Enhanced visible light photocatalytic activity of g-C3N4 decorated ZrO2-x nanotubes heterostructure for degradation of tetracycline hydrochloride

Journal of Hazardous Materials 384 (2020) 121275

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Enhanced visible light photocatalytic activity of g-C3N4 decorated ZrO2-x nanotubes heterostructure for degradation of tetracycline hydrochloride ⁎

Qingling Chena, Wulin Yanga,b, , Jiajun Zhua, Licai Fua, Deyi Lia, Lingping Zhoua, a b

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College of Materials Science and Engineering, Hunan University, Changsha, 410082, China Hunan Province Key Laboratory for Spray Deposition Technology and Application, Hunan University, Changsha, 410082, China

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

A R T I C LE I N FO

A B S T R A C T

Editor: Danmeng Shuai

Photocatalytic degradation is considered as a promising strategy to address the environmental threat caused by antibiotics abuse. Visible light driven g-C3N4 decorated ZrO2-x nanotubes heterostructure photocatalysts for antibiotic degradation were successfully synthesized by anodic oxidation and following a thermal vapor deposition method. Compared with pure g-C3N4 or ZrO2-x nanotubes, the composite photocatalysts exhibited more extended visible light response and higher separation rate of photo-generated electron-holes pairs. The optimized heteroctructure with 7.1 wt.% g-C3N4 exhibited 90.6% degradation of tetracycline hydrochloride (TC-H) under 1 h visible light irradiation. The mainly active species of TC-H degradation were photo-generated h+ and % − O2 . The pathway of charge migration in the g-C3N4/ZrO2-x NTs system was also studied and a possible photocatalytic mechanism was proposed for TC-H degradation. Constructing the g-C3N4/ZrO2-x nanotubes heterostructure is anticipated to be an effective strategy for photocatalytic degradation of antibiotics.

Keywords: Band structure Charge transfer Heterostructure Antibiotic photocatalysis

1. Introduction With the widespread application of antibiotics, the incomplete wastewater treatment leads to the continuous discharge of pharmaceutical residues into aquatic environments. The residual antibiotics pose a serious threat to ecosystem (Zheng et al., 2017; Zeng et al., 2018a). In order to remove these pharmaceutical chemicals, various

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conventional methods, including biological treatment, chlorination and adsorption, have been employed (Yu et al., 2016). However, the high cost, the risk of toxic byproducts and insufficient efficiency etc. of these methods limit their application. Hence, it is urgent to find a more direct and effective approach to dealing with antibiotic pollutants. Low-cost semiconductor photocatalysis considered as one of the most promising green techniques has attracted much attention recently,

Corresponding authors at: College of Materials Science and Engineering, Hunan University, Changsha, 410082, China. E-mail addresses: [email protected] (W. Yang), [email protected] (L. Zhou).

https://doi.org/10.1016/j.jhazmat.2019.121275 Received 24 June 2019; Received in revised form 19 September 2019; Accepted 20 September 2019 Available online 26 September 2019 0304-3894/ © 2019 Elsevier B.V. All rights reserved.

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(HF, AR), glycerine (C3H8O3, AR), isopropanol alcohol (IPA, AR), disodium ethylenediaminetetraacetate dehydrate (EDTA-2Na, AR), benzoquinone (BQ, AR), sodium sulfate (Na2SO4, AR), melamine (C3H6N6, AR), were purchased from Sinopharm Group CO., Ltd, Shanghai, China. Tetracycline hydrochloride (TC-H, AR) was purchased from Aladdin Reagent Company, China. All regents used in the experiments without further purification.

which can completely degrade antibiotic pollutants in water under visible light irradiation with no harmful byproducts (Anku et al., 2016; Vattikuti and Byon, 2017; Zeng et al., 2017). ZrO2 photocatalyst has attracted extensive research interest due to its remarkable physicochemical stability and unique electronic band structures (Zhang et al., 2017, 2016; Alalm et al., 2016; Bailon-Garcia et al., 2017). However, the ZrO2 photocatalyst can only absorb the ultraviolet light that occupies no more than 4% of the solar spectrum due to the wide band gap of ZrO2 (around 5.1 eV), which restricts the practical application of ZrO2 as a photocatalyst. In addition, the low separation rate of the photogenerated carriers in the material also limits the photocatalytic activity of ZrO2. Among the various strategies for suppressing the recombination of photo-generated carriers and enhancing light harvesting, coupling ZrO2 with other narrow band gap semiconductor, such as TiO2, MoS2, Ag3PO, g-C3N4 etc. (Anwer and Park, 2018; Vattikuti et al., 2016; Guerrero-Araque et al., 2017; Ke et al., 2014; Wang et al., 2014), is the most effective way. Notably, however, ZrO2 only serves as either a catalyst carrier or an intermediary for maintaining strong redox abilities of photo-generated electrons or holes in most case. With persevering efforts, extending the spectral response of ZrO2 from ultraviolet to visible region has come true. Not only ion doped ZrO2 but also oxygen-deficient ZrO2-x exhibits reduced band gap for visible light photocatalysis (Zhang et al., 2018; Wang et al., 2018a; Kumar and Ojha, 2015; Mzoughi et al., 2016). Coupling visible light responsive zirconia with other semiconductor will have a profound value for improving light utilization and promoting photocatalytic activity of the material, however, little attention has been paid to this field. Among the various semiconductors, g-C3N4 stands out due to its great solar utilization, non-toxicity and easy-preparation (Vattikuti et al., 2018a; Li et al., 2018; Dong et al., 2018). Especially, the high conduction band position suggests that g-C3N4 maybe sensitize ZrO2 very well. However, small specific surface area and high recombination efficiency of photo-generated carriers are the main shortcomings of gC3N4 (Chen et al., 2019a; Nagajyothi et al., 2017; Vattikuti et al., 2018b). Coupling g-C3N4 with nano-structured ZrO2 can enlarge specific surface area and improve charge carrier separation. It has been found that oxygen-deficient ZrO2-x nanotubes possess the ability of visible light absorption, high specific surface area, short diffusion length of photo-generated carriers and excellent charge transport merits (Chen et al., 2019b). Additionally, its high negative value of the conduction band, superb photocorrosion stability, non-toxicity and low cost-environmental friendly nature have aroused great attention in photocatalysis field. Taking above considerations into account, we constructed a novel heterostructure photocatalyst by combining oxygen-deficient zirconia nanotubes with g-C3N4 for further improving visible light photocatalytic activity. Herein, the g-C3N4 decorated ZrO2-x nanotubes (g-C3N4/ZrO2-x NTs) heterostructure photocatalysts were prepared via anodic oxidation and following a thermal vapor deposition method. A series of fundamental characterizations of the photocatalysts, including light absorption capacity, band structures, phase structure and surface chemical states etc., were investigated in detail. Visible light photocatalytic activity was evaluated by photocatalytic degradation of TC-H in water under visible light irradiation. Moreover, photoelectrochemical measurements, radical species trapping experiments, HPLC-MS, and in-situ XPS were carried out for further illustrating the photocatalytic mechanism. The results indicated that g-C3N4/ZrO2-x NTs photocatalysts might be a promising candidate for treatment of antibiotic water pollution.

2.2. Preparation of photocatalysts Firstly, the original ZrO2 NTs were prepared by anodic oxidation of zirconium foil in two-electrode system at room temperature (approximately 25 °C). The zirconium foil having a diameter of 20 mm was used as a working electrode and graphite sheet was utilized as a counter electrode, operating at 50 V for 2 h in a glycerol-based electrolyte (0.35 M NH4F, 2.5 vol% H2O and 2 vol% HF). Subsequently, the original ZrO2 NTs were rinsed with ethanol thoroughly and then dried under a stream of nitrogen. For the synthesis of the g-C3N4/ZrO2-x NTs photocatalysts, different mass (0.03 g, 0.06 g, 0.09 g) of melamine was placed at the bottom of a corundum crucible, and the original ZrO2 NTs were placed top-down with a distance of 1 cm away from the bottom (Raziq et al., 2017). To avoid the influence of oxygen atmosphere on preparing the ZrO2-x NTs with narrowed band gap and visible light absorption, the whole crucible was annealed at 550 °C for 3 h in argon. According to the different mass of melamine, the composite photocatalysts were marked as 0.03 CN/ZrO2-x NTs, 0.06 CN/ZrO2-x NTs, and 0.09 CN/ZrO2-x NTs respectively. For comparative study, ZrO2-x NTs and ZrO2 NTs-Air were prepared by annealing the original ZrO2 NTs without melamine addition in argon and air atmosphere, respectively. In addition, g-C3N4 sample was also synthesized under the same annealing process by melamine pyrolysis in Ar atmosphere. 2.3. Characterization The UV–Vis diffuse reflectance spectra (DRS) were obtained using a Scan UV–Vis spectrophotometer (UV-2600, Shimadzu) equipped with an integrating sphere assembly, using BaSO4 as the reference sample. Xray diffraction (XRD) was carried out in parallel mode (2θ varied from 10° to 90°) using a Bruker D8 Advance X-ray diffractometer (Cu Kα radiation, λ = 1.5406 Å). Specific surface area (BET method) and pore size distributions (BJH method) were determined by a physisorption apparatus (JW-BK200C, JWGB Sci.&Tech) at 77 K. Samples were heated to 200 °C for 2 h under vacuum to remove adsorbates before tests. The thermogravimetry (TG) was carried out in a flow of air (10 mL/min) at a heating rate of 10 °C/min (HCT-4, Henven). The detected range of temperature was from room temperature to 800 °C. The chemical composition and chemical states were analyzed by X-ray photoelectron spectroscopy (XPS, ESCALAB 250Xi, Thermo Scientific Escalab). All of the binding energy values were calibrated by using C 1s =284.6 eV as a reference. Peak fitting and semi-quantitation of XPS spectra were performed by using XPS Peak Fit software. In in-situ XPS, a 300 W Xenon lamp (CEL-PE300L-3A, 100 mW cm−2) with a cutoff filter (420 nm–780 nm) was used as the visible light source at room temperature. The distance between the lamp and samples was 50 cm. The microstructure of materials was examined via Scanning electron microscope (SEM) operating at 5.00 kV (Hitachi S-4800). The structural characterization was realized via Cs-corrected STEM (Titan G2 60-300). The photoluminescence (PL) spectra tests were carried out using a confocal microscope system (alpha-300, WlTec) with 532 nm continuous wave laser as the excitation. The Fourier Transform Infrared (FTIR) spectra were recorded on Tensor 27, Bruker. Electron spin response (ESR) spectra of active species captured by 5, 5-dimethyl-lpyrroline N-oxide (DMPO) were examined on an JES-FA300 spectrometer under visible-light illumination (λ > 420 nm). The oxygen vacancy of the photocatalyst was also detected by ESR method without light irradiation.

2. Experimental 2.1. Materials A zirconium foil (99.9% purity, 0.2 mm in thickness) was purchased from Purui Advanced Material Technology Co., Ltd, Beijing. The chemicals, including ammonium fluoride (NH4F, AR), hydrofluoric acid 2

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However, the peak intensities are different, indicating that the contents of tetragonal and monoclinic phases in ZrO2 NTs-Air and ZrO2-x NTs are different. Specifically, diffraction peaks of the tetragonal phase are more obvious in ZrO2-x NTs sample, whereas more monoclinic phase can be found in ZrO2 NTs-Air sample. The 0.03 CN/ZrO2-x NTs, 0.06 CN/ZrO2-x NTs and 0.09 CN/ZrO2-x NTs exhibit similar characteristic diffraction peaks to ZrO2-x NTs, indicating that g-C3N4 has little effect on the crystal structure of ZrO2-x NTs after coupling. In addition, no characteristic diffraction peaks of g-C3N4 are observed in composites, which may be due to amorphous and low amount of g-C3N4 (Ma et al., 2016). However, the FTIR spectra confirm the existence of g-C3N4 in the composites, as shown in Fig. 1b. The spectrum of g-C3N4 shows a broad band of the stretching mode of NH2 groups at 3178 cm−1, a group of multiple bands with characteristic stretching modes of CeN heterocycles at 1635, 1570, 1404, 1325, 1243 cm−1, and a typical breathing mode of the tri-s-triazine units at 806 cm−1 (Yu et al., 2014; Khabashesku et al., 2000). The spectrum of ZrO2-x NTs shows a broad band of OH stretching at 3420 cm−1 and 1616 cm−1 from the adsorbed H2O molecules (Shu et al., 2013). The band at 1417 cm−1 is represented the CO32- species after adsorption of CO2 (Bianchi et al., 1993). Moreover, a group of multiple bands at 780, 580, 490 cm−1 are associated with vibration of Zr-O band in the zirconia (Tian et al., 2019; Wang et al., 2007). With the increased content of coupled g-C3N4, the characteristic bands of g-C3N4 and ZrO2-x NTs simultaneously appear in the composites. Notably, the composites show a new band at around 2046 cm−1. The new band may generate from the interaction of g-C3N4 and ZrO2-x NTs. The similar phenomenon has also been observed in gC3N4/TiO2 (Qu et al., 2016). The SEM images of ZrO2-x NTs, 0.03 CN/ZrO2-x NTs, 0.06 CN/ZrO2-x NTs and 0.09 CN/ZrO2-x NTs are shown in Fig. 2a. The ZrO2-x NTs show typical nanotubes morphology with a clean surface. The diameter of the nanotubes is approximately 50 nm. The surface of 0.03 CN/ZrO2-x NTs remains clean without covering, while with the increase of the precursor mass, some flocculent g-C3N4 is deposited on the top of nanotubes in 0.06 CN/ZrO2-x NTs. The nanotubes’ structure is completely covered in 0.09 CN/ZrO2-x NTs. Fig. 2b shows the TEM image of 0.06 CN/ZrO2-x NTs. The contour of the nanotubes is clearly visible. Corresponding elemental mapping of 0.06 CN/ZrO2-x NTs (as shown in Fig. 2c) detects four elements (Zr, O, C and N) and their distribution, where C and N elements distribute in the same testing area. The TEM images of ZrO2-x NTs and a single nanotube of 0.06 CN/ ZrO2-x NTs are also given in Fig. S1. The ZrO2-x NTs present regular and clean tubular morphology, whereas the 0.06 CN/ZrO2-x NTs show tubular morphology with apparent attachments. The HRTEM image of 0.06 CN/ ZrO2-x NTs is shown in Fig. 2d. The values of lattice spacing of 0.2961 nm and 0.3650 nm are associated with the (011) lattice planes

Electrochemical measurements, including Mott-Schottky technology (M-S plots), electrochemical impedance spectroscopy (EIS) and transient photocurrent response (I-t plots), were implemented by an electrochemical workstation (CHI660E, Chenhua) in a conventional three electrode cell. Platinum wire and a saturated calomel electrode acted as a counter electrode and a reference electrode in 0.5 M Na2SO4 solution, respectively. The M-S plots were measured at a fixed frequency of 1 kHz. The EIS tests were conducted in a frequency range from 0.1 Hz to 100 kHz with an AC amplitude of 5 mV. The I-t curves were measured at 1.0 V bias potential under visible light irradiation. 2.4. Evaluation of photocatalytic activity The photocatalytic degradation test of TC-H was carried out under visible light irradiation. A 300 W Xenon lamp (CEL-PE300L-3A, 100 mW cm−2) with a cutoff filter (420 nm–780 nm) was used as the visible light source at room temperature. The distance from the lamp to samples was 10 cm. About 2 mg of photocatalyst was added into 5 mL of 10 ppm TC-H solution in a 25 mL quartz reaction beaker. The adsorption-desorption equilibrium was obtained by 30 min dark treatment before photocatalysis. The concentrations of TC-H and intermediate products were measured using HPLC-MS (Alilent 6120, USA). The degradation efficiency was calculated by the following formula: Degradation efficiency %= (1－C/C0) × 100%

(1)

Where the C0 and C were the concentrations of TC-H at irradiation time zero and any irradiation time, respectively. The UV–vis absorption curves were analyzed by using a UV–vis spectrometer (UV-2550, Shimadzu). The volume of the solution for HPLC-MS and UV–vis absorption tests was 2 mL. For photocatalytic degradation experiments with different irradiation time, multiple samples with the same synthetic parameters were prepared. The examination of reactive radical species was carried out by introducing different scavengers into the TCH solution prior to addition of the photocatalysts. Total organic carbon (TOC, Shimadzu TOC-VCPH) was also measured to study the mineralization of TC-H during the photocatalytic processes. 3. Results and discussion The phase structures of ZrO2 NTs-Air, ZrO2-x NTs, g-C3N4 and the composites were determined by XRD. As shown in Fig. 1a, two major diffraction peaks at 12.9° and 27.3° in the g-C3N4 are correspond to the inter-layer structural packing (100) and the characteristic inter-planar staking peaks of aromatic systems (002), respectively (Han et al., 2016). Both ZrO2 NTs-Air and ZrO2-x NTs are composed of tetragonal zirconia (JCPDS No. 50-1089) and monoclinic zirconia (JCPDS No. 86-1451).

Fig. 1. (a) The X-ray diffraction of ZrO2 NTs-Air, ZrO2-x NTs, g-C3N4 and the composites; (b) the FTIR spectra of ZrO2-x NTs, g-C3N4 and the composites. 3

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Fig. 2. (a) The SEM images of ZrO2-x NTs, 0.03 CN/ZrO2-x NTs, 0.06 CN/ZrO2-x NTs and 0.09 CN/ZrO2-x NTs; (b) the TEM image, (c) HAADF-STEM image with elemental mapping and (d) the HRTEM image of 0.06 CN/ZrO2-x NTs.

surface defects formed under heat treatment (Wang et al., 2018b). Owing to abundant defects states (such as oxygen vacancies etc.) and lattice disorder (Chen et al., 2011), the light absorption range of ZrO2-x NTs extends to visible light region. More importantly, the 0.06 CN/ ZrO2-x NTs show obviously enhanced visible light absorption. The influence of hereostructure on optical behavior of the composite is obvious. Considering the negligible enhancement of specific surface area in 0.06 CN/ZrO2-x NTs, the huge absorption in visible light region more likely originates from the intrinsic absorption of g-C3N4 and ZrO2-x NTs and the deep energy levels (Giannakopoulou et al., 2017) formed during the coupling process. It is beneficial to generate electron-hole pairs and improve photocatalytic performance of the hereostructure (Shi et al., 2017). Based on the Kubelka-Munk equation (Chen et al., 2017):

of tetragonal zirconia (t-ZrO2) and (110) lattice planes of monoclinic zirconia (m-ZrO2), respectively. Some nanosheets with blurred edges are dispersed on the wall of nanotubes (signed by dashed), but no obvious lattice fringes are observed. It suggests the amorphous nature of g-C3N4, which is in agreement with other studies (Wang et al., 2017a). The above results demonstrate that g-C3N4 has successfully deposited into ZrO2-x NTs. The TG curves further confirm the content of g-C3N4 in 0.06 CN/ZrO2-x NTs, as shown in Fig. S2. The g-C3N4 decomposes completely at 744 °C. Considering the increased weight (1.4 wt.%) of ZrO2-x NTs due to oxidation and the total weight loss (5.8 wt.%) of 0.06 CN/ZrO2-x NTs, the real content of g-C3N4 in 0.06 CN/ZrO2-x NTs is around 7.1 wt.%. The N2 adsorption-desorption isotherm and pore size distribution of g-C3N4, 0.06 CN/ZrO2-x NTs and ZrO2-x NTs are shown in Fig. S3. The isotherm curves of the samples exhibit a characteristic type IV hysteresis loop of mesoporous materials. The pore size in ZrO2-x NTs is mainly 44.6 nm. After coupled with g-C3N4, small pore size is witnessed in 0.06 CN/ZrO2-x NTs. The BET surface area of 0.06 CN/ZrO2-x NTs (29.4 m2/ g) is close to that of ZrO2-x NTs (29.1 m2/ g) and much larger than that of g-C3N4 (8.9 m2/ g). It demonstrates that the g-C3N4 is tightly attached on tube wall of the nanotubes in 0.06 CN/ZrO2-x NTs. UV–Vis absorption spectra of ZrO2 NTs-Air, ZrO2-x NTs, g-C3N4 and 0.06 CN/ZrO2-x NTs are shown in Fig. 3. The g-C3N4 can absorb UV light and a small amount of visible light. The light harvesting of ZrO2 NTs-Air is mainly in the UV light region, resulting from the intrinsic band gap absorption of ZrO2 material. The weak absorption before 400 nm is ascribed to the light scattering caused by pores or cracks and

(αhυ) n = k(hυ-Eg)

(2)

Where α represents the absorption coefficient, υ is the light frequency, k is a constant, Eg is the band gap energy and n depends on the characteristics of the transition in a semiconductor. The values of n for ZrO2 and g-C3N4 are determined to 2 and 1/2, respectively (Bailon-Garcia et al., 2017; Han et al., 2016). The inset in Fig. 3a shows that the optical band gaps of ZrO2 NTs-Air, ZrO2-x NTs and g-C3N4 are 5.15 eV, 2.97 eV and 2.78 eV, respectively. The corresponding absorption edges of the three samples are 241 nm, 417 nm and 446 nm, respectively. Mott-Schottky method was employed to verify the semiconductor type and the flat band potentials (Vfb). As shown in Fig. 3b, the positive slopes of the plots reveal the n-type semiconductor feature of ZrO2 NTs4

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Fig. 3. (a) The UV–Vis absorption spectra of ZrO2 NTs-Air, ZrO2-x NTs, g-C3N4 and 0.06 CN/ZrO2-x NTs. The inset is the plots of the (αhν) n vs photon energy (hν) for ZrO2 NTs-Air, ZrO2-x NTs and g-C3N4; (b) the M-S plots of ZrO2 NTs-Air, ZrO2-x NTs and g-C3N4.

285.8 eV and the Ce(N)3 groups at 287.6 eV (Liao et al., 2015). The N 1s of g-C3N4 can be described as four peaks: the sp2-hybridized nitrogen in triazine rings at 398.0 eV, the tertiary nitrogen N-(C)3 groups at 399.2 eV, the amino NeH bonds at 400.5 eV and the positively charged g-C3N4 heterocycles at 403.9 eV (Giannakopoulou et al., 2017; Wang et al., 2017b). After coupled g-C3N4 with ZrO2-x NTs, the C 1s and N 1s in 0.06 CN/ZrO2-x NTs shift positively towards the higher binding energy. By contrast, both the Zr 3d and O 1s in the composite shift negatively towards the lower binding energy (see Fig. 4(e) and (f)). It indicates that the electrons migrated from g-C3N4 to ZrO2-x NTs after coupling in order to getting Fermi level equilibrium (Yu et al., 2015). The XPS analysis of 0.06 CN/ZrO2-x NTs under light irradiation will be discussed in the following photocatalytic mechanism section. Fig. 5a presents the results of TC-H degradation under visible light irradiation. After 1 h visible light irradiation, the ZrO2-x NTs and the gC3N4 show 51.5% and 40.4% degradation of TC-H, respectively. While introduced g-C3N4 to ZrO2-x NTs, the degradation efficiencies are improved in different degrees. In the composites, both ZrO2-x NTs and gC3N4 can be excited by visible light, resulting in a significant increase in the number of photo-generated electron-hole pairs. Meanwhile, the gC3N4/ZrO2-x NTs heterostructure improves the separation efficiency of photo-generated electron-hole pairs. Thus, the photocatalytic activities of the composites are greatly enhanced. The 0.06 CN/ZrO2-x NTs exhibit 90.6% degradation of TC-H that much better than other ZrO2 catalysts for degradation of TC-H in the same condition (Zhang et al., 2018; Gao et al., 2017). The 0.03 CN/ZrO2-x NTs and 0.09 CN/ZrO2-x NTs display 80.8% and 86.1% degradation of TC-H, respectively. Compared with the 0.03 CN/ZrO2-x NTs, the higher photocatalytic efficiency of 0.06 CN/ZrO2-x NTs is mainly due to the increase of heterogeneous interface. However, excessive g-C3N4 loading will cover the nanotubes, reducing the light absorption of ZrO2-x NTs in 0.09 CN/ZrO2-x NTs and limiting its photocatalytic efficiency subsequently. The photocatalytic degradation kinetics can be regarded as a pseudo-first-order kinetics reaction when the concentration of solution is within the millmolar range (Kanagaraj and Thiripuranthagan, 2017):

Air, ZrO2-x NTs and g-C3N4. By extrapolating the linear part of the M-S plot, the Vfb values of ZrO2 NTs-Air, ZrO2-x NTs and g-C3N4 are estimated to be -1.13 V, -1.00 V and -1.28 V (vs NHE, pH = 0), respectively. Since the flat band potential of n-type semiconductor equals to the Fermi level (Gong et al., 2018), the Fermi level of g-C3N4 is higher than ZrO2-x NTs. Fig. 4a and b show the XPS survey spectra of g-C3N4, ZrO2-x NTs, ZrO2 NTs-Air and the composites. The C, N and a small amount of O elements are detected in pure g-C3N4. Zr, O and C elements are detected in ZrO2-x NTs and ZrO2 NTs-Air. In 0.03 CN/ZrO2-x NTs, 0.06 CN/ZrO2-x NTs and 0.09 CN/ZrO2-x NT, all peaks of C, N, Zr and O elements are found. Notably, a new peak of F element is accidentally detected in all the composites. The fluorine element was derived from the oxyfluoride, which generated in the original ZrO2 NTs. Based on our previous research (Chen et al., 2019b), after annealing at 550 °C, the fluorine element can completely vanish from the nanotubes due to the decomposition of oxyfluoride. In the composites, however, the decomposing might be suppressed by the g-C3N4 on the tube wall of naontubes. The residual fluorine element in catalysts has been proved to facilitate the light absorption and photocatalytic performance of oxides (Chen et al., 2019b; Gao et al., 2019; Bao et al., 2018; Liu et al., 2017). After excluded the content of adventitious C, adsorbed O and F, the element component analysis of the composites based on XPS is shown in Table S1. Calculating from the atomic ratio of each element, the mass contents of g-C3N4 in 0.03 CN/ZrO2-x NTs, 0.06 CN/ZrO2-x NTs and 0.06 CN/ZrO2-x NTs are calculated to be 4.2 wt.%, 6.5 wt.% and 9.0 wt.%, respectively. For a better understanding of different light-harvesting ability of ZrO2-x NTs and ZrO2 NTs-Air, the Zr 3d and O 1s spectra of them are also provided in Fig. S4. Compared with the Zr 3d spectrum of ZrO2 NTs-Air, the one of ZrO2-x NTs shifts negatively towards the lower binding energy, suggesting the partial reduction of Zr4+ to Zrx+ (0 < x < 4) is happened in the nantotubes. Meanwhile, the peak fittings of O 1s spectrum in ZrO2 NTs-Air and ZrO2-x NTs suggest that the concentration of oxygen vacancies in ZrO2-x NTs is more than ZrO2 NTAir (Zeng et al., 2018b). The existence of oxygen vacancies and lattice disorders plays a vital role in narrowing band gap of ZrO2-x NTs (Chen et al., 2011). As complementary, the ESR spectrum of the ZrO2-x NTs is given to detect the signal of oxygen vacancies, as shown in Fig. S5. The signal at g = 2.004 represents the oxygen vacancies (Hou et al., 2017; Wang et al., 2018c), which further verifies the existence of oxygen vacancies in the ZrO2-x NTs. C 1s and N 1s spectra of the g-C3N4 and the 0.06 CN/ZrO2-x NTs under different irradiation conditions are given in Fig. 4(c) and (d). Without visible light irradiation, the C 1s of g-C3N4 splits into three peaks: the adventitious carbon at 284.6 eV, the CeNeC groups at

-ln(C/C0) = kt

(2)

Where the C0 is the concentration of TC-H at irradiation time zero, C is the concentration of TC-H at any irradiation time, k is the first-order kinetics rate constant. Fig. 5b shows the linear relationship between -ln (C/C0) and irradiation time. The kinetics rate constant and the correlation coefficient (R2) of all the catalysts are also listed in the figure. The 0.06 CN/ZrO2-x NTs exhibit the highest rate constant among the above samples, giving 3.7 and 5.2 times higher than that of ZrO2-x NTs and g-C3N4. For further understanding the enhancement of photocatalytic 5

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Fig. 4. (a)The survey spectra of g-C3N4, ZrO2-x NTs and ZrO2 NTs-Air; (b)The survey spectra of 0.03 CN/ZrO2-x NTs, 0.06 CN/ZrO2-x NTs and 0.09 CN/ZrO2-x NTs; (c) C 1s spectra, (d) N 1s spectra, (e) Zr 3d spectra and (f) O 1s spectra of g-C3N4, ZrO2-x NTs and 0.06 CN/ZrO2-x NTs with or without irradiation.

Additionally, the EIS Nyquist plots of the ZrO2-x NTs and the 0.06 CN/ ZrO2-x NTs are shown as Fig. S6. The diameter of arc radius of the 0.06 CN/ZrO2-x NTs is much smaller than that of ZrO2-x NTs, representing that the composite has more effective separation rate of photo-generated electron-hole pairs and stronger interfacial charge transfer capacity (Wang et al., 2018d). Thus, the better photocatalytic performance of g-C3N4/ZrO2-x composites can be anticipated. The UV–vis absorption curves and HPLC-MS spectra of the TC-H degradation intermediate products in the presence of the 0.06 CN/ZrO2-

activity, the photo-generated carrier transfer and separation in the samples were explored via I-t plots and PL spectra. Corresponding results are shown in Fig. 6. The photocurrent density of 0.06 CN/ZrO2-x NTs is about 3.5 times and 15 times as much as that of ZrO2-x NTs and gC3N4. The higher photocurrent density implies faster separation and transport rate of the photo-generated electron-hole pairs. In PL spectra, 0.06 CN/ZrO2-x NTs exhibit the lowest photoluminescence intensity in comparison with other samples, manifesting faster separation rate and lower recombination rate of photo-generated carriers in the material. 6

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Fig. 5. (a) The photocatalytic activities and (b) the pseudo-first-order kinetics of g-C3N4, ZrO2-x NTs and the composites for TC-H degradation under visible light irradiation.

Fig. 6. (a) I-t plots of g-C3N4, ZrO2-x NTs and 0.06 CN/ZrO2-x NTs; (b) PL spectra of ZrO2-x NTs, 0.03 CN/ZrO2-x NTs, 0.06 CN/ZrO2-x NTs and 0.09 CN/ZrO2-x NTs.

2 mM) and benzoquinone (BQ, 1 mM) acted as the sacrificial reagents of holes (h+), hydroxyl radicals (%OH) and %O−2 , respectively. The degradations of TC-H with different sacrificial reagents are shown in Fig. 9. The degradation efficiency decreases considerably after adding BQ and attenuates after injecting EDTA-2Na, suggesting that the %O−2 and h+ are main contributors for TC-H degradation. However, the effect of IPA addition is not obvious, suggesting that the %OH is not the main reactive specie. The DMPO spin-trapping ESR spectra for detection of %O−2 and %OH over 0.06 CN/ZrO2-x NTs are shown as Fig. 10. Both the characteristic signals of DMPO-%OH and DMPO-%O−2 appearunder visible light irradiation. In 0.06 CN/ZrO2-x NTs, the conduction band (CB) potential of g-C3N4 is much more negative than the %O−2 /O2 potential (about -0.33 eV, vs. NHE (Li et al., 2016)), which can generate %O−2 . However, the valence conduction (VB) position of ZrO2-x NTs is on the top of the % OH/H2O potential (about 2.38 eV, vs. NHE (Zheng et al., 2016)), restricting the oxidation ability of photo-generated holes to emerge %OH. The DMPO-%OH signals are more likely to come from the reaction of %O−2 and hydrogen ions (Wang et al., 2018d; Wu et al., 2019; Tang et al., 2018; Jiang et al., 2012). This also can explain that why the addition of IPA caused a little inhibitory effect for the photocatalytic activity of 0.06 CN/ZrO2-x NTs. Based on the band structures of g-C3N4 and ZrO2-x NTs, Fig. 11 shows two possible heterostructure of g-C3N4/ZrO2-x NTs photocatalysts: (a) the conventional type II heterojunction; (b) the Z-scheme heterojunction. ZrO2-x NTs and g-C3N4 can concurrently produce photo-

x NTs photocatalyst at different irradiation time are shown in Fig. 7. The intensity of TC-H characteristic peak at 357 nm in UV–vis absorption curves (as shown in Fig. 7a) obviously decreases with increasing irradiation time. Similarly, the intensity of prominent signal at m/z = 445 which agrees well with deprotonated TC-H decreases drastically with increasing irradiation time to 60 min (as shown in Fig. 7b–d). Meanwhile, some new mass signals appear due to the intermediate products of TC-H degradation. The mineralization rate of TC-H was determined by TOC method. With the 0.06 CN/ZrO2-x NTs photocatalyst, the mineralization rate of TC-H was 24.7% after 1 h visible light irradiation. It indicates that the TC-H was not completely degraded into H2O and CO2 but produced other intermediate organic products. The possible pathway of TC-H degradation is proposed in Fig. 8. First, under visible light irradiation, the photo-generated active species attacked TC-H molecules, which led to the demethylation process and hydroxylation process (Chen and Liu, 2016; Deng et al., 2018). As the reaction continued, more such groups detached (m/z = 327), and then the cyclic hydrocarbon structure would be opened by positive holes (m/ z = 267). Undergoing a series of redox reaction, low-molecular-weight organic compounds (m/z = 242, 235, 223) were generated. Finally, TCH and these intermediate compounds would be further decomposed into CO2 and H2O. Radical species trapping experiments were carried out by the addition of various sacrificial reagents into the photocatalytic system in the presence of 0.06 CN/ZrO2-x NTs. In particular, ethylenediaminetetraacetic acid disodium salt (EDTA-2Na, 2 mM), isopropyl alcohol (IPA,

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Fig. 7. (a)The UV–vis absorption curves and (b) - (d) HPLC-MS spectra of the TC-H degradation intermediate products in the presence of the 0.06 CN/ZrO2-x NTs photocatalyst at different irradiation time.

to identify the photocatalytic mechanism by the radical species trapping experiments. In order to identify the type of heterojunction and further understand the underlying mechanism for the enhanced photocatlytic performance of g-C3N4/ZrO2-x NTs photocatalysts, it needs more profound discussion based on the experimental results. The M-S results suggest that the Fermi level of g-C3N4 (-1.28 V, vs. NHE) is higher than that of ZrO2-x NTs (-1.13 eV, vs. NHE). Thus, the electrons will flow from gC3N4 to ZrO2-x NTs for getting Fermi level equilibrium after coupling. As shown in Fig. 12a and b, the band bending occurs on the contact interface between g-C3N4 to ZrO2-x NTs and the directed build-in electric field (from g-C3N4 to ZrO2-x NTs) is formed. For one thing, the interfacial band bending restrains the migration of photo-generated electrons in the CB of g-C3N4 to the CB of ZrO2-x NTs as well as photo-

generated electron-hole pairs under visible light irradiation. In the conventional type II heterojunction, the photo-generated electrons can transfer from the CB of g-C3N4 to that of ZrO2-x NTs due to more negative CB position of g-C3N4. The photo-generated holes can transfer from the VB of ZrO2-x NTs to that of g-C3N4 due to the more positive VB position of ZrO2-x NTs. Both the reducibility of electrons and the oxidizability of holes will decrease. However, in the Z-scheme heterojunction, the interfacial charge migration pathway is the opposite of the type II heterojunction. The photo-generated electrons in the CB of ZrO2x NTs and the photo-generated holes in the VB of g-C3N4 will be consumed, while the photo-generated electrons in the CB of g-C3N4 and the holes in the VB of ZrO2-x NTs will be reserved. The mainly active species in the type II heterojunction or the Z-scheme heterojunction are both photo-generated h+ and %O−2 in g-C3N4/ZrO2-x NTs system, so it is hard

Fig. 8. The possible pathway of photocatalytic degradation of TC-H. 8

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Fig. 11. Schematic diagram of the possible heterostructure formed by g-C3N4 and ZrO2-x NTs.

and the %O−2 from the CB of g-C3N4 are the mainly active species in degradation of TC-H into smaller molecules.

Fig. 9. The species trapping experiments for TC-H degradation of 0.06 CN/ ZrO2-x NTs under visible light irradiation.

generated holes in the VB of ZrO2-x NTs to the VB of g-C3N4. For another, the direction of build-in electric field accelerates the recombination of photo-generated electrons in the CB of ZrO2-x NTs and photo-generated holes in the VB of g-C3N4. Considering that the shift of binding energies in XPS can reveal the electron migration in heterojunction photocatalysts (Li et al., 2016), in-situ XPS characterization under light irradiation was utilized for verifying the electron-migration pathway in our work (as shown in Fig. 4c–f). Compared with no visible light irradiation case, the C 1s and N 1s in 0.06 CN/ZrO2-x NTs shift negatively towards the lower binding energy under visible light irradiation. Meanwhile, Zr 3d and O 1s in 0.06 CN/ZrO2-x NTs shift positively towards the higher binding energy under the irradiation. The change of binding energies demonstrates that the photo-generated electrons migrate from ZrO2-x NTs to g-C3N4 in the hereostructure under visible-light irradiation, which is in accordance with the interfacial charge migration pathway of Z-scheme heterojunction. Based on the above analysis, the schematic diagram of g-C3N4/ZrO2-x NTs for TCH degradation is proposed, as shown in Fig. 12c. Under visible light irradiation, both ZrO2-x NTs and g-C3N4 are excited. Due to the interfacial band bending and directed build-in electric field in the band structure of composites, the photo-generated electrons in the CB of ZrO2-x NTs will combine with the photo-generated holes in the VB of gC3N4. The photo-generated electrons in CB of g-C3N4 with stronger reducibility and the photo-generated holes in VB of ZrO2-x NTs with stronger oxidizability are reserved. Considering the VB potential of ZrO2-x NTs is hard to emerge the %OH, the h+ from the VB of ZrO2-x NTs

4. Conclusion In summary, a new Z-scheme photocatalytic system based on the gC3N4/ZrO2-x NTs heterostructure were successfully constructed by chemical vapor deposition of g-C3N4 into ZrO2-x NTs. The g-C3N4/ZrO2x NTs photocatalysts possessed broad visible-light harvest and superior charge separation efficiency, which were caused by the well-matched band structures between g-C3N4 and ZrO2-x NTs. Therefore, the photocatalytic activities of the heterojunction photocatalysts for TC-H degradation were significantly enhanced. The optimized photocatalytic degradation performance was achieved by 90.6% under 1 h visible light irradiation. It verified that the active radicals of %O−2 and h+ played a critical role during the degradation of TC-H. The profound research of g-C3N4/ZrO2-x NTs heterojunction photocatalysts provides a new thought for the photodegradation of antibiotics and organic dyes.

Author contributions The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript.

Declaration of Competing Interest The authors declare no competing financial interest.

Fig. 10. The DMPO spin-trapping ESR spectra for (a) DMPO-%O−2 and (b) DMPO-%OH over 0.06 CN/ZrO2-x NTs. 9

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Fig. 12. Energy band diagrams of g-C3N4 and ZrO2-x NTs (a) before and (b) after contact; (c) the schematic diagram of TC-H degradation in g-C3N4/ZrO2-x NTs photocatalytic system.

Acknowledgements

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