Journal of Hazardous Materials 278 (2014) 444–453

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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

h i g h l i g h t s

g r a p h i c a l

a b s t r a c t

• 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.

a r t i c l e

i n f o

Article history: Received 19 March 2014 Received in revised form 10 June 2014 Accepted 11 June 2014 Available online 23 June 2014 Keywords: Hierarchical Photocatalyst Visible-light Anti-photocorrosion Stainless-steel wire mesh ZnO

a b s t r a c t 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. © 2014 Elsevier B.V. All rights reserved.

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

∗ Corresponding author. Tel.: +886 4 24517250x3678; fax: +886 4 24510890. E-mail address: [email protected] (C.-J. Chang). http://dx.doi.org/10.1016/j.jhazmat.2014.06.038 0304-3894/© 2014 Elsevier B.V. All rights reserved.

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

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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, 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. 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 ␮m, respectively. The screen openings of the wire for M60 and M400 wire mesh are 60 and 400 ␮m, 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 0.02 M(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). 2.4. Nomenclature The samples are denoted as MwTxPSy. Mw represents that the distance between edges of adjacent wires of stainless steel meshes

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Scheme 1. (a) Experimental setup of the photocatalytic reactor, and chemical structures of (b) Food Black 2 dye (c) methyl orange dye.

is w ␮m. 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 K␣ (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. 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 (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 ◦ 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–vis absorbance spectrometer (measuring the absorbance of FB2 dye at 589 nm

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Fig. 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).

and MO dye at 463 nm). The visible light was shut off during the absorbance monitoring procedure. After the measurement of UV–vis 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 max of the dyes. The decolorization (%) can be calculated as decolorization (%) =

C0 − C × 100 (%) C0

where C0 is the initial concentration of dye and C is the concentration of dye after irradiation of UV light or visible light.

3. Results and discussion 3.1. Morphology and surface wettability 3.1.1. Surface treatment In Fig. 1(a) and (b), the pristine stainless-steel M60 wire mesh exhibited a smooth surface. In Fig. 1(b), the water contact angle on M60 mesh is 132◦ . 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) shows the FESEM image of M60T6 samples prepared without O2 plasma treatment. The water contact angle on M60T6 mesh is 110◦ . There is nearly no nanorod grown on the wire mesh. Seed layer and nanorods cannot be grown on the

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Fig. 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 (0 0 2) peak.

Fig. 2. Ag-doped ZnO nanorods on wire mesh hierarchical photocatalysts (a) M60T6PS1 (b) M60T6PS2 (c) M60T6PS3 prepared with different dopant precursor concentration.

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 (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 0◦ . 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 (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 ␮m, respectively (Fig. 1e and g). 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. Fig. 2a and b 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 0.02 M, 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 concentrations [18,19]. In this study, similar trend was observed for Ag-doped ZnO. Ashfold et al.

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Fig. 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.

proposed the following chemical reactions for the growth of ZnO nanorods [20]: (CH2 )6 N4 + 6H2 O ↔ 4NH3 + 6HCHO

(1)

NH3 + H2 O ↔ NH4 + + OH−

(2)

Zn2+ + 2OH− → Zn(OH)2 → ZnO(s) + H2 O

(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 Fig. 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 (0 0 2) 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 sub˚ with larger Ag+ ions stitution of Zn2+ ions (ionic radius 0.74 A) ˚ (ionic radius 1.15 A). Similar trends for the Ag doped ZnO thin films were also observed by Kong et al. [23]. In addition, Ag(1 1 1) and Ag(2 0 0) 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

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precursor concentration among the three samples exhibited strong Ag peaks. 3.3. HRTEM and SAED patterns Fig. 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 (0 0 2) plane is 0.260 nm. The result is consistent with the reported one by Fan et al. [24]. As shown in Fig. 4b and c, 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. Fig. 3a shows the existence of silver clusters with Ag(1 1 1) and Ag(2 0 0) 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 to 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%. 3.4. XPS spectra The O1s spectra of undoped ZnO and Ag-doped ZnO nanorods samples are shown in Fig. 5a and 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 b, 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 Agdoped 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.

Fig. 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).

[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

3.5. DRS and band gap Fig. 6 exhibits the diffuse reflectance spectra (DRS) of undoped ZnO and 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.

Fig. 6. The diffuse reflectance spectra of the undoped ZnO and Ag-doped ZnO photocatalysts.

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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 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 Cedoped 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. Fig. 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.

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 Fig. 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

3.7. Photocatalytic activity 3.7.1. Effect of wire mesh density Fig. 8a and b shows 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 ␮m, 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 optimum level of 0.015 M. The M60T6PS2

Fig. 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.

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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 Agdoped 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 + ln C 0 , 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 d shows 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, 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 Fig. 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]. Fig. 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 decoloration, since it affects the surface-charge-properties of the photocatalysts [36]. As shown

Fig. 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.

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

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Fig. 10. The photodegradation of 10 ppm Food Black 2 with different radical scavengers by M60T6PS2 photocatalyst under visible light irradiation.

Fig. 11. Repeated photocatalytic decolorization of MO dye by various kinds of recycled photocatalysts during three tests (pH = 7).

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 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 h. 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 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. 4. Conclusion

Scheme 2. Proposed schematic mechanism for the photocatalytic decolorization of organic dye by the hierarchical photocatalyst.

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 = 132◦ ) to superhydrophilic (CA = 0◦ ). 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

M.-H. Hsu, C.-J. Chang / Journal of Hazardous Materials 278 (2014) 444–453

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. Acknowledgements The authors would like to thank the financial support from the National Science Council Taiwan under the contract of NSC1022221-E-035-090. The authors appreciate the Precision Instrument Support Center of Feng Chia University in providing the measurement facilities. References [1] H.B. Fu, S.C. Zhang, T.G. Xu, Y.F. Zhu, J.M. Chen, Photocatalytic degradation of RhB by fluorinated Bi2 WO6 and distributions of the intermediate products, Environ. Sci. Technol. 42 (2008) 2085–2091. [2] C.Y. Tsay, K.S. Fan, Y.W. Wang, C.J. Chang, Y.K. Tseng, C.K. Lin, Transparent semiconductor zinc oxide thin films deposited on glass substrates by sol–gel process, Ceram. Int. 36 (2010) 1791–1795. [3] C.J. Chang, M.H. Tsai, Y.H. Hsu, C.S. Tuan, Morphology and optoelectronic property of ZnO rod array/conjugated polymer hybrid films, Thin Solid Films 516 (2008) 5523–5526. [4] P. Rai, Y.S. Kim, H.M. Song, M.K. Song, Y.T. Yu, The role of gold catalyst on the sensing behavior of ZnO nanorods for CO and NO2 gases, Sens. Actuators, B: Chem. 165 (2012) 133–142. [5] C.J. Chang, S.T. Hung, C.K. Lin, C.Y. Chen, E.H. Kuo, Selective growth of ZnO nanorods for gas sensors using ink-jet printing and hydrothermal processes, Thin Solid Films 519 (2010) 1693–1698. [6] M.S. Zhu, P.L. Chen, M.H. Liu, Graphene oxide enwrapped Ag/AgX (X = Br, Cl) nanocomposite as a highly efficient visible-light plasmonic photocatalyst, ASC Nano 5 (2011) 4529–4536. [7] Y.C. Qiu, W. Chen, S. Yang, B. Zhang, X.X. Zhang, Y.C. Zhong, Hierarchical hollow spheres of ZnO and Zn1−x Cox O: directed assembly and room-temperature ferromagnetism, Cryst. Growth Des. 10 (2010) 177–183. [8] Z.G. Xiong, J.Z. Ma, W.J. Ng, T.D. Waite, X.S. Zhao, Silver-modified mesoporous TiO2 photocatalyst for water purification, Water Res. 45 (2011) 2095–2103. [9] C.Y. Wang, C.P. Liu, H.W. Shen, Y.J. Chen, C.L. Kuo, T.Y. Wang, Growth and valence excitations of ZnO:M(Al, In, Sn) hierarchical nanostructures, J. Phys. Chem. C 114 (2010) 18031–18036. [10] C.J. Chang, M.H. Hsu, Y.C. Weng, C.Y. Tsay, C.K. Lin, Hierarchical ZnO nanorodarray films with enhanced photocatalytic performance, Thin Solid Films 528 (2013) 167–174. [11] S.T. Hung, C.J. Chang, M.H. Hsu, Improved photocatalytic performance of ZnO nanograss decorated pore-array films by surface texture modification and silver nanoparticle deposition, J. Hazard. Mater. 198 (2011) 307–316. [12] G. Marban, I. Lopez, T. Valdes-Solis, A.B. Fuertes, Highly active structured catalyst made up of mesoporous Co3 O4 nanowires supported on a metal wire mesh for the preferential oxidation of CO, Int. J. Hydrogen Energy 33 (2008) 6687–6695. [13] L. del Rio, G. Marban, Stainless steel wire mesh-supported potassium-doped cobalt oxide catalysts for the catalytic decomposition of nitrous oxide, Appl. Catal., B: Environ. 126 (2012) 39–46. [14] T.T. Vu, L. del Rio, T. Valdes-Solis, G. Marban, Stainless steel wire meshsupported ZnO for the catalytic photodegradation of methylene blue under ultraviolet irradiation, J. Hazard. Mater. 246–247 (2013) 126–134. [15] H. Fu, T. Xu, S. Zhu, Y. Zhu, Photocorrosion inhibition and enhancement of photocatalytic activity for ZnO via hybridization with C60, Environ. Sci. Technol. 42 (2008) 8064–8069.

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Ag-doped ZnO nanorods coated metal wire meshes as hierarchical photocatalysts with high visible-light driven photoactivity and photostability.

Ag-doped ZnO nanorods were grown on stainless-steel wire meshes to fabricate the hierarchical photocatalysts with excellent visible light driven activ...
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