Vertically-aligned ZnO@ZnS nanorod chip with improved

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Vertically-aligned ZnO@ZnS nanorod chip with improved photocatalytic activity for antibiotics degradation Bo Ji, Jixiang Zhang, Cheng Zhang, Nian Li, Tingting Zhao, Fa Chen, Lihe Hu, Shudong Zhang, and Zhenyang Wang ACS Appl. Nano Mater., Just Accepted Manuscript • DOI: 10.1021/acsanm.7b00242 • Publication Date (Web): 09 Jan 2018 Downloaded from http://pubs.acs.org on January 9, 2018

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ACS Applied Nano Materials

Vertically-aligned ZnO@ZnS nanorod chip with improved photocatalytic activity for antibiotics degradation Bo Ji,†,‡,§Jixiang Zhang,*,†Cheng Zhang,‡,§ Nian Li,‡ Tingting Zhao,‡ Fa Chen,†,‡ Lihe Hu,†,‡ Shudong Zhang‡ and Zhenyang Wang*,‡ †

School of Mechanotronics & Vehicle Engineering, Chongqing Jiaotong University, Chongqing

400074, China. ‡

CAS Center for Excellence in Nanoscience, Institute of Intelligent Machines, Chinese Academy

of Sciences, Hefei, Anhui 230031, China. KEYWORDS: ZnO@ZnS, nanorod arrays, photocatalysis, antibiotics degradation, chip device

ACS Paragon Plus Environment

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ABSTRACT. The photocatalytic degradation for the environmentally hazardous substances has been widely explored with the use of various photocatalyts and techniques. It is still remains a challenge to achieve efficient degradation and removal of residual antibiotics in the water environment. Here, we report an improved photocatalytic activity chip for degradation of antibiotics tetracycline hydrochloride by perpendicular ZnS-coated ZnO nanorod arrays (ZnO@ZnS NAs). The enhanced photocatalytic activity was contributed to the polycrystalline ZnS shell to effectively inhibit the recombination of photogenerated electron-hole pairs. Meanwhile, vertically-aligned ZnO@ZnS nanorod arrays can increase the light harvesting ability by enhancing scattering of light among ZnO@ZnS NAs. On the basis of these findings, an improved photocatalytic activity ZnO@ZnS NAs chip has been fabricated by growth of ZnO NAs on a piece of silicon wafer and the further sulfurization. More importantly, ZnO@ZnS NAs chips have been utilized to construct a ladder-like device, purifying antibiotics wastewater in one step, to effectively degrade tetracycline hydrochloride with enhanced the photocatalytic efficiency in flowing contaminated water. Besides, the ZnO@ZnS NAs device exhibited excellent recyclability in multiple repeated cycles. The ZnO@ZnS NAs chip provided a convenient and fast strategy for removal of antibiotics, pharmaceutical residues in wastewater.

1. Introduction. Antibiotics are the world's largest production and use of drugs, which play an important role in the prevention and treatment of the disease1-3. Accompanied by the increasing in the use of antibiotics, a large number of antibiotics discharged into the environment including pharmaceutical waste water and aquaculture waste water year by year. Residual antibiotics cause a serious threat to the ecological environment balance and human health4-6. In order to efficiently


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remove the remaining antibiotics in environmental water, the current technology is mainly dependent on activated carbon adsorption7, coagulation-flotation8, ultrasound degradation9, and membrane bioreactor10. These methods are still confronted with the difficulty of long timeconsuming, energy consumption and poor efficiency. Clearly, there is a strong demand for fast and inexpensive method for the removal of antibiotics from contaminated water. Semiconductor photocatalysis has attracted intense attentions due to their wide application to environmental remediation, especially for organic pollutants degradation11-16. This technique is based on the generation of electrons (e-) and holes (h+) from the semiconductor catalyst under light irradiation. The photo-generated e- and h+ will be transformed into superoxide radical anions (O2·-) or hydroxyl radical (OH·) by reaction with absorbed O2 and H2O to oxidize organic molecular17-21. As a versatile photocatalyst, zinc oxides (ZnO) have been extensively investigated and widely used in this field owing to the rapid generation of electron-hole pairs upon light-excitation. To date, ZnO with different morphologies, phase, and structure for photocatalysis have been extensively reported22-29. Importantly, vertically-aligned ZnO nanorod arrays can be conveniently prepared, which provides significant prerequisites for semiconductor device manufacture30. Photocatalytic efficiency relies heavily on the recombination rate of photoinduced electronhole charge pairs. A reasonable method to inhibit this recombination is coupling noble metal or other semiconductor31-33. The coupling of two semiconductors creates heterogeneous structure to enhance charge separation and transfer at the interface. Thus, the depressing of photogenerated carriers recombination can significantly improve the catalytic performance. Zinc sulfide (ZnS) with a wider band gap has been widely used in photocatalysts34-38. Notably, converting ZnO


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partial into ZnS by chemical reaction to construct heterogeneous interface provide a convenient way for efficient photocatalysis. Herein, we report an improved photocatalytic activity chip for degradation of antibiotics tetracycline hydrochloride by perpendicular ZnS-coated ZnO nanorod arrays (ZnO@ZnS NAs). The enhanced photocatalytic activity was contributed to the polycrystalline ZnS shell to effectively inhibit the recombination of photogenerated electron-hole pairs. Meanwhile, vertically-aligned ZnO@ZnS nanorod arrays can increase the light harvesting ability by enhancing scattering of light among ZnO@ZnS NAs. More importantly, ZnO@ZnS nanorod array chips have been utilized to construct a ladder-like device, purifying wastewater in one step, to effectively degrade antibiotics tetracycline hydrochloride with enhanced the photocatalytic efficiency in flowing contaminated water. Besides, the ZnO@ZnS NAs device exhibited excellent recyclability in multiple repeated cycles. The ZnO@ZnS NAs chip provided a convenient and fast strategy for removal of antibiotics, pharmaceutical residues in waste water. 2. Experimental section. 2.1 Materials. All chemicals were analytical grade and used as received without further purification.

Zinc acetate dihydrate [Zn(CH3COO)2·2H2O], Zinc nitrate hexahydrate

[Zn(NO3)2·6H2O], hexamethylene tetramine (HMTA), thioacetamide (TAA), sulphuric acid, hydrogen peroxide, acetone and Congo Red (CR) were purchased by Sinopharm Chemical Reagent Co.Ltd. (Shanghai, China). Tetracycline hydrochloride (TC), oxytetracycline (OC), and chlorotetracycline (CC) were purchased from Aladdin. The ultrapure water was obtained using Millipore purification system. 2.2 Synthesis of nanorods arrays on substrate.


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Silicon wafers or glass substrate (3×3 cm) were treated with concentrated sulphuric acid and hydrogen peroxide (H2SO4 : 30% H2O2 = 3 : 1) for 30 min at 90 °C, and then rinsed with deionized water and ethanol. Well-aligned ZnO nanorods arrays were grown on Si substrate using a two-step method. In the first step, 0.005 M ethanol solution of Zn(CH3COO)2 were droped onto the Si [100] substrate and annealed at 130 °C for 15 min, the process were repeated 5 times to form ZnO seed layer. In the growth step, the Si wafer with seed layer was suspended vertically into a glass bottle containing 200 mL of a mixed solution of 0.05 M Zn(NO3)2 and 0.05 M HMTA at 90 °C for 5 h in an oven. Then, the wafer was removed from solution, cleaned ultrasonically in ethanol, rinsed with deionized water, and then dried. To synthesize ZnO@ZnS core@shell structures, the obtained ZnO nanorods arrays on Si substrate were transferred to a glass bottle containing 100 mL of 0.03 M TAA and heated to 90 °C for 4 h. Finally, the wafer was removed from solution, rinsed repeatedly with deionized water and dried for further use. 2.3 Photocatalytic activity test. A piece of silicon wafer (3×3 cm) with ZnO@ZnS nanorods arrays was added into 15 mL of 10 mg/mL tetracycline hydrochloride solution. Before irradiation, the solution was stirred for 30 min to reach an adsorption-desorption balance between the surface of wafer and TC in the dark. The above solution was irradiated by a 500 W xenon lamp in a photocatalytic chamber under successive stirring. In the course of this photocatalytic experiment, the concentration of TC was monitored within a fixed time interval (20 min) with a UV-vis spectrophotomer. 2.4 Characterization. The morphology and structure of the arrays were identified by scanning electron microscope (SEM, Quanta 200 FEG, FEI) and transmission electron microscope (TEM, JEOL-2010,


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operated at 200 kV). The crystal structure was analyzed by powder X-ray diffraction (XRD) on a Philips X’pert diffractometer equipped with Cu-Kα radiation (λ=0.154 nm). The UV-visible spectra were monitored on Shimadzu UV 2550 spectrophotomer. The PL spectra were detected using a Fluorolog-3 fluorescence spectrophotometer (Horiba) equipped with a R928 PMT. 3. Results and discussion. 3.1 Design and preparation of array substrate. In our design, p-Si wafers [100] were chosen as a substrate to fabricate nanorods arrays. As shown in Figure 1a, a seed layer of ZnO was first grown on the Si substrate through calcination of Zn(CH3COO)2 (Figure S1). The ZnO seeds were covered the whole substrate with the thickness of ~100 nm. Besides, the thickness of the ZnO seed could be modulated by the dispensing and calcination times. We found that when the dispensing and calcination reached 5 times, the obtained ZnO nanorods array is uniform (Figure S2). Then, well-aligned ZnO nanorods arrays were grown in aqueous solution including equimolar Zn(NO3)2 and HMTA at 90 °C for 5 h. Figure 1b and c showed the SEM images of the ZnO nanorod arrays and the crosssectional image of the arrays. The obtained ZnO nanorod arrays stand vertically and cover the whole Si substrate with high density from the images. The ZnO nanorods were about 80-100 nm in diameters and 1-1.5 µm in lengths. The diameter of the ZnO nanorods was dependent on the concentration of Zn(NO3)2/HMTA mixed solution. Figure S3 showed SEM images of ZnO nanorod arrays obtained from different concentration of growth solution. The optimal concentration of Zn(NO3)2 and HMTA mixed solution to form orderly array is 50 mM. When the concentration of mixture exceeds 50 mM, the ZnO nanorods have larger and ununiform


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diameters. On the other hand, the ZnO nanorod array was not able to form when the concentration was below 50 mM due to the insufficient growth materials.

Figure 1. (A) Schematic illustration for the synthesis process of the ZnO@ZnS core-shell nanorods arrays. Plan-view SEM image (B) and cross-sectional SEM image (C) of the ZnO NAs on Si substrate. Plan-view SEM image (D) and cross-sectional SEM image (E) of the ZnO@ZnS NAs on Si substrate. To inhibit the recombination of photoinduced electron-hole charge and enhance the photocatalytic efficiency, a layer of ZnS was covered by surface replacement of ZnO to ZnS. We transferred the ZnO nanorod arrays substrate into a solution containing 0.03 M TAA at 90 °C for


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4 h, where the TAA solution provides S2- to react with Zn2+ to from ZnS nanorods arrays. The obtained ZnO@ZnS vertical nanorods were about 100-120 nm in diameters and 1-1.5 µm in lengths. The design of this array structure not only provides efficient photocatalytic active sites, but also facilitates the recovery of catalysts in practical applications.

(112) (201)

(103)

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ZnO No:36-1451

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ZnS No:05-0566

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Figure 2. XRD patterns of as-synthesized ZnO and ZnO@ZnS NAs. X-ray diffraction (XRD) patterns in Figure 2 reveal that both the ZnO and ZnO@ZnS nanorods exhibit the crystalline structure. A sharp peak at 69° can be assigned to p-Si [100] substrate. The diffraction peaks of hexagonal ZnO (JCPDF: 36-1451) at 31.7°, 34.4°, 36.2°, 47.5°, 62.8° and 67.9° indicated the (100), (002), (101), (102), (103) and (112) planes. Notably, the peak of (002) plane is very strong, which confirm the vertical growth process of nanorods. After the deposition of ZnS, several new diffraction peaks at 28.6°, 47.5° and 56.3° can be indexed to blende ZnS (JCPDF: 05-0566), implying that the ZnO was partially changed into ZnS. Other peaks of impurity were not observed in the patterns, which showed that all of the assynthesized samples have high phase purity.


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Figure 3. TEM and HRTEM images of the as-synthesized ZnO (A, B) and ZnO@ZnS (C, D) nanorod. Further structure characterization of the as-synthesised ZnO@ZnS NAs with TEM and HRTEM images are shown in Figure 3. The ZnO nanorod exhibits a smooth surface due to the growth of single crystal. Combined the d-spacing of 0.26 nm in the Figure 3B, we believe the nanorod is single crystal and growth along the [001] direction. While, the ZnO@ZnS core-shell nanorod displays a rough surface, which imply the polycrystalline shell formation. With the increase in the degree of sulfuration (from 2 to 8 h), the nanorods became more and more rough and arranged tightly (Figure S4). In order to further confirm the coating of ZnS shell, we photographed the HRTEM image at the edge of the nanorod. The obtained interplanar spacing of


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about 0.31 nm can be attributable to the (111) plane of ZnS. All these experimental results confirmed the design of our core/shell catalyst structure. 3.3 Improved photocatalytic activity. 0.5

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Figure 4. Light-induced photodegradation of tetracycline hydrochloride as a function of irradiation time: visible absorption spectra showing (A) photolysis of tetracycline hydrochloride, (B) degradation by ZnO NAs chip, and (C) degradation by ZnO@ZnS NAs chip. (D) C/C0 as a function of degradation time. To investigate the photocatalytic performances of as-synthesized nanorods array chip, we carried out a series of control experiments. Tetracycline hydrochloride (TC), a most widely used


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antibiotics, was utilized as a simulated pollutant39-40. Here, we use a 3×3 cm photocatalytic chip to deal with 15 mL of TC (10 mg/mL) under Xenon light irradiation (500 W). Figure 4 showed the absorption spectra of TC as a function of irradiation time. The characteristic absorption bands did not induce obvious intensity decrease in the absence of photocatalyst even after 140 min, so that TC can be considered stable against Xenon light inducing photolysis (Figure 4A). Meanwhile, the adsorption of TC by our chip is negligible (Figure S5). On the contrary, TC was progressively degraded in presence of both ZnO NAs and ZnO@ZnS NAs (Figure 4B and C). It can be found that the intensity of the characteristic peak at 357 nm decreased severely with the increasing of irradiation time. Notably, the slight red shift of absorption peak may be interpreted as coordination between catabolite and Zn2+ ions. To quantitatively comprehend the photodegradation of TC over different as-prepared samples, we analyzed the variation of TC concentration (C/C0, where C0 represent the initial absorption intensity of TC before light irradiation) as a function of irradiation time in Figure 4D. For pure ZnO NAs, about 69.8% of TC was degraded after irradiation for 140 min. A higher extent of TC photodegradation about 80.9% was obtained for using ZnO@ZnS NAs chip at the same irradiation condition. Besides, we calculated the rate constant of photodegrade reaction using the Langmuir-Hinshelwood equation24 (lnC/C0=kt, where k is the reaction rate constant) in Figure S6. It showed that the ZnO@ZnS NAs are more efficient than the bare ZnO NAs (kZnO/ZnS=0.0119 min-1 and kZnO=0.0085 min-1). The ZnO@ZnS NAs chip for antibiotics degradation is quite different from traditional semiconductor powder catalyst. Owing to the vertically-aligned nanorod structure possesses a direct electrical pathway for the rapid transport of photogenerated carriers, the degradation efficiency of nanorod chip is much higher than powder catalyst (Figure S7).


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Figure 5. (A) Photocatalytic efficiency of the ZnO@ZnS NAs with different sulfuration condition: 20, 30, 40 and 50 mM of TAA was used. (B) Scheme of the photogenerated carriers transfer process and photocatalytic mechanism. To understand the major active radical in the photocatalytic reaction, radical-trapping experiments were performed through control experiment using three scavengers: N2 purging (superoxide anion radical scavenger), disodium ethylenediaminetetraacetate (EDTA-Na2, hole scavenger) and tert-butanol (hydroxyl radical scavenger)41-43. As shown in Figure S8, when EDTA-Na2 and N2 purging were added, the TC degradation were obvious suppressed, the results indicated that the hole and superoxide anion radical are major active radical in the photocatalytic reaction. Subsequently, we investigated the photocatalytic properties of core-shell nanorods with different degrees of sulfuration. The ZnO nanorod arrays were transferred into a solution containing different concentration of TAA (20, 30, 40 and 50 mM) at 90 °C. We found that the optimum reaction condition of TAA is 30 mM (Figure 5A). According to the EDX analysis in Figure S9, the spectrum confirmed the presence of Zn, O, and S. Meanwhile, the atomic percent confirmed that 55% of ZnO transferred into ZnS under 30 mM TAA treatment for 4 h. As shown


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in Figure S10, excessive sulfuration resulted in a reduction in the effective surface area of the nanorod array, which reduced the surface catalytic sites. The enhanced photocatalytic efficiency by ZnS shell coating can be explained from the charge-transfer process. The potentials of conduction band and valence band for semiconductor can be calculated by the following equations: ECB=X-Ec-1/2Eg (1); EVB=ECB+Eg (2), where X represent the absolute electronegativity of the semiconductor, Ec is the energy of free electrons on hydrogen scale (4.5 eV), Eg is the band gap44. As shown in Figue 5B, the conduction band of ZnS possess more negative potential power (-1.04 eV) than that of ZnO (-0.31 eV). Moreover, the valence band of ZnO (2.89 eV) possess more positive power than that of ZnS (2.66 eV)45. Consequently, with the generation of electron-hole pairs under light irradiation, the electrons will move to the ZnO side and holes will move to the ZnS side, facilitating the formation of a charge transfer state and the spatial separation of the photogenerated carriers. The spatial separation of the photogenerated carriers decreases the recombination of the electron-hole pairs. PL spectra of the ZnO and ZnO@ZnS NAs were measured in the Figure S11. Both the ZnO and ZnO@ZnS NAs comprise two main emission bands in the UV and visible regions under the excitation of 320 nm. The narrow UV emission peak is assigned to the band edge emission of ZnO, while the broad visible emission originates from defects and surface states. The PL intensity in the UV region decreased after the ZnS shell coating, which demonstrated the efficient charge separation inhibit the recombination of photogenerated electron-hole pairs. Thus, the catalytic performance of ZnO@ZnS NAs can be significantly improved, which provide an opportunity to photodegrade many kinds of antibiotics and organic dyes, such as oxytetracycline, chlorotetracycline and Congo Red (Figure S12 and S13).


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Figure 6. Photocatalytic degradation cycles of the ZnO@ZnS NAs chip. Each cycle time is 140 min. To research the recyclability of photocatalytic performance under Xenon light irradiation, the ZnO@ZnS NAs chip was employed to degrade TC in five repeated cycles, and the results are exhibited in Figure 6. It was remarkable that the chip still keep high photocatalytic activity after repeated use, where the photocatalytic efficiency decreased below 5% after five cycles. It indicated that the as-synthesized photocatalytic chip can be reuse in actual applications. 3.4 Degradation of antibiotics in stage device.


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Figure 7. (A) Image of the three-stage flowing device for antibiotics degradation. (B) Photodegradation of tetracycline hydrochloride using the ladder-like device with different cycles. Finally, we demonstrated that the ZnO@ZnS NAs chip can be used as an effective method for antibiotics photodegradation combined with a flowing device. We designed a simple device in Figure 7A to enhance its phohocatalytic rate in our research. This is a three-stage flowing device including three square grooves (each size is 31×31×4 mm), flowing channels and outflow channels. The ZnO@ZnS NAs chip were put in square grooves in order. A peristaltic pump is used as power source to introduce the undegraded water samples, which will be droped onto the substrate surface in the highest square groove at a rate of 0.5 mL/min. Since the surface of array substrate is highly hydrophilic (Figure S14), the water droplets spread evenly over its surface to degradation. When the first groove is submerged, the solution flows into the second sample groove and into the third groove through flowing channels. When the third sample is submerged,


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the solution flows through outflow channels. We placed a beaker at the outflow channel to collect the final solution. The whole experimental process is carried out under the 500 W lamp as a light source. Detailed experimental results of tetracycline degradation are shown in Figure 7B. The solution flow through the three piece of substrates to the beaker as a cycle. The tetracycline was degraded about 50% for the first cycle, and about 78.8% of TC was degraded after the third cycle using the rate of 0.5 mL/min. Compared with photodegradation in common beaker, using the stage device can shorten 35% of the degradation time due to the improved photocatalytic activity and adequate contact between ZnO@ZnS NAs chip and wastewater. Additionally, if we can reasonably choose the number of steps and the flow rate of the liquid, the purified water will be obtained. 4. Conclusion. In summary, we have successfully fabricated ZnO and ZnO@ZnS NAs on Si substrate via a hydrothermal method. The as-synthesized ZnO@ZnS nanorods showed efficient photocatalytic activity for tetracycline hydrochloride under Xenon light irradiation. The enhanced photocatalytic activity and improved photostability could be attributed to the unique nanorod arrays structure, the superior crystallinity of ZnO nanorods, and the coated ZnS shell. In addition, we demonstrated an easy and cost-effective flow-device to degradate tetracycline hydrochloride using the ZnO@ZnS NAs chip. Thus, our chip can be utilized as an efficient photodegradation catalyst and may have potential application for wastewater disposal. ASSOCIATED CONTENT Supporting Information.


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Structure characterization of ZnO seed, SEM images of as-obtained samples with different growth condition, photolysis and adsorption of TC, kinetic curves for TC degradation, radicaltrapping experiments, EDS spectrum, optical properties of chip, multiple antibiotics degradation, contact angle of as-synthesized chip. AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] *E-mail: [email protected] Author Contributions The manuscript was written through contributions of all authors. All authors have given approval §

to the final version of the manuscript. Bo Ji and Cheng Zhang contributed equally. ACKNOWLEDGMENT This work was financially supported by the National Natural Science Foundation of China (No. U1432132, 61605222 and 21703255), Natural Science Foundation of Anhui Province (Grant 1708085MB35), the Major/Innovative Program of Development Foundation of Hefei Center for Physical Science and Technology (2014FXZY001) and the Basic and Frontier Research Program of Chongqing (cstc2015jcyjBX0140). Notes The authors declare no competing financial interest.


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Vertically-aligned ZnO@ZnS nanorod chip with improved

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