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Electrodeposition of flexible stainless steel mesh supported ZnO nanorod arrays with enhanced photocatalytic performance
Ceramics International 43 (2017) 6460–6466
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Ceramics International journal homepage: www.elsevier.com/locate/ceramint
Electrodeposition of flexible stainless steel mesh supported ZnO nanorod arrays with enhanced photocatalytic performance
MARK
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Xiaofei Wanga, Hui Lub, Wenwu Liua, Min Guoa, , Mei Zhanga a b
School of Metallurgical and Ecological Engineering, University of Science and Technology Beijing, Beijing 100083, PR China School of Materials Science and Engineering, Beifang University of Nationalities, Yinchuan, Ningxia 750021, PR China
A R T I C L E I N F O
A BS T RAC T
Keywords: ZnO nanorod arrays Electrodeposition Al-doped ZnO films Substrate pre-treatment Photocatalytic degradation Renewable photocatalytic ability
Large scale well oriented ZnO nanorod arrays (ZNRAs) were electrodeposited on flexible stainless steel mesh (SSM) substrate pre-treated by Al doped ZnO (AZO) seed layers. The effects of substrate pre-treatment conditions such as Al doping and spin coating times of the colloid on the morphology characteristics and photocatalytic properties of as-prepared ZNRAs were systematically studied. The results showed that by introducing Al into ZnO colloid solution, well aligned ZNRAs with relatively higher specific surface area (higher growth density and smaller rod diameter) could be obtained on the premodified SSM substrate. In addition, increasing spin coating times of AZO colloid solution would decrease the average diameter of ZNRAs. Under the optimum preparing conditions, the formed flexible SSM supported ZNRAs exihibited enhanced photocatalytic performance of 93.42% and remarkable photocatalytic stability under the UV-lamp for degradation of Rhodamine B.
1. Introduction Contaminations of ground water systems by organic chemicals pose a serious environmental threat. Nanostructured metal oxide semiconductor photocatalyst has been widely used in dealing with such environmental problems due to its suitable energy band gap, high specific surface area and structure stability [1–3]. Although TiO2 has been thoroughly investigated as an effective photocatalyst [4–6], ZnO has also been considered as a suitable alternative because of its simple fabrication process, lower production cost and various novel structures [7,8]. In some cases, ZnO may exhibit a better efficiency than TiO2 in photo-catalytic degradation for some dye [9–11]. However, ZnO nanoparticles aggregate easily to decrease surface specific area during photocatalytic process, more seriously, it is difficult to be separated from waste water, thus resulting in photocatalyst loss and secondary pollution [12,13]. Moreover, the depth of penetration of UV light is limited because of strong absorption by both catalyst particles and dissolved dyes [14]. To overcome the disadvantages, the substrate supported photocatalyst with oriented nanostructure (e. g. nanorod/ nanowire arrays) may improve the recovery rate of the catalyst, and more importantly, the support may interact with the catalyst to inhibit the electron-hole recombination, thus enhancing the photocatalytic activity [15]. Stainless steel mesh (SSM) with double-faced structure for high catalyst loading, is considered as a desirable support due to its
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light weight, good flexibility and low cost. Till now, various methods have been used for synthesis of ZnO with different morphologies on SSM [16–20]. However, to date, electrodeposition of oriented ZnO nanorod/wire arrays on the flexible SSM is scarcely reported. Ren et al. [21] synthesized hierarchical ZnO/Cu2O nanorods films on steel mesh substrates via electrodeposition route. The nanocomposites showed photocatalytic performance of 64.3% for photodegradation of MO solution under visible-light irradiation. Lu et al. [22] fabricated ZnO nanorod arrays (ZNRAs) on flexible SSM by using electrodeposition method. The effects of electrochemical parameters on the orientation, morphology and density of ZNRAs were systematically investigated. The photodegradation efficiency of ZNRAs improved from 89.4% to 98.3% with deposition times from one to six times under the UV-lamp for Rhodamine B. However, the problems of tedious operation procedures, rigorous control over reaction conditions still existed, which were unfavorable for low-cost and large-scale production. It is well known that a photocatalytic reaction occurs at the interface between catalyst and organic pollutants, so the efficiency of photocatalyst should strongly depend on the surface structure. More importantly, a larger surface area benefits the photocatalytic activity as it provide enough active sites to allows more organic substances to be attached to the surface of the photocatalyst and the smaller size of nanomaterials can also decrease the diffuse reflection of light, thus improving the utilization of light. Notwithstanding the improvement,
Corresponding author. E-mail address: [email protected] (M. Guo).
http://dx.doi.org/10.1016/j.ceramint.2017.02.061 Received 25 December 2016; Received in revised form 6 February 2017; Accepted 14 February 2017 Available online 16 February 2017 0272-8842/ © 2017 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
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Fig. 1. Top view SEM images of ZNRAs electrodeposited on SSM substrates under different pre-treatments: (a) unmodified SSM substrate, (b) pre-coated with 0.5 mol dm−3 ZnO colloid solution for three times, then annealed at 300 °C. (c) pre-coated with 0.5 mol dm−3 Al doped ZnO colloid for three times, then annealed at 300 °C. Insets: side view SEM images. (other preparing conditions: −1.0 V, 1800 s, T=80 °C, CZn2+=0.0005 mol dm−3, three deposition times).
photocatalytic enhancement have also been discussed. Additionally, the optimum sample has been used as a probe reaction to evaluate the photocatalytic performance of the ZNRAs for degradation of rhodamine B.
Table 1 The influence of substrate pre-treatment on average growth density, diameter and length of as-electrodeposited ZNRAs. Sample ZNRAs ZNRAs-3ZnO ZNRAs-3AZO
Diameter (nm) 115 ± 5 81 ± 5 63 ± 5
Density (rod cm−2) 9
(2.42 ± 0.21)×10 (4.22 ± 0.21)×109 (6.72 ± 0.21)×109
Length (µm) 0.94 ± 0.1 1.00 ± 0.1 1.34 ± 0.1
2. Experimental section 2.1. Materials All chemicals were of analytical reagent grade and used without further purification, and all aqueous solutions were prepared using deionized water. The SSM which was tailored into rectangular shape with dimensions of 1 cm×2.5 cm was used as substrate. The SSM was firstly put into diluted hydrochloric acid (0.0001 mol dm−3) for 10 s to remove the rust, then it was cleaned and degreased successively with ultrasonic in acetone and absolute ethyl alcohol for 10 min respectively, finally rinsed with deionized water.
further increasing specific surface area of ZNRAs by a simple way is still of great necessity. Given that the morphology characteristics and loading capacity of ZNRAs strongly depend on the conditions of the underlying seed layers, substrate pre-treatment by seed layers can be an effective way to solve the problem. Nayeri et al. [23] prepared the ZnO nanowires on an indium tin oxide (ITO) coated glass using sputter-deposited aluminum doped zinc oxide (AZO) and ZnO seed layers through chemical bath deposition process. The results showed that the density of the nanowires grown on AZO thin film was 8 times larger, and the average diameter was 50% smaller in comparison with the growth on ZnO film. Song et al. [24] investigated the effect of seed layer on the growth of ZnO nanorods during hydrothermal synthesis. It was found that the smaller crystal size of the seed layers facilitated a higher surface area of the corresponding ZnO nanorods. Although there are a few reports available, which deal with the structural and physical properties of AZO films, the thickness-dependent change in the growth characteristics and photocatalytic activity of the resulting ZnO NRs evolving from AZO seed layers is scarcely reported. In addition, few efforts have focused on the recyclability of SSM supported ZNRAs photocatalyst till now [25,26]. In this paper, we reported a facile electrodeposition approach to fabricate large-area single crystalline ZNRAs photocatalyst on flexible SSM pre-modified by ZnO and AZO seed layers. The effects of certain seeding and growth parameters on the resulting morphology and properties of ZNRAs, specifically size, density, length and alignment, have been systematically studied and the influencing factors for the
2.2. SSM substrate pre-treatment 2.2.1. Preparation of ZnO seed layers on SSM substrate The ZnO films were prepared by the sol-gel spin coating method. Zinc acetate dihydrate [Zn(CH3COO)2·2H2O] as a starting material were used. 2-Methoxyethanol (C3H8O2) and monoethanolamine (MEA) were used as a solvent and stabilizer, respectively. The molar ratios of MEA to [Zn(CH3COO)2·2H2O] were maintained at 1.0 under a constant 0.5 mol dm−3 of zinc acetate. The accurate amount of materials was added to the solution, which was stirred at 60 °C for 3 h to yield a clear and homogeneous solution. The solutions were then transformed onto the substrate using a spin coater which was rotated at two steps (1000 rpm for 5 s, 3000 rpm for 30 s) simultaneously. For each layer, the sample was preheated at 300 °C for 10 min to evaporate the solvent and remove organic residuals. The coating procedure was repeated from one to three times in order to get a film thickness of approximately 150 nm and the as-prepared ZNRAs on SSM pre-coated
Fig. 2. Low magnification top view SEM images of SSM substrates under different pre-treatments: (a) unmodified SSM substrate, (b) pre-coated with 0.5 mol dm−3 ZnO colloid solution for three times, then annealed at 300 °C. (c) pre-coated with 0.5 mol dm−3 Al doping ZnO colloid for three times, then annealed at 300 °C. Inset: high magnification top view SEM images of SSM substrates.
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Fig. 3. Top view SEM images of ZNRAs electrodeposited on SSM substrates pre-coated with different layers of AZO films: (a) 0.5 mol dm−3 AZO colloid solution for one times, then annealed at 300 °C. (b) three times, then annealed at 300 °C. (c) five times, then annealed at 300 °C. Insets: side view SEM images. (other preparing conditions: −1.0 V, 1800 s, T=80 °C, CZn2+=0.0005 mol dm−3, three deposition times).
was maintained at −1.0 V for 1800 s.
Table 2 Influence of spin coating times of AZO seed layers on average growth density, diameter and length of as-electrodeposited ZNRAs. Sample ZNRAs-1AZO ZNRAs-3AZO ZNRAs-5AZO
Diameter (nm) 92 ± 6 57 ± 6 47 ± 6
Density (rod cm−2) 9
(3.53 ± 0.20)×10 (7.77 ± 0.20)×109 (8.90 ± 0.20)×109
Length (µm) 0.92 ± 0.1 1.29 ± 0.1 1.00 ± 0.1
O2+2H2O+4e-→4OH-
(1)
Zn2++xOH-↔Zn(OH)x2−x
(2)
Zn(OH)x2−x↔ZnO+H2O+(x-2)OH-
(3)
During the electrodeposition process, hydroxide ions which were firstly generated at the surface of SSM by reduction of oxygen (reaction (1)), can react with Zn2+ ions in the solution to form Zn(OH)x2−x (reaction (2)), then, Zn(OH)x2−x was spontaneously dehydrated into ZnO particles on the working electrode with the temperature over 34 °C (reaction (3)).
with ZnO seed layers for three times were named as ZNRAs-3ZnO. 2.2.2. Preparation of Al-doped ZnO seed layers on SSM substrate In the synthesis of Al-doped ZnO (AZO) colloid solution process, aluminum nitrate nonahydrate (Al(NO3)3·9H2O) was added into the above-formed solution as a dopant source. The molar ratios of MEA to (Al(NO3)3·9H2O) was maintained at 1.0 and the doping concentrations was 1 at%. The remaining steps for preparing AZO seed layers on SSM substrate are similar to the process of ZnO seed layers preparation. The as-prepared ZNRAs on SSM pre-coated with AZO seed layers for one to five times were named as ZNRAs-1AZO, ZNRAs-3AZO and ZNRAs5AZO, repectively.
2.4. Characterization and analysis The surface morphologies of as-prepared ZNRAs were characterized by a field emission scanning electron microscope (FESEM, Zeiss Supra-55, operated at 10 kV). The ultraviolet and visible (UV–vis) absorption spectrum was measured by Doublebeam UV–vis spectrophotometer (Beijing Purkinje General Instrument Co., Ltd., TU-1901) using BaSO4 as background and creating the baseline from 230 to 900 nm.
2.3. Preparation of ZNRAs on pre-modified SSM substrate The precursor solutions for fabrication of WNRAs were The electrodeposition of ZnO was performed with an electrochemical analytical instrument (CHI760C) in a three electrode system with the SSM pre-coated by seed films as the working electrode (cathode), a Pt spiral wire as the counter electrode and a saturated calomel electrode (SCE) as the reference electrode. In the text, all of the potentials were quoted versus SCE. The electrolyte was an aqueous solution of ZnCl2 (0.0005 mol dm−3) used as the Zn2+ precursor and KCl (0.1 mol dm−3) served as a supporting electrolyte, saturated with bubbling oxygen 10 min before and during the experiment. The electrochemical cell was held at 80 °C and the applied potential
2.5. Photocatalysis tests The photocatalytic activity was investigated using an RhB aqueous solution as a probe and a 350 mL quartz beaker as the photo reactor. The reaction system containing 60 mL of RhB solution with an initial concentration of 5.0×10−5 mol dm−3 and three pieces supported catalysts (1 cm×2.5 cm strips) of ZnO samples was magnetically stirred in the dark for 30 min to reach adsorption equilibrium. The solution was then exposed to UV irradiation from a 500 W high-pressure Hg lamp at room temperature. Solutions were collected every 30 min to measure the RhB degradation by UV spectrophotometer.
Fig. 4. Low magnification top view SEM images of SSM substrates pre-coated with different layers of AZO films: (a) 0.5 mol dm−3 Al doping ZnO colloid solution for one times, then annealed at 300 °C. (b) three times, then annealed at 300 °C. (c) five times, then annealed at 300 °C. Inset: high magnification top view SEM images of SSM substrates.
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Fig. 5. (a) UV–visible spectra of ZNRAs electrodeposited on SSM pre-coated with AZO seed layers for none, one and three times, then annealed at 300 °C for 10 min and (b) (ɑhγ)2 is plotted as a function of hγ from which the band gap energy is obtained by Tauc plot (Preparation conditions: −1.0 V, 1800 s, T=80 °C, CZn2+=0.0005 mol dm−3, three deposition times).
Fig. 6. Absorbance spectra of Rhodamine B aqueous solutions after UV irritation in the presence of ZNRAs during different UV irritation times on SSM (a) pre-coated with 0.5 mol dm−3 Al-doped ZnO colloid for three times, then annealed at 300 °C (b) one times, then annealed at 300 °C (c) unmodified SSM substrate (d) Degradation efficiencies of Rhodamine B after UV irritation in the presence of ZNRAs on SSM pre-coated by AZO seed layers for different times. (Preparation conditions: −1.0 V, 1800 s, T=80 °C, CZn2+=0.0005 mol dm−3, three deposition times).
3. Results and discussion
The variation of RhB concentration with irradiation time was measured using a UV spectrophotometer. The intensity of the absorption band peak (553 nm) was recorded at a certain time interval. The degradation rate was estimated as C/C0, where C0 (mol dm−3) was the equilibrium concentration before UV irradiation and C (mol dm−3) was the concentration at the sampling time. According to the Beer-Lambert law, the concentration of RhB was linearly proportional to the absorbance value (A, a.u.) at 553 nm, thus C/C0=A/A0.
3.1. Effect of substrate pre-treatment on preparation of ZNRAs Fig. 1 showed the SEM images of ZNRAs electrodeposited on SSM substrates under different pre-treatments. It can be seen that except in the case of ZnO NRAs on bare SSM, the verticality of the NRs was reasonably good, with the best case in the NRs evolving from 3AZO seed layers. No matter which pre-coated SSM was used as substrate, the growth density and length of as-prepared ZnO nanorods increased
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length of as-electrodeposited ZNRAs were summed up in detail in Table 1. When the SSM substrate was modified with Al doped ZnO seed layers, ZNRAs with relatively higher density, smaller rods diameter and well orientation vertical to substrates could be obtained, which was benefit for being used as photocatalyst to degrade waste water. All these phenomena could be well explained by the following reasons: When depositing ZnO rods on bare SSM which has a lower nucleation density, the lateral growth can not be effectively suppressed, resulting in larger-diameter and smaller-size rods. When depositing ZnO rods on pre-modified substrates, large numbers of ZnO nucleus caused high consumption rate for precursors, thus decreasing the growth rate on single rod and resulting in high density and small size of the ZnO rods. Fig. 2 gave the SEM images of bare and pre-treated SSM substrates. Obviously, by introducing Al into the seed layer, the formed film became more smooth and the average size of nanoparticles became smaller compared with ZnO seed layers, further confirming that substrate pretreatment played a key role in determining the morphology of the obtained ZnO nanorods.
Fig. 7. Photocatalytic kinetics of the ZNRAs on SSM pre-coated with different AZO seed layers for Rhodamine B photodegradation (Preparation conditions: −1.0 V, 1800 s, T=80 °C, CZn2+=0.0005 mol dm−3, three deposition times).
3.2. Effect of spin coated times on preparation of ZNRAs Fig. 3 illustrated the SEM images of as-prepared ZNRAs electrodeposited on SSM substrates spin coated with 0.5 mol dm−3 AZO colloid solution for once, three and five times. And the average diameter, growth density and length were summed up in Table 2. It can be seen that well aligned ZNRAs vertical to the substrates were obtained on all the pre-treated SSM substrates. However, when the substrate was spin coated once, the orientation of the ZNRAs was worse than that of the other two. With the spin-coated times increasing from one to five, the growth density and length of as-prepared ZnO nanorods increased from (3.53 ± 0.20)×109 to (8.90 ± 0.20)×109 rod cm−2, 0.92 ± 0.1 to 1.29 ± 0.1 µm, respectively, while the size of the nanorods decreased from 92 ± 0.6 to 47 ± 0.6 nm, which demonstrated that more spin-coated times, namely, thicker AZO seed layer on SSM substrates, was beneficial for electrodeposition of ZNRAs with higher specific surface area. The SEM images of the substrates spin coated with 0.5 mol dm−3 AZO colloid solution for different times at 300 °C were shown in Fig. 4. From Fig. 4 insets, it is indicated that the spin coating times had a great influence on the surface characteristics of the AZO seeds. The size of the crystal decreased and the density multiplied as the spin coating times increased. More importantly, it was clear that the ZNRAs morphology characteristics variations were on account of a seed layers effect which played the role of a template for the ZNRAs growth. Therefore, it can be concluded that the decrease in the diameter and increase in the density of the ZnO NRAs with increasing spin coating times was due to the smaller crystal size and well dispersibility of the AZO seed layer, which led to an increase in the total surface area of the nanorods. Apparently, the well orientation of nanorods might be caused by the nanorods’ higher density. In order to investigate the effect of the spin coating times of AZO seed layers on the optical properties of ZNRAs, UV–vis–NIR spectrophotometer was used to characterize the optical properties. Fig. 5(a) gave the UV–vis absorption spectra of ZNRAs on different seed layers. As expected, ZnO showed the characteristic spectrum with its fundamental absorption sharp edge rising at 400 nm, while the ZNRAs on AZO seed layers exhibited a red-shift in the absorption edge and ZNRAs evolving from 3AZO seed layers absorbed highest in the whole ultraviolet and visible region. It is also well known that ZnO is a direct band-gap material and the energy gap (Eg) can thus be estimated by assuming direct transition between conduction band and valance bands. Theory of optical absorption gives the relationship between the absorption coefficients ɑ and the photon energy hγ for direct allowed transition as (ɑhγ)2=A(hγ-Eg) where A is a function of the index of refraction and hole/electron effective masses [27,28]. Therefore, the band gap energy of different ZNRAs can be measured by extrapolating the linear portion of the plots of (ɑhγ)2 vs hγ, as
Fig. 8. UV–vis absorbance spectra of dyes solution desorbed from the corresponding sensitized ZNRAs on SSM pre-coated with different AZO seed layers (Preparation conditions: −1.0 V, 1800 s, T=80 °C, CZn2+=0.0005 mol dm−3, three deposition times).
Fig. 9. Cyclic photodegradation of rhodamine B solutions in the presence of the ZNRAs on SSM pre-coated with 0.5 mol dm−3 AZO colloid for three times, then annealed at 300 °C for three cycles (Preparation conditions: −1.0 V, 1800 s, T=80 °C, CZn2+=0.0005 mol dm−3, three deposition times).
from (2.42 ± 0.21)×109 to (6.72 ± 0.21)×109 rods cm−2, 0.94 ± 0.1 to 1.34 ± 0.1 µm, respectively, while the size of the nanorods decreased from 115 ± 5 to 63 ± 5 nm, suggesting that substrate pretreatment played an important role in determining the size, density and length of the obtained ZnO nanorods. The general characteristics of the influence of substrate pre-treatment on average growth density, diameter and 6464
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Fig. 10. Low magnification top SEM views of ZNRAs on SSM pre-coated with 0.5 mol dm−3 AZO colloid for three times, then annealed at 300 °C after 3 recycle experiments for degradation of RhB (a) initial photodegradation experiment (b) first cycle of the photodegradation experiment. (c) second cycle of the photodegradation experiment (d) third cycle of the photodegradation experiment. Insets: high magnification top view SEM images of ZNRAs on SSM substrates (Preparation conditions: −1.0 V, 1800 s, T=80 °C, CZn2+=0.0005 mol dm−3, three deposition times).
fied SSM led to the decreases in the dye adsorption and then to the poor photocatalytic performance. Similarly, as shown in Fig. 6(d), the absorption of rhodamine B for the samples in the dark decreased along with the layer thickness of AZO films reducing could affirm this conclusion. Furthermore, with the existence of the problem of poor adhesion between the ZNRAs and SSM substrate due to the interfacial tensile stress during calcination, there was considerable chance that the ZNRAs cracked or peeled away from the bare substrate. Thus, the efficient surface areas decreased, resulting in the decrease in the photodegradation efficiency. The ZnO product obtained by pre-spreading film between the nanorods arrays and the substrate could avoid such unwanted troubles due to the enhanced adhesion ability and stability, such as chemical stability against dissolution and mechanical stability against breakage and detachment of nanorods under the stresses, thus enhancing the photocatalytic activity to some degree.
depicted in Fig. 5(b). The band gap energies of these ZNRAs were estimated to be around 3.08 eV, 3.00 eV and 3.00 eV on SSM precoated with bare, one and three AZO seed layers respectively. The narrowed band gap of ZNRAs-3AZO was probably ascribed to the chemical bonding between ZNRAs and AZO seed layers, which was similar to the previous findings obtained for ZNRAs-RGO composite materials [29]. In general, it was noticeable that there was an obvious correlation between the introduction of AZO seed layers and the corresponding UV–vis spectrum change of ZNRAs. The incorporation of AZO seed layers which enhanced the light absorption and narrowed the band gap of the tailored ZNRAs was expected to improve the photocatalytic activity of ZNRAs evolving from them. 3.3. Evaluation of the photocatalytic activities of ZNRAs on SSM premodified with AZO seed layers A series of experiments were conducted to see if the AZO seed layer thickness had significant influence on the application of the nanorod arrays for which they are designed, e.g. photocatalysis. Fig. 6 showed the UV–vis absorption spectra of the aqueous solutions of RhB (initial concentration 5.0×10−5 mol dm−3, 60 mL) with different ZnO samples as photocatalysts and exposure to ultraviolet light for various durations. The characteristic absorption of RhB at 553 nm decreased rapidly with extension of the exposure time. It can be seen that 93.42% of RhB was degraded by the ZNRAs-3AZO for 2 h under UV irradiation, in contrast, 82.34% and 81.81% of RhB were removed by the ZNRAs-1AZO and ZNRAs, respectively. Obviously, the ZNRAs3AZO exhibited the best photocatalytic activity. As shown in Fig. 7, the exponential decay profile of the plot between C/C0 (where C0 and C are the initial and actual concentration of RhB at time t, respectively) versus time “t” suggested that the photodecomposition reaction followed first-order rate law and the rate constant was calculated to be 0.02055, 0.01527 and 0.01340 min−1 for ZNRAs on SSM pre-coated with three, one and bare AZO seed layers, repectively. It was clear that the ZNRAs on bare SSM showed a much slower degradation rate of RhB (k=0.01340 min−1) compared to that of our as-synthesized ZNRAs-3AZO catalyst (k=0.02055 min−1). The photocatalytic degradation mechanism of ZnO photocatalyst has been widely studied and can be illustrated as follows: Generally, when ZnO nanocrystals are irradiated by UV light, electron-hole pair is generated on the surfaces of ZnO. Holes can be trapped by H2O to form highly reactive hydroxyl radicals which are powerful oxidants to decompose RhB molecules. Therefore, in the photocatalytic system, photo-induced molecular transformation or reaction takes place at the surface of the catalyst. The large surface area of the catalyst provides enough active reaction sites to adsorb the dyes and offers more possibilities for diffusion and mass transportation of RhB molecules. In order to compare specific surface area of different samples, the dye adsorbed amounts of them were estimated. As shown in Fig. 8, the dye adsorption of ZnO nanorods increased from 0.4×10−9 to 0.6×10−9 mol cm−2 with increasing AZO colloid spin coating times. Obviously, relatively low specific surface areas for ZNRAs on unmodi-
3.4. Renewable photocatalytic behavior of ZNRAs on SSM pre-coated with three AZO seed layers To evaluate whether the prepared ZNRAs on SSM is stable enough for effective photocatalysis, its use as a recyclable photocatalyst was further studied for three recycles under the same measurement conditions. After the former photodegradation experiment finished, the substrates were rinsed and dried to wipe off superfluous dye molecules and again immersed into fresh solutions with same concentrations for another cycle of the photodegradation experiment. The results, given in Fig. 9, demonstrated that these ZNRAs could still maintain relatively high photocatalytic activity. After 3 cycles, a slight decline in the degradation rate was observed due to a possible loss of some nanorods during the substrate rinse as illustrated in Fig. 10. More importantly, it should be noted that the ZNRAs after three photodegradation cycles did not show noticeable change in morphologies (Fig. 10), indicating that ZNRAs on AZO seed layers were not photocorroded and highly stable even after recycles.
4. Conclusion In this work, we reported a facile electrodeposition approach to fabricate large-area well-oriented ZNRAs photocatalyst on flexible SSM pre-modified by ZnO and AZO seed layers. The results showed that the morphology of ZNRAs was strongly affected by the type and crystal size of pre-coated seed layer, and well aligned ZNRAs with relatively higher specific surface area, in virtue of the introduction of Al into ZnO seed layers, could be obtained on the premodified SSM substrate. Under the optimum preparing conditions, the as-prepared ZNRAs exihibited enhanced photocatalytic performance of 93.42% and good photocatalytic stability under the UV-lamp for degradation of Rhodamine B. The recovery of the photocatalysts immobilized on substrates is compatible with industrial feasibility in practical use and environmental protection. 6465
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Acknowledgements The work was financially supported by the National Natural Science Foundation of China (Nos. 51572020 and 51272025), and the National Basic Research Program of China (Nos. 2014CB643401, 2013AA032003). References [1] B. İkizler, S.M. Peker, Synthesis of TiO2 coated ZnO nanorod arrays and their stability in photocatalytic flow reactors, Thin Solid Films 605 (2016) 232–242. [2] M. Samadi, M. Zirak, A. Naseri, E. Khorashadizade, A.Z. Moshfegh, Recent progress on doped ZnO nanostructures for visible-light photocatalysis, Thin Solid Films 605 (2016) 2–19. [3] D. Chen, D. Wang, Q. Ge, G. Ping, M. Fan, L. Qin, L. Bai, C. Lv, K. Shu, Graphenewrapped ZnO nanospheres as a photocatalyst for high performance photocatalysis, Thin Solid Films 574 (2015) 1–9. [4] C.S. Chua, O.K. Tan, S.T. Man, X. Ding, Photocatalytic activity of tin-doped TiO2 film deposited via aerosol assisted chemical vapor deposition, Thin Solid Films 544 (2013) 571–575. [5] S.A. Darko, E. Maxwell, S. Park, Photocatalytic activity of TiO2 nanofilms deposited onto polyvinyl chloride and glass substrates, Thin Solid Films 519 (2010) 174–177. [6] F. He, J. Li, T. Li, G. Li, Solvothermal synthesis of mesoporous TiO2: the effect of morphology, size and calcination progress on photocatalytic activity in the degradation of gaseous benzene, Chem. Eng. J. 237 (2014) 312–321. [7] A. Sapkota, A.J. Anceno, S. Baruah, O.V. Shipin, J. Dutta, Zinc oxide nanorod mediated visible light photoinactivation of model microbes in water, Nanotechnology 22 (2011) 215703–215709. [8] B. Neppolian, S. Sakthivel, B. Arabindoo, M. Palanichamy, V. Murugesan, Degradation of textile dye by solar light using TiO and ZnO photocatalysts, J. Environ. Sci. Health Part A Toxic./Hazard. Subst. Environ. Eng. 34 (1999) 1829–1838. [9] Bircan Dindar, Si. Içli, Unusual photoreactivity of zinc oxide irradiated by concentrated sunlight, J. Photochem. Photobiol. A Chem. 140 (2001) 263–268. [10] A.A. Khodja, T. Sehili, J.F. Pilichowski, P. Boule, Photocatalytic degradation of 2phenylphenol on TiO2 and ZnO in aqueous suspensions, J. Photochem. Photobiol. A Chem. 141 (2001) 231–239. [11] S. Sakthivel, B. Neppolian, M. Palanichamy, Banumathi Arabindoo, V. Murugesan, Photocatalytic degradation of leather dye, Acid green 16 using ZnO in the slurry and thin film forms, Indian J. Chem. Technol. 6 (1999) 161–165. [12] S. Baruah, M.A. Mahmood, M.T.Z. Myint, T. Bora, J. Dutta, Enhanced visible light photocatalysis through fast crystallization of zinc oxide nanorods, Beilstein J. Nanotechnol. 1 (2010) 14–20. [13] Z. Liu, Y.E. Miao, M. Liu, Q. Ding, W.W. Tjiu, X. Cui, T. Liu, Flexible polyanilinecoated TiO2/SiO2 nanofiber membranes with enhanced visible-light photocatalytic
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Electrodeposition of flexible stainless steel mesh supported ZnO nanorod arrays with enhanced photocatalytic performance
MARK
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Xiaofei Wanga, Hui Lub, Wenwu Liua, Min Guoa, , Mei Zhanga a b
School of Metallurgical and Ecological Engineering, University of Science and Technology Beijing, Beijing 100083, PR China School of Materials Science and Engineering, Beifang University of Nationalities, Yinchuan, Ningxia 750021, PR China
A R T I C L E I N F O
A BS T RAC T
Keywords: ZnO nanorod arrays Electrodeposition Al-doped ZnO films Substrate pre-treatment Photocatalytic degradation Renewable photocatalytic ability
Large scale well oriented ZnO nanorod arrays (ZNRAs) were electrodeposited on flexible stainless steel mesh (SSM) substrate pre-treated by Al doped ZnO (AZO) seed layers. The effects of substrate pre-treatment conditions such as Al doping and spin coating times of the colloid on the morphology characteristics and photocatalytic properties of as-prepared ZNRAs were systematically studied. The results showed that by introducing Al into ZnO colloid solution, well aligned ZNRAs with relatively higher specific surface area (higher growth density and smaller rod diameter) could be obtained on the premodified SSM substrate. In addition, increasing spin coating times of AZO colloid solution would decrease the average diameter of ZNRAs. Under the optimum preparing conditions, the formed flexible SSM supported ZNRAs exihibited enhanced photocatalytic performance of 93.42% and remarkable photocatalytic stability under the UV-lamp for degradation of Rhodamine B.
1. Introduction Contaminations of ground water systems by organic chemicals pose a serious environmental threat. Nanostructured metal oxide semiconductor photocatalyst has been widely used in dealing with such environmental problems due to its suitable energy band gap, high specific surface area and structure stability [1–3]. Although TiO2 has been thoroughly investigated as an effective photocatalyst [4–6], ZnO has also been considered as a suitable alternative because of its simple fabrication process, lower production cost and various novel structures [7,8]. In some cases, ZnO may exhibit a better efficiency than TiO2 in photo-catalytic degradation for some dye [9–11]. However, ZnO nanoparticles aggregate easily to decrease surface specific area during photocatalytic process, more seriously, it is difficult to be separated from waste water, thus resulting in photocatalyst loss and secondary pollution [12,13]. Moreover, the depth of penetration of UV light is limited because of strong absorption by both catalyst particles and dissolved dyes [14]. To overcome the disadvantages, the substrate supported photocatalyst with oriented nanostructure (e. g. nanorod/ nanowire arrays) may improve the recovery rate of the catalyst, and more importantly, the support may interact with the catalyst to inhibit the electron-hole recombination, thus enhancing the photocatalytic activity [15]. Stainless steel mesh (SSM) with double-faced structure for high catalyst loading, is considered as a desirable support due to its
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light weight, good flexibility and low cost. Till now, various methods have been used for synthesis of ZnO with different morphologies on SSM [16–20]. However, to date, electrodeposition of oriented ZnO nanorod/wire arrays on the flexible SSM is scarcely reported. Ren et al. [21] synthesized hierarchical ZnO/Cu2O nanorods films on steel mesh substrates via electrodeposition route. The nanocomposites showed photocatalytic performance of 64.3% for photodegradation of MO solution under visible-light irradiation. Lu et al. [22] fabricated ZnO nanorod arrays (ZNRAs) on flexible SSM by using electrodeposition method. The effects of electrochemical parameters on the orientation, morphology and density of ZNRAs were systematically investigated. The photodegradation efficiency of ZNRAs improved from 89.4% to 98.3% with deposition times from one to six times under the UV-lamp for Rhodamine B. However, the problems of tedious operation procedures, rigorous control over reaction conditions still existed, which were unfavorable for low-cost and large-scale production. It is well known that a photocatalytic reaction occurs at the interface between catalyst and organic pollutants, so the efficiency of photocatalyst should strongly depend on the surface structure. More importantly, a larger surface area benefits the photocatalytic activity as it provide enough active sites to allows more organic substances to be attached to the surface of the photocatalyst and the smaller size of nanomaterials can also decrease the diffuse reflection of light, thus improving the utilization of light. Notwithstanding the improvement,
Corresponding author. E-mail address: [email protected] (M. Guo).
http://dx.doi.org/10.1016/j.ceramint.2017.02.061 Received 25 December 2016; Received in revised form 6 February 2017; Accepted 14 February 2017 Available online 16 February 2017 0272-8842/ © 2017 Elsevier Ltd and Techna Group S.r.l. All rights reserved.
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Fig. 1. Top view SEM images of ZNRAs electrodeposited on SSM substrates under different pre-treatments: (a) unmodified SSM substrate, (b) pre-coated with 0.5 mol dm−3 ZnO colloid solution for three times, then annealed at 300 °C. (c) pre-coated with 0.5 mol dm−3 Al doped ZnO colloid for three times, then annealed at 300 °C. Insets: side view SEM images. (other preparing conditions: −1.0 V, 1800 s, T=80 °C, CZn2+=0.0005 mol dm−3, three deposition times).
photocatalytic enhancement have also been discussed. Additionally, the optimum sample has been used as a probe reaction to evaluate the photocatalytic performance of the ZNRAs for degradation of rhodamine B.
Table 1 The influence of substrate pre-treatment on average growth density, diameter and length of as-electrodeposited ZNRAs. Sample ZNRAs ZNRAs-3ZnO ZNRAs-3AZO
Diameter (nm) 115 ± 5 81 ± 5 63 ± 5
Density (rod cm−2) 9
(2.42 ± 0.21)×10 (4.22 ± 0.21)×109 (6.72 ± 0.21)×109
Length (µm) 0.94 ± 0.1 1.00 ± 0.1 1.34 ± 0.1
2. Experimental section 2.1. Materials All chemicals were of analytical reagent grade and used without further purification, and all aqueous solutions were prepared using deionized water. The SSM which was tailored into rectangular shape with dimensions of 1 cm×2.5 cm was used as substrate. The SSM was firstly put into diluted hydrochloric acid (0.0001 mol dm−3) for 10 s to remove the rust, then it was cleaned and degreased successively with ultrasonic in acetone and absolute ethyl alcohol for 10 min respectively, finally rinsed with deionized water.
further increasing specific surface area of ZNRAs by a simple way is still of great necessity. Given that the morphology characteristics and loading capacity of ZNRAs strongly depend on the conditions of the underlying seed layers, substrate pre-treatment by seed layers can be an effective way to solve the problem. Nayeri et al. [23] prepared the ZnO nanowires on an indium tin oxide (ITO) coated glass using sputter-deposited aluminum doped zinc oxide (AZO) and ZnO seed layers through chemical bath deposition process. The results showed that the density of the nanowires grown on AZO thin film was 8 times larger, and the average diameter was 50% smaller in comparison with the growth on ZnO film. Song et al. [24] investigated the effect of seed layer on the growth of ZnO nanorods during hydrothermal synthesis. It was found that the smaller crystal size of the seed layers facilitated a higher surface area of the corresponding ZnO nanorods. Although there are a few reports available, which deal with the structural and physical properties of AZO films, the thickness-dependent change in the growth characteristics and photocatalytic activity of the resulting ZnO NRs evolving from AZO seed layers is scarcely reported. In addition, few efforts have focused on the recyclability of SSM supported ZNRAs photocatalyst till now [25,26]. In this paper, we reported a facile electrodeposition approach to fabricate large-area single crystalline ZNRAs photocatalyst on flexible SSM pre-modified by ZnO and AZO seed layers. The effects of certain seeding and growth parameters on the resulting morphology and properties of ZNRAs, specifically size, density, length and alignment, have been systematically studied and the influencing factors for the
2.2. SSM substrate pre-treatment 2.2.1. Preparation of ZnO seed layers on SSM substrate The ZnO films were prepared by the sol-gel spin coating method. Zinc acetate dihydrate [Zn(CH3COO)2·2H2O] as a starting material were used. 2-Methoxyethanol (C3H8O2) and monoethanolamine (MEA) were used as a solvent and stabilizer, respectively. The molar ratios of MEA to [Zn(CH3COO)2·2H2O] were maintained at 1.0 under a constant 0.5 mol dm−3 of zinc acetate. The accurate amount of materials was added to the solution, which was stirred at 60 °C for 3 h to yield a clear and homogeneous solution. The solutions were then transformed onto the substrate using a spin coater which was rotated at two steps (1000 rpm for 5 s, 3000 rpm for 30 s) simultaneously. For each layer, the sample was preheated at 300 °C for 10 min to evaporate the solvent and remove organic residuals. The coating procedure was repeated from one to three times in order to get a film thickness of approximately 150 nm and the as-prepared ZNRAs on SSM pre-coated
Fig. 2. Low magnification top view SEM images of SSM substrates under different pre-treatments: (a) unmodified SSM substrate, (b) pre-coated with 0.5 mol dm−3 ZnO colloid solution for three times, then annealed at 300 °C. (c) pre-coated with 0.5 mol dm−3 Al doping ZnO colloid for three times, then annealed at 300 °C. Inset: high magnification top view SEM images of SSM substrates.
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Fig. 3. Top view SEM images of ZNRAs electrodeposited on SSM substrates pre-coated with different layers of AZO films: (a) 0.5 mol dm−3 AZO colloid solution for one times, then annealed at 300 °C. (b) three times, then annealed at 300 °C. (c) five times, then annealed at 300 °C. Insets: side view SEM images. (other preparing conditions: −1.0 V, 1800 s, T=80 °C, CZn2+=0.0005 mol dm−3, three deposition times).
was maintained at −1.0 V for 1800 s.
Table 2 Influence of spin coating times of AZO seed layers on average growth density, diameter and length of as-electrodeposited ZNRAs. Sample ZNRAs-1AZO ZNRAs-3AZO ZNRAs-5AZO
Diameter (nm) 92 ± 6 57 ± 6 47 ± 6
Density (rod cm−2) 9
(3.53 ± 0.20)×10 (7.77 ± 0.20)×109 (8.90 ± 0.20)×109
Length (µm) 0.92 ± 0.1 1.29 ± 0.1 1.00 ± 0.1
O2+2H2O+4e-→4OH-
(1)
Zn2++xOH-↔Zn(OH)x2−x
(2)
Zn(OH)x2−x↔ZnO+H2O+(x-2)OH-
(3)
During the electrodeposition process, hydroxide ions which were firstly generated at the surface of SSM by reduction of oxygen (reaction (1)), can react with Zn2+ ions in the solution to form Zn(OH)x2−x (reaction (2)), then, Zn(OH)x2−x was spontaneously dehydrated into ZnO particles on the working electrode with the temperature over 34 °C (reaction (3)).
with ZnO seed layers for three times were named as ZNRAs-3ZnO. 2.2.2. Preparation of Al-doped ZnO seed layers on SSM substrate In the synthesis of Al-doped ZnO (AZO) colloid solution process, aluminum nitrate nonahydrate (Al(NO3)3·9H2O) was added into the above-formed solution as a dopant source. The molar ratios of MEA to (Al(NO3)3·9H2O) was maintained at 1.0 and the doping concentrations was 1 at%. The remaining steps for preparing AZO seed layers on SSM substrate are similar to the process of ZnO seed layers preparation. The as-prepared ZNRAs on SSM pre-coated with AZO seed layers for one to five times were named as ZNRAs-1AZO, ZNRAs-3AZO and ZNRAs5AZO, repectively.
2.4. Characterization and analysis The surface morphologies of as-prepared ZNRAs were characterized by a field emission scanning electron microscope (FESEM, Zeiss Supra-55, operated at 10 kV). The ultraviolet and visible (UV–vis) absorption spectrum was measured by Doublebeam UV–vis spectrophotometer (Beijing Purkinje General Instrument Co., Ltd., TU-1901) using BaSO4 as background and creating the baseline from 230 to 900 nm.
2.3. Preparation of ZNRAs on pre-modified SSM substrate The precursor solutions for fabrication of WNRAs were The electrodeposition of ZnO was performed with an electrochemical analytical instrument (CHI760C) in a three electrode system with the SSM pre-coated by seed films as the working electrode (cathode), a Pt spiral wire as the counter electrode and a saturated calomel electrode (SCE) as the reference electrode. In the text, all of the potentials were quoted versus SCE. The electrolyte was an aqueous solution of ZnCl2 (0.0005 mol dm−3) used as the Zn2+ precursor and KCl (0.1 mol dm−3) served as a supporting electrolyte, saturated with bubbling oxygen 10 min before and during the experiment. The electrochemical cell was held at 80 °C and the applied potential
2.5. Photocatalysis tests The photocatalytic activity was investigated using an RhB aqueous solution as a probe and a 350 mL quartz beaker as the photo reactor. The reaction system containing 60 mL of RhB solution with an initial concentration of 5.0×10−5 mol dm−3 and three pieces supported catalysts (1 cm×2.5 cm strips) of ZnO samples was magnetically stirred in the dark for 30 min to reach adsorption equilibrium. The solution was then exposed to UV irradiation from a 500 W high-pressure Hg lamp at room temperature. Solutions were collected every 30 min to measure the RhB degradation by UV spectrophotometer.
Fig. 4. Low magnification top view SEM images of SSM substrates pre-coated with different layers of AZO films: (a) 0.5 mol dm−3 Al doping ZnO colloid solution for one times, then annealed at 300 °C. (b) three times, then annealed at 300 °C. (c) five times, then annealed at 300 °C. Inset: high magnification top view SEM images of SSM substrates.
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Fig. 5. (a) UV–visible spectra of ZNRAs electrodeposited on SSM pre-coated with AZO seed layers for none, one and three times, then annealed at 300 °C for 10 min and (b) (ɑhγ)2 is plotted as a function of hγ from which the band gap energy is obtained by Tauc plot (Preparation conditions: −1.0 V, 1800 s, T=80 °C, CZn2+=0.0005 mol dm−3, three deposition times).
Fig. 6. Absorbance spectra of Rhodamine B aqueous solutions after UV irritation in the presence of ZNRAs during different UV irritation times on SSM (a) pre-coated with 0.5 mol dm−3 Al-doped ZnO colloid for three times, then annealed at 300 °C (b) one times, then annealed at 300 °C (c) unmodified SSM substrate (d) Degradation efficiencies of Rhodamine B after UV irritation in the presence of ZNRAs on SSM pre-coated by AZO seed layers for different times. (Preparation conditions: −1.0 V, 1800 s, T=80 °C, CZn2+=0.0005 mol dm−3, three deposition times).
3. Results and discussion
The variation of RhB concentration with irradiation time was measured using a UV spectrophotometer. The intensity of the absorption band peak (553 nm) was recorded at a certain time interval. The degradation rate was estimated as C/C0, where C0 (mol dm−3) was the equilibrium concentration before UV irradiation and C (mol dm−3) was the concentration at the sampling time. According to the Beer-Lambert law, the concentration of RhB was linearly proportional to the absorbance value (A, a.u.) at 553 nm, thus C/C0=A/A0.
3.1. Effect of substrate pre-treatment on preparation of ZNRAs Fig. 1 showed the SEM images of ZNRAs electrodeposited on SSM substrates under different pre-treatments. It can be seen that except in the case of ZnO NRAs on bare SSM, the verticality of the NRs was reasonably good, with the best case in the NRs evolving from 3AZO seed layers. No matter which pre-coated SSM was used as substrate, the growth density and length of as-prepared ZnO nanorods increased
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length of as-electrodeposited ZNRAs were summed up in detail in Table 1. When the SSM substrate was modified with Al doped ZnO seed layers, ZNRAs with relatively higher density, smaller rods diameter and well orientation vertical to substrates could be obtained, which was benefit for being used as photocatalyst to degrade waste water. All these phenomena could be well explained by the following reasons: When depositing ZnO rods on bare SSM which has a lower nucleation density, the lateral growth can not be effectively suppressed, resulting in larger-diameter and smaller-size rods. When depositing ZnO rods on pre-modified substrates, large numbers of ZnO nucleus caused high consumption rate for precursors, thus decreasing the growth rate on single rod and resulting in high density and small size of the ZnO rods. Fig. 2 gave the SEM images of bare and pre-treated SSM substrates. Obviously, by introducing Al into the seed layer, the formed film became more smooth and the average size of nanoparticles became smaller compared with ZnO seed layers, further confirming that substrate pretreatment played a key role in determining the morphology of the obtained ZnO nanorods.
Fig. 7. Photocatalytic kinetics of the ZNRAs on SSM pre-coated with different AZO seed layers for Rhodamine B photodegradation (Preparation conditions: −1.0 V, 1800 s, T=80 °C, CZn2+=0.0005 mol dm−3, three deposition times).
3.2. Effect of spin coated times on preparation of ZNRAs Fig. 3 illustrated the SEM images of as-prepared ZNRAs electrodeposited on SSM substrates spin coated with 0.5 mol dm−3 AZO colloid solution for once, three and five times. And the average diameter, growth density and length were summed up in Table 2. It can be seen that well aligned ZNRAs vertical to the substrates were obtained on all the pre-treated SSM substrates. However, when the substrate was spin coated once, the orientation of the ZNRAs was worse than that of the other two. With the spin-coated times increasing from one to five, the growth density and length of as-prepared ZnO nanorods increased from (3.53 ± 0.20)×109 to (8.90 ± 0.20)×109 rod cm−2, 0.92 ± 0.1 to 1.29 ± 0.1 µm, respectively, while the size of the nanorods decreased from 92 ± 0.6 to 47 ± 0.6 nm, which demonstrated that more spin-coated times, namely, thicker AZO seed layer on SSM substrates, was beneficial for electrodeposition of ZNRAs with higher specific surface area. The SEM images of the substrates spin coated with 0.5 mol dm−3 AZO colloid solution for different times at 300 °C were shown in Fig. 4. From Fig. 4 insets, it is indicated that the spin coating times had a great influence on the surface characteristics of the AZO seeds. The size of the crystal decreased and the density multiplied as the spin coating times increased. More importantly, it was clear that the ZNRAs morphology characteristics variations were on account of a seed layers effect which played the role of a template for the ZNRAs growth. Therefore, it can be concluded that the decrease in the diameter and increase in the density of the ZnO NRAs with increasing spin coating times was due to the smaller crystal size and well dispersibility of the AZO seed layer, which led to an increase in the total surface area of the nanorods. Apparently, the well orientation of nanorods might be caused by the nanorods’ higher density. In order to investigate the effect of the spin coating times of AZO seed layers on the optical properties of ZNRAs, UV–vis–NIR spectrophotometer was used to characterize the optical properties. Fig. 5(a) gave the UV–vis absorption spectra of ZNRAs on different seed layers. As expected, ZnO showed the characteristic spectrum with its fundamental absorption sharp edge rising at 400 nm, while the ZNRAs on AZO seed layers exhibited a red-shift in the absorption edge and ZNRAs evolving from 3AZO seed layers absorbed highest in the whole ultraviolet and visible region. It is also well known that ZnO is a direct band-gap material and the energy gap (Eg) can thus be estimated by assuming direct transition between conduction band and valance bands. Theory of optical absorption gives the relationship between the absorption coefficients ɑ and the photon energy hγ for direct allowed transition as (ɑhγ)2=A(hγ-Eg) where A is a function of the index of refraction and hole/electron effective masses [27,28]. Therefore, the band gap energy of different ZNRAs can be measured by extrapolating the linear portion of the plots of (ɑhγ)2 vs hγ, as
Fig. 8. UV–vis absorbance spectra of dyes solution desorbed from the corresponding sensitized ZNRAs on SSM pre-coated with different AZO seed layers (Preparation conditions: −1.0 V, 1800 s, T=80 °C, CZn2+=0.0005 mol dm−3, three deposition times).
Fig. 9. Cyclic photodegradation of rhodamine B solutions in the presence of the ZNRAs on SSM pre-coated with 0.5 mol dm−3 AZO colloid for three times, then annealed at 300 °C for three cycles (Preparation conditions: −1.0 V, 1800 s, T=80 °C, CZn2+=0.0005 mol dm−3, three deposition times).
from (2.42 ± 0.21)×109 to (6.72 ± 0.21)×109 rods cm−2, 0.94 ± 0.1 to 1.34 ± 0.1 µm, respectively, while the size of the nanorods decreased from 115 ± 5 to 63 ± 5 nm, suggesting that substrate pretreatment played an important role in determining the size, density and length of the obtained ZnO nanorods. The general characteristics of the influence of substrate pre-treatment on average growth density, diameter and 6464
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Fig. 10. Low magnification top SEM views of ZNRAs on SSM pre-coated with 0.5 mol dm−3 AZO colloid for three times, then annealed at 300 °C after 3 recycle experiments for degradation of RhB (a) initial photodegradation experiment (b) first cycle of the photodegradation experiment. (c) second cycle of the photodegradation experiment (d) third cycle of the photodegradation experiment. Insets: high magnification top view SEM images of ZNRAs on SSM substrates (Preparation conditions: −1.0 V, 1800 s, T=80 °C, CZn2+=0.0005 mol dm−3, three deposition times).
fied SSM led to the decreases in the dye adsorption and then to the poor photocatalytic performance. Similarly, as shown in Fig. 6(d), the absorption of rhodamine B for the samples in the dark decreased along with the layer thickness of AZO films reducing could affirm this conclusion. Furthermore, with the existence of the problem of poor adhesion between the ZNRAs and SSM substrate due to the interfacial tensile stress during calcination, there was considerable chance that the ZNRAs cracked or peeled away from the bare substrate. Thus, the efficient surface areas decreased, resulting in the decrease in the photodegradation efficiency. The ZnO product obtained by pre-spreading film between the nanorods arrays and the substrate could avoid such unwanted troubles due to the enhanced adhesion ability and stability, such as chemical stability against dissolution and mechanical stability against breakage and detachment of nanorods under the stresses, thus enhancing the photocatalytic activity to some degree.
depicted in Fig. 5(b). The band gap energies of these ZNRAs were estimated to be around 3.08 eV, 3.00 eV and 3.00 eV on SSM precoated with bare, one and three AZO seed layers respectively. The narrowed band gap of ZNRAs-3AZO was probably ascribed to the chemical bonding between ZNRAs and AZO seed layers, which was similar to the previous findings obtained for ZNRAs-RGO composite materials [29]. In general, it was noticeable that there was an obvious correlation between the introduction of AZO seed layers and the corresponding UV–vis spectrum change of ZNRAs. The incorporation of AZO seed layers which enhanced the light absorption and narrowed the band gap of the tailored ZNRAs was expected to improve the photocatalytic activity of ZNRAs evolving from them. 3.3. Evaluation of the photocatalytic activities of ZNRAs on SSM premodified with AZO seed layers A series of experiments were conducted to see if the AZO seed layer thickness had significant influence on the application of the nanorod arrays for which they are designed, e.g. photocatalysis. Fig. 6 showed the UV–vis absorption spectra of the aqueous solutions of RhB (initial concentration 5.0×10−5 mol dm−3, 60 mL) with different ZnO samples as photocatalysts and exposure to ultraviolet light for various durations. The characteristic absorption of RhB at 553 nm decreased rapidly with extension of the exposure time. It can be seen that 93.42% of RhB was degraded by the ZNRAs-3AZO for 2 h under UV irradiation, in contrast, 82.34% and 81.81% of RhB were removed by the ZNRAs-1AZO and ZNRAs, respectively. Obviously, the ZNRAs3AZO exhibited the best photocatalytic activity. As shown in Fig. 7, the exponential decay profile of the plot between C/C0 (where C0 and C are the initial and actual concentration of RhB at time t, respectively) versus time “t” suggested that the photodecomposition reaction followed first-order rate law and the rate constant was calculated to be 0.02055, 0.01527 and 0.01340 min−1 for ZNRAs on SSM pre-coated with three, one and bare AZO seed layers, repectively. It was clear that the ZNRAs on bare SSM showed a much slower degradation rate of RhB (k=0.01340 min−1) compared to that of our as-synthesized ZNRAs-3AZO catalyst (k=0.02055 min−1). The photocatalytic degradation mechanism of ZnO photocatalyst has been widely studied and can be illustrated as follows: Generally, when ZnO nanocrystals are irradiated by UV light, electron-hole pair is generated on the surfaces of ZnO. Holes can be trapped by H2O to form highly reactive hydroxyl radicals which are powerful oxidants to decompose RhB molecules. Therefore, in the photocatalytic system, photo-induced molecular transformation or reaction takes place at the surface of the catalyst. The large surface area of the catalyst provides enough active reaction sites to adsorb the dyes and offers more possibilities for diffusion and mass transportation of RhB molecules. In order to compare specific surface area of different samples, the dye adsorbed amounts of them were estimated. As shown in Fig. 8, the dye adsorption of ZnO nanorods increased from 0.4×10−9 to 0.6×10−9 mol cm−2 with increasing AZO colloid spin coating times. Obviously, relatively low specific surface areas for ZNRAs on unmodi-
3.4. Renewable photocatalytic behavior of ZNRAs on SSM pre-coated with three AZO seed layers To evaluate whether the prepared ZNRAs on SSM is stable enough for effective photocatalysis, its use as a recyclable photocatalyst was further studied for three recycles under the same measurement conditions. After the former photodegradation experiment finished, the substrates were rinsed and dried to wipe off superfluous dye molecules and again immersed into fresh solutions with same concentrations for another cycle of the photodegradation experiment. The results, given in Fig. 9, demonstrated that these ZNRAs could still maintain relatively high photocatalytic activity. After 3 cycles, a slight decline in the degradation rate was observed due to a possible loss of some nanorods during the substrate rinse as illustrated in Fig. 10. More importantly, it should be noted that the ZNRAs after three photodegradation cycles did not show noticeable change in morphologies (Fig. 10), indicating that ZNRAs on AZO seed layers were not photocorroded and highly stable even after recycles.
4. Conclusion In this work, we reported a facile electrodeposition approach to fabricate large-area well-oriented ZNRAs photocatalyst on flexible SSM pre-modified by ZnO and AZO seed layers. The results showed that the morphology of ZNRAs was strongly affected by the type and crystal size of pre-coated seed layer, and well aligned ZNRAs with relatively higher specific surface area, in virtue of the introduction of Al into ZnO seed layers, could be obtained on the premodified SSM substrate. Under the optimum preparing conditions, the as-prepared ZNRAs exihibited enhanced photocatalytic performance of 93.42% and good photocatalytic stability under the UV-lamp for degradation of Rhodamine B. The recovery of the photocatalysts immobilized on substrates is compatible with industrial feasibility in practical use and environmental protection. 6465
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