The enhanced photocatalytic activity and self-cleaning properties of mesoporous SiO2 coated Cu-Bi2O3 thin films

Article abstract of DOI:10.1039/c5dt00381d

Mesoporous SiO₂ coated Cu-Bi₂O₃ thin films (meso-SiO₂/Cu-Bi₂O₃) were prepared on glass substrates using a simple sol-gel/spin-coating method. The structure and optical properties were characterized using X-ray diffraction, X-ray photoelectron spectroscopy, a UV-vis spectrophotometer and a water contact angle meter. The photocatalytic activity and self-cleaning properties of the films were investigated through the degradation of methyl orange and stearic acid, respectively. It was found that the meso-SiO₂/Cu-Bi₂O₃ thin films were highly transparent and showed excellent superhydrophilicity even in the dark. The thin films exhibited enhanced photocatalytic activity and self-cleaning properties compared to pure Bi₂O₃ films, which was attributed to the cooperation of the interfacial charge transfer between Bi₂O₃ and surface Cu species as well as the unique mesoporous SiO₂ structure. The results showed that the films can be used as promising self-cleaning and antifogging materials.

Full text of DOI:10.1039/c5dt00381d

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PAPER
The enhanced photocatalytic activity and self-
cleaning properties of mesoporous SiO coated
2
Cite this: Dalton Trans., 2015, 44,
7428
Cu-Bi O thin films†
2
3
Wenjie Shan, Yun Hu,* Mengmeng Zheng and Chaohai Wei
2 2 3 2 2 3
Mesoporous SiO coated Cu-Bi O thin films (meso-SiO /Cu-Bi O ) were prepared on glass substrates
using a simple sol–gel/spin-coating method. The structure and optical properties were characterized
using X-ray diffraction, X-ray photoelectron spectroscopy, a UV-vis spectrophotometer and a water
contact angle meter. The photocatalytic activity and self-cleaning properties of the films were investi-
gated through the degradation of methyl orange and stearic acid, respectively. It was found that the
meso-SiO
the dark. The thin films exhibited enhanced photocatalytic activity and self-cleaning properties compared
to pure Bi films, which was attributed to the cooperation of the interfacial charge transfer between
Bi and surface Cu species as well as the unique mesoporous SiO structure. The results showed that
the films can be used as promising self-cleaning and antifogging materials.
2 2 3
/Cu-Bi O thin films were highly transparent and showed excellent superhydrophilicity even in
Received 28th January 2015,
Accepted 9th March 2015
2 3
O
DOI: 10.1039/c5dt00381d
2
O
3
2
www.rsc.org/dalton
good as expected due to the fast recombination of the photo-
induced electrons and holes in Bi . Thus, much effort has
1
. Introduction
2 3
O
Photocatalytic degradation of organic pollutants by semicon- been taken to overcome this problem, including microstruc-
ductor photocatalysts is an efficient method for environmental ture control, metal doping and surface deposition of noble
1
–3
17–22
pollution remediation and solar energy conversion.
Like metals (Pt or Au).
other semiconductors, bismuth oxide (Bi O ) is also one of the
Pt loading on the Bi O surface significantly increased the
2
3
2 3
important metal oxide semiconductors with high refractive photocatalytic efficiency by capturing the excited electrons
index, high dielectric permittivity, and good photoconducti- from the conduction band of Bi , which prevented the
2 3
O
4
vity. It has been widely used for various applications such as recombination of excited electrons and holes and prolonged
sensor technology, optical coatings, and electrochromic the lifetime of photo-generated charge carriers. The noble
5
–8
materials. Bi
2 3
O is an important p-type semiconductor with metals are good cocatalysts for the catalysts, but these cocata-
five main polymorphic forms denoted as α-, β-, γ-, δ- and lysts are expensive. Thus, it is necessary to find a kind of low-
9
,10
2 3
ε-Bi O ,
and it is already well-known that its energy gap may cost additive to replace noble metal ions for the development
change from about 2 to 3.96 eV depending on the preparation of highly efficient and cost-effective photocatalysts. More
1
1–13
technology.
Moreover, Bi O is a prospective candidate for recently, Irie et al. fabricated a Cu(II) grafted TiO2 catalyst
2 3
photodegrading the environmentally relevant organic pollu- which exhibited a higher photocatalytic oxidation activity than
2
3
tants and purifying water or air, due to its unique characteri- pure TiO
2
and metal or nonmetal doped TiO
2
,
and Miyauchi
stics including the predominant high oxidation power of et al. found that Cu(II) grafted TiO2 thin films exhibited a
1
4–16
valence holes and the nontoxic nature as TiO
pure Bi as a photocatalyst has been found not to be as easier hydrophilic conversion than TiO
the interfacial charge transfer (IFCT), which involves the trans-
fer of electrons in the conduction band of TiO to Cu(II),
forming Cu(I) as well as holes in the valence band of TiO
The electrons transferred to Cu ions may reduce oxygen into
2
.
However, higher oxidation activity than nitrogen doped TiO
2
and an
2
4
2
O
3
2
.
This is ascribed to
2
2
5,26
2
.
The Key Lab of Pollution Control and Ecosystem Restoration in Industry Clusters,
Ministry of Education, School of Environment and Energy, South China University of
Technology, Guangzhou 510006, P. R. China. E-mail: huyun@scut.edu.cn;
Fax: +86-20-39380588; Tel: +86-20-39380573
2
7
2 2 2
H O and H O through multi-electron reduction reaction.
Thus, the photo-generated charge carriers can be separated
efficiently, leading to the enhancement of the photocatalytic
activity. According to these reports, we supposed that the Cu
grafting will also be an effective way to enhance the photo-
†
Electronic supplementary information (ESI) available: The cross-section SEM of
the sample, degradation kinetics of MO over different films, XRD patterns of the
fresh and used samples, and the movie of the superhydrophilic experiment. See
DOI: 10.1039/c5dt00381d
7428 | Dalton Trans., 2015, 44, 7428–7436
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catalytic performance of Bi
2
O
3
. To the best of our knowledge,
meso-SiO
2
/Cu-Bi
2
O
3
films. SiO
2
sol was prepared by a sol–
there are no existing reports on the preparation and investi- gel process. Briefly, tetraethyl orthosilicate (TEOS), anhydrous
R
gation of Cu modified Bi
2
O
3
photocatalysts.
ethanol (EtOH), hydrochloric acid (HCl) and Brij 30 were used
as the precursor, solvent, catalyst and structure-directing
Furthermore, most of the studies reported to date on Bi
2 3
O
photocatalysts are based on the powder system. However, the agent, respectively. The molar ratio of the reagents was
R
thin film form is very important for practical application, since (TEOS) : (EtOH) : (Brij 30) : (HCl) = 8 : 50 : 0.9 : 0.8. A clear solu-
it is easy to be immobilized and used as self-cleaning products tion was obtained after stirring for 10 min at 30 °C. The sol
for window glasses, automotive glasses, tiles, mirrors, and was coated on the Cu-Bi O thin film by a spin-coating
2
3
2
8
plastics. In recent years, transparent and colorless self-clean- method. Then a double layered film was obtained after calcina-
ing thin films have attracted wide attention. As we know, the tion at 450 °C for 5 h with a heating rate of 1 °C min and
−1
self-cleaning properties are related to the photocatalytic reac- denoted as meso-SiO /(x) Cu-Bi O .
2
2 3
tivity and the surface properties such as superhydrophilicity or
superhydrophobicity. So the surface hydrophilic conversion
2
9
2.2. Characterization
plays an important role in the improvement of surface self- The crystal phase structures of the thin films were identified
cleaning properties of the films. For this reason, we designed a by X-ray diffraction (XRD) (Bruker, D8) using Cu Kα (λ =
mesoporous SiO
2 2 3
thin film coated on Cu-Bi O films. The 0.15418 nm) radiation. The scan range of 2θ was 10°–80°.
mesoporous SiO film is expected to possess superhydrophili- Transmission measurements were performed on a UV–vis
2
2 3 4
city even under dark conditions and protect Cu-Bi O from Cu spectrophotometer (Shimadzu, UV-2550) and BaSO was used
dissociation into the environment. Thus, in this work, we as the reflectance standard. The thickness of the film was
report a class of novel mesoporous SiO coated Cu-Bi O thin measured on a field emission scanning electron microscope
2
2 3
films as effective self-cleaning materials. Thin films with (FE-SEM, Hitachi, S-4800). The binding energy was identified
different Cu/Bi molar ratios were prepared by a simple sol–gel/ by X-ray photoelectron spectroscopy (XPS) with Mg Kα radi-
spin-coating method. The enhanced photocatalytic activity and ation (Axis Ultra DLD). The XPS peaks were calibrated with the
self-cleaning properties of meso-SiO /Cu-Bi O films were evalu- C 1s peaks derived from a surface contaminating hydrocarbon
2 2 3
ated by degradation of methyl orange (MO) and stearic acid that had a binding energy of 284.4 eV. Water contact angles
as model pollutants under weak UV irradiation, respectively. were obtained at room temperature using a commercial
In addition, the role of Cu in the Cu modified Bi
2 3
O thin film contact angle meter (Dataphysics OCA 15 plus) with ±1° accu-
was revealed based on the results of various physicochemical racy. A droplet of water was injected on the surface of the film
characterization techniques.
with a 1 μL micro-injector. The photoluminescence (PL)
spectra were recorded with a fluorescence spectrophotometer
(
Edinburgh Instruments, FLS900) using a Xe lamp as the exci-
tation source with optical filters.
2
. Experimental section
2
.1. Sample preparation
2.3. Photocatalytic activity measurements
Cu-Bi O films. All chemicals used in this work were For the evaluation of the photocatalytic activity of the prepared
2
3
analytical grade reagents without further purification. Com- films, a 125 W high pressure mercury lamp with a light inten-
mercial glass plates were used as substrates after being washed sity at 1 mw cm− was used as the UV light source. MO was
with ethanol, a sodium hydroxide solution (10 wt%) and dis- used as a model pollutant to evaluate the photocatalytic
2
tilled water, followed by drying at 100 °C for 3 h. The Cu-Bi
2
O
3
activity of the prepared samples. The photocatalytic degra-
thin films were synthesized by a sol–gel/spin-coating method. dation of MO was carried out in a home-made reactor, two
2
Typically, 5 g of Bi(NO ) ·5H O was dissolved in 4.2 mL of a pieces of the sample (the total area is 12.5 cm ) were added in
3
3
2
nitric acid solution. Then 2 mL of polyethylene glycol (PEG) 100 mL of an MO solution (20 mg L− ). At a given time inter-
1
2
00, 2 g of citric acid and 10 mL of acetone were mixed and val, about 3 mL of MO was collected and then analyzed using
added in the above solution, while 4 mL of triton™ X-100 (OP) a UV-vis spectrometer. The characteristic absorption peak of
was added as the surfactant. A certain amount of CuSO ·5H MO at 464 nm was used to determine the extent of its degra-
4
2
O
was added after stirring for 5 h at 20 rpm and then stirring dation. The removal ratio (η) of MO was calculated as η(%) =
was continued for 3 h. The obtained sol was coated onto a (1 − C/C ) × 100%, where C and C are the concentrations of MO
0
0
cleaned commercial glass plate by spin-coating at 4000 rpm after and before the reaction, respectively.
for 1 min, repeated 3 times, followed by calcination in air at
5
50 °C for 3 h with a heating rate of 1 °C min−1. Then a Cu-
2.4.
Self-cleaning property measurements
Bi
Bi
2
O
O
3
3
thin film was produced. According to this method, Cu- The degradation of stearic acid was carried out under UV
thin films with different Cu/Bi molar ratios were illumination as a model reaction to evaluate the self-cleaning
2
obtained and denoted as x% Cu-Bi O , where x = 0.5, 1, 3, 5, 7, properties of the prepared thin films. A methyl alcohol solu-
2
3
2 3
O
thin film was also tion of stearic acid (1.5 × 10− mol L ) was dropped onto the
3
−1
and 10. For comparison, a pure Bi
prepared under the same conditions in the absence of samples and dried at 353 K for 2 h. The change in the stearic
CuSO ·5H O. acid layer thickness on the surface of the meso-SiO /Cu-Bi O
2 3
4
2
2
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films was recorded by measuring the infrared absorption spec- pure Bi
2
O
3
. When the Bi
2
O
3
films were modified by the Cu
trum with a FT-IR spectrometer (Nicolet-6700). The absor- ions, the main crystal phase structure of Bi O was changed
2
3
bance at 2917 cm− was transformed to a thickness of the from the β to the α phase with the increase in the Cu content
1
stearic acid layer, and the absorbance intensity of 0.01 corre- from 0.5% to 5%, indicating that Cu is helpful in the phase
3
0
sponds to a thickness of 12.5 nm.
transition of Bi O from β to α, namely from the high-tempera-
2
3
ture metastable phase to a low-temperature stable phase.
Moreover, the XRD peak intensity of Bi remarkably
2 3
O
3
. Results and discussion
increased and the diffraction peaks appeared to be sharper
with the increase in Cu content, suggesting that the crystalliza-
tion of Bi O was obviously enhanced. However, these
2 3
3
.1 XRD analysis
XRD measurements were used to confirm the crystal phase
structure and purity of the as-synthesized thin films. The XRD
patterns of Cu-Bi O with different Cu contents and Bi O
2 3 2 3
films are shown in Fig. 1a. The amorphous signal is attributed
to the SiO2 of the substrates since we used the thin film
samples in all the test processes. It was clearly observed that
all the films included mixed phases of monoclinic α-Bi
tetragonal β-Bi O along with the presence of the tetragonal
Bi
phenomena disappeared and the intensity of the main diffrac-
tion peaks weakened with the continuous increase in the Cu
content from 5% to 10%, suggesting the decrease in the crys-
tallinity of Bi O . This is probably because the excessive Cu
2
3
inhibited the crystallization of Bi
2 3
O or destructed the struc-
ture of Bi . The peaks belonging to the Cu species were not
2 3
O
2 3
O and
detected in all of the thin films owing to a high dispersion of
copper salt or its oxide phase as well as the low content of
2
3
SiO
5
phase. The existence of Bi
2
SiO
5
can be due to the high
32
2
copper (0.14–2.94 wt%) below the detection limit.
temperatures facilitating the melting of the glass substrates,
which resulted in a solid reaction between SiO and Bi O . It
is clearly observed that the β phase is the main phase in the
Fig. 1b shows the low angle XRD patterns of the films. The
3
1
2
2 3
single (100) diffraction peak of meso-SiO
and meso-SiO /Cu-Bi films at 2θ = 2–3° is attributed to the
presence of an ordered hexagonal mesoporous structure.
Moreover, no peaks could be observed in the low angle range
on the other films without mesoporous SiO coating.
2 2 2 3
, meso-SiO /Bi O ,
2
2 3
O
3
3
2
3.2 XPS analysis
The above XRD results cannot afford direct evidence for the
oxidation state of Cu. XPS with a high sensitivity is an
effective method to confirm the surface composition and
chemical states of solid catalysts, as shown in Fig. 2. The
binding energy for the C 1s peak at 284.6 eV was used as a
reference. Fig. 2a shows the survey spectrum of the 5%Cu-
Bi O thin film. It was confirmed that Bi, O and Cu existed in
2 3
the composite film. High resolution spectra for Bi 4f are
shown in Fig. 2b. The binding energies at 164.2 and 158.9 eV
of pure Bi
ively, which is a feature of Bi . The binding energies of Bi
5/2 and Bi 4f7/2 shifted to 163.8 and 158.5 eV, respectively,
after Bi was modified with Cu (5%Cu-Bi ). The shift of
2 3
O are attributed to Bi 4f5/2 and Bi 4f7/2, respect-
3
+ 34
4
f
2
O
3
2 3
O
the binding energy to the lower position indicated the change
of the chemical environment surrounding Bi and implied the
doping of Cu into the host lattice. A similar phenomenon was
3
5
reported to occur on the fluorinated Bi O . Moreover, the
2
3
sol–gel method is beneficial to the doping of metal ions into
the host lattice. Meanwhile, as shown in Fig. 2c, the high
resolution spectra of O 1s made it clear that there are two
surface oxygen species in the films. The binding energy in
S
the range of 531–532 eV is ascribed to the surface oxygen (O )
presented as the hydroxyl group (–OH) bound to the surface,
while the binding energy in the range of 529–530 eV is attri-
3
6
buted to the lattice oxygen (O
L
). On account of the relative
XPS area, the atomic ratio of the surface oxygen (O ) to the
S
S L
total oxygen (O + O ) sharply decreased from 57.79% to
31.11% after the Cu modification. The decrease in the content
Fig. 1 XRD patterns of the films (a) wide angle and (b) low angle.
of surface oxygen (O ) may be due to the fact that the surface
S
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Fig. 3 (a) Photograph of (A) meso-SiO
meso-SiO /Bi thin film, (D) Cu-Bi
Bi thin film; (b) UV-vis spectrum of different samples.
2
thin film, (B) Bi
2
O
3
thin film, (C)
/Cu-
2
2
O
3
2
O
3
thin film, (E) meso-SiO
2
2
O
3
glass slides decreased a little due to the surface films, the thin
films on the glass slides were transparent and colorless as the
blank glass slide. The colorless and transparent thin films can
be widely used as self-cleaning materials on the surfaces of
glass, ceramic tiles, mirrors, plastics, etc., without any adverse
effect on the appearance of the substrate materials. The thick-
ness of the synthesized meso-SiO /Cu-Bi O film was about
2
2 3
400 nm. There are no obvious interfaces between the layers, as
can be seen from the cross-sectional SEM image (Fig. S1†).
The optical transparence spectra of the thin films are shown in
Fig. 3b. All the films exhibited a high transmittance in the
visible light range. Moreover, the transmittance of Bi O and
2
3
Cu-Bi
50 nm after the mesoporous SiO
of Bi O and Cu-Bi O films. This could be attributed to the
2
O
3
films was found to be improved between 300 and
4
2
was coated on the surface
Fig. 2 XPS spectra of (a) survey spectrum; (b) Bi 4f and (c) O 1s for pure
Bi
2
O
3
and 5%Cu-Bi
2
O
3
thin films; (d) Cu 2p for the 5%Cu-Bi
2
O
3
thin film.
2
3
2 3
2
mesoporous structure of the surface SiO which could reduce
3
9
the reflection and increase the transmission of light. This is
beneficial to improve the utilization rate of light. Thus the
photocatalytic activity of the films could be enhanced.
hydroxyl group (–OH) was substituted by Cu species. Fig. 2d
shows the high resolution spectrum of Cu 2p for 5%Cu-Bi O .
2
3
3
.4 Photocatalytic degradation of MO
The binding energies at 933.8 and 953.6 eV are attributed to the
2+ 37
Cu 2p3/2 and Cu 2p1/2 of Cu . The obvious characteristic satel- To investigate the influence of Cu on the photocatalytic activity
lite peak around 942.0 eV further confirmed the presence of of the Bi O films, the photocatalytic performance of Cu-Bi O
2 3
2
3
2
+
38
Cu in the 5%Cu-Bi
2 3
O thin film. According to the results of thin films with different Cu contents for the degradation of
2
+
XPS analyses, it indicated that the Cu ions were doped into MO was evaluated under UV light irradiation, as shown in
the Bi O lattice and also existed on the Bi O crystal surface.
Fig. 4a. The blank experiment demonstrated that only 20% of
MO was degraded, due to its self-photolysis, in the absence of
the catalyst under UV light irradiation for 180 min. The pure
2
3
2 3
3
.3 Optical properties
The photographs of the synthesized thin films on the glass Bi
slides are shown in Fig. 3a. Though the transparency of the about 40% of MO was degraded after irradiation for 180 min.
2 3
O thin film showed a poor photocatalytic activity and
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acted as a sink to collect the electrons from the conduction
band of Bi O , thus the photo-induced carriers of Bi O were
2
3
2 3
separated efficiently and the photocatalytic activity was
enhanced, but the photocatalytic activity of the Cu-Bi thin
film decreased when the Cu content was increased from 5% to
0%. Combined with the XRD results and analysis, the rela-
tively low activity of 7% and 10% Cu-Bi films could be
attributed to the negative effect of excess Cu on the structure
of the catalysts. For 7% and 10% Cu-Bi films, the XRD peak
intensities of Bi greatly decreased and the widths of the
XRD peaks increased, suggesting that the crystal structure of
Bi was destroyed partly with the excess Cu loading. These
2 3
O
1
2 3
O
2 3
O
2 3
O
2 3
O
results indicated that Cu is an efficient cocatalyst for enhan-
cing the activity of Bi O , while the photocatalytic activity of
2
3
Cu-Bi
more, it can be observed that the activity of the 5%Cu-Bi
film was improved after the film was coated with a meso-
2 3
O was strongly influenced by the Cu content. Further-
2 3
O
porous SiO
surface SiO
2
film, since the unique mesoporous structure of
increased the light utilization, which was con-
2
firmed by the UV-vis spectra results. The experimental data
were fitted in the pseudo first-order kinetic equation −ln(C/C ) =
kt, as shown in Fig. S2.† The rate constant of MO photode-
0
−
1
gradation is about 0.00605 min
for 5%Cu-Bi O and
2 3
.00704 min− for meso-SiO
1
/Cu-Bi . The optimum photo-
0
2
2 3
O
catalytic activity of meso-SiO
2
/Cu-Bi O is more than 5 times
2 3
−
1
higher than that of Bi O (0.00125 min ). Moreover, the
2
3
2 3
actual Cu content in Cu-Bi O films was investigated. The film
was scraped from the glass substrate and dissolved in a HNO
3
solution (about 16 mol L− ). Then the Cu content was
1
measured using an atomic absorption spectrophotometer. The
result showed that the Cu content was 1.40 wt%, which was
Fig. 4 (a) Photocatalytic activity of the films (pure Bi
2
O
3
films and Cu consistent with the initial dosage (5% molar ratio is equal to
modified Bi
2
O
3
films with different values) for degradation of MO under 1.44 wt% mass ratio). This is because there was no loss of Cu
UV illumination; (b) UV-vis spectrum of MO on the meso-SiO
thin film with different times under UV light irradiation.
2 2 3
/Cu-Bi O
during the sol–gel preparation and the film coating processes.
Fig. 4b shows a gradual decrease in the absorbance of the
2 2 3
MO aqueous solution in the presence of a meso-SiO /Cu-Bi O
thin film under UV irradiation. The characteristic absorption
peak at 464 nm is assigned to the conjugated structure consist-
This is due to the fast recombination of photo-induced elec- ing of a benzene ring and azo groups. The absorption peak at
4
2
trons and holes. The 0.5% Cu-Bi O thin film showed an 270 nm is ascribed to the benzene ring. The absorption peak
2
3
enhanced photocatalytic activity for the degradation of MO. at 464 nm decreased significantly with the increasing
This is probably because suitable Cu doping could form a irradiation time, indicating the destruction of the conjugated
shallow potential well which can capture the electrons and structure. Moreover, the reduction in the absorption peak at
accelerate the separation of photo-generated electrons and 270 nm was attributed to the ring cleavage reaction. These
4
0
holes, thus improving the photocatalytic activity. When the absorption peaks disappeared after irradiation for 6 h,
Cu content was increased from 0.5% to 1%, the photocatalytic suggesting that complete decolorization of the MO solution
activity of the Cu-Bi
2 3
O thin film decreased. This is because was achieved.
excess dopants might act as the new recombination centers for
4
1
3.5 Superhydrophilic and self-cleaning properties
thin film was enhanced again The superhydrophilicity of various films was estimated using a
when the Cu content was increased from 1% to 5%. More than commercial water contact angle meter. The lower contact angle
0% of MO was degraded within 180 min on 5%Cu-Bi O
corresponds to the higher hydrophilicity, and the superhydro-
photo-induced electrons and holes. However, the photo-
catalytic activity of the Cu-Bi
2 3
O
7
2
3
which showed the highest activity. According to the XPS philicity is realized when it is below 5°, which plays an impor-
results, we could speculate that some Cu species were de- tant role in the self-cleaning properties. Fig. 5a shows the results
posited on the surface of the Bi O thin film, which enhanced of water contact angles of various films under the dark con-
2
3
the photocatalytic activity of Bi
2
O
3
. The surface Cu species ditions. It can be observed that the water contact angle of glass
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Fig. 5 (a) The images of water contact angles of different samples
under dark conditions; (b) fogging behavior of a glass slide coated with
2 2 3
a meso-SiO /Cu-Bi O film (left) and an uncoated glass slide (right).
was 35.3°, and the contact angle increased to 62.7° after the
glass was coated with a Bi thin film, showing a decrease in
hydrophilicity. This suggested that the hydrophilic groups
such as hydroxyl groups on the Bi thin film were fewer than
that on the glass slide. Moreover, the Cu-Bi film showed
higher hydrophilicity (23.9°) than the pure Bi O film, prob-
2 3
O
2 3
O
2 3
O
2
3
ably because the Cu species affected the roughness of the film
surface. The surface roughness plays an important role in
hydrophilic transfer. Interestingly, the films changed to be
superhydrophilic after the surfaces were coated with a meso-
Fig. 6 (a) Evolution of the FT-IR spectra of a meso-SiO
under UV irradiation; (b) photocatalytic degradation of stearic acid on
pure Bi and Cu-Bi with different Cu contents under UV irradiation
for 150 min.
2 2 3
/Cu-Bi O film
2
O
3
2 3
O
2
SiO film and the water contact angles all changed to 0° within
approximately 1 s even under dark conditions (see the movie
in the ESI†). Fig. 5b illustrates the antifogging behavior of the
prepared thin films. The glass slides were cooled in a refriger-
The self-cleaning properties of the prepared films were eval-
ator at about −18 °C for 24 h and then moved into humid air. uated by monitoring the degradation of stearic acid under UV
The uncoated glass slide was fogged immediately, while the light irradiation. Fig. 6a shows the FT-IR spectra of stearic acid
slide coated with the meso-SiO
2
/Cu-Bi
2
O
3
thin film remained on the meso-SiO
2
/Cu-Bi
2
O
3
film surface with different
transparent. The superior superhydrophilicity of the meso-SiO2 irradiation times under UV light. The peaks at 2850 and
film may be attributed to the improved surface roughness and 2917 cm− are the typical –CH
1
stretching vibrations and the
peak at 2965 cm is the typical –CH stretching vibration of
2
−
1
capillarity induced by the mesoporous structure of the SiO
2
3
43
30
film. According to the early theoretical work by Wenzel and stearic acid. All the absorption peaks decreased with the
Quere et al., the void fraction and surface roughness would sig- irradiation time and disappeared completely after irradiation
4
4,45
nificantly affect the wettability of a surface with water.
Our for 150 min, indicating that stearic acid on the film surfaces
previous studies proved that a high level of wettability of SiO2 was degraded by the photocatalysts under UV irradiation. The
with the mesoporous nature establishes ideal conditions for self-cleaning properties of the various films were estimated
4
6
extreme wetting behaviors. Therefore, the superhydrophili- based on the thickness variation of stearic acid after UV
city of meso-SiO /Cu-Bi O may be due to the increases in the irradiation for 150 min. As can be seen in Fig. 6b, the variation
2
2 3
2 3
number of voids and pores or in the surface roughness, and all tendency of self-cleaning properties of Cu-Bi O films with
the increases would lead to a larger contact area between the various Cu contents is in accordance with the photocatalytic
water droplet and the surface coated mesoporous SiO , thus activity of the films (Fig. 4a). The highest self-cleaning ability
2
4
7,48
improving the hydrophilicity of the films.
effective antifogging properties can be achieved on the meso- of the degraded stearic acid was more than 50 μm, which
SiO /Cu-Bi O films. was 2 times as high as that on the pure Bi thin film.
2 3
Therefore, was obtained over the 5%Cu-Bi O thin film, and the thickness
2
2 3
2 3
O
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As suggested in the previous sections, this may be ascribed to
the Cu species deposited on the surface of the Bi O thin film,
2
3
which can promote the separation of photo-induced carriers
and lead to an improvement of activity. Moreover, it can also
be found that the self-cleaning ability of 5%Cu-Bi O was
2
3
2
improved after it was coated with a mesoporous SiO film,
which showed photocatalytic properties for the degradation of
4
9
MO. Thus, the meso-SiO /Cu-Bi O film can be applied as an
2
2 3
antifogging film in the dark or rainy days and also can be used
as a self-cleaning product under weak light conditions.
3
.6 Reaction mechanism
On the basis of the above results, the reaction mechanism of
organic pollutant degradation on the meso-SiO /Cu-Bi films
2
2 3
O
was proposed and is shown in Fig. 7. The schematic represen-
tation of the Cu-Bi O film is shown in Fig. 7a. In this system,
the Bi O could be excited under UV light irradiation to gene-
2 3
2
3
Fig. 8 PL spectra of (a) 1%Cu-Bi
Cu-Bi film.
2 3 2 3
O film, (b) pure Bi O film and (c) 5%
2
O
3
rate electrons in the valence band (VB), and then these elec-
trons transferred to the conduction band (CB) of Bi O . These
2
3
electrons may have two main paths for the next step responses:
1) the electrons recombined with the holes in the VB; (2)
other electrons transferred to Cu(II) supported on the surface
of the Bi thin film based on interfacial charge transfer
IFCT). The electrons transferred to Cu(II) and formed Cu(I),
which reduced oxygen molecules through two-electron reaction
(
could decompose most organic compounds. Based on the pre-
vious reports, the photogenerated holes in the VB of Bi-system
2
O
3
−
•
photocatalysts cannot oxidize OH to form OH, as the stan-
(
V
III
dard redox potential of Bi /Bi (1.59 eV), which decides the
oxidation potential of holes in Bi O , is more negative than the
2
3
(
reaction (1)) and formed Cu(II), so the electrons were
•
−
standard redox potential of OH/OH (1.99 eV). So the photo-
generated holes in the VB of Bi O are capable of directly
5
0,51
consumed.
2
3
3
5
CuðiÞ þ O2 þ 2Hþ ! 2CuðiiÞ þ H2O2
decomposing organic substances. Thus the electrons and
holes were separated effectively and the photocatalytic activity
was enhanced. In addition, the structure schematic of the
2
ð1Þ
ð2Þ
•
H2O2 ! 2 OH
meso-SiO /Cu-Bi O thin films is shown in Fig. 7b. The surface
Moreover, the hydrogen peroxide formed by two-electron
reduction generated the hydroxyl radicals (reaction (2)) which
2
2 3
•
contaminant can be degraded by H
2 2
O , OH or holes produced
from the Cu-Bi O thin film under UV light. Some pollutants
2
3
decomposed incompletely may be taken away by water, which
is attributed to the superhydrophilicity of the surface meso-
SiO film.
2
Furthermore, the better separation of the electrons and
holes in the Cu-Bi
2 3
O films was confirmed from PL spectra, as
shown in Fig. 8. It can be observed that the 5%Cu-Bi O film
2
3
exhibited much lower emission intensity than the pure Bi O
2
3
film, indicating that the recombination of the photogenerated
electrons and holes was inhibited after Cu modification.
However, the 1%Cu-Bi O film showed the highest emission
2
3
intensity, demonstrating that the recombination of the elec-
trons and holes was enhanced. Thus the photocatalytic activity
of the 1%Cu-Bi O film would be decreased, which was con-
2
3
sistent with the measurement results of the photocatalytic
activity and self-cleaning property.
3.7 Reusability
It is necessary to estimate the stability of the prepared films
during photocatalytic reaction. Six successive reaction experi-
ments were carried out by using a meso-SiO /Cu-Bi O thin
2 2 3
film into an unreacted MO solution under UV irradiation, as
shown in Fig. 9. It can be seen that the film retained high
Fig. 7 (a) Schematic illustration of photocatalytic degradation of MO
over Cu-Bi O ; (b) structure scheme of a meso-SiO /Cu-Bi O thin film.
2 3 2 2 3
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rated electrons and holes based on the interface charge trans-
fer induced by the surface Cu species. Moreover, the surface
mesoporous SiO enhanced the transmittance and hydrophili-
2
city of the film due to the unique mesoporous structure and
the surface hydroxyl of the SiO . This is helpful for the
2
improvement of the surface self-cleaning properties of the film
and the meso-SiO
2 2 3
/Cu-Bi O thin films showed superhydrophi-
licity even in the dark. The present meso-SiO /Cu-Bi O thin
2
2 3
films can be applied as antifogging films in the dark or rainy
days and also be used as self-cleaning materials under weak
light irradiation.
Acknowledgements
This work was supported by the National Natural Science
Foundation of China (no. 21277051, 21037001), the Science
and Technology Planning Project of Guangzhou City, China
2 2 3
Fig. 9 Recyclability of the meso-SiO /Cu-Bi O for the degradation of
MO under UV irradiation.
(
no. 12C62081602), and the Fundamental Research Funds for
the Central Universities, SCUT (no. 2012ZZ0049).
activity for MO photodegradation after six cycling reactions
and the removal rate of MO in the solution remained constant
for each cycle. We also measured the Cu ions released in the References
solution during and after the photocatalytic reaction using an
1
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