Accepted Manuscript

Title: Ag-doped ZnO nanorods coated metal wire meshes ashierarchical photocatalysts with high visible-light drivenphotoactivity and photostability

Author: Mu-Hsiang Hsu Chi-Jung Chang

PII: S0304-3894(14)00495-6DOI: http://dx.doi.org/doi:10.1016/j.jhazmat.2014.06.038Reference: HAZMAT 16047

To appear in: Journal of Hazardous Materials

Received date: 19-3-2014Revised date: 10-6-2014Accepted date: 11-6-2014

Please cite this article as: M.-H. Hsu, C.-J. Chang, Ag-doped ZnO nanorodscoated metal wire meshes as hierarchical photocatalysts with high visible-lightdriven 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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Ag-doped ZnO nanorods coated metal wire meshes as hierarchical

photocatalysts with high visible-light driven photoactivity and photostability

Mu-Hsiang Hsu, Chi-Jung Chang*

Department of Chemical Engineering, Feng Chia University,

100, Wenhwa Road, Seatwen, Taichung 40724, Taiwan, ROC

*E-mail: changcj@fcu.edu.tw

Tel: 886-4-24517250 ext 3678

Fax: 886-4-24510890

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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

oC by magnetic stir bar under the visible light (214 mW/cm

2) 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).

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Figure 1

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Figure 2

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Figure 3

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Figure 4

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Figure 5

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Figure 6

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Figure 7

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Figure 8

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Figure 9

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Figure 10

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Figure 11

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Scheme 1

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Scheme 2