ZnO@C Nanocable with Dual-Enhanced Photocatalytic

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Pt/ZnO@C Nanocable with Dual-Enhanced Photocatalytic Performance and Superior Photostability Peng Zhang, Yang Chen, Xiaoyan Yang, Jianzhou Gui, Yi Li, Hailong Peng, Dan Liu, and Jieshan Qiu Langmuir, Just Accepted Manuscript • DOI: 10.1021/acs.langmuir.7b00995 • Publication Date (Web): 14 Apr 2017 Downloaded from http://pubs.acs.org on April 14, 2017

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Pt/ZnO@C Nanocable with Dual-Enhanced Photocatalytic Performance and Superior Photostability Peng Zhanga,b, Yang Chenc, Xiaoyan Yanga, Jianzhou Gui*a,c, Yi Lic, Hailong Pengc, Dan Liuc, and Jieshan Qiu*b

a

State Key Laboratory of Separation Membranes & Membrane Processes, School of

Material Science and Engineering, Tianjin Polytechnic University, Tianjin 300387, China b

Liaoning Key Lab for Energy Materials and Chemical Engineering, State Key Lab

of Fine Chemicals, School of Chemical Engineering, Dalian University of Technology, Dalian 116024, China c

School of Environmental & Chemical Engineering, Tianjin Polytechnic University,

Tianjin 300387, China

KEYWORDS: hydrothermal carbon, ZnO, photocatalysis, degradation

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ACS Paragon Plus Environment

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ABSTRACT

To improve the photocatalytic activity and photostability of ZnO, the novel cable-like Pt/ZnO@C composite is successfully fabricated by coating a 3-5 nm hydrothermal carbon (HTC) layer on the surface of Pt nanoparticle-modified ZnO nanowire. After investigating the optical and photoelectrochemical performance in detail, it is found that the Pt/ZnO@C nanocable shows a dual-enhancing migration efficiency for the photoinduced surface electrons, distributing to the modified Pt nanoparticles and the coated HTC layer. Consequently, the Pt/ZnO@C nanocable exhibits a dual-enhanced photocatalytic activity for the degradation of various organic pollutants under the UV light irradiation. The coated HTC layer can also play a role in suspending the ZnO photocorrosion, and significantly improves the photostability of the Pt/ZnO@C nanocable. Furthermore, the photocatalysis and photocorrosion mechanism of the Pt/ZnO@C nanocable is proposed and discussed in terms of its structural feature and photoelectrochemical property. The resultant Pt/ZnO@C nanocable with the unique HTC layer-coated structure will probably stimulate to design and synthesize more HTC-hybridized composites with the superior photocatalytic or photoelectrocatalytic performance.

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INTRODUCTION

Nowadays, the water pollution released from textile and fine chemical industries has become a global environmental issues. Although many wastewater treatment methods have been developed, photocatalysis attracts substantial interest due to its continuity, simplicity and high efficiency.1 Up to now, many semiconductors with a large band gap have been found to show great photocatalytic performance under the UV light irradiation.2 Of various alternative semiconductor materials, TiO2 and ZnO are regarded as two ideal catalysts with good photocatalytic performance, owning to the abundant resource, low cost, and environmental sustainability.3-6 Benefiting from versatile synthesis strategies, morphologies and structures, ZnO exhibits more broad application prospects than TiO2.7-8 More importantly, it is easier to combine ZnO with other materials, such as SiO2,9 C3N4,10 Fe2O3,11 to fabricate ZnO-type multifunctional composite with the great photocatalytic or photoelectrochemical performance. However, on the one hand, the photocatalytic activity of ZnO is still needed to be improved to arrive the practicality of its implementation. On the other hand, the ZnO photocorrosion occurred in the UV-light illumination process would destroy the crystal structure of ZnO photocatalysts, further making the dramatic decline of its photocatalytic activity.12 Therefore, it is highly necessary to design and prepare the ZnO-based composite materials with both the high photocatalytic activity and the great photostability. As electron reservoirs, the high electrical conductivity materials (metal and carbon materials) have been proved to be capable of greatly improving the photocatalytic 3


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performance of photocatalysts.13-19 Recently, many ZnO/C or ZnO/metal composites have been successfully synthesized and widely studied, such as ZnO/graphene composite,20 ZnO@graphene nanosphere,21 ZnO-Au nanopyramid,22 and dendrite-like ZnO@Ag nanocrystals.23 Besides, it is also found that many carbon materials can effectively suspend the ZnO photocorrosion and enhance the stability of the photocatalysts.12 Among various carbon materials, the biomass-derived hydrothermal carbon (HTC) catches tremendous attention,24 which features not only the cheap and nontoxic feedstock, the facile and controllable synthetic approach, but also the excellent bonding capability with other materials.25 Zhu et al.26 reported a ZnO/C composite via hybridizing ZnO particles with glucose-derived HTC, and found that the HTC was able to impede the ZnO photocorrosion. Moreover, by studying the rod-like ZnO@C product with a surface HTC layer, our group27 certified that the core/shell architecture is the optimized structure for improving the photocatalytic stability of ZnO. Although enormous efforts have been devoted, there are still many problems needed to be resolved: Can the HTC layer also promote the photocatalytic activity of ZnO as other carbon materials? If so, will the photostability of ZnO decrease accordingly? As for the existing ZnO/HTC photocatalysts, their photocatalytic activities are still not good enough to meet the industrial requirement for the wastewater treatment. Therefore, it is highly desired to develop a new strategy to synthesize the ZnO/HTC composite materials with the superior photocatalytic performance and photostability.

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Scheme 1. Schematic illustration of the synthesis process of the Pt/ZnO@C nanocable.

Herein, we present a two-step preparation route to obtain the cable-like Pt/ZnO@C composite with a core/shell structure, and the detailed process is shown in Scheme 1: (I) Using ZnO nanowires as the matrix, nanosized Pt particles are located on the surface to synthesis the Pt/ZnO nanowire; (II) Under the hydrothermal situation, a HTC layer with 3-5 nm thickness is uniformly coated on the Pt/ZnO nanowire to finally fabricate the Pt/ZnO@C nanocable. The optical, photoelectrochemical, and photocatalytic properties of the Pt/ZnO@C nanocable are investigated in details. It is demonstrated that both the surface-modified Pt particles and the coated HTC layer can effectively transfer the photogenerated electrons, further dual-promoting the photocatalytic activity of the Pt/ZnO@C nanocable for the degradation of four organic pollutants under UV light irradiation. Furthermore, the coated HTC layer also plays an important role in suppressing the ZnO photocorrosion and improving the photostability of the Pt/ZnO@C nanocable. EXPERIMENTAL

Preparation of photocatalysts In the present work, all reagents involved were of analytical grade and used without further purification. The ZnO nanowires were synthesized as described in the previous 5


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paper.28 In a typical procedure, 0.2 g zinc chloride, 1.5 g sodium dodecyl sulfate (SDS) and 20 g sodium carbonate were added into 40 mL deionized water, and completely mixed for 30 min under magnetic stirring. The resulted white emulsion was transferred into a 50 mL Teflon-lined autoclave and kept at 140 °C for 12 h. After the hydrothermal reaction, the autoclave was naturally cooled back to the room temperature. The precipitates were collected by filtering, rinsed with deionized water and absolute ethanol for three times, and then dried at 80 °C for 12 h under vacuum. Finally, the obtained white powders were the ZnO nanowires used in this work, which hereafter were denoted as ZnO NW. By adding 0.1 g of the ZnO nanowires and 0.4 mL H2PtCl6 aqueous solution (10 g/L) into 1 mL deionized water, a reaction mixture was formed after completely dispersed by ultrasonic treatment and aged for 10 h at the room temperature. The powder-like products collected were then loaded into a tube furnace and heated at 300 °C in H2/Ar (1:9, v/v) for 2 h with a heating rate of 5 °/min. After cooled back to the room temperature, the light-yellow products prepared were the ZnO nanowires supported by Pt nanoparticles and denoted as Pt/ZnO in this work. 0.2 g of the as-obtained Pt/ZnO nanowires were ultrasonically dispersed in 40 mL glucose aqueous solution (5 g/L), yielding a suspension. This suspension was rapidly transferred into a 50 mL Teflon-lined autoclave that maintained at 180 °C for 12 h. After that, the hydrothermal precipitates were collected by centrifuging, washed with deionized water and absolute ethanol for three times, and then dried at 80 °C for 12 h under vacuum. Finally, the black products were the HTC-coated Pt/ZnO nanocables, 6


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which were hereafter denoted as Pt/ZnO@C. For comparison, the HTC-coated ZnO nanocables were also synthesized in the present work by hydrothermally treating the ZnO nanowires under the identical experimental conditions as the Pt/ZnO@C. These comparative samples were denoted as ZnO@C. Characterization of photocatalysts The morphologies and structures of different samples were observed by field-emission scanning electron microscopy (FESEM, Hitachi S4800), transmission electron microscopy (TEM, Hitachi H7650), and high resolution transmission electron microscopy (HRTEM, JEOL JEM-2100). The X-ray diffraction (XRD) was performed on a Bruker D8 Advance diffractometer with Cu-Kα radiation, operated at 40 kV and 40 mA. The energy-dispersive X-ray (EDX) spectrometry of the Pt/ZnO@C was obtained using a Hitachi S4800. The surface chemistry of four photocatalysts was analyzed

by

Fourier

transform

infrared

(FTIR,

Thermo

Nicolet

6700).

Thermogravimetric analysis (TG) was carried out on a Netzsch STA 449F3 in air, in which the samples were heated from 50 to 800 °C with a ramping rate of 10 °/min. UV-vis diffuse reflectance spectra (DRS) were recorded in a range of 200-800 nm on a Shimadzu UV2700 with an integrating sphere attachment. The photoluminescence (PL) spectra were measured using a Hitachi F-7000 with an excitation light of 340 nm. Photocatalytic experiments The methylene blue (MB) degradation was firstly chosen as the probe reaction to 7


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evaluate the photocatalytic performance of photocatalysts under the UV light illumination, of which the UV light was supplied by a 500 W high-pressure Hg lamp with average light intensity of 150-200 mW/cm2. In a typical catalytic experiment, 20 mg of photocatalysts were put into a 50 mL quartz reactor containing 40 mL of MB aqueous solution (10 mg/L) to yield a suspension. Prior to the UV light illumination, this reaction suspension was magnetically stirred in dark for 30 min to establish the adsorption/desorption equilibrium. When the photocatalytic reaction starts, about 4 mL of suspension was extracted at interval of 10 min, and centrifuged to remove the photocatalyst.

Absorbance

of

the

filtrate

was

measured

by

the

UV-vis

spectrophotometer (Thermo Evolution 300) in a range of 450-800 nm. The MB degradation processes of different photocatalysts were monitored by the corresponding C/C0 ratio, where C0 is the absorbance of the initial MB aqueous solution at 664 nm, and C is the absorbance of the filtrates at different times. The stability and repeatability of four photocatalysts were also measured by the typical catalytic process described above, but only analyzed the absorbance of the reaction solution at 60 min. When the reaction was finished, the photocatalysts were recovered by centrifugation, and reused for the next cycle after fully washed with absolute ethanol and dried at 80 °C under vacuum. In addition, three antibiotics were adopted to investigate the photocatalytic activity of the Pt/ZnO@C for decomposing the colorless organics. They were tetracycline (TC, 20 mg/L), enrofloxacin hydrochloride (ENRH, 20 mg/L), and ciprofloxacin (CIP, 10 mg/L), respectively. Their photodegradation procedures were identical with the MB 8


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degradation, and the corresponding maximum absorbance were 357 nm (TC), 273 nm (ENRH), and 273 nm (CIP). In the photocatalytic process of the Pt/ZnO@C, the reactive species generated could be detected by trapping experiments, where scavengers were introduced into the standard MB photodegradation. In the present case, tert-butylalcohol (t-BuOH) and ethyl-enediaminetetraacetic acid disodium salt (EDTA-2Na) were chosen as the hydroxyl radical scavenger and hole scavenger, respectively. Photoelectrochemical measurements All of photoelectrochemical measurements were performed on a electrochemical work station (CHI 760D, Chenhua Instruments Co., China) in 0.1 M Na2SO4 aqueous solution, using the standard three-electrodes system. A platinum wire and saturated calomel electrode (SCE) were used as counter electrode and reference electrode, respectively. The working electrodes were prepared as follows: 4 mg of photocatalyst was ultrasonically dispersed in 2 mL of ethanol for 10 min to generate the slurry. Subsequently, the slurry was carefully dipped into a groove (10 mm × 10 mm) on an indium-tin oxide (ITO) glass. After totally dried at 80 °C for 12 h, the photocatalyst/ITO electrodes were heated at 200 °C for 8 h in N2. A 500 W high-pressure Hg lamp was used as the exciting light source for the UV irradiation. Both the photocurrent and electrochemical impedance spectra (EIS) were measured at 0.0 V under ambient conditions, of which the Nyquist plots were recorded in the frequency range of 0.05-100 Hz with 5 mV of the sinusoidal AC perturbation. RESULTS AND DISCUSSIONS 9


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Structure and morphology of photocatalysts As shown in Figure 1a, the morphology of the first-step hydrothermal product is in a typical wire-shaped structure. From the SEM image (Figure 1a), diameter distribution (Figure S1) and TEM image (inset, Figure 1a), it is found that the ZnO nanowires have the uniform sizes (ca. 30-110 nm) and smooth surface. In the present work, these high-quality ZnO nanowires are utilized as the backbone to support Pt nanoparticles, and the resultant Pt/ZnO nanowires are displayed in Figure S2. Compared with the ZnO NW, the as-obtained Pt/ZnO shows a negligible variation on appearance, which is also consisted of the regular nanowires with smooth surface. TEM image (Figure 1b) of the Pt/ZnO reveals that the ZnO nanowire is modified by Pt particles with ca. 3-5 nm on the surface. After coated by the HTC, the final product, Pt/ZnO@C, still maintains the primary wire-shaped morphology (Figure 1c), whereas manifests a coarse surface (Figure 1d). From TEM image of the Pt/ZnO@C (Figure 1e), the cable-like Pt/ZnO@C composite could be clearly observed, in which a HTC thin layer with 3-5 nm thickness is uniformly coated on the surface of the Pt/ZnO nanowire. Owning to the possible decomposition in the second-step hydrothermal reaction, the unstable ZnO on the Pt/ZnO surface is dissolved to yield the rough surface in the Pt/ZnO@C (Figure 1d and 1e). In addition, the similar rough surface also appears on the ZnO@C used as the contrast product (Figure S3), further proving the decomposition of ZnO nanowires during the hydrothermal process.

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Figure 1. (a) SEM image of the ZnO NW, inset is its TEM image; (b) TEM image of the Pt/ZnO; (c) SEM, (d)

magnified SEM, (e) TEM, and (f) HRTEM images of the Pt/ZnO@C.

In order to confirm the composition and structure, the Pt/ZnO@C is also characterized by HRTEM, as shown in Figure 1f. The measured adjacent lattice spacings are about 0.265 nm and 0.23 nm, corresponding to the interplanar spacing of ZnO (111) and Pt (111), respectively. The well-resolved 2D lattice fringes in ZnO nanowire indicants a single-crystal nature and high crystallinity. Besides, the disorderly fringes wrapping on the side edge of the ZnO crystal and Pt nanoparticles could be distributed to the coated HTC layer on the surface of the Pt/ZnO@C. In spite of different structures, four samples showcase the identical featured peaks in the XRD patterns (Figure 2), attributing to the wurtzite structure of ZnO (JCPDS no. 36-1451).29 For the Pt/ZnO, ZnO@C and Pt/ZnO@C, the absence of diffraction peaks of Pt and HTC mainly results from the slight content in samples. However, the EDX spectrum of the Pt/ZnO@C (Figure S4) shows several strong peaks indexed to 11


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C, O, Zn and Pt species, evidenced the existence of Pt and C. To further explore the surface carbon materials, the as-obtained products are also analyzed by FTIR and the corresponding spectrums have been shown in Figure 3a. Compared to the smooth curves of the ZnO NW and Pt/ZnO, the FTIR spectra of the ZnO@C and Pt/ZnO@C possess a series of absorption bands in the wide range of 450-4000 cm-1. Particularly, the bands at ca. 3550-3300, 3020-2880, 1740-1560, 1430-1250, 1150-980, and 830-750 cm-1 could be ascribed to the stretching vibrations of O-H, C-H, C=O, OC-O, C-OH, and C-H, respectively.30-31 Consequently, the abundant oxygen-containing groups distributed on the surface of the ZnO@C and Pt/ZnO@C further demonstrate the formation of glucose-derived HTC during the hydrothermal process.

Figure 2. XRD patterns of the ZnO NW, Pt/ZnO, ZnO@C and Pt/ZnO@C.

The HTC contents in the ZnO@C and Pt/ZnO@C are qualitatively confirmed by the TG analysis in air, as shown in Figure 3b. The ZnO NW and Pt/ZnO are also measured as the contrastive samples, and two slight weight-loss variations could be 12


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found in the continuous range of 200-900 °C. It is indicated that only a little amount of adsorbed water and lattice water are removed during the heating process. As for the ZnO@C and Pt/ZnO@C, the obvious weight loss can be observed from 300 °C to 500 °C, attributing to the combustion of surface HTC layers. As a result, we can infer that the weight percentages of HTC in the ZnO@C and Pt/ZnO@C are 11 % and 11.4 %, respectively.

Figure 3. (a) FTIR spectra and (b) TG analysis of the ZnO NW, Pt/ZnO, ZnO@C and Pt/ZnO@C.

Optical property of photocatalysts UV-vis DRS and PL spectra are utilized to investigate the optical property of the ZnO NW, Pt/ZnO, ZnO@C and Pt/ZnO@C (Figure 4 and 5). In the UV-vis DRS, four samples exhibit the equivalent strong absorption in UV region (below 400 nm), whereas different absorptions in the visible region (400-800 nm). The minimal visible absorption implies that the ZnO NW cannot adsorb any visible light. After modified by Pt nanoparticles, the Pt/ZnO shows an enhanced adsorption in the visible range due to the increasing surface defect, and the details will be further analyzed by the PL 13


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spectra below. When the ZnO NW and Pt/ZnO are wrapped by a thin HTC layer, tremendous improvement of the visible absorption can be found in the ZnO@C and Pt/ZnO@C. This variation of visible absorbance is also visually depicted by the color of photocatalysts (inset of Figure 4): white, gray, brown and black are for the ZnO NW, Pt/ZnO, ZnO@C and Pt/ZnO@C, respectively. Additionally, it is noteworthy that the similar UV absorbance of the four photocatalysts suggests that the effect of the hybridized materials on the UV-light adsorption of ZnO nanowires is neglectable.

Figure 4. UV-vis DRS of the ZnO NW, Pt/ZnO, ZnO@C and Pt/ZnO@C. Insets are the corresponding digital

photographs.

Figure 5 is the PL spectra of the as-synthesized photocatalysts using an excitation wavelength of 340 nm at the room temperature. As shown, all of PL spectra include a strong UV-vis emission peak at 360-450 nm and series of weak peaks in the visible range of 450-550 nm. Particularly, the ZnO NW has the strongest UV emission, 14


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indicating the fastest combination of the photogenerated electrons and holes.32 The decreasing UV emission intensity of the Pt/ZnO demonstrates the slower recombining rate of excited electron-hole pairs, resulting from the electrons transfer from ZnO surface to Pt nanoparticles. After coated by the HTC layer, the UV-emission peak of the ZnO@C and Pt/ZnO@C further declines, suggesting that both the coated HTC layer and the deposited Pt particles accelerate the electrons migration. Hence, it is reasonable to expect that the hybridized Pt nanoparticles and HTC layer can remarkably improve the photocatalytic performance of the Pt/ZnO@C. On the other hand, it is believed that the visible emissions are generally caused by the electron transitions in surface defect states.32 The augment of the visible emissions of the Pt/ZnO implies that the native defects in ZnO nanowires are increased after the H2 treatment and Pt modification, which is consistent with the UV-vis DRS result above. For the ZnO@C and Pt/ZnO@C, the coated HTC layer could rapidly trap the surface electrons migrated from the ZnO nanowires and Pt particles, thus showing the weakest visible emissions.

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Figure 5. PL spectra of the ZnO NW, Pt/ZnO, ZnO@C and Pt/ZnO@C.

Photocatalytic performance of photocatalysts In the present case, the photocatalytic performance of the resultant catalysts is firstly evaluated by the MB decomposition under the UV irradiation. Prior to the photodegradation reaction, the adsorption/desorption equilibrium is established in dark. The adsorption experiment (Figure 6c) illustrates that two HTC-coated samples, i.e. Pt/ZnO@C and ZnO@C, exhibit the remarkable adsorbing improvement for MB molecules, due to the strong adsorption capacity of the HTC materials for organics.33-34 It should be noted that the saturated adsorption amount of the Pt/ZnO@C is equal with that of the ZnO@C, while the Pt/ZnO is slightly larger than the ZnO NW. It is indicated that the high-temperature H2 treatment produces the surface defects on the Pt/ZnO, which will conduce to the improved surface adsorption for organics. Not only the adsorption capacities, four photocatalysts involved in this 16


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work also show very different photocatalytic activities after being exposed to the UV light. The photodegrading processes are monitored at intervals of 10 min, and the corresponding absorbance of reaction solutions have been recorded in Figure 6a and S5. To expediently analyze the catalytic efficiency of various samples, C/C0 is adopted and shown in Figure 6b, where C and C0 are the absorbance of reaction solution and initial solution at 664 nm, respectively. By comparison with the blank experiment, we can find that all the as-synthesized photocatalysts have good photocatalytic activities for the MB decomposition reaction under the UV irradiation. Particularly, the photocatalytic activity of the Pt/ZnO@C is much higher than the others and finishes the full degradation of MB within 40 min. As the electron reservoirs, Pt particles have been proved to efficiently transfer the surface electrons by the PL spectra, making the separated photogenerated carriers. These long-life electrons and holes will contribute to the elevation of the photocatalytic efficiency. Hence, in this work, the Pt/ZnO@C and Pt/ZnO manifest the higher photocatalytic activities in comparison with the ZnO@C and ZnO NW, respectively. Meanwhile, the reaction kinetics plots of various photocatalysts are fitted to further distinguish the contribution between the Pt particles and the HTC layer to the activity improvement of the Pt/ZnO@C. As shown in Figure 6d, the reaction rates of the Pt/ZnO@C and ZnO@C are obviously higher than that of the Pt/ZnO and ZnO NW, of which their reaction constants are orderly 0.052, 0.020, 0.037, and 0.007 min-1, respectively. It is indicated that the surface HTC layer can also play an important part in the outstanding photocatalytic performance of the Pt/ZnO@C nanocables. 17


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Figure 6. (a) Time-dependent absorbance of MB solution over the Pt/ZnO@C under UV light irradiation; (b)

Photocatalytic degradation of MB solution over the ZnO NW, Pt/ZnO, ZnO@C, Pt/ZnO@C and no catalyst; (c)

Adsorption ratios and (d) kinetics plots of the MB photodegradation over different samples.

Furthermore, the Pt/ZnO@C also exhibits the great photocatalytic activity for the degradation of various colorless organic pollutants under the UV-light irradiation. Figure 7 shows the decomposition process of three antibiotics (TC, ENRH, and CIP) over the Pt/ZnO@C. It is found that all the organics have been rapidly removed by the Pt/ZnO@C, and only 17 % of TC, 8 % of ENRH, and 19 % of CIP are remained after physical adsorption and UV-irradiated photodegradation.

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Figure 7. Photocatalytic degradation of three antibiotics (TC, ENRH, and CIP) over the Pt/ZnO@C under UV

light irradiation.

The good photocatalytic stability is also of importance for the practical application of ZnO photocatalysts, and the photocatalytic activities of various samples are repeatedly investigated in five cycle reactions under the illumination of UV light, as shown in Figure 8. It should be noted that the surface HTC layer endows two HTC-coated products (the Pt/ZnO@C and ZnO@C) with great photostability, and their photocatalytic activities show no clear drop in five cycle reactions. In contrast, the decreasing degradation efficiency could be found in the Pt/ZnO and ZnO NW. This result adequately illuminates that the HTC layer coated on the surface can promote the catalytic stability of ZnO photocatalysts.

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Figure 8. Recycle photocatalytic performance of the ZnO NW, Pt/ZnO, ZnO@C, Pt/ZnO@C for the UV-irradiated

degradation of MB in 60 min.

To figure out the reason of the great stability of HTC-coated photocatalysts, the Pt/ZnO and Pt/ZnO@C were collected after five repeating experiments and measured by XRD (Figure S6). In both XRD patterns, the strong diffraction peaks match well with the wurtzite phase of ZnO, suggesting that the Pt/ZnO and Pt/ZnO@C could maintain the main crystal structure during five cycle photodegradations. Nevertheless, TEM image of the reused Pt/ZnO (Figure 9a) reveals that the surface structures have been badly destroyed owing to the photocorrosion of ZnO. The recovered Pt/ZnO@C exhibits the identical uniform surfaces with the fresh sample (Figure 9b). Meanwhile, the similar result can be supported from TEM images of the ZnO NW and ZnO@C, as shown in Figure S7. Obviously, the coated HTC layer successfully suppresses the ZnO photocorrosion and facilitates the enhanced stability of photocatalysts, and the 20


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detailed mechanism will be discussed in this work below.

Figure 9. TEM images of the (a) Pt/ZnO and (b) Pt/ZnO@C after 5 cycles.

Photocatalytic mechanism of the Pt/ZnO@C nanocable To explore the photocatalytic mechanism of the Pt/ZnO@C, the photoelectrochemical performance is investigated by the photocurrent responses and EIS. Figure 10a reveals the fast and uniform photocurrent responses of the ZnO NW, Pt/ZnO and Pt/ZnO@C electrodes for three on/off cycles of UV irradiation. For three electrodes, their photocurrent intensities remain constant while the light is turned on, which is resulted from the efficient separation of photoinduced carriers in photocatalysts. Both Pt nanoparticles and HTC layer endow the Pt/ZnO@C a faster migration rate of surface electrons compared with the ZnO NW and Pt/ZnO, thus exhibiting the dramatically dual-enhancement on photocurrent intensity. Besides, it can be clearly seen that the photocurrent intensity of the Pt/ZnO is higher than that of the ZnO NW, indicating an increasing electron transfer by the Pt particles. 21


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Figure 10. (a) Photocurrent responses and (b) Nyquist plots of the ZnO NW, Pt/ZnO, and Pt/ZnO@C electrodes in

0.1 M Na2SO4 solution under UV light irradiation.

EIS is a powerful tool in studying the surface charge migration in electrode materials. Figure 10b is the EIS Nyquist plots of three resultant photocatalysts in 0.1 M Na2SO4 aqueous solution under the illumination of UV light. As shown, it is believed that the semicircles at high frequency represent the double-layer capacitance and the charge-transfer resistance at the contact interface between the electrode and electrolyte solution. Generally, the depressed semicircle at high frequencies reflects an effective separation process of the photogenerated charge carriers and a fast interfacial charge migration. After the modification of Pt nanoparticle, the surface electron transfer on ZnO nanowires is increased, and further accelerated after coated the HTC layer on the surface. Additionally, we also compare the EIS Nyquist plots of the Pt/ZnO@C in dark and under UV light irradiation, as shown in Figure S8. The charge migration efficiency of the Pt/ZnO@C is improved when exposed in UV light, revealing the important role of light-irradiation in the charge separation of the 22


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photocatalysts. Consequently, the photocurrent and EIS results are consistent with the trend of the photocatalytic performance, indicating that both Pt nanoparticles and surface HTC layer can increase the separation efficiency of the photoinduced electron/hole pairs, which is considered as the key factor in affecting the photocatalytic activity of semiconductor catalysts.

Figure 11. Photocatalytic degradation of MB solution over the Pt/ZnO@C with or without the addition of hole and

hydroxyl radical scavenger under UV light irradiation.

By trapping experiments, the reactive species involved in the photocatalytic process were detected to further understand the reaction mechanism. In particular, t-BuOH and EDTA-2Na are chosen as the hydroxyl radical scavenger and hole scavenger, respectively. As shown in Figure 11, the photocatalytic activity of the Pt/ZnO@C has been remarkably suppressed when EDTA-2Na is added, whereas only slightly decreased in presence of t-BuOH. It is demonstrated that the photogenerated holes are 23


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the main oxidative species for the Pt/ZnO@C, although the hydroxyl radicals are also found to decompose a little amount of MB. It is known that the semiconductor material ZnO can capture the photons in the UV light, and be excited to produce conduction band electrons (e-) and valence band holes (h+), as illustrated in Scheme 2 (reaction a). Most of the photogenerated h+ will rapidly recombine with the photogenerated e- within extremely short time (reaction b), leading to the low quantum yield and poor photocatalytic activity of ZnO. However, the residual h+, on the one hand, will oxidize the organic molecules absorbed on the surface to produce many fragmented molecules (reaction c), meanwhile the photogenerated e- reacts with dissolved oxygen to form hydroxyl radicals (HO•, reaction e), which can also degrade the organic molecules (reaction f). Thus, ZnO can effectively catalyze the degradation of organics under the UV light irradiation. On the other hand, the h+ can also be trapped by the surface ZnO in aqueous solution (reaction d), and the resulting Zn2+ is divorced from the material surface. Therefore, as exposed to the UV-light for a long time, the structure of ZnO catalysts will be destroyed, further making the decrease of the photocatalytic property.

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Scheme 2. Proposed mechanism of the photocatalytic organics degradation and the photocorrosion of the

Pt/ZnO@C nanocable under UV light irradiation.

In terms of the PL result and the photoelectrochemical analysis above, we can conclude that both the deposited Pt nanoparticles and the surface HTC layer are able to effectively capture the photogenerated e- to suspend the reaction rate of b, thus dual-increasing the surface concentrations of h+ and e- on the Pt/ZnO@C nanocables. From the adsorption data, it is found that the coated HTC layer significantly elevates the organic adsorption on the surface (inset of Scheme 2). As a result, the increasing concentration of h+, e- and organics in the Pt/ZnO@C nanocables inevitably accelerate the degradation processes including reaction c, e and f, finally improving its overall photocatalytic performance. Moreover, it can be noticed that the photogenerated h+ participates in reaction c and d simultaneously (Scheme 2). The accelerated reaction c will necessarily decrease the reaction rate of d during the photocatalytic process, thus 25


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impeding the photocorrosion of ZnO nanowires. Therefore, the Pt/ZnO@C nanocable exhibits the great photocatalytic stability compared with the ZnO NW and Pt/ZnO, benefiting from the coated HTC layer on the surface. CONCLUSIONS

In summary, the novel Pt/ZnO@C nanocable is successfully fabricated via depositing Pt nanoparticles and then coating the uniform HTC layer with 3-5 nm thickness on the as-obtained ZnO nanowires. From the PL spectra and the photoelectrochemical analysis, it is found that both Pt nanoparticles and coated HTC layer can effectively transfer the surface electrons and prolong the lifetime of the photogenerated charge carriers. Furthermore, due to the great adsorption capacity for organic molecules, the coated HTC layer can also remarkably increase the surface adsorption of the Pt/ZnO@C nanocable. As a result, both the Pt nanoparticles and the HTC layer endow the Pt/ZnO@C nanocable with the dual-enhanced photocatalytic performance for degrading organic pollutants under the UV-light irradiation, while the HTC layer can also improve the photocatalytic stability of the Pt/ZnO@C nanocable. Consequently, the as-obtained Pt/ZnO@C nanocable with the superior photocatalytic performance and great stability can be expected to be a promising catalyst for the industrial wastewater treatment. The functionalized HTC layer would also stimulate the fabrication of other composite photocatalysts with the similar structure and the higher photocatalytic performance.

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Supporting Information. Diameter distribution of the ZnO NW; SEM image of the Pt/ZnO; SEM and TEM images of the ZnO@C; EDX spectrum of the Pt/ZnO@C; Time-dependent UV-vis absorbance of MB solution over the no catalysts, ZnO NW, Pt/ZnO, and ZnO@C; XRD patterns of the ZnO NW, Pt/ZnO, ZnO@C and Pt/ZnO@C after 5 cycles; TEM images of ZnO NW and ZnO@C after 5 cycles; Nyquist plots of the Pt/ZnO@C electrodes in dark and under UV light irradiation.

AUTHOR INFORMATION

Corresponding Author Prof. Jianzhou Gui: [email protected] Prof. Jieshan Qiu: [email protected] Notes The authors declare no competing financial interest.

ACKNOWLEDGMENTS

This work was partly supported by the National Natural Science Foundation of China (No. 21576211) and the China Postdoctoral Science Foundation (No. 2016M590204).

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ZnO@C Nanocable with Dual-Enhanced Photocatalytic

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