Enhancement of photo-catalytic degradation of formaldehyde through loading anatase TiO2 and silver nanoparticle films on wood substrates

Article abstract of DOI:10.1039/c5ra06390f

Since formaldehyde is considered to be a potential risk for human health, its emission must be eliminated in an effective way. In this study, anatase TiO₂ particles with silver nanoparticles were doped on wood substrates through a two-step method and used as a photo-catalyst for formaldehyde degradation. The effect of Ag dopant on the formaldehyde degradation of the TiO₂-treated wood was investigated. The results showed that the formaldehyde response of the TiO₂ film was drastically improved by doping with Ag nanoparticles under visible-light irradiation. In this heterostructured system, because the Fermi level of Ag was lower than the conduction band of TiO₂, Ag nanoparticles can act as electron scavenging centers to cause electron and hole pair separation, leading to enhanced photo-catalytic activity of TiO₂.

Full text of DOI:10.1039/c5ra06390f

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Enhancement of photo-catalytic degradation of
formaldehyde through loading anatase TiO and
2
Cite this: RSC Adv., 2015, 5, 52985
silver nanoparticle films on wood substrates†
a
a
a
b
a
Likun Gao, Wentao Gan, Shaoliang Xiao, Xianxu Zhan* and Jian Li*
Since formaldehyde is considered to be a potential risk for human health, its emission must be eliminated in
an effective way. In this study, anatase TiO particles with silver nanoparticles were doped on wood
substrates through a two-step method and used as a photo-catalyst for formaldehyde degradation. The
effect of Ag dopant on the formaldehyde degradation of the TiO -treated wood was investigated. The
results showed that the formaldehyde response of the TiO film was drastically improved by doping with
Ag nanoparticles under visible-light irradiation. In this heterostructured system, because the Fermi level
of Ag was lower than the conduction band of TiO , Ag nanoparticles can act as electron scavenging
centers to cause electron and hole pair separation, leading to enhanced photo-catalytic activity of TiO
2
2
Received 10th April 2015
Accepted 10th June 2015
2
DOI: 10.1039/c5ra06390f
2
www.rsc.org/advances
2
.
pollutants, but also fundamentally modied the wood
substrates.
Introduction
As a signicant volatile organic compound (VOCs), formalde-
Since Fujishima reported UV irradiation induced redox
hyde (HCHO) is considered to be one of the most important chemistry on TiO
pollutants in indoor environment. Wood-based products are interest as a green and sustainable solution for energy and
under the category of natural materials due to their friendly environmental issues.
2
, photo-catalysis has attracted the most
1,2
9,10
As an n-type semiconductor oxide,
properties towards health, aesthetic and environmental aspects. TiO is widely used as photo-catalyst in the applications such as
2
However, because urea-formaldehyde adhesives in wooden self-cleaning, self-degradation, etc., because it is relatively
panels could produce HCHO gas during application, the fear of inexpensive and can degrade toxic contaminants without
3
11,12
formaldehyde emission limits the use-value of wooden panels.
requiring other reagents.
Up to now, an increasing attention
Although the elimination of formaldehyde is very difficult, has been paid to improve its physical and chemical properties,
photo-decomposition was proved to be a promising method to which would further facilitate the profound understanding of
4,5
remove gaseous pollutants.
Furthermore, due to the the dependence of material properties. Driven by this require-
increasing concern on HCHO in the indoor environment, the ment, great amount of innovative approaches have been con-
abatement of HCHO is of signicant practical interest at low ducted to develop novel semiconductor-based composites with
temperature, especially at room temperature. Very recently, desired properties and functions for special applications. In
many studies on the reduction of HCHO by adsorbents or order to endow the TiO
catalysts were reported.
2
-based materials with extraordinary
However, the reclamation of the properties and superb photocatalytic performance in practical
catalysts, such as powder and particles, plays a critical role in applications, the method of metal doped TiO , such as Ag/TiO
6
–8
2
2
the practical applications, which would inuence the develop- composites, has become one of the most attractive candidate
7
,13,14
15
16,17
ment of environmental technology for decontamination, puri- solution.
Among numerous metals, such as Ru, Au,
18,19
20,21

cation, and deodorization of the atmospheres. Based on above Bi,
Pt,
etc., Ag is particularly suitable for industrial
consideration, the wood-based products composited with a lm applications because of its low cost and nontoxic properties
14,22
of catalysts were proposed. The planned treatment would not compared to other noble metals.
In addition, Ag has a good
only produce a catalyst for decomposition of the toxic organic light absorption capability, extending the response of TiO
2
to
23,24
visible light.
In this study, because formaldehyde was considered as a
main indoor pollutant, it was chosen as the aim pollutant, and
wood was selected as substrates coated by TiO particles and
2
a
Material Science and Engineering College, Northeast Forestry University, Harbin
1
50040, P.R. China. E-mail: gaolknefu@163.com; nefulijian@163.com; Fax: +86-
51-82192399; Tel: +86-451-82192399
silver nanoparticles. The photo-catalytic degradation characters
of HCHO by the TiO -treated wood and the Ag-doped TiO /wood
4
b
2
2
Dehua TB New Decoration Material Co., Ltd, Huzhou, 313200, P.R. China
under visible-light irradiation was characterized by the scan-
ning electron microscopy (SEM), transmission electron
†
Electronic supplementary information (ESI) available. See DOI:
0.1039/c5ra06390f
1
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microscopy (TEM), X-ray diffraction (XRD), X-ray photoelectron
spectroscopy (XPS), and UV-vis diffuse reectance spectroscopy
(UV-vis DRS). The analogue experiments of formaldehyde
degradation were performed and the mechanism was explored
simultaneously.
Methods
Materials
Scheme 1 Schematic representation of the formation of the Ag-
2
doped TiO /wood.
All chemicals supplied by Shanghai Boyle Chemical Company
Limited were of analytical reagent-grade quality and used
without further purication. The formaldehyde used in our
experiment was an analytical reagent. Deionized water was used
throughout the study. The wood blocks of 20 mm (R) ꢀ 20 mm
absorbed on the TiO particles. And then the silver ammonia
complex loaded TiO -treated wood was transferred into the
2
2
+
solution of glucose and [Ag(NH
seeds on the TiO
glucose, more and more silver ions were absorbed onto the
TiO -treated wood surface and reduced into silver. The silver
3
)] was in situ reduced into silver
(
T) ꢀ 30 mm (L) were obtained from the sapwood sections of
+
2
3
particles. With the addition of [Ag(NH )] and
poplar (Populus ussuriensis Kom), which is one of the most
common tree species in the northeast of China. Aer ultra-
sonically rinsing in deionized water for 30 min, the wood
2
seeds grew larger into silver nanoparticles (Ag NPs) and
ꢁ
specimens were oven-dried (24 h, 103 C) to a constant weight,
attached to each other forming a compact lm on the TiO -
2
and the weights were determined.
25
treated wood.
Preparation of anatase TiO lm on the wood surface
2
Characterization
The ammonium uorotitanate (0.4 M) and boric acid (1.2 M)
were dissolved in the distilled water in a 500 mL glass container
at a room temperature with vigorous magnetic stirring. A
solution of 0.3 M hydrochloric acid was added until the pH
value reached approximately 3. Then, the 75 mL adjusted
solution was transferred into a 100 mL Teon container. Wood
specimens were subsequently placed into the solution, sepa-
The morphology of the wood surfaces was observed through the

eld emission scanning electron microscopy (SEM, Quanta 200,
FEI, Holland) operating at 12.5 kV in combination with EDS
Genesis, EDAX, Holland). The transmission electron micros-
(
copy (TEM) experiment was performed on a Tecnai G20 electron
microscope (FEI, USA) with an acceleration voltage of 200 kV.
Carbon-coated copper grids were used as the sample holders.
The X-ray diffraction (XRD, Bruker D8 Advance, Germany) was
employed to analyze the crystal structures of all samples
applying graphite monochromatic with Cu Ka radiation (l ¼
ꢁ
rately. The autoclave was sealed and maintained at 90 C for 5 h,
and then naturally cooled to room temperature. Finally, the
treated samples were removed from the solution, ultrasonically
ꢁ
rinsed with deionized water for 30 min, and dried at 45 C for
˚
ꢁ ꢁ
1
.5418 A) in the 2q range from 5 to 80 and a position-sensitive
more than 24 h in a vacuum chamber. For the comparative
purpose, un-treated wood samples were also examined. As a
ꢁ
ꢁ
ꢂ1
detector using a step size of 0.02 and a scan rate of 4 min
.
Further evidence for the composition of the product was
inferred from the X-ray photoelectron spectroscopy (XPS,
Thermo ESCALAB 250XI, USA), using an ESCALab MKII X-ray
photoelectron spectrometer with Mg-Ka X-rays as the excita-
tion source. Optical properties of the materials were charac-
terized by the UV-vis diffuse reectance spectroscopy (UV-vis
DRS, Beijing Purkinje TU-190, China) equipped with an inte-
2
result, TiO surfaces on the treated wood contained OH groups,
which improved the surface hydrophilicity through the TiO
hydroxylation.
2
Loading of the TiO
nanoparticles
2
-treated wood surface with Ag
Aqua ammonia (28 wt%) was added dropwise into a 0.5 M
AgNO aqueous solution with stirring until a transparent col-
ourless [Ag(NH )] solution was formed. The TiO -treated wood
4
grating sphere attachment, which BaSO was the reference.
3
+
3
2
Photo-catalytic degradation of HCHO
samples were soaked to the above solution for 1 h, then trans-
ferred to a 0.1 M glucose stock solution. Aer 5 min, the A schematic diagram of the experimental system for photo-
+
3
residual [Ag(NH )] solution was also poured into the glucose oxidation is shown in Scheme 2. The experiments were per-
solution. The reaction was continued for 15 min. Finally, the formed in an obturator with the size of 500 mm (L) ꢀ 300 mm
prepared samples were removed from the solution, ultrasoni- (W) ꢀ 300 mm (H). A at-type LED-light (wavelength is in the
ꢁ
cally rinsed with deionized water for 30 min, and dried at 45 C range of 400–760 nm, light intensity is approximately 3.6 mW
ꢂ2
for more than 24 h in a vacuum chamber. The detailed proce- cm ) installed in the open central region was used to simulate
dure for the synthesis of the Ag-doped TiO /wood hetero- visible light. Initially, there was no HCHO in the obturator,
2
structures is illustrated in Scheme 1. In this work, the TiO2 which was detected by a formaldemeter. Then, the desired
particles with abundant hydrophilic groups (OH) on the wood amount of HCHO was injected into the obturator. The gaseous
surface was dipped into the silver ammonia complex solution, formaldehyde was allowed to reach adsorption and desorption
+
the complexes of [Ag(NH
3
)] were easily and abundantly equilibrium for 30 min. Aer equilibrium, the initial
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Scheme 2 Schematic diagram of experimental set-up (1) a flat-type
LED-light; (2) the treated wood sample; (3) formaldemeter; (4) obtu-
rator (500 mm (L) ꢀ 300 mm (W) ꢀ 300 mm (H)).
concentration of formaldehyde in the obturator was controlled
ꢂ3
in the region of approximately 1.5–2.5 mg m . Then, the LED
light was turned on to irradiate the sample. Each set of degra-
dation experiment under LED irradiation lasted for 10 h. HCHO
concentration in the obturator was determined by the
formaldemeter.
Results and discussion
Fig. 1 SEM images of the surfaces of (a) the pristine wood, (b) the
TiO
magnifications, and (e and f) EDS spectrums of the TiO
and the Ag-doped TiO /wood, respectively.
2
-treated wood, (c and d) the Ag-doped TiO
2
/wood at different
The SEM images of the un-treated and treated wood samples are
presented in Fig. 1. Fig. 1a shows the micrographs of un-treated
poplar wood surface, which clearly reveals that the poplar wood
is a type of heterogeneous and porous material. Fig. 1b shows
2
-treated wood
2
that the TiO particles deposit on the wood cell walls and ll
2
2
into the pits of the wood surface. The TiO particles with the
average diameter of approximately 1.5 mm are adhered onto
wood surface through the chemical bonds between hydroxyl
groups of wood surface and TiO
the morphology of pristine TiO
nanoparticles doped one, it was observed that a compact layer Fig. 2 (a) TEM image, (b) the selected electron diffraction (SAED)
2
particles. As a comparison of
2
/wood with that of the Ag
2
patterns, and (c) HRTEM image of the Ag-doped TiO /wood.
composed by Ag nanoparticles deposited on its surface (Fig. 1c).
The magnied image (Fig. 1d) of the Ag-doped TiO /wood
2
demonstrates that the lm is approximately 1.8 mm thick and
the sphere particles stacked over the wood surface is approxi- TiO
mately 130 nm in diameter. The surface chemical elemental large quantity of Ag nanoparticles are attached onto the surface
compositions of the treated wood are determined via the energy- of the TiO -treated wood matrix, and the size distribution of Ag
dispersive X-ray spectroscopy (EDS), and the results are pre- nanoparticles is wide. The corresponding selected area electron
sented in Fig. 1e and f. The evidence of only uorine, titanium, diffraction (SAED) pattern of the Ag-doped TiO /wood is shown
gold, and carbon elements could be detected in Fig. 1e, while in Fig. 2b. The SAED pattern of TiO microparticles in anatase
/wood. As shown in Fig. 2a, the TEM image shows that a
2
2
2
2
the additional presence of silver element exists in Fig. 1f. The phase and Ag nanoparticles in cubic phase shows that the
gold element is originated from the coating layer used for SEM diffraction patterns and rings are co-existent, and the different
observation, the uorine element may be originated from the crystal planes are identied as (101), (200) and (211) diffraction
precursor ammonium uorotitanate, and carbon element was planes of anatase TiO
from the wood substrate. Since no other elements were detec- diffraction planes of cubic Ag, respectively. To further inves-
ted, the composition of the microspheres on the TiO
-treated tigate the distribution of Ag and TiO particles, the HRTEM
wood sample was conrmed to be TiO
as well as revealing the image is displayed in Fig. 2c. The clear lattice fringe of
loading of metallic Ag on the Ag-doped TiO /wood sample.
d-0.35 nm matches that of the (101) plane of anatase phase of
Fig. 2 shows the representative TEM image, the selected TiO , while the fringe of d-0.24 nm corresponds to the (111)
as well as (111), (200), (220) and (311)
2
26
2
2
2
2
2
7
electron diffraction (SAED) pattern and high-resolution trans- plane of cubic phase of Ag. No other impurities such as silver
mission electron microscopy (HRTEM) image of the Ag-doped oxides are detected.
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4
+
Fig. 3 shows the XRD patterns of the pristine wood, the 459.3 eV, attributed to a Ti species, and the other located at
3
+
TiO -treated wood and the Ag-doped TiO /wood. Apparently, the 458.9 eV, assigned to a Ti species (in Fig. 4c), further indi-
2
2
ꢁ
ꢁ
diffraction peaks at 14.8 and 22.5 belonging to the (101) and cating the strong interaction formed between Ag and TiO
002) crystal planes of cellulose in the wood are observed in both species. Certainly, the presence of Ti oxide with narrow band
2
3
+
(
the spectrum of the pristine wood and the treated wood gap and energy level located between the valence and the
27
samples. It can be found that the diffraction peaks are well conduction band of TiO
indexed to the standard diffraction pattern of anatase phase photocatalytic activity of the Ag-doped TiO
TiO (JCPDS le no. 21-1272) and the face-centered cubic (fcc) structures driven by visible light. Moreover, the defect sites of
phase of Ag (JCPDS le no. 04-0783), indicating that the present TiO
2
may be advantageous to the higher
2
/wood hetero-
2
3
+
2
surface induced by light, that is, Ti species, are necessary
synthesis strategy successfully achieves Ag/TiO2 hierarchical for adsorption and photo-activation of oxygen, which are
heterostructures with high crystallinity on wood substrate. The necessary for the photo-oxidation of pollutants.
curve in Fig. 3b shows that the diffraction peaks of pure anatase
The high resolution XPS spectra of O 1s in the TiO -treated
2
ꢁ
ꢁ
ꢁ
ꢁ
ꢁ
TiO
2
ꢁ
particles center at 2q ¼ 25.2 , 38.0 , 47.8 , 54.2 , 62.5 , wood and the Ag-doped TiO
2
/wood are provided in Fig. 4d. The
ꢁ
68.8 , and 74.9 , which agree with (101), (004), (200), (211), wide and asymmetric O 1s spectra suggests that there would be
(
204), (116) and (215) crystal planes of anatase TiO
2
, respec- more than one component. Using the XPS Peak tting program,
14
tively. Aer doped with Ag nanoparticles (Fig. 3c), all of the version 4.1, the spectra can be further tted into three peaks
new diffraction peaks at 38.1 , 44.2 , 64.4 , and 77.4 , except the including crystal lattice oxygen attributed to the peak of OTi–O at
ꢁ
ꢁ
ꢁ
ꢁ
diffraction peaks of TiO , can be perfectly indexed to (111), 530.3 eV, surface hydroxyl groups assigned to the peak of OO–H
2
(
200), (220) and (311) reections of the face-centered cubic (fcc) at 532.8 eV, and adsorbed O in molecular water or C–O bonds
2
7
29
phase of Ag, which is in agreement with SAED pattern results in (at 533.2 eV).
Fig. 2b. Fig. 4e represents the C 1s XPS spectra of the TiO
Fig. 4 ascertains more detailed information concerning the wood and the Ag-doped TiO /wood as well as their tting
2
-treated
2
elemental and chemical state of the resulting samples. The results. The spectrum is de-convoluted into four separated
fully scanned spectra (Fig. 4a) shows that the Ti, O, and C Gaussian distributions. The peak around 294.8 eV is assigned to
elements exist on the surface of the pure TiO -treated wood C–C or C–H bonds, and the peak observed at 286.5 eV corre-
2
sample, while Ti, O, Ag, and C elements exist on the surface of sponds to C–O bonds. The other two peaks around 288.0 eV and
the Ag-doped TiO /wood sample. The C element can be 289.0 eV are attributed to C]O and O–C]O bonds, respec-
2
ascribed to the wood substrate or the adventitious carbon- tively, which may be typical for adsorbed carbon on the
30
based contaminant.
samples.
As shown in Fig. 4f, the Ag 3d XPS spectra with two peaks at
2
From Fig. 4b, the Ti 2p XPS spectra of the TiO -treated wood
with two peaks at binding energies of 459.0 eV and 464.8 eV, binding energies of 367.6 eV and 373.6 eV, corresponding to the
corresponding to the Ti 2p3/2 and Ti 2p1/2 peaks, respectively. Ag 3d5/2 and Ag 3d3/2 peaks, respectively. The gap of 6.0 eV
The gap of 5.8 eV between the two peaks indicates the existence between the two peaks is also indicative of metallic Ag, and
4
+
28
+
of the Ti oxidation state. However, the peak position for Ti 2p there is no evidence for the presence of Ag , indicating that the
in the Ag-doped TiO /wood sample shis to a higher binding Ag ions in the composites are fully reduced to metallic Ag.
2
energy band than that in the pure TiO -treated wood sample. Additionally, it is obvious that the peaks of Ag 3d shi to the
2
This conrms a lower electron density of the Ti atoms aer lower position compared with bulk Ag (368.3 eV for Ag 3d5/2
,
doped with Ag nanoparticles, and there is a strong interaction and 374.3 eV for Ag 3d3/2), suggesting that the electrons may
31
between metallic Ag and TiO
addition, the Ti 2p3/2 XPS peak of the Ag-doped TiO
sample can be tted into two components, one located at TiO
In brief, it can be demonstrated that Ag nanoparticles are
2
in the Ag-doped TiO
2
/wood. In migrate from the TiO
/wood that there is a strong interaction between Ag nanoparticles and
particles in the interface of heterostructures.
2
particles to metallic Ag, which reveals
2
2
grown on the TiO -treated wood matrix, and there is a strong
2
interaction between Ag and TiO2 in the interface of the
Ag-doped TiO /wood heterostructures. Furthermore, Ag nano-
2
particles can act as electron acceptors and be advantageous for
separating the photo-excited electron–hole pairs, the hetero-
structures can inhibit the recombination of excited electrons
and holes and then enhance the photo-catalytic activity of the
products.
The probable scheme diagram for the photo-catalytic
process of the Ag-doped TiO /wood sample and a simplied
2
reaction scheme for the catalytic oxidation of HCHO on the
surface of the Ag-doped TiO /wood sample are illustrated in
2
Scheme 3. As shown in Scheme 3a, during visible-light irradia-
tion, the Ag nanoparticles are photo-excited owing to the plas-
mon resonance, which the photo-excited electrons are
Fig. 3 XRD patterns of (a) the pristine wood, (b) the TiO
2
-treated
wood and (c) the Ag-doped TiO /wood.
2
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2 2
Fig. 4 (a) Survey scan and (b) Ti 2p XPS spectra of TiO -treated wood and the Ag-doped TiO /wood, (c) peaking-fitting results of Ti 2p XPS
2 2 2
spectra of the Ag-doped TiO /wood, (d) O 1s and (e) C 1s XPS spectra of TiO -treated wood and the Ag-doped TiO /wood, (f) Ag 3d XPS spectra
of the Ag-doped TiO /wood.
2
subsequently migrated from the surface of the Ag nanoparticles (VB') to be excited to the conduction band (CB) of TiO . There-
2
to the conduction band (CB) of TiO
to the high crystallinity of the Ag nanoparticles, the resistance of centers in the conduction band of TiO
2
particles. In addition, due fore, the Ag nanoparticles can be regarded as electrons trapping
, leading to a decrease in
2
the electron migration is decreased leading to a reduction in the the concentration of photo-induced charge carriers and an
recombination of excited electrons and holes. The electrons on enhancement for the photo-catalytic activity of the samples.
the conduction band (CB) of TiO
2
are scavenged by dissolved That is, the migration of charges is enhanced and the recom-
oxygen molecules (O ) and the holes on the valence band (VB) of bination of excited electrons and holes is suppressed as a result
2
ꢂ
14
TiO2 absorb water molecules (H O) or hydroxyl ions (OH ), of the well-known Schottky barrier effect. The proposed initial
2
which yields abundant of highly oxidative species including elementary reactions are listed in eqn (1)–(4) as the followings:
ꢂ
superoxide radical anion (cO
2
) and hydroxyl radical (cOH). This
ꢂ
+
TiO
2
/Ag + hv / e + h
(1)
(2)
(3)
(4)
causes an improvement of the photo-catalytic performance and
aggressive oxidation of the surface-adsorbed toxic organic
pollutants (HCHO) to convert into CO and water. Furthermore,
2
as revealed by the XPS results, the production of Ti species is
ascribed to the strong interaction between Ag and TiO . The
ꢂ
+
TiO + hv / e + h
2
3
+
ꢂ
2
+ O / cO
2ꢂ
e
2
3
+
energy level of Ti species is located between the valence band
VB) and conduction band (CB) of TiO , which is highly
+
ꢂ
+
h + OH / cOH + H
(
2
advantageous to promote the electrons in the new valence band
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Scheme
migration process in the Ag-doped TiO
light irradiation, and (b) possible scheme for HCHO adsorption and
3
(a) Proposed photo-induced charge separation and
2
/wood sample under visible-
2 2
surface oxidation to CO on the surface of Ag-doped TiO /wood
sample.
Moreover, published literatures have described that the
1,32,33
mechanism of TiO
Based on the discussion above, a scheme for HCHO adsorption
and surface oxidation to CO on the surface of Ag-doped
2
lm on decomposition of HCHO.
2
2
Fig. 5 (a) UV-vis absorption spectra of the TiO -treated wood and the
TiO
2
/wood sample is postulated in Scheme 3b. During the Ag-doped TiO /wood, and (b) comparison of adsorption curve and
2
photo-catalytic decomposition curve of HCHO over the pristine wood
and the Ag-doped TiO /wood.
2
HCHO degradation in the photo-catalytic process, formic acid
HCOOH) is identied as intermediate. The related reactions of
(
HCHO destruction are shown with eqn (5)–(9) as the followings:
Fig. 5b presents comparison of adsorption curve and photo-
catalytic degradation curve of HCHO over the pristine wood and
the Ag-doped TiO /wood for every 2 hours. For the pristine
2
wood, both the adsorption and photo-catalytic degradation rate
of HCHO is about 2% aer 10 h, possibly because the effect of
the porous structure of the cell wall of wood is similar to that of
ꢂ
HCHO + e / Hc + HCO
HCHO + Oc / OHc + HCO
(5)
(6)
(7)
2
HCHO + OHc / HCOc + H O
HCOc + OHc / HCOOH
(8) active carbon. For the Ag-doped TiO /wood, in the rst two
2
hours, the concentrations of HCHO decreased fast in both
cases, and the treatment rate of HCHO by photo-catalytic
degradation is still superior to that of HCHO by absorption.
Finally, the treatment rate of HCHO by photo-catalytic degra-
dation is 94.9%, while the treatment rate of HCHO by absorp-
tion is 21%. The obvious difference between two curves
conrms that the photo-catalytic reaction plays an important
role in the degradation of HCHO. Aer 10 h, the absorption
curve variation of HCHO is less than 1% and tends to zero. The
result indicates the absorption of HCHO over the Ag-doped
+
HCOOH + h / CO + 2Hc
(9)
2
In order to investigate the light absorbance of the samples,
the UV-vis diffuse reection spectra of the TiO -treated wood
and the Ag-doped TiO /wood are depicted in Fig. 5a. As for the
pure TiO -treated wood, it presents prominent adsorption
below 350 nm wavelength. Whereas the Ag-doped TiO /wood
exhibits a much higher absorption in the region 380–800 nm,
indicating the absorption of Ag-doped TiO /wood is signi-
cantly red-shied to the visible due to the loading of the Ag
nanoparticles onto the surface of the TiO
Theoretically, the wide visible absorption should be attributed
to the characteristic absorption of surface plasmon resonance
originating from the metallic Ag nanoparticles in the Ag-
doped TiO /wood samples. Moreover, it is worth noting that
the Ag-doped TiO /wood heterostructures with absorptions in
2
2
2
2
2
TiO
2
/wood reached the adsorption equilibrium. Furthermore,
ꢂ3
2
-treated wood.
the initial concentrations of the HCHO are 2.14 mg m for the
Ag-doped TiO
tion. It is obvious that, for the Ag-doped TiO
2
/wood in the photo-catalytic degradation reac-
/wood, the
2
degradation rate of HCHO is dramatically increased, aer 6
hours reaction, the degradation rate is slightly decreased but
still rising, indicating the photo-catalytic reaction of HCHO is
continuing. As well-known, the national regulation illustrates
that, for the indoor formaldehyde standard, the HCHO of public
2
2
the visible region may implicate higher activity in photo-
catalysis.
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Ag nanoparticles dopant on the surface of TiO
2
/wood. The
TiO -treated wood and the Ag-doped TiO /wood were used in an
2
2
obturator for HCHO decomposition under visible light. The
results indicate that the Ag-doped TiO /wood exhibits an
2
enhanced photo-catalytic activity in the decomposition of
HCHO driven by the visible light, which is mainly ascribed to
the strong interaction between TiO
2
and Ag, and the surface
plasmon resonances of Ag nanoparticles excited by visible-light
3
+
irradiation. Meanwhile, the presence of Ti in the Ag-doped
TiO /wood is advantageous to higher photo-catalytic activity,
2
which extends the photo-response of the products to the visible
light. Since the Ag-doped TiO /wood enhanced the photo-
2
catalytic degradation of formaldehyde, it would greatly
promote the application of the modied wood in a fast elimi-
nation of HCHO. However, the stability of the photo-catalyst
based on wood substrate requires more exploration in the
future researches.
Fig. 6 Five cycles of photo-catalytic degradation of HCHO over the
Ag-doped TiO /wood.
2
ꢂ3
places should be below 0.12 mg m , and the HCHO of resi-
ꢂ3
dential indoors should be below 0.08 mg m . Apparently, the
results indicate that the removal efficiency of HCHO by the Acknowledgements
2
Ag-doped TiO /wood under visible-light irradiation could reach
This work was nancially supported by the National Natural
Science Foundation of China (grant no. 31470584).
the national regulation for the indoor formaldehyde standard of
public places, and approximate to the national regulation for
indoor formaldehyde standard of residential indoor.
In addition, we further investigated the analogue experiment
Notes and references
of formaldehyde degradation by the Ag-doped TiO /wood under
2
the ultraviolet-germicidal-lamp illumination (365 nm wave-
length and approximately 1.5 mW cm light intensity), as
shown in ESI.† Most interestingly, the experimental results
2
present that, for the Ag-doped TiO /wood, the degradation rate
1
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1
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52992 | RSC Adv., 2015, 5, 52985–52992
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