can keep stable for more than two months. PEG cannot influence the crystal type and size of TiO₂, which can be well controlled by the introduction of HCl in the reaction system. The photocatalytic activity of PEG-capped TiO₂ was evaluated by monitoring the degradation of methyl orange, which is better than that of commercial P25 TiO₂ mainly due to the good dispersion in water. However, PEG molecular chains are detrimental to the transfer of photogenerated electrons and reactive oxygen species while the increasing dispersion in water with the increase of PEG molecular weight can increase the photocatalytic activity of TiO₂. As a result, PEG400 capped TiO₂ presents the best photocatalytic activity while PEG2000 capped TiO₂ exhibits the worst photocatalytic activity. The photocatalytic activity begins to increase when the molecular weight of PEG is larger than 2000.

Full text of DOI:10.1039/c6ra12988a

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Molecular weight effects of PEG on the crystal
structure and photocatalytic activities of PEG-
capped TiO2 nanoparticles
Cite this: RSC Adv., 2016, 6, 83366
*
Meizhou Zhang, Jinfeng Lei, Yifeng Shi, Lina Zhang, Youxin Ye, Defu Li
*
and Changdao Mu
Polyethylene glycol (PEG) was used as stabilizer to prepare water-soluble anatase titanium dioxide (TiO2).
The molecular weight effects of PEG on the crystal structure and photocatalytic activities of PEG-capped
TiO2 nanoparticles were systematically studied. The results show that the steric hindrance effect of PEG
molecular chains will hinder the PEG molecules from being tied to the surface of TiO2, resulting in the
decrease of PEG molecules capped on the surface of TiO2 with the growth of PEG molecular chains. It is
significant that PEG can effectively promote the dispersion of TiO2 nanoparticles in water, which
becomes better and better with the increase of molecular weight of PEG. Moreover, the PEG-capped
TiO2 aqueous solutions can keep stable for more than two months. PEG cannot influence the crystal
type and size of TiO2, which can be well controlled by the introduction of HCl in the reaction system.
The photocatalytic activity of PEG-capped TiO2 was evaluated by monitoring the degradation of methyl
orange, which is better than that of commercial P25 TiO2 mainly due to the good dispersion in water.
However, PEG molecular chains are detrimental to the transfer of photogenerated electrons and reactive
oxygen species while the increasing dispersion in water with the increase of PEG molecular weight can
increase the photocatalytic activity of TiO2. As a result, PEG400 capped TiO2 presents the best
photocatalytic activity while PEG2000 capped TiO2 exhibits the worst photocatalytic activity. The
photocatalytic activity begins to increase when the molecular weight of PEG is larger than 2000.
Received 19th May 2016
Accepted 29th August 2016
DOI: 10.1039/c6ra12988a
www.rsc.org/advances
by using an organic media to reduce the sensitivity of precursor
to moisture.25–27 However, the surface of produced TiO2 NPs is
1. Introduction
oen hydrophobic due to the hydrophobicity of capped organic
media like oleic acid, which limits the applications of TiO2 NPs
since the most applications are carried out in aqueous solu-
tion.24 Recently, Wang and co-workers24 have successfully
prepared water-soluble anatase TiO2 NPs using ethylene glycol
as a surfactant. Yan and co-workers28 have reported a new and
facile route for the controllable synthesis of water-soluble
anatase TiO2 NPs using polyethylene glycol 400 (PEG400) as
a solvent and a stabilizer, with phases well controlled by
introduction of HCl in the reaction system. The results indi-
cated that PEG was an effective media for the preparation of
water-soluble anatase TiO2 NPs. It is known that the capped
surfactant should impact the surface activity of TiO2 NPs, which
has been thought to be correlated with its photovoltaic and
photocatalytic properties.24 However, to the best of our knowl-
edge, only ethylene glycol and PEG400 have been used as
stabilizers to prepare water-soluble anatase TiO2 NPs. The
effects of molecular weight of PEG on the structure and pho-
tocatalytic activity of TiO2 NPs have not attracted enough
attention, which has not been discussed in detail.
Titanium dioxide (TiO2), one of the most promising photo-
catalysts, has been widely investigated for decades since
Fujishima discovered the photocatalytic splitting of water on
TiO2 electrodes.1–4 TiO2 nanoparticles (NPs) have multiple
advantages such as good structural stability, abundance, non-
toxicity and low cost.5–7 Hence, TiO2 NPs have been used in
many areas for instance solar energy conversion,8–10 photo-
catalytic self-cleaning,11–15 sensors16–18 and photochromic
devices.19,20
TiO2 has three crystallographic phases: anatase, brookite
and rutile.21 Among them, the anatase TiO2 has been conrmed
to have excellent photoactivity and optoelectronic proper-
ties.22,23 However, the high purity anatase TiO2 is difficult to be
prepared by traditional methods, because that the hydrolytic
process of many traditional methods for the preparation of TiO2
is hard to be controlled at room temperature and titanium
alkoxides are sensitive to moisture.24 Nowadays, new routes
have been developed to prepare high quality anatase TiO2 NPs
Department of Pharmaceutics and Bioengineering, School of Chemical Engineering,
Sichuan University, Chengdu 610065, China. E-mail: lidefu@scu.edu.cn; cdmu@scu.
edu.cn
In this work, a series of PEG with different molecular weight
were used as stabilizers to prepare water-soluble TiO2 NPs. The
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anatase phase of TiO2 was controlled by introduction of HCl in 700 ꢀC at a heating rate of 10 ꢀC minꢁ1 and under nitrogen
the reaction system. Then the crystal structures and physico- atmosphere to avoid thermo-oxidative reactions.
chemical properties of the PEG-capped TiO2 NPs were investi-
gated. The photocatalytic activities of the TiO2 NPs were
evaluated by monitoring the degradation of methylene blue
(MB) and methyl orange (MO). Our aim is to understand the
effects of molecular weight of PEG on the crystal structure and
2.4 Photocatalytic activity evaluation of the PEG-capped
TiO2 NPs
The photocatalytic activities of the PEG-capped TiO2 NPs were
evaluated by monitoring the degradation of methylene blue
(MB) and methyl orange (MO). 30 mg of TiO2 NPs was sus-
pended in 15 mL aqueous solution of MB or MO (10 mg Lꢁ1) to
photocatalytic activities of PEG-capped TiO2 nanoparticles.
obtain the reaction slurry. The slurries were stirred for 6 h in the
dark to assure the establishment of an adsorption–desorption
equilibrium. Then, the slurries were irradiated by a 90 W UV
lamp (maximum emission at 302 nm). The photocatalytic
2. Experimental section
2.1 Materials
Titanium butoxide ($99.0%) and polyethylene glycol (PEG)
were purchased from Aladdin Industrial Corporation
(Shanghai, China). Hydrochloric acid (HCl, 36.0–38.0%) was
degradation of MB was examined by measuring its absorbance
at 664 nm every 30 minutes. The photocatalytic degradation of
purchased from Kelong Chemical Reagent Company (Chengdu,
MO was examined by measuring its absorbance at 464 nm every
China). Commercial P25 TiO2 powder was purchased from
Sigma-Aldrich Corporation.
30 minutes. The measurements were carried out for ve times
and the mean values were used.
2.2 Preparation of the PEG-capped TiO2 NPs
2.5 Statistical analyses
TiO2 NPs capped by PEG with different molecular weight were
prepared according to previous method with a slight modi-
cation.28 3 mL of titanium butoxide was mixed with 6 g of PEG
under magnetic stirring at 65 ꢀC to form a white titanium
alkoxide complex. Then, 1 mL of HCl was dropped into the
titanium alkoxide complex. The formed transparent slight
yellow solution was then transferred to a stainless poly (tetra
The SPSS version 17.0 soware was used for statistical analysis
of experimental data. Data were assessed with one-way analysis
of variance (ANOVA) by using Duncan's multiple range test. A
value of p < 0.05 was considered to be signicant.
3. Results and discussion
uoroethylene) (Teon)-lined autoclave (50 mL) and heated in 3.1 Preparation and characterization of the PEG-capped
a vacuum drying oven at 150 ꢀC for 5 h. The tawny TiO2 products TiO2 NPs
were separated by high-speed centrifugation (10 000 rpm),
The photographs of the PEG-capped TiO2 NPs aqueous solu-
tions are presented in Fig. 1. It can be seen that the series of
thoroughly washed with hot ethanol for 2–3 times and dried in
ꢀ
the vacuum drying oven at 50 C for 24 h. 5 types of PEG with
TiO2 NPs capped by PEG with different molecular weight all
molecular weight of 400, 2000, 4000, 6000 and 10 000 were used
have good dispersion in water. Furthermore, we found that the
to prepare TiO2 NPs. The corresponding resulting products were
PEG-capped TiO2 NPs aqueous solutions can maintain stable
named as TP400, TP2000, TP4000, TP6000 and TP10000,
for more than two months without obvious precipitation. It is
respectively.
worth noting that the transparency of the TiO2 NPs aqueous
solutions is increasing from TP400 to TP10000. The result
indicates that the dispersion of TiO2 NPs in water is getting
better with the increase of molecular weight of PEG. To further
understand the water solubility of PEG-capped TiO2 NPs, the
particle sizes of the PEG-capped TiO2 NPs in aqueous solutions
were measured and showed in Table 1. It is obvious that the
particle size of TiO2 NPs decreases from 76.2 to 49.2 nm with the
growth of PEG molecular chains. The result is agreed with
previous works that PEG can promote the dispersion of TiO2
2.3 Characterization of the PEG-capped TiO2 NPs
The particle size of TiO2 NPs in aqueous solution was measured
using a Malvern Zetasizer Nano – ZS ZEN3600 instrument at
a detection angle of 90ꢀ at 25 ꢀC. The concentration of TiO2 NPs
aqueous solution was 2 g Lꢁ1. The mean particle sizes were
obtained in the cumulant mode using the built-in Malvern
soware. The crystal structures of the TiO2 NPs were identied
by an 18 kW X-ray diffractometer (X'Pert Pro, Philips, Almelo,
˚
Netherlands) with a xed CuKa radiation (l ¼ 1.54 A). The
scanning range of the diffraction angle (2q) was from 10ꢀ to 80ꢀ
with a rate of 2ꢀ minꢁ1. The morphology and microstructure of
the TiO2 NPs were characterized by transmission electron
microscopy (TEM) (JEM-2100, JEOL, Japan) and scanning elec-
tron microscopy (SEM) (JSM-7500F, JEOL, Japan). Thermogra-
vimetric analysis (TGA) of the samples was carried out on
a NETZSCH thermal analyzer (TG209F1, NETZSCH, Germany).
The samples were dried in a vacuum drying oven at 105 ꢀC for 24
h before the test. The measurements were running from 50 ꢀC to Fig. 1 Photographs of the PEG-capped TiO2 NPs aqueous solutions.
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Table 1 Particle sizes of the PEG-capped TiO2 NPs in aqueous
solutions
Samples
Particle size (nm) 76.2
TP400 TP2000 TP4000 TP6000 TP10000
68.6 60.9 56.3 49.2
NPs in water.28 Furthermore, the dispersion of TiO2 NPs in water
can become better when capped by PEG with longer molecular
chain.
To study the crystallographic phases of as-prepared PEG-
capped TiO2 NPs, the typical XRD patterns of the PEG-capped
TiO2 NPs were measured and showed in Fig. 2. There is
almost no difference among the ve samples. They exhibit
distinctive characteristic peaks at 2 theta 25.2ꢀ, 37.8ꢀ, 47.8ꢀ,
53.9ꢀ, 62.2ꢀ and 69.7ꢀ, corresponding to the anatase phase
planes of (101), (004), (200), (105), (204) and (116) according to
JCPDS card no. 21-1272.29 The result indicates that the
produced PEG-capped TiO2 NPs are anatase phase. Yan and co-
workers have successfully prepared water-soluble anatase TiO2
NPs under mild solution conditions using PEG400 as a stabi-
lizer and HCl as a phase controlling reagent. The results showed
that the amount of concentrated HCl in the reaction system was
found to have a strong inuence on the TiO2 crystal phases. For
the low HCl content, the pure anatase phase could be
produced.28 In our work, PEG with different molecular weight
were used as stabilizers while the low HCl content was selected.
The water-soluble anatase TiO2 NPs were successfully prepared
too. The result indicates that the molecular weight of PEG has
no inuence on the crystal structures of the TiO2 NPs, which can
be controlled by the introduction of HCl in the reaction system.
The morphology and microstructure of the PEG-capped TiO2
NPs were characterized by TEM and SEM to further study the
crystallographic phases of as-prepared PEG-capped TiO2 NPs.
Fig. 3 shows the TEM images of the PEG-capped TiO2 NPs. It can
be observed that the TiO2 NPs exhibit almost spheroidal shapes
with the average grain sizes in the range of 5–10 nm, which is
consistent with the shapes of anatase TiO2 reported in previous
works.12 Note that the grain size of the TiO2 in Fig. 3 is much
smaller than that in aqueous solutions (Table 1). Moreover, the
grain sizes of the TiO2 are basically the same while the particle
sizes of TiO2 NPs in water decrease as the growth of PEG
Fig. 3 TEM images of the PEG-capped TiO2 NPs. (a) TP400, (b)
TP2000, (c) TP4000, (d) TP6000 and (e) TP10000.
molecular chains. The results indicate that TiO2 is present in
the form of aggregates in water. PEG can facilitate the disper-
sion of TiO2 NPs in water, but cannot inuence the crystal types
and size of TiO2 NPs. Fig. 4 shows the SEM images of the PEG-
capped TiO2 NPs. As seen from Fig. 4, the samples present
spherical shapes, which is consistent with the results gotten
from TEM images. However, the morphology of TiO2 NPs
gradually becomes unclear with the increase of the molecular
weight of PEG. The reason may be that PEG capped on the
surface of TiO2 NPs with high molecular weight can make TiO2
NPs present the morphology of polymers.
TGA was used to evaluate whether PEG was capped on the
surface of TiO2 NPs according to the published method.30 Fig. 5
shows the TGA and DTG curves of the PEG-capped TiO2 NPs.
ꢀ
The weight loss of the PEG-capped TiO2 NPs at 700 C and the
ratio of weight loss to molecular weight of PEG were calculated
and showed in Table 2. It is found that the weight losses for
TP400, TP2000, TP4000, TP6000 and TP10000 at 700 ꢀC are
24.70, 17.31, 18.03, 23.57 and 27.61 wt%, respectively. The
weight losses are mainly attributed to the decomposition of
organic groups on the surface of TiO2, which veries that PEG
has been successfully capped on the surface of TiO2 NPs. Table
2 shows that the weight losses of TiO2 NPs at 700 ꢀC are
Fig. 2 XRD patterns of the PEG-capped TiO2 NPs.
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Fig. 4 SEM images of the PEG-capped TiO2 NPs. (a) TP400, (b) TP2000, (c) TP4000, (d) TP6000 and (e) TP10000.
irregular. However, it is interesting that the ratio of weight loss recover in 150 minutes. The same phenomenon can be observed
to molecular weight of PEG is decreasing with the increase of for TP2000. The phenomenon of reversible color change of MB
molecular weight of PEG. The result indicates that the number has been reported by previous works, which is attributed to the
of PEG molecules capped on the surface of TiO2 is reduced with reduction–oxidation behavior of MB.31,32 The oxidized form of
the growth of PEG molecular chains. The reason may be that the MB presents blue and the reduced form is colorless. The
growth of PEG molecular chain leads to the increase of steric oxidation–reduction potential of MB is lower than the conduc-
hindrance to result in the decrease of reaction site for PEG tion band edge of TP400 and TP2000. Hence, the photo-
molecules.
generated electrons transferred from TP400 and TP2000 to the
MB molecules reduce the MB molecules from blue to the
colorless state. However, the reversible color phenomenon
cannot be observed for TP4000, TP6000 and TP10000. It may be
because that the conduction band edge of TP4000, TP6000 and
TP10000 is lower than the oxidation–reduction potential of MB.
This reversible color phenomenon of MB is very interesting,
but is not convenient for the present purpose. To solve this
problem, methyl orange (MO) was used as a probe to evaluate
the photocatalytic activities of the PEG-capped TiO2 NPs again.
Fig. 7 shows the photocatalytic degradation of MO in the pres-
ence of PEG-capped TiO2 under UV light irradiation and the
3.2 Photocatalytic activity of the PEG-capped TiO2 NPs
The photocatalytic activities of the PEG-capped TiO2 NPs were
evaluated by monitoring the degradation of methylene blue
(MB) and methyl orange (MO). Fig. 6 shows the photocatalytic
degradation of MB in the presence of PEG-capped TiO2 under
UV light irradiation and the corresponding photographs versus
irradiation time. It shows that the degradation of MB takes
place induced by PEG-capped TiO2 NPs. The reaction slurry of
TP400 becomes colorless aer the UV-irradiation for 30
minutes. However, it is interesting that the color begins to
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corresponding photographs versus irradiation time. It is
obvious that the photocatalytic activity of all the PEG-capped
TiO2 NPs is better than that of commercial P25 TiO2. The
result illustrates that the presence of PEG on the surface of TiO2
NPs can increase the photocatalytic activity of TiO2 mainly due
to the resulting good dispersion in aqueous solution. The same
results have been reported by previous work.28 Fig. 7 shows that
TP400 presents the best photocatalytic activity and MO can be
absolutely degraded in about 60 minutes. TP2000 exhibits the
worst photocatalytic activity and it needs 300 minutes to
completely degrade MO. The photocatalytic activity of the PEG-
capped TiO2 NPs begins to increase with the growth of PEG
molecular chain when the molecular weight of PEG is larger
than 2000.
Fig. 8 shows the tentative photodegradation mechanism
diagram of MB and MO on the surface of PEG-capped TiO2 NPs.
The electrons of TiO2 NPs are stimulated under the UV-
irradiation and then transfer from the valence band (VB) to
the conduction band (CB).33 Then the photogenerated electrons
are captured directly by O2 and formed superoxide anion (O2ꢁ).
Meanwhile, the valence band hole can oxidize water or OHꢁ to
form cOH radicals.34 These reactive oxygen species would
decompose organic dye to CO2 and water. The results in Fig. 7
show that the PEG molecular chains capped on the surface of
TiO2 are detrimental to the transfer of photogenerated electrons
and reactive oxygen species. Previous works have supposed that
the strongly bonded surfactant should passivate the surface
activity of TiO2 NPs, resulting in the decrease of the photovoltaic
and photocatalytic properties.35 So the TP400 capped by PEG
with the shortest molecular chain presents the best photo-
catalytic activity. But then, the photocatalytic activity of TiO2
NPs is not only dependent on the photogenerated electrons, but
also dependent on the particle size and surface area.32 The
smaller the particles, the larger will be its specic surface area
and the higher the photocatalytic activity. So the photocatalytic
activity of the PEG-capped TiO2 NPs begins to increase when the
molecular weight of PEG is larger than 2000, which is mainly
due to the increasing dispersion of the PEG-capped TiO2 NPs in
aqueous solutions.
Fig. 5 TGA (A) and DTG (B) curves of the PEG-capped TiO2 NPs.
Table 2 Weight loss of the PEG-capped TiO2 NPs at 700 ꢀC and the
ratio of weight loss to molecular weight of PEG
Weight loss
(%)
Weight loss/molecular
weight of PEG
Samples
TP400
24.70
17.31
18.03
23.57
27.61
0.062
0.009
0.005
0.004
0.003
TP2000
TP4000
TP6000
TP10000
Fig. 6 Photocatalytic degradation of MB in the presence of PEG-capped TiO2 under UV light irradiation (A) and the corresponding photographs
versus irradiation time (B).
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Fig. 7 Photocatalytic degradation of MO in the presence of PEG-capped TiO2 under UV light irradiation (A) and the corresponding photographs
versus irradiation time (B).
Research Funds for the Central Universities of China
(2015SCU04A34).
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