1. Accepted Manuscript
Title: Ag-doped ZnO nanorods coated metal wire meshes as
hierarchical photocatalysts with high visible-light driven
photoactivity and photostability
Author: Mu-Hsiang Hsu Chi-Jung Chang
PII: S0304-3894(14)00495-6
DOI: http://dx.doi.org/doi:10.1016/j.jhazmat.2014.06.038
Reference: HAZMAT 16047
To appear in: Journal of Hazardous Materials
Received date: 19-3-2014
Revised date: 10-6-2014
Accepted date: 11-6-2014
Please cite this article as: M.-H. Hsu, C.-J. Chang, Ag-doped ZnO nanorods
coated metal wire meshes as hierarchical photocatalysts with high visible-light
driven photoactivity and photostability, Journal of Hazardous Materials (2014),
http://dx.doi.org/10.1016/j.jhazmat.2014.06.038
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Graphical Abstract (for review)
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Highlight
● Ag-doped ZnO nanorods on stainless-steel wire mesh as hierarchical photocatalyst.
● Hierarchical photocatalyst with anti-photocorrosion and visible light driven activity.
● Conductive mesh helps the separation of photogenerated carriers.
● Porous mesh structure helps the contact between pollutants and photocatalysts.
● Almost no photoactivity loss after three repeated photocatalytic tests.
*Highlights (for review)
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Abstract
Ag-doped ZnO nanorods were grown on stainless-steel wire meshes to fabricate
the hierarchical photocatalysts with excellent visible light driven activity and
anti-photocorrosion property. Effects of Ag doping and the surface structure on the
surface chemistry, surface wetting properties, absorption band shift,
photoelectrochemical response, and photocatalytic decolorization properties of the
hierarchical photocatalysts, together with the stability of photocatalytic activity for
recycled photocatalysts were investigated. Ag doping leads to red-shift in the
absorption band and increased visible light absorption. Nanorods coated wire meshes
hierarchical structure not only increases the surface area of photocatalysts but also
makes the surface hydrophilic. The photocatalytic activity enhancement and reduced
photocorrosion can be achieved because of increased surface area, enhanced
hydrophilicity, and the interaction between the metal wire/ZnO and Ag/ZnO
heterostructure interface which can improve the charge separation of photogenerated
charge carriers.
*Abstract
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Abstract
Ag-doped ZnO nanorods were grown on stainless-steel wire meshes to fabricate
the hierarchical photocatalysts with excellent visible light driven activity and
anti-photocorrosion property. Effects of Ag doping and the surface structure on the
surface chemistry, surface wetting properties, absorption band shift,
photoelectrochemical response, and photocatalytic decolorization properties of the
hierarchical photocatalysts, together with the stability of photocatalytic activity for
recycled photocatalysts were investigated. Ag doping leads to red-shift in the
absorption band and increased visible light absorption. Nanorods coated wire meshes
hierarchical structure not only increases the surface area of photocatalysts but also
makes the surface hydrophilic. The photocatalytic activity enhancement and reduced
photocorrosion can be achieved because of increased surface area, enhanced
hydrophilicity, and the interaction between the metal wire/ZnO and Ag/ZnO
heterostructure interface which can improve the charge separation of photogenerated
charge carriers.
Keywords:
Hierarchical, photocatalyst, visible-light, anti-photocorrosion, stainless-steel wire
mesh, ZnO.
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1. Introduction
ZnO can be used for many applications, such as photocatalysts [1], transparent
conducting layer [2], photoconductor [3], and gas sensors [4,5]. ZnO based
photocatalysts have attracted much attention due to their applications in
decolorization of hazardous pollutants such as dyes, chemicals, and toxic gases. Since
UV light accounts for only 3–5% of the sunlight, the wide band gap character of pure
ZnO photocatalyst limits the utilization of complete solar energy. Visible-light driven
photocatalysts have attracted much attention recently [6]. Metal doped ZnO
photocatalyst exhibited improved photocatalytic property, including Co doped ZnO
hollow microsphere [7], Ag modified ZnO nanostructures [8], and Al, Sn, and Ce
doped ZnO nanobrushes [9]. Silver can interact with visible-light by means of the
resonance of the free electrons within the particles.
Hierarchical structures with high degree of order had improved physical and
chemical properties over that of their single component [10,11]. Marban et al reported
that the use of stainless steel wire mesh-supported catalysts for the preferential
oxidation of CO [12], nitrous oxide decomposition [13], and catalytic
photodegradation of methylene blue under ultraviolet irradiation [14]. In the present
study, we try to prepare wire-mesh based hierarchical photocatalysts for the
photodegradation of dye solutions under visible light exposure.
Materials such as C60-hybridized ZnO and Ag modified ZnO have been studied
to inhibit the photocorrosion of ZnO based photocatalysts [15,16]. The goal of this
study is to develop a hierarchical photocatalyst with good visible-light driven activity
and effectively reduce the photocorrosion problem. Three-dimensional hierarchical
photocatalysts were synthesized by growing Ag-doped ZnO nanorods on
stainless-steel wire meshs through a hydrothermal process to increase the surface area,
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change the band gap, and reduce the photocorrosion problem of the photocatalysts.
The effect of surface texture and Ag doping on the surface wettability, absorption
spectra, photoluminance spectra, photocurrent, photocatalytic decolorization and
photocorrosion properties of the hierarchical photocatalysts were investigated. In this
work, the as-prepared Ag doped ZnO hierarchical photocatalysts display an efficient
photodegradation of organic dye under visible light irradiation.
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2. Experimental
2.1. Materials
Zinc acetate dihydrate and zinc nitrate 6-hydrate were provided by J. T. Baker.
Silver nitrate nonahydrate (Showa), hexamethylenetetramine (Riedel-de Haen),
methyl orange (MO) dye, and Food Black 2 (FB2) dye were used as received..
2.2. Seed Solution
For the fabrication of seed, 0.01 M zinc acetate was dissolved in the deionized
(DI) water at room temperature for 30 min. After being coated with the seed solution,
the stainless-steel nanowire mesh were dried at room temperature and then annealed
at 400 °C for 2 h to make a seed layer on the stainless-steel wire surface. The
stainless-steel wire meshes with and without the O2 plasma treatment are used for
comparison. For the O2 plasma treatment, mesh samples were exposed to oxygen
plasma (50 W) for 5 min. The diameters of the wire for M60 and M400 wire mesh are
35 and 195 µm respectively. The screen openings of the wire for M60 and M400 wire
mesh are 60 and 400 µm respectively.
2.3. Preparation of doped hierarchical photocatalyst
For synthesizing the doped ZnO nanorods decorated mesh sheets, different
amounts of silver nitrate nonahydrate were added to equimolar aqueous solutions of
zinc nitrate hydrate and hexamethylenetetramine as an Ag source to fix its
concentration at 0.01(S1), 0.015(S2) and 0.02M(S3), respectively. Doped ZnO
nanorods decorated mesh sheets was grown at 95°C by immersing the modified
stainless-steel mesh in the aqueous solution with different ZnO growth time (3, 6,and
9 h).
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2.4. Nomenclature
The samples are denoted as MwTxPSy. Mw represents that the distance between
edges of adjacent wires of stainless steel meshes is w μm. Tx means that the
hydrothermal reaction time for the growth of undoped and Ag-doped ZnO nanorods is
x h. P indicates that the wire mesh substrates is treated by O2 plasma before the
coating of the seed solution on the wire mesh substrates. Sy represents the silver
nitrate dopant precursor concentration where the precursor concentrations for S1, S2,
and S3 are 0.01, 0.015, and 0.02 M, respectively.
2.5. Characterization
The crystallite structures of the samples were investigated by X-ray Diffraction
(XRD). An MXP3 diffractometer (Mac Science) with a Cu Kα (0.154 nm) X-ray
source, a current of 40 mA, and a voltage of 40 kV was used for the XRD analysis.
Field emission scanning electron microscope (FESEM) experiments were carried out
by an energy dispersive X-ray (EDX) with a HITACH S-4800 FESEM. HRTEM
experiments were performed on a Transmission Electron Microscope (JEOL
JEM-2010). The absorbance spectra were measured by the PL 2006 multifunctional
spectrometer (Labguide Co.). The photoelectrochemical (PEC) measurements were
carried out in a glass cell with 0.2 M NaOH electrolyte solution using PC-controlled
PEC-SECM (photoelectrochemical scanning electrochemical microscopy, CHI model
900C, CHI Instruments). The wire mesh photocatalysts with surface area of 2.25 cm2
were used as the working electrode, while the Pt electrode and Ag/AgCl electrode
were used as the counter and reference electrodes, respectively.
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2.6. Photocatalytic decolorization
Experimental setup of the photocatalytic reactor was shown in Scheme 1(a). The
chemical structures of Food Black 2 dye and methyl orange dye were shown in
Scheme 1(b) and 1(c), respectively. Doped or undoped ZnO nanorods decorated mesh
sheets were added into a testing vessel with 10 mL aqueous dye solution
(concentration is 10 mg/L).The FB2 or MO solutions was continuously stirred at 25
o
C by magnetic stir bar under the visible light (214 mW/cm2
) irradiation. 3.5 mL FB2
or MO aqueous solution was taken per 30 min to monitor the absorbance spectra. The
decolorization process was monitored by the UV–Visible absorbance spectrometer
(measuring the absorbance of FB2 dye at 589 nm and MO dye at 463 nm). The
Visible light was shut off during the absorbance monitoring procedure.
After the measurement of UV–Visible absorbance, 3.5 mL FB2 or MO aqueous
solution was poured into the testing vessel. The visible light irradiation toward the dye
solution continued. The absorption spectra were recorded and the rate of
decolorization was observed in terms of change in intensity at λmax of the dyes. The
decolorization (%) can be calculated as
Decolorization (%) = (Co-C/Co) × 100 (%)
where Co is the initial concentration of dye and C is the concentration of dye
after irradiation of UV light or visible light.
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3. Results and discussion
3.1. Morphology and surface wettability
3.1.1 Surface treatment
In Fig. 1(a) and 1(b), the pristine stainless-steel M60 wire mesh exhibited a
smooth surface. In Fig 1(b), the water contact angle on M60 mesh is 132o
. The surface
of pristine stainless-steel M60 mesh is hydrophobic. A seed layer coating and a
hydrothermal rod growing process were applied on the mesh without O2 plasma
treatment. Fig. 1(c) and (d) show the FESEM image of M60T6 samples prepared
without O2 plasma treatment. The water contact angle on M60T6 mesh is 110o
. There
is nearly no nanorod grown on the wire mesh. Seed layer and nanorods cannot be
grown on the hydrophobic mesh substrate. Then, stainless-steel M60 wire mesh was
exposed to oxygen plasma (50 W) for 5 min. After the surface treatment, the mesh
substrate surface changed from hydrophobic to hydrophilic. As shown in Fig 1(e) and
1(f), the surface of the stainless steel mesh M60T6P with oxygen plasma treatment are
uniformly covered by hexagonal ZnO nanorods. The ZnO nanorods are observed
perpendicular to the surface and grow in a very high density over the entire wire mesh
substrates. The diameters of the ZnO nanorods arrays are ranging from 65 to 90 nm.
The M60T6P sample showed superhydrophilic surface. The water contact angle on
M60T6P mesh is 0o
. Lin [17] reported that the hydrophilic surface of SUS304
stainless steel can be achieved by applying the atmospheric pressure Ar/N2/O2 plasma.
The addition of small quantities of oxygen to the Ar/N2 plasma leads to the formation
of oxygen functional groups on the treated surface. As a result, the surface polarity
was enhanced and the surface energy was correspondingly increased.
The surface wettability of hierarchical M60T6P and M400T6P photocatalysts
were characterized through sequence images to investigate the effects of wire density
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(amounts of wires per unit area of photocatalyst) on the wetting behavior. The higher
the wire density of the photocatalyst is, the shorter the distance between two adjacent
wires. The distances between adjacent wires for M400 and M60 based substrates are
400 and 60 μm, respectively (Fig. 1e and 1g). Hierarchical M60T6P and M400T6P
photocatalysts exhibited different wetting properties. A drop of water was released on
to the hierarchical photocatalyst. It immediately floated into the M60T6P, adsorbing
and dispersing as soon as it contacted with the M60T6P photocatalyst (Fig. 1f). On
the other hand, when a water droplet contacted with M400T6P, it passed and adhered
to the M400T6P photocatalyst (Fig. 1h). Since the hierarchical photocatalyst is
designed for decolorization of organic dye in aqueous solution, the surface of the
photocatalyst should be hydrophilic. Then, the aqueous dye solution can contact with
the photocatalyst to achieve high photocatalytic activity. Both the M400T6P and
M60T6P series photocatalysts can be used for the decolorization of organic dye in
aqueous solution.
3.1.2 Effect of the AgNO3 precursor concentration on the morphology
Ag-doped ZnO nanorods on wire mesh hierarchical photocatalysts M60T6PS1
and M60T6PS2 were prepared with 0.01 M and 0.015 M silver nitrate precursor,
respectively. Figure 2a and 2b showed the nanorods on the M60T6PS1 and
M60T6PS2 photocatalysts are uniformly distributed, with the average nanorod
diameter of about 60 nm and 110 nm. As the silver nitrate precursor concentration
increased to 0.02M, the average nanorod diameter of hierarchical photocatalyst
M60T6PS3 became 135 nm. In addition, large aggregates (indicated by yellow arrow)
were observed locating among the nanorods of M60T6PS3 (Fig. 2c). Diameters of the
copper-doped and Ga-doped ZnO nanorod increased with increasing dopant
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concentrations [18,19]. In this study, similar trend was observed for Ag-doped ZnO.
Ashfold et al. proposed the following chemical reactions for the growth of ZnO
nanorods [20]:
(CH2)6N4 + 6 H2O 4 NH3 + 6 HCHO (1)
NH3 + H2O NH4
+
+ OH－ (2)
Zn2+
+ 2 OH－ Zn(OH)2 ZnO (s) + H2O (3)
When the concentrations of Zn2+
and OH–
ions in the growth solution exceed the
critical values, continuous aggregation of ZnO nuclei from the precipitation of
Zn(OH)2 resulted in the formation of crystallized ZnO nanorods. As shown in Eq. (3),
as the concentration of silver nitrate precursor increases, Ag+
ions can substitute the
Zn sites during the growth process. The density of ZnO nanorod arrays may decrease
because of the decrease in the heterogeneous nucleation. An increase in the average
diameter due to doping is related to the decreased density of ZnO nuclei [21,22].
3.2. X-ray diffraction patterns
Figure 3 shows the XRD patterns of undoped and Ag doped ZnO nanorods grown
on meshes with different Ag concentrations. The sharp diffraction peaks in the XRD
patterns indicate that the ZnO nanorods were highly crystallized. The ZnO (002)
peaks shown in curves of undoped M400T6P, and doped (M400T6PS1, M400T6PS2
and M400T6PS3) samples located at 34.45°, 34.41°, 34.40° and 34.38°, respectively
(Fig. 3b). Compared with undoped M400T6P photocatalyst, the slight shifts toward
smaller angle found in doped M400T6PS1, M400T6PS2 and M400T6PS3
photocatalysts are due to the increase of their lattice constants which are caused by
substitution of Zn2+
ions (ionic radius 0.74 Å) with larger Ag+
ions (ionic radius 1.15
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Å). Similar trends for the Ag doped ZnO thin films were also observed by Kong et al.
[23]. In addition, Ag(111) and Ag(200) peaks were observed in the diffraction
patterns of Ag-doped ZnO (Fig. 3a), indicating the formation of crystalline silver
clusters. M400T6PS3 prepared by the highest silver nitrate precursor concentration
among the three samples exhibited strong Ag peaks.
3.3. HRTEM and SAED patterns
Figure 4 shows the HRTEM images and SAED patterns of Ag doped ZnO
nanorod with different Ag concentrations. For the undoped ZnO nanorods (Fig. 4a),
the lattice spacing along the (002) plane is 0.260 nm. The result is consistent with the
reported one by Fan et al [24]. As shown in Fig. 4b and 4c, the lattice spacing of
M400T6PS2 and M400T6PS3 are about 0.264 nm and 0.269 nm, which are slightly
larger than that of undoped ZnO nanorod. The increasing lattice spacing of Ag doped
ZnO nanorod indicates that Ag could be doped in the ZnO lattice due to the big ionic
radius of Ag [25]. From the EDS analysis, the silver contents of Ag-doped
M400T6PS2 and M400T6PS3 photocatalysts are about 0.14% and 0.26 atom %
respectively.
Figure 3a shows the existance of silver clusters with Ag(111) and Ag(200) peaks
on the XRD spectra of Ag-doped samples. The HRTEM image of M60T6PS2 (Fig.4d)
reveals the formation of small Ag particles on the ZnO nanorods with the particle
diameter ranging from 3-7 nm. The EDS analysis of nanorods on M60T6PS3 (Fig. 4e)
shows that no Ag peak at 3.5 keV is observed. On the contrary, the Ag peak at around
3.5 keV is observed from the large aggregates among the Ag-doped ZnO nanorod of
M400T6PS3 sample (Fig. 4f). The silver content reaches 13.82%.
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3.4. XPS spectra
The O1s spectra of undoped ZnO and Ag-doped ZnO nanorods samples are shown
in Fig 5(a) and 5(b). The 530.3 eV peak belongs to the crystal lattice oxygen in ZnO,
while the 532.1 eV peak can be assigned to the hydroxyl species on the catalyst
surface [26]. The surface hydroxyl group plays an important role in the photocatalytic
process [27]. The holes generated under irradiation can oxidize other substrate or be
caught by the surface hydroxyl groups to form hydroxyl radicals. Electron–hole
recombination can be suppressed by these surface hydroxyl group. Comparing Fig. 5a
and Fig. 5b, the amount of hydroxyl group increased after incorporation of Ag dopant.
For undoped ZnO photocatalyst, the atomic ratio (peak area ratio) of oxygen from
hydroxyl group to total oxygen contributions was calculated to be 27.3%. For
Ag-doped ZnO photocatalyst, the atomic ratio of hydroxyl group (532.7 eV) to total
oxygen contributions on Ag-doped ZnO sample was 36.0%, higher than that of
undoped ZnO. The peaks at 368.2 and 374.2 eV which correspond to the Ag 3d5/2 and
Ag 3d3/2 peaks for Ag [28] were observed for the Ag-doped sample (Fig. 5c). Ag was
successfully doped into the photocatalyst. In Fig. 5c, the Ag 3d5/2 and Ag 3d3/2 peaks
of the Ag-doped hierarchical ZnO sample shifted to the lower binding energy when
compared with the standard binding energies. Yildirim et al. [29] reported that such
shift can be attributed to the interaction between Ag and ZnO crystal, which leads to
the adjustment of their Fermi level. Tunneling of the free electrons on the new Fermi
level of Ag through the empty region within the conduction band of the ZnO crystal
leads to the formation of a higher valance for Ag.
3.5. DRS and band gap
Figure 6 exhibits the diffuse reflectance spectra (DRS) of undoped ZnO and
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Ag-doped ZnO samples. The Ag-doped ZnO sample exhibits a broad absorption. The
band gap of undoped M60T6 sample is 3.26 eV. Doping not only causes red-shift in
the absorption band but also improves the absorption of the photocatalysts. Band gaps
of the M60T6PS1, M60T6PS2, and M60T6PS3 photocatalysts are 3.22, 3.16, and
3.10 eV respectively. The band gaps of the photocatalysts decreased after
incorporation of Ag dopant. Zheng et al. [30] reported that the work function of Ag
which lied between the valence band and conduction band of ZnO facilitated the light
absorption capacity of Ag/ZnO heterostructure nanocatalyst. In this study, the red
shift and increased absorption will be attributed to the increased formation rate of
electron-hole pairs on the photocatalyst surface. The Ag-doped ZnO photocatalyst can
be used under visible light irradiation. The reduction of the band-gap means that
lower energy is required for the electron-hole pair generation. It is consistent with the
XPS results (Fig.5). Applying the same energy, more hydroxyl radicals can be
generated by doped ZnO than undoped ZnO with wider band-gaps.
3.6. Chopped photocurrent-time transient response
Figure 7 shows the chopped photocurrent-time transient responses of undoped
M60T6P and three Ag-doped ZnO samples M60T6PS1, M60T6PS2, and M60T6PS3
with different dopant precursor concentrations (0.01 M, 0.015 M and 0.02 M). The
photocurrent of the Ag-doped hierarchical photocatalyst M60T6PS2 is about 7 times
that of undoped hierarchical photocatalyst M60T6P. It is reported that the transition
metal ions can improve the electron scavenging mechanism, which is attributed to
their behavior as scavengers for the photo-induced electron [31]. Thus, the separation
of photogenerated electron–hole pairs is enhanced. In this study, when more Ag ions
were doped into ZnO nanorods, the photocurrent increased due to the formation of the
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dopant energy levels between the valence band and the conduction band. At first, the
photocurrent increased with the increase of silver nitrate precursor concentration, but
declined later when the precursor concentration reached an optimum level. The
Ag-doped ZnO hierarchical photocatalyst prepared with 0.015 M precursor exhibited
the highest photocurrent. Similar influences of dopant amounts on photocurrents were
also observed by Moshfegh et al. in the Ce-doped ZnO photocatalysts [32] and Au
doped TiO2 [33]. Based on the measured transient time, when the amount of added
dopant is higher the optimum value, there will be defect scattering and/or
recombination which may cause a negative effect on the charge separation efficiency.
3.7. Photocatalytic Activity
3.7.1. Effect of wire mesh density
Figure 8a and 8b show the decolorization of 10 ppm Food Black 2 dye solution
using undoped and Ag-doped ZnO nanorods on M400 and M60 wire-mesh substrates
prepared with different silver nitrate concentration. The distances between adjacent
wires are M400 and M60 wire-mesh substrates are 400 and 60 μm, respectively. The
latter has higher wire density than the former. Comparing the decolorization of Food
Black 2 dye solution by using undoped hierarchical M60T6P and M400T6P
photocatalysts with different mesh, the photocatalytic activity of M60T6P is higher
than M400T6P under visible light irradiation. Such result is mainly due to the high
effective photocatalytic surface area of M60T6P. Besides, the photocatalytic
decolorization of all Ag-doped hierarchical ZnO photocatalysts prepared with
different silver nitrate concentrations is faster than their undoped analogs. At first, the
enhancement of decolorization rate increased with increasing silver nitrate
concentration, but declined later when the silver nitrate concentration reached an
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optimum level of 0.015 M. The M60T6PS2 photocatalyst prepared with 0.015 M
silver nitrate exhibited the fastest decolorization of FB2 dye. The decolorization
efficiency of Ag-doped hierarchical ZnO photocatalyst changes with different silver
nitrate concentration. For the Ag-doped hierarchical ZnO photocatalysts, the presence
of Ag nanoparticles on the surfaces of ZnO nanorods promoted the separation of
photoinduced electron-hole pairs and thus enhanced the photocatalytic activity. The
relatively small Ag particles on M60T6PS2 can result in more interfacial interaction
between ZnO nanorods and Ag nanoparticles which helps the transfer of
photogenerated charge carriers from ZnO. In order to achieve an efficient Ag-doped
ZnO nanorods/ stainless-steel wire mesh based hierarchical photocatalysts for
visible-light driven photocatalytic decolorization applications, controlling the particle
size of Ag is critical to optimize the interaction between Ag and ZnO nanorods. The
electron conductive silver can extend the lifetime of photogenerated electron–hole
pairs from ZnO nanorods. M60T6PS3 which has larger aggregates exhibits lower
photocatalytic activity than M60T6PS2. The concentration of silver nitrate precursor
should be controlled to prevent the formation of large aggregates.
The first-order kinetic model was introduced to compare the reaction rate
among different catalysts.
ln C = －kt + lnC0,
where k is the apparent reaction rate constant, C0 is the initial concentration, and
C is the dye concentration at time t. We assume that the dye concentration after
desorption–adsorption equilibrium is the initial concentration C0. Fig. 8c and 8d show
the kinetics of decolorization of 10 ppm Food Black 2 dye solution using undoped and
Ag-doped ZnO nanorods on M400 and M60 meshes. The rate constants k are found to
be 0.00967, 0.01877, 0.02789, 0.01292 min-1
for pure M60T6P, M60T6PS1,
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M60T6PS2, M60T6PS3, respectively. Therefore, incorporating the dopant can
improve the activity of photocatalyst. M60T6PS2 exhibits the largest k. Similar trend
was observed for the M400T6P based photocatalysts. For the doped photocatalysts,
new impurity levels were introduced between the conduction and valence band when
Zn2+
was replaced by Ag+
in ZnO. The electrons can be promoted from the valence
band to these impurity levels. More photogenerated electrons and holes can be
induced to participate in the photocatalytic reactions.
3.7.2. Effect of dye structure and concentration
Figure 9a shows the decolorization of different dye solutions (FB2 and MO
dyes) by M60T6PS2 photocatalyst. The decolorization of 10 ppm MO dye by
M60T6PS2 can be completed within 60 min under visible light irradiation. The
decolorization of the MO dye occurs faster on the photocatalyst Ag-doped ZnO than
the FB2 dye. The MO dye and FB2 dye are monoazo and diazo dyes, respectively.
Monoazo dyes are easier oxidized than diazo dyes which may in turn be easier than
triazo dyes [34,35]. That may explain why the decolorization of the MO dye occurs
faster on the photocatalyst Ag-doped ZnO in comparison with the FB2 dye. The
mechanism of azo dye degradation by photocatalyst by hydroxyl or superoxides
radicals has been reported by Konstantinou [34]. Figure 9b shows the decolorization
of MO solutions with different dye concentration by M60T6PS2 photocatalyst. Using
fixed amount of photocatalyst, the dye decolorization rate gets slower as the MO dye
content increases.
3.7.3. Effect of pH
The effect of pH on the photocatalytic activity of the M60T6PS2 photocatalysts was
shown in Fig. 9c. pH is an important parameter governing the rate of photocatalytic
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decoloration, since it affects the surface-charge-properties of the photocatalysts [36].
As shown in Fig. 9c, the decolorization efficiency of MO decreased significantly
when pH became 9. Pan [37] reported that the isoelectric point (where the
zetapotential is 0) occurs at acid range. When pH was higher than point of zero charge,
the ZnO surface became negatively charged. Since MO dye contained negatively
charged sulfonate groups, thus, the electrostatic repulsion between the catalyst surface
and the dye cations increases. It resulted in a strong adsorption of the dye cations on
the ZnO surface. That may explain why the decolorization rate declined at higher pH.
3.8. Photocatalytic decoloration mechanism
To investigate the decolorization mechanism of the Ag-doped hierarchical
photocatalysts, FB2 was degraded by M60T6PS2 under visible light irradiation in the
presence of different radical scavengers (Fig. 10). The scavengers used in our work
were iso-butanol for hydroxyl radical scavenging [38] and 1,4-benzoquinone for
superoxide radical scavenging. The FB2 dye can be hardly degraded when iso-butanol
is added as a hydroxyl radical scavenger. The decomposition of FB2 is through redox
reaction by hydroxyl radicals or superoxide radicals. In addition, the presence of 0.05
mM of 1,4-benzoquinone reduced the photocatalytic activity of FB2. The 1,
4–benzoquinone can inhibit the produced O2 radicals, hence reduce the amounts of O2
radicals available for decolorization of FB2. The hydroxyl radicals and superoxide
radicals play an important role for the decolorization of FB2 dye.
A schematic reaction mechanism for photocatalytic decolorization of organic
dyes by the Ag-doped ZnO nanorods/stainless-steel wire mesh photocatalyst is
proposed in Scheme 2. Under visible light irradiation, the electron–hole pairs are
generated when the photocatalysts catched photons with energy equal to or higher
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than the energy band gap of the photocatalyst. The photocatalytic efficiency will
decrease if the electron-hole pairs cannot be separated effectively. Electric conductive
substrates such as indium tin oxide and copper plates [39,40] play an active role in the
catalytic process by favoring separation of photogenerated electron–hole pairs. In this
study, the conductive stainless-steel wire mesh has two functions in the catalytic
process. The first is enhancing the separation of photogenerated electron–hole pairs.
The second is that the aqueous dye solution can wet and pass through the wire-mesh
based hierarchical photocatalysts, as shown in Fig. 1f and 1h. That is impossible when
the zinc oxide is grown on nonporous conductive supports such as ITO glass or
copper plates. The excellent electron conductivity and hydrophilic porous structure of
ZnO nanorods decorated stainless-steel wire mesh helps not only the transfer of
photogenerated electrons from ZnO to the wire mesh, but also the contact between
dye molecule and the photocatalysts. The photogenerated electrons react with O2 or
oxygen species to produce superoxide anion radicals (•O2
−
) and the photogenerated
holes react with water molecules to produce hydroxyl radicals (•OH). These radicals
can decompose organic compounds such as FB2 dye.
3.9. Photocatalyst recycling and photostability
Repeated photocatalytic decolorization of MO dye by recycled photocatalysts
during three tests (Fig. 11) was conducted to evaluate the influences of conductive
materials (Ag nanoparticles and stainless-steel wire mesh) and doping on the
photocorrosion of photocatalysts. The photocorrosion will lead to photoactivity loss
during the recycled experiments. During the three repeated photocatalytic
decolorization experiments for recycled photocatalysts, the Ag-doped ZnO nanorods
coated stainless-steel wire mesh photocatalyst M60T6PS2 shows almost no
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photoactivity loss. However, the photocorrosion phenomenon is still observed for
undoped ZnO nanorods decorated stainless-steel wire mesh photocatalyst M60T6P.
There is about 8% photocatalytic activity loss for M60T6P photocatalyst after three
recycled experiments. The photogenerated electrons from ZnO nanorods can transfer
to the stainless-steel wire mesh or Ag nanoparticles through the interfacial interaction
between the stainless-steel/ZnO and Ag/ZnO interface. It can enhance the carrier
lifetime and reduce the recombination of electron–hole pairs, which is evidenced by
the above-mentioned photoelectrochemical analysis (Fig. 7). It enhances the
photocatalytic activity and reduces the photocorrosion problem. ZnO nanorods were
grown on the nonconductive glass substrate to make ZnOT6S2 photocatalyst which
was used as a control sample. There is about 33% photocatalytic activity loss for
ZnOT6S2 photocatalyst (ZnO nanorods on glass) after three recycled experiments.
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4. Conclusion
Ag-doped ZnO nanorods coated stainless-steel wire meshes can act as efficient
visible-light driven hierarchical photocatalysts with high activity and stability.
Nanorods coated wire meshes hierarchical structure not only increases the surface
area of photocatalysts but also changes the surface from hydrophobic (CA=132o
) to
superhydrophilic (CA=0o
). The aqueous dye solution can contact with the
photocatalyst to achieve good photocatalytic activity. Ag doping enhances red-shift in
the absorption band and improves the visible light absorption capacity. Besides,
introducing certain amount of Ag precursor leads to the formation of Ag/ZnO
heterostructure. The enhanced photocatalytic activity and reduced photocorrosion of
the hierarchical photocatalysts can be achieved because of the interfacial interaction
between the stainless-steel/ZnO and Ag/ZnO heterostructure interface which can help
the transfer of photogenerated charge carriers from ZnO under visible light irradiation.
The decolorization of 10 ppm MO dye by M60T6PS2 completed within 60 min under
visible light irradiation. During the three repeated photocatalytic decolorization
experiments for recycled photocatalysts, the Ag-doped ZnO nanorods coated
stainless-steel wire mesh photocatalyst M60T6PS2 shows almost no photoactivity
loss. After being rinsed with water, these hierarchical photocatalysts can be recycled
and repeatedly utilized.
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Acknowledgements
The authors would like to thank the financial support from the National Science
Council under the contract of NSC102-2221-E-035-090. The authors appreciate the
Precision Instrument Support Center of Feng Chia University in providing the
measurement facilities.
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Figure caption
Scheme 1. (a) Experimental setup of the photocatalytic reactor, and chemical
structures of (b) Food Black 2 dye (c) methyl orange dye.
Scheme 2. Proposed schematic mechanism for the photocatalytic decolorization of
organic dye by the hierarchical photocatalyst
Figure 1. (a) FESEM image and (b) contact angle and enlarged SEM view of pristine
stainless-steel wire mesh M60; (c) FESEM image and (d) contact angle and
enlarged SEM view of M60T6 samples prepared without O2 plasma
treatment; (e) FESEM image and (f) contact angle and enlarged SEM view of
ZnO nanorods on wire mesh with O2 plasma treatment (M60T6P) (g)
FESEM image and (h) contact angle and enlarged SEM view of ZnO
nanorods on wire mesh with O2 plasma treatment (M400T6P).
Figure 2. Ag-doped ZnO nanorods on wire mesh hierarchical photocatalysts (a)
M60T6PS1 (b) M60T6PS2 (c) M60T6PS3 prepared with different dopant
precursor concentration.
Figure 3. (a) X-ray diffraction patterns of undoped ZnO, and Ag-doped ZnO nanorods
coated stainless-steel wire meshs with different silver nitrate precursor
concentrations (*: peaks related to stainless steel) (b) Enlarged X-ray
diffraction patterns of (002) peak.
Figure 4. HRTEM images and SAED patterns of Ag-doped ZnO nanorods on
M60T6PS1, M60T6PS2, M60T6PS3 samples with different Ag
concentrations (a) 0 M (b) 0.015 M (c) 0.02 M respectively, (d) HRTEM
image of nanorod on M60T6PS2, and the EDS analysis of (e) nanorods on
M60T6PS3 and (f) the large aggregate beside nanorods of M60T6PS3.
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Figure 5. XPS O1s spectra of (a) undoped ZnO and (b) Ag-doped ZnO sample (S2).
The Ag3d XPS spectra for Ag-doped ZnO sample (S2) is shown in (c).
Figure 6. The diffuse reflectance spectra of the undoped ZnO and Ag-doped ZnO
photocatalysts.
Figure 7. Chopped current-time transient response of undoped M60T6P photocatalyst
and Ag-doped hierarchical M60T6PS1, M60T6PS2, and M60T6PS3
photocatalysts prepared with 0.01, 0.015 and 0.02 M silver nitrate precursor.
Figure 8. The decolorization of 10 ppm Food Black 2 dye solution using undoped and
Ag-doped ZnO nanorods on (a) M400 (b) M60 meshes prepared with
different silver nitrate concentration (0.01, 0.015, and 0.02 M), kinetics of
decolorization of 10 ppm Food Black 2 dye solution using undoped and
Ag-doped ZnO nanorods on (c) M400 (d) M60 meshes.
Figure 9. (a) decolorization of MO and FB2 dye solutions (10 ppm), decolorization of
MO dye solution (b) with different dye concentration (10, 20, and 30 ppm) (c)
at different pH = 5, 7, 9 (MO dye, 10 ppm) by M60T6PS2 photocatalyst
under visible light irradiation.
Figure 10. The photoegradation of 10ppm Food Black 2 with different radical
scavengers by M60T6PS2 photocatalyst under visible light irradiation.
Figure 11. Repeated photocatalytic decolorization of MO dye by various kinds of
recycled photocatalysts during three tests (pH = 7).
