TiO2 NBs with enhanced photocatalytic activity towards moxifloxacin degradation

Chemical Engineering Journal 389 (2020) 124476

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Construction of immobilized 0D/1D heterostructure photocatalyst Au/CuS/ CdS/TiO2 NBs with enhanced photocatalytic activity towards moxifloxacin degradation

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Qinghua Chen , Mengmeng Zhang, Jingying Li, Guodong Zhang, Yanjun Xin, Chao Chai College of Resources and Environment, Qingdao Agricultural University, Qingdao Engineering Research Center for Rural Environment, Qingdao, PR China

H I GH L IG H T S

G R A P H I C A L A B S T R A C T

0D/1D Au/CuS/CdS/ • Immobilized TiO NBs heterostructure photo2

• • • •

catalyst was synthesized. The construction of Au/CuS/CdS/TiO2 NBs effectively extended the visible light response. Au/CuS/CdS/TiO2 NBs improved the separation and transfer efficiency of electrons. Au/CuS/CdS/TiO2 NBs exhibited enhanced photocatalytic activity on degradation of MOX. There are eight by-products for the photocatalytic degradation of MOX were detected.

A R T I C LE I N FO

A B S T R A C T

Keywords: 0D/1D heterostructure photocatalyst Photocatalytic activity Moxifloxacin degradation Mechanism Degradation products

Strong absorption in a wide wavelength range and effective charge separation are main factors for improving photocatalytic activity of semiconductor photocatalysts. Herein, immobilized zero-dimensional/one-dimensional (0D/1D) Au/CuS/CdS/TiO2 nanobelts (Au/CuS/CdS/TiO2 NBs) heterostructure photocatalyst with innovative morphology and appropriate band-gap configuration was synthesized for the first time. For this construction, TiO2 NBs provide the 1D structure for a large superficial area and fast electron mobility, 0D CdS and CuS nanoparticles are the visible-light responsive nanomaterials, and Au nanoparticles serve as the surface plasmon resonance effect and electron transfer. The Au/CuS/CdS/TiO2 NBs showed improved photocatalytic performance towards moxifloxacin (MOX) degradation, which can be attributed to the promoted photoabsorption contribution and enhanced separation and transfer of photoexcited electrons and holes. The morphology, constitution, optical and electrochemical properties of the prepared photocatalyst were investigated comprehensively. Moreover, the photocatalytic degradation mechanism and degradation by-products of moxifloxacin were studied. Results showed that the %OH, %O2− and h+ are the main chemical oxidation species responsible for the degradation of MOX. Eight different photocatalytic degradation products were detected. The stability assessment showed that photodissolution plays the primary role in the photocatalytic performance decrease of Au/CuS/CdS/TiO2 NBs. This work gives a promising strategy to rational design efficient hybrid metal-semiconductor photocatalysts for solar energy conversion.

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Corresponding author. E-mail address: [email protected] (Q. Chen).

https://doi.org/10.1016/j.cej.2020.124476 Received 10 October 2019; Received in revised form 12 February 2020; Accepted 15 February 2020 Available online 17 February 2020 1385-8947/ © 2020 Elsevier B.V. All rights reserved.

Chemical Engineering Journal 389 (2020) 124476

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1. Introduction

can overcome the Schottky barrier to inject into semiconductor, which promotes the photocatalysis efficiency. Fang et al [11]. reported Au@TiO2-CdS ternary nanostructures, which showed excellent photocatalytic performance on H2 generation under visible-light illumination due to the convenient transfer path of photoexcited electrons. Yuan et al. [12] prepared hollow mesoporous CdS@TiO2@Au microspheres, and the results showed that under visible-light irradiation this composite photocatalyst possesses outstanding photocatalytic activity and stability on hydrogen evolution. Compared with bulk materials, 0D and 1D materials are favorable for the rapid migration of photogenerated charge carriers [13]. 1D TiO2 nanostructures such as nanowires [14], nanotubes [15], and nanobelts [16] have shown some inherent advantages because of the high specific surface area, abundant exposed edge sites, and fast collection of photoexcited electrons. Therefore, 1D TiO2 nanostructures have exhibited enhanced photocatalytic activity and attracted much attention. To further improve the photocatalytic performance of TiO2, a good strategy for coupling interfaces of two different dimensional nanostructures by loading 0D semiconductor photocatalysts on 1D TiO2 nanostructures (0D/1D) was developed. The formation of 0D/1D nanostructure favors the exciton dissociation and has large contact area, which contribute to the charge carriers transfer and migration [17]. Therefore, in this work, the 0D Au, CuS, and CdS were combined with 1D TiO2 NBs to construct 0D/1D Au/CuS/CdS/TiO2 NBs heterostructure photocatalyst. For this quaternary composition, there are three key advantages: TiO2 NBs provide the 1D structure for a large superficial area and fast electron mobility, CdS and CuS are the visiblelight responsive nanomaterial, and the Au NPs serve as the surface plasmon resonance (SPR) effect and electrons transfer. This composite material was immobilized on the surface of titanium sheets, which is benefits for the recycle and can reduce the energy and material cost. The physical, chemical, and photocatalytic characters of Au/CuS/CdS/ TiO2 NBs heterostructure were systematically investigated. The results demonstrate that this quaternary heterostructure has stronger absorption of sunlight and higher separation efficiency of photoexcited electron-hole pairs, leading to the higher photocatalytic activity on degradation of antibiotics MOX. This present strategy could be used as a basis to synthesize hierarchical 0D/1D heterostructure photocatalyst with high activity on degradation purification of organic pollutants.

In the past few years, the antibiotics released into environment have been considered as emerging pollutants, which has aroused the attention around the world. Fluoroquinolones are essential antibiotics to use for both human beings and animals. Moxifloxacin (MOX) is a thirdgeneration synthetic fluoroquinolone antibiotic agent, which is only be partially metabolized in the body. The MOX residues in natural waters are potential threats to human health and environmental ecosystems. Therefore, the fate and degradation of MOX are the matter of increasing concern in recent years. Photocatalysis is a promising technique to eliminate organic contaminants from environment. TiO2, as one of the most attractive semiconductor photocatalysts, has been widely utilized to degrade organic contaminants [1]. However, the photocatalytic efficiency of TiO2 is limited by two main drawbacks. On the one hand, narrow spectral response interval confines its absorption spectrum under UV region, which only accounts for less than 5% of the whole solar spectrum. On the other hand, the high charge carrier recombination leads to poor quantum yield. Therefore, several methods have been applied to boost the performance and application potential of TiO2 photocatalyst. For example, coupling TiO2 with narrow band gap semiconductors to construct heterojunction structure or loading with noble metal to design a novel reaction system can effectively improve the photoresponse under visible light irradiation and transfer of photoexcited electrons and holes between interfaces [2,3]. Recently, metal chalcogenide nanomaterials have drawn enormous attention for using as cocatalysts owing to their outstanding electronic and optical properties and appropriate band gap potential. Modifying TiO2 by metal chalcogenide semiconductor with narrow band gap would improve the light response and separation and transfer of photogenerated charge carriers [4,5]. Cadmium sulfide (CdS) is an interesting semiconductor with unique energy levels and a narrow band gap about 2.4 eV. This material can absorb and utilize visible light. Unfortunately, several drawbacks significantly obstruct the practical application of bare CdS, including low quantum efficiency and photocorrosion problem. Coupling semiconductor with well-matched energy levels to form heterostructure is a simple and inexpensive means to promote the electron-hole separation by efficient interfacial charge transfer and extended solar utilization. So, the photocatalytic performance will be enhanced after combining the CdS and TiO2, which can improve the separation efficiency of electron-hole pairs and broaden light absorption to visible-light range [6]. CuS is a p-type semiconductor material, which can be used to improve the photocatalytic performance because of its narrow band gap (2.2 eV), abundance, and low cost. Using CuS to construct proper heterojunction with staggered CB and VB positions can facilitate separation of photoinduced charge carriers and improve photochemical activity [7]. Through optimizing the energy band positions the photogenerated electrons of CuS can migrate to the combined semiconductor with a lower CB, while the photogenerated holes of combined semiconductor can transfer to the VB of CuS, leading to a higher quantum efficiency [8]. Song et al [9]. reported that synthesized CuS/ WO3 heterostructure by in situ solution method reveals much higher photocatalytic performance than individual WO3 and CuS. Especially, acting as a cocatalyst CuS can increase the visible light absorption, which makes it a promising light-sensitive material to utilize the sunlight effectively [4]. Plasmonic photocatalysis is an effective method to enhance the photocatalytic activity of semiconductor. Recently, act as cocatalyst Au nanoparticles (Au NPs) have been combined with semiconductor photocatalysts to suppress the charge recombination and supply reactive sites for photocatalysis [10]. Furthermore, the localized surface plasmon resonance (LSPR) of Au NPs can improve the light harvesting ability and local electric field intensity of compound photocatalyst. For Au-semiconductor nanocomposites, the plasmon-induced hot electrons

2. Experimental 2.1. Chemicals The titanium foils were purchased from Dongguan Futai Metal Material Co., Ltd. The ethylene glycol ((CH2OH)2) was purchased from Tianjin Yongda Chemical Reagent Co., Ltd. MOX hydrochloride monohydrate (C21H24FN3O4·HCl·H2O), sodium sulfide nonahydrate (Na2S·9H2O), and benzoquinone (C6H4O2) were provided by Aladdin (China). Ammonium fluoride (NH4F), cadmium nitrate tetrahydrate (Cd (NO3)2·4H2O), copper nitrate trihydrate (Cu(NO3)2·3H2O), Tetrachloroauric acid tetrahydrate (HAuCl4·4H2O), ammonium oxalate monohydrate (C2H8N2O4·H2O), and t-butyl alcohol (C4H10O) were purchased from Sinopharm Chemical Reagent Co., Ltd. 2.2. Synthesis TiO2 NBs were produced by anodization in a 100 ml ethylene glycol solution with deionized (DI) water (5.0 vol%) and NH4F (0.5 wt%)[18]. The Titanium foil (100 mm × 10 mm × 0.5 mm) was used as working electrode and platinum foil was used as the cathode. The voltage, temperature, and reaction time was 60 V, 25℃, and 3 h, respectively. After anodization, the TiO2 NBs were annealed in a furnace in air atmosphere at 550 ℃ for 2 h. The CuS/CdS/TiO2 NBs were synthesized by successive ionic layer adsorption and reaction (SILAR) method. The prepared TiO2 NBs was 2

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Fig. 1. Schematic illustration of the synthesis of Au/ CuS/CdS/TiO2 NBs.

solution (35 ml, 5 mg L−1). Firstly, the reaction system was stored in dark for 30 min under stir to ensure the adsorption-desorption equilibrium. The illumination light source was provided by a 35 W Xenon lamp (simulating sun light) with distance of 3 cm away from photocatalysts. The residual of MOX was analyzed by a Shimadzu LC-10A high performance liquid chromatography (HPLC) at a given time interval. A ZORBAX SB-C18 column (4.6 mm × 150 mm, 5 μm) with mobile phase containing water (HCOOH 0.1%, v/v) and acetonitrile (85:15) was used for the chromatographic analysis of MOX. The analysis wavelength and flow rate were 296.0 nm and 1.2 ml min−1, respectively. To explore the active species responsible for the photocatalytic oxidization decomposition of MOX by Au/CuS/CdS/TiO2 NBs photocatalyst, the hydroxyl radical (%OH), superoxide radicals (%O2−), and holes (h+) were captured by tert-butylalcohol (TBA, 2 ml), p-benzoquinone (BQ, 2 mg), and ammonium oxalate (AO, 0.02 g), respectively. The experiment process was same to the photocatalytic activity test except for the addition of scavengers. The quantitative results and identification of degradation products of MOX were obtained by HPLC/ MS/MS (Orbitrap Fusion Lumos, Thermo Scientific and Agilent 1290 Infinity/6460) with a C18 (150 × 4.6 mm, 5 μm) column. The photocatalytic treatment time was 150 min. The mobile phase was water (HCOOH 0.5%, v/v) – acetonitrile (85:15). The flow rate was 1.2 ml min−1. The column temperature and injection volume were 30 ℃ and 5 μL, respectively. After degradation, the reaction solution was sampled to measure the concentration of released Cd2+ and Cu2+ by inductively coupled plasma-optical emission spectrometry (ICP, CPOES-Optima 8x00, America PE).

immersed in ethanol solution of Cd(NO3)2 (0.01 M) for 5 min followed by rinsing with ethanol, and then dipped into methanol solution of Na2S (0.01 M) for 5 min, rinsed with methanol. All these procedures were termed as one SILAR cycle. Nine cycles were carried out to construct CdS decorated TiO2 NBs (CdS/TiO2 NBs). Subsequently, dipping the CdS/TiO2 NBs into ethanol solution of Cu(NO3)2 (0.01 M) and methanol solution of Na2S (0.01 M) for 5 min each, respectively, and this procedure was repeated 9 times to get CuS/CdS/TiO2 NBs. The preparation of CuS/TiO2 NBs was similar with CdS/TiO2 NBs except for ethanol solution of Cu(NO3)2 (0.01 M). Electrodeposition was used to assemble Au NPs on the surface of CuS/CdS/TiO2 NBs. The reaction system was a three-electrode system. The working electrode was CuS/CdS/TiO2 NBs, the counter electrode was a platinum foil, and the reference electrode was saturated calomel electrode (SCE). The bath solution was 0.2 mM HAuCl4 aqueous solution containing 0.05 M Na2SO4. The operating voltage and time were −0.2 V and 10 s, respectively. The synthesis process was shown in Fig. 1. 2.3. Characterization The scanning electron microscopy (SEM) images were recorded by a field emission scanning electron microscope (SEM, FEI Quanta 200F) with energy dispersive X-ray spectroscopy (EDX). The images of transmission electron microscopy (TEM) and high-resolution transmission electron microscopy (HRTEM) were obtained using a field emission transmission electron microscope (TEM, Tecnai F-30ST). The elemental mapping details were confirmed by a field emission scanning electron microscope (Hitachi S-4800). X-ray photoelectron spectroscopy (XPS) measurement was performed using Thermo Fisher ESCALAB 250Xi with an Al Kα X-ray radiation source (hν = 1486.6 eV). Raman spectrum was recorded by a Raman spectrometer (Thermo Scientific DXR) equipped with a 532-nm laser. The measurement of UV–vis diffuse reflectance spectrum (UV–vis DRS) was carried out using a UV–vis spectrophotometer (Shimadzu UV-2550) with BaSO4 as the standard reference. The photoluminescence properties were investigated on a fluorescence spectrometer (F-4600, Hitachi) with excitation wavelength of 315 nm. The open-circuit photovoltage (Voc) and electrochemical impedance spectroscopy (EIS) measurements were carried out by an electrochemical workstation (AUTOLAB PGSTAT128N, Metrohm) in a standard three-electrode quartz cell. The electrolyte was 0.5 M Na2SO4 aqueous solution. The working electrode was prepared photocatalyst. The counter electrode and reference electrode were platinum foil and Ag/AgCl electrode, respectively. The EIS measurement was performed with frequency range from 100 kHz to 0.01 Hz. A 35 W Xenon lamp system was used as the irradiation light source. The concentration of Cd2+, Cu2+, and Au3+ were detected by inductively coupled plasmaoptical emission spectrometry (ICP, CP-OES-Optima 8x00, America PE).

3. Results and discussion 3.1. Characterization of photocatalyst The morphology and microstructures of Au/CuS/CdS/TiO2 NBs were investigated by SEM and TEM. In Fig. 2a, we can see that the asprepared TiO2 NBs show 1D belt-like structure with width of 30–50 nm, length of 2–3 μm, and thickness of several nm. It was clear that there are no impurities on the surface of TiO2 NBs. After modification, the entire surface of TiO2 NBs was covered by dense Au/CuS/CdS NPs (As shown in Fig. 2b), revealing that the compound photocatalyst Au/CuS/ CdS/TiO2 NBs were synthesized successfully. To measure the yield of CdS, CuS, and Au, the ion concentration of reaction solution before and after deposition reaction were detected. Results showed that the deposition amount of CdS, CuS, and Au is about 0.0036 g·cm−2, 0.0019 g·cm−2, and 0.00020 g·cm−2, respectively. The TEM measurement further displays the hierarchical structure of Au/CuS/CdS/TiO2 NBs. As shown in Fig. 2c, the Au/CuS/CdS NPs were distributed over the TiO2 NBs, which possess 0D nanospheres structure with a diameter around 3–10 nm. Furthermore, the HRTEM image (Fig. 2d) testifies sufficiently the heterostructure of Au/CuS/CdS/TiO2 NBs, which resulting in the formation of heterojunction interface. The measured lattice spacing 0.235 nm is in accord with the Au (1 1 1) plane [19]. The lattice spacing of 0.304 nm is assigned to the (1 0 2) lattice plane of CuS [20]. The lattice fringes with d-spacing of about 0.316 nm can be ascribed to the (1 0 1) plane of CdS [21]. And the lattice spacing of 0.350 nm corresponds to the (1 0 1) plane of anatase TiO2 [16]. The HRTEM results showed that Au, CuS, and CdS NPs are closely

2.4. Photocatalytic activity measurement The degradation of antibiotic MOX was performed to evaluate the photocatalytic performance of prepared photocatalysts. The photocatalytic reactions were carried out in a quartz reactor. The as-prepared photocatalysts (area: 4 cm2; thickness: 0.5 mm; weight: 0.89 g, including oxide layer and matrix) was immersed into the MOX aqueous 3

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Fig. 2. SEM images of TiO2 NBs (a) and Au/CuS/CdS/TiO2 NBs (b). TEM (c) and HRTEM (d) images of Au/CuS/CdS/TiO2 NBs.

Room temperature Raman spectra of Au/CuS/CdS/TiO2 NBs were recorded to analyze their composition, and the results were shown in Fig. 5. The peak positions located at about 144 cm−1, 399 cm−1, 514 cm−1, and 637 cm−1 are consistent with the Eg, B1g, A1g, and Eg modes of the anatase phase TiO2 [26]. A sharp characteristic peak appears at 468 cm−1 can be assigned to hexagonal crystal structure of CuS [4,27]. However, the characteristic peaks of CdS were not be found according to the Raman patterns, which could because of the shielding effect of CuS deposited on the surface of CdS NPs.

assembled on TiO2 NBs. It can be concluded that the hierarchical 0D/ 1D heterostructure was formed between the four components, which would promote the transfer of photoexcited electrons. To further verify the composition of heterostructure photocatalyst Au/CuS/CdS/TiO2 NBs, XPS was used to analyze the surface elemental composition and chemical states. The XPS survey spectrum (Fig. 3a) shows the existence of S, Cu, Cd, Ti, and O elements. Moreover, the high-resolution XPS spectra were applied to analyze the chemical valence state of S, Cd, Cu, and Au elements, and the XPS spectra of S 2p, Cd 3d, Cu 2p, and Au 4f were displayed in Fig. 3b–e, respectively. As revealed in Fig. 3b, two typical peaks of S 2p were detected at 162.3 and 163.5 eV, which are assigned to the binding energies of S 2p3/2 and S 2p1/2 states of S2−, respectively [22,23]. At binding energies of 405.8 and 412.5 eV, there are doublet peaks with a spin–orbit separation of 6.7 eV (Fig. 3c), which are in accordance with the characteristic peaks of Cd 3d5/2 and Cd 3d3/2 orbits, confirming the existence of Cd2+ [24]. From Fig. 3d, the binding energies recorded at 932.2 and 952.2 eV are the characteristic peaks of Cu 2p3/2 and Cu 2p1/2, respectively [9]. Fig. 3e presents the characteristic doublet peaks located at binding energy of 84.5 and 88.2 eV, which can be ascribed to Au 4f7/2 and Au 4f5/2 of zero valent Au [25]. In addition, the elemental composition of Au/CuS/CdS/TiO2 NBs was detected by EDX and elemental mapping. It can be seen from Fig. 3f and Fig. 4 that the EDX and elemental mapping spectra also confirm the presence of S, Au, Cd, Cu, O and Ti elements. These results suggested that the Au/CuS/CdS/TiO2 NBs composite materials were successfully constructed.

3.2. Photocatalytic performance To evaluate the photocatalytic degradation activity of Au/CuS/CdS/ TiO2 NBs composite material, the MOX, a typical antibiotic pollutant, was photocatalytic decomposed under simulated solar light irradiation. It can be seen from Fig. 6 that the photolysis was almost negligible. The photocatalytic purification effect of MOX by Au/CuS/CdS/TiO2 NBs photocatalyst was higher than that by its counterparts. Specifically, after reaction of 60 min the removal efficiency was 75.4% for Au/CuS/ CdS/TiO2 NBs, higher than those achieved by TiO2 NBs (55.3%), CdS/ TiO2 NBs (59.6%), CuS/TiO2 NBs (63.1%), and CuS/CdS/TiO2 NBs (69.5%). These results indicate that combining Au/CuS/CdS and TiO2 NBs could enhance the photocatalytic degradation performance. It is well known that there are three major factors which determine the photocatalysis efficiency. They are the utilization efficiency of light, migration of photoexcited charge carriers, and recombination of 4

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Fig. 3. XPS spectra of Au/CuS/CdS/TiO2 NBs: (a) full-scale survey spectrum and (b–e) high-resolution XPS spectra of (b) S 2p, (c) Cd 3d, (d) Cu 2p, and (e) Au 4f. (f) EDX spectrum of Au/CuS/CdS/TiO2 NBs.

[30]. It is important to figure out that the SPR absorption of Au NPs is too weak to be detected due to the few deposition quantity [31]. Moreover, the band gaps (Eg) of the as-synthesized samples were calculated according to the intercept of the tangents of the plots from Kubelka-Munk function [αhν]1/2 versus the photon energy [22,30,32]. The calculated Eg values for Au/CuS/CdS/TiO2 NBs, CuS/CdS/TiO2 NBs, CuS/TiO2 NBs, CdS/TiO2 NBs, and TiO2 NBs were 2.50, 2.63, 2.82, 2.87, and 2.95 eV, respectively. Results showed that there is strong interaction or contact interfaces between composite Au/CuS/CdS NPs and the support TiO2 NBs, which effectively extend the visible light response [33]. Therefore, it can be expected that the Au/CuS/CdS/TiO2 NBs heterostructure would possess higher photocatalytic activity

electrons and holes [28,29]. Therefore, to understand the improvement of photocatalytic performance of Au/CuS/CdS/TiO2 NBs some characterizations aim at above three factors were conducted. The light absorption property of semiconductors is a key parameter in the assessment of their photocatalytic activity. Thus, to explore the raised photocatalytic efficiency of Au/CuS/CdS/TiO2 NBs hybrid system, the characterization of light absorption was measured by UV–vis diffuse reflection spectroscopy. As shown in Fig. 7, the absorption intensity of Au/CuS/CdS/TiO2 NBs heterostructure in UV-light and visible light region was improved compared with other samples. The reasons for enhanced light absorption may be that the sensitization of Au, CuS, and CdS NPs leads to the better light harvesting capacity

5

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Fig. 4. EDX elemental mapping of Au/CuS/CdS/TiO2 NBs.

the Voc of Au/CuS/CdS/TiO2 NBs is more negative than other samples, manifesting that coupling of Au/CuS/CdS and TiO2 NBs can improve the separation of e−-h+ pair and electron accumulation on the surface of Au/CuS/CdS/TiO2 NBs. For TiO2, CdS, and CuS, the relative CB position is different, which can form the gradient band gap structure [18]. So, when TiO2, CdS, and CuS NPs are attached to construct composite photocatalyst, a local electric field will be formed. Consequently, the photogenerated electrons can migrate quickly from the CB of CuS into CdS, then to the CB of TiO2. Therefore, for Au/CuS/CdS/ TiO2 NBs the excited electrons can be separated and collected more effectively, leading to the improvement of Voc. EIS is also a powerful tool to investigate the interior resistance and charge transfer kinetics of photocatalysts [37]. As displayed in Fig. 8b. the Au/CuS/CdS/TiO2 NBs heterostructure photocatalyst reveals the smallest semicircle at high frequencies under irradiation of xenon lamp (35 w), indicating that the Au/CuS/CdS/TiO2 NBs possess more efficient interfacial transfer and separation of photoexcited charges than that of other samples [25,38]. The results of Voc and EIS measurement demonstrate definitely that

because of the enhanced utilization efficiency for solar light, which means more photoexcited charge carriers participating in photocatalytic reaction. The photoelectrochemical measurement is an efficient method to explore the properties of separation and transfer about photoexcited charges in composite heterostructure photocatalysts, hence, the Voc and EIS were detected. Under illumination, the electrons located at valence band (VB) can be excited and transfer to conduction band (CB) of photocatalyst. For the nanostructured semiconductor films, the photogenerated electrons accumulate at the photocatalyst surface under open-circuit conditions. The Voc corresponds to the difference in Fermi levels between the photoanode and the counter electrode. The Fermi levels would shift to a more negative potential because of the electron accumulation, which leads to the increase of Voc [34,35]. Therefore, the measurement of Voc can be used to investigate the transfer and separation of electrons. The increase of Voc means a better separation of e−-h+ pair and electron accumulation in semiconductor–semiconductor heterojunctions [36]. As displayed in Fig. 8a, 6

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Fig. 5. Raman spectrum of Au/CuS/CdS/TiO2 NBs.

Fig. 7. (a) UV–vis DRS and plots of (αhν)1/2 versus photon energy (hν).

cascade energy level structure was formed and a strong overlapping of band structure was constructed in hybrid heterostructures. The perfectly matched energy level between TiO2 NBs, CdS, and CuS accelerates the electron migration due to its higher driving force, which contributes to the efficient separation of photogenerated charges [40]. Based on the above analyses, the migration of photogenerated carriers for Au/CuS/CdS/TiO2 NBs heterostructure was illustrated in Fig. 10. Under irradiation, the electrons are excited and transited from VB to CB. Due to the potential difference, that is, CB of CuS and CdS is more negative than that of TiO2, the interface electron transfer is promoted. So, the photogenerated electrons can easily jump in the CB from CuS and CdS to TiO2 NBs. Meanwhile, photogenerated holes transfer in the VB from TiO2 NBs to CuS and CdS. In addition, Au NPs were evenly distributed on the surface of CuS/CdS/TiO2 NBs, which can improve the production and transfer of photogenerated electrons. On the one hand, the Au NPs can act as light sensitizers to enhance light-harvesting efficiency and inject photogenerated electrons to the CB of CuS [41]. On the other hand, the photoexcited electrons accumulated in the CB of TiO2 NBs can partly inject to Au NPs, which suppresses the electronhole recombination effectively [11,12,28,42]. The photoexcited electrons located at the CB of TiO2 NBs and Au NPs can be captured by absorbed O2 to produce %O2−. Moreover, the residual holes in the VB of TiO2 NBs can yield %OH through reaction with OH−. Finally, the holes, %O2−, and ·OH radicals all directly or indirectly take part in the oxidative degradation of MOX. To confirm the above analysis the control experiments adding radicals trapping agents into reaction system were carried out. Here, TBA was introduced to scavenge %OH, AO was used as the scavenger of h+, and BQ was adopted as the scavenger of %O2−

Fig. 6. Photocatalytic activity assessment of Au/CuS/CdS/TiO2 NBs by degrading moxifloxacin.

coupling Au/CuS/CdS and TiO2 NBs to make heterostructure could improve the separation and interfacial transport efficiency of the photogenerated electrons, which can promote photocatalytic performance. The PL signal represents the recombination of photoexcited electrons and holes. It is well known that the lower PL intensity signifies the higher separation efficiency of electrons and holes [39]. Hence, the PL spectra of photocatalysts were used to estimate the photogenerated electron-hole separation capacity. Fig. 9 shows the PL spectra detected under excitation wavelength of 315 nm. It can be observed that the Au/ CuS/CdS/TiO2 NBs present the lowest PL intensity, suggesting the highest separation efficiency of electrons and holes. This result indicates that the recombination of electron and hole was weakened because of the formation of 0D/1D Au/CuS/CdS/TiO2 NBs heterostructure, which is beneficial for promoting the photocatalysis [31]. 3.3. Possible photocatalytic mechanism The reasonable mechanism for the interfacial electron transfer process and degradation reactions over Au/CuS/CdS/TiO2 NBs heterostructure was proposed. As we know that the band edge potential of semiconductor is the key property which determine the photocatalytic activity of heterostructure. The CB and VB potential of the composite Au/CuS/CdS/TiO2 NBs were displayed in Fig. 10. It is obvious that the 7

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Fig. 10. Schematic illustration of possible photocatalytic mechanism of Au/ CuS/CdS/TiO2 NBs.

Fig. 8. (a) Open-circuit voltage response curves and (b) Nyquist plots of electrochemical impedance spectroscopy (EIS).

Fig. 11. Photocatalytic degradation curves of moxifloxacin under trapping of radical species.

3.4. Identification of degradation products In this section the major degradation products of MOX were determined by HR-MS. There were eight degradation products be detected during MOX photocatalytic degradation. The proposed chemical structures of the detected products are shown in Fig. S1. On a basis of the proposed chemical structures, pathways for the photocatalytic transformation of MOX are elucidated in Fig. 12. It can be found that MOX molecule degradation can occur at different functional moieties, and the degradation may proceed on three different pathways. Among them, compound P1 represents defluorination and cleavage of the piperazine moiety, which further transforms to compound P2 through demethylation and dehydroxylation. In the next step, loss of carboxyl group and cyclopropyl group leads to compound P3*. In the other path, firstly, degradation occurs at the F substituent, forming the compound P4. Further, loss of the piperazine moiety leads to compound P5. There are two possible paths for the subsequent degradation of compound P5. On the one hand, demethylation leads to the production of compound P6*. Further degradation may occur by decarboxylation, yielding the compound P7*. On the other hand, compound P8* is proposed arise from the degradation removal of methoxy group. And further oxidative degradation through dehydroxylation generates compound P9*. Compound P10 is result from the opening of the piperazine ring and

Fig. 9. PL emission spectra of Au/CuS/CdS/TiO2 NBs, CuS/CdS/TiO2 NBs, CdS/ TiO2 NBs, CuS /TiO2 NBs, and TiO2 NBs.

[43,44]. As shown in Fig. 11, compared with the contrastive system the removal efficiency of MOX decreased after introducing TBA, AO, or BQ. Therefore, we can conclude that the %OH, h+, and %O2− are the active species responsible for the degradation of MOX, of which, %O2− is the main active species, h+ is the secondary active species, and ·OH is the tertiary active species for the photocatalytic decomposition purification of MOX by Au/CuS/CdS/TiO2 NBs nanocomposite. 8

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Fig. 12. The proposed transformation pathway for the photocatalytic degradation of moxifloxacin by Au/CuS/CdS/TiO2 NBs (The detected products in this work were labelled by asterisk).

Fig. 14. The photocatalytic stability of Au/CuS/CdS/TiO2 NBs on degradation of moxifloxacin.

Fig. 13. Peak areas of photocatalytic degradation products detected in the liquid phase as a function of photocatalytic treatment time.

leads to the compound P13*. In addition, Fig. 13 shows the quantitative evolution and decay curves of MOX degradation by-products. It can be found that the content of by-products P3*, P6*, P7*, P8*, and P9* increased firstly and then decreased. The concentration of by-product P11* kept continuous growth from 30 to 150 min. However, the by-

subsequent stepwise oxidative degradation [45]. The compound P11* arises by oxidative demethylation of the aliphatic amine moiety [46], and its further demethylation generates the compound P12*. Further, reductive dehalogenation combined with loss of cyclopropyl group 9

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fluorine conveys enhanced DNA Gyrase potency and cell penetration. Moreover, most of the identified degradation products possess increased hydrophilicity, leading to the passive diffusion of their uncharged structures through the cell membrane might decrease when compared with uncharged MOX [45,47]. 3.5. Photocatalyst stability For photocatalyst, the stability is an important factor to assess its performance. Herein, the stability of Au/CuS/CdS/TiO2 NBs was estimated by cyclic decomposition of MOX. As displayed in Fig. 14, the photocatalytic activity of Au/CuS/CdS/TiO2 NBs has an obvious decrease after three cycles, which may be ascribed to the photocorrosion or photodissolution occurred on the surface of photocatalyst [48–50]. In order to further explore the reasons for the unsatisfactory stability of Au/CuS/CdS/TiO2 NBs, the XRD and XPS spectra of both fresh and used samples were obtained. It can be found from Fig. 15 that there is no obvious deviation for the locations of diffraction peaks, however, there is slight decline for the peak intensity of CdS and CuS. Furthermore, the comparison of XPS patterns (Fig. 16) showed that there is no distinguishable shift for the binding energy after three cycles of photocatalytic reaction, while the intensity of S 2p, Cu 2p, Cd 3d, and Au 4f after three recycle experiments has evident decrease. From the results, we can speculate that the main reason for the decline of photocatalytic performance of Au/CuS/CdS/TiO2 NBs is photodissolution. To confirm the photodissolution of Au/CuS/CdS/TiO2 NBs during photocatalytic reaction, ICP measurement of released Cd2+ and Cu2+ concentration in reaction solution was performed [51,52]. Results showed that after photocatalytic reaction for 60 min, the concentration of Cd2+ and Cu2+ in reaction solution were 5590.40 and 64.81 μg L−1, respectively.

Fig. 15. Comparison of XRD patterns for the fresh and used (three cycles) Au/ CuS/CdS/TiO2 NBs.

products P12* and P13* were below detection limit in the first 90 and 120 min, respectively, and then increased rapidly. It is reported that the antimicrobial activity of degraded products decreases due to the transformation and/or degradation of the auxiliary groups of parent compound. Some degradation products, especially those conserving fluoroquinolone structure, possess antimicrobial activity. Defluorination was observed for some degradation products, which can significantly decrease the MOX biological activity because

Fig. 16. Comparison of XPS patterns for the fresh and used (three cycles) Au/CuS/CdS/TiO2 NBs. 10

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Therefore, the loss of CdS and CuS was calculated according to the ion concentration of Cd2+ and Cu2+ in reaction solution, results showed that the loss of CdS and CuS was 1.747 and 0.0448%, respectively. Evidently, the ICP measurement results agree with the above conjecture, suggesting that photodissolution plays the primary roles in the photocatalytic activity decrease of Au/CuS/CdS/TiO2 NBs.

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4. Conclusions In summary, a novel immobilized 0D/1D Au/CuS/CdS/TiO2 NBs heterostructure photocatalyst was designed and fabricated by anodic oxidation, SILAR, and electro-deposition. The as-prepared TiO2 NBs showed belt-like 1D structure and the Au/CuS/CdS NPs possessing 0D nanospheres structure with a diameter around 3–10 nm were distributed over the TiO2 NBs. The characterizations of XPS, EDX, elemental mapping, and Raman spectra suggested that the Au/CuS/CdS/ TiO2 NBs composite material was successfully constructed. This heterostructure photocatalyst showed higher photocatalytic efficiency for MOX degradation because this quaternary heterostructure has enhanced utilization efficiency for solar light and higher separation and interfacial transport efficiency for photogenerated electrons. Under irradiation, the electrons were excited and transited in the CB from CuS and CdS to TiO2 NBs. Meanwhile, photogenerated holes transferred in the VB from TiO2 NBs to CuS and CdS. Au NPs can improve the production and transfer of photogenerated electrons. The ·OH, h+, and % O2− were the main active species responsible for the photocatalytic oxidization decomposition of MOX. Eight different degradation products during MOX photocatalytic oxidization decomposition were identified and quantitative analysis. Unfortunately, the photocatalytic stability of Au/CuS/CdS/TiO2 NBs needs to further improve. Declaration of Competing Interests The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. Acknowledgments This work was supported by the Natural Science Foundation of Shandong Province (No. ZR2019MD012), Support Plan on Youth Innovation Science and Technology for Higher Education of Shandong Province (No. 2019KJD014), and National Natural Science Foundation of China (No. 51678323). The authors sincerely thank the Central Laboratory of Qingdao Agricultural University for providing the support. Appendix A. Supplementary data Supplementary data to this article can be found online at https:// doi.org/10.1016/j.cej.2020.124476. References [1] Q.H. Chen, S.N. Wu, Y.J. Xin, Synthesis of Au-CuS-TiO2 nanobelts photocatalyst for efficient photocatalytic degradation of antibiotic oxytetracycline, Chem. Eng. J. 302 (2016) 377–387. [2] C.W. Lai, S. Sreekantan, Study of WO3 incorporated C-TiO2 nanotubes for efficient visible light driven water splitting performance, J. Alloy. Compd. 547 (2013) 43–50. [3] T. Nogawa, T. Isobe, S. Matsushita, A. Nakajima, Ultrasonication effects on the visible-light photocatalytic activity of Au-modified TiO2 powder, Mater. Lett. 90 (2013) 79–82. [4] J. Wu, B.B. Liu, Z.X. Ren, M.Y. Ni, C. Li, Y.Y. Gong, W. Qin, Y.L. Huang, C.Q. Sun, X.J. Liu, CuS/RGO hybrid photocatalyst for full solar spectrum photoreduction from UV/Vis to near-infrared light, J. Colloid Interf. Sci. 517 (2018) 80–85. [5] J.Y. Do, R.K. Chava, S.K. Kim, K.Y. Nahm, N.K. Park, J.P. Hong, S.J. Lee, M. Kang, Fabrication of core@interface:shell structured CuS@CuInS2:In2S3 particles for highly efficient solar hydrogen production, Appl. Surf. Sci. 451 (2018) 86–98.

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