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Dispersed GaOOH rods loaded on the surface of ZnBiNbO5 particles with enhanced photocatalytic activity toward enrofloxacin

A GaOOH/ZnBiNbO5 composite was constructed by loading dispersed GaOOH rods on the surface of ZnBiNbO5 particles and characterizations, including SEM-EDS, XRD, FT-IR spectroscopy, XPS, and UV-Vis DRS, were performed to analyze the morphology, structure and optical properties of the GaOOH/ZnBiNbO5 composite. The characterization results showed that ZnBiNbO5 was not destroyed by a high temperature and high pressure in the solvothermal process and that GaOOH was successfully dispersed on the surface of ZnBiNbO5. Simultaneously, there was a red-shift of the absorbance edge for GaOOH/ZnBiNbO5 compared with those of pure ZnBiNbO5 and pure GaOOH. The band gaps of ZnBiNbO5, GaOOH and GaOOH/ZnBiNbO5 were calculated to be 2.96 eV, 4.76 eV and 2.93 eV, respectively. The photocatalytic activity of GaOOH/ZnBiNbO5 was explored by degrading enrofloxacin under illumination. After ultraviolet light irradiation for 60 min, the removal rate of enrofloxacin with GaOOH/ZnBiNbO5 as a photocatalyst was 15.11% higher than that of pure ZnBiNbO5 and was 29.29% higher than that of pure GaOOH. In addition, the contribution of the free radicals in the photocatalytic process was confirmed to be ·O2− > ·OH > h+. The construction of the GaOOH/ZnBiNbO5 composite improved the performance of the single ZnBiNbO5 photocatalyst and single GaOOH photocatalyst, thereby increasing their practical application potential.


Introduction
In recent years, veterinary drugs have been widely and commonly used to treat many animal diseases. [1][2][3] Antibiotics are among the most widely used veterinary drugs because antibiotics can not only treat and prevent animal diseases, but also promote animal growth and improve the nutritional value of animal foods. 4,5 Among these, enrooxacin is a uoroquinolone antibiotic that is the most commonly used antibacterial compound in veterinary applications. 6 The extensive use of enrooxacin has caused serious environmental pollution mainly due to the use of contaminated livestock waste as a natural fertilizer. 7,8 In addition, since in most cases, the equipment in sewage treatment plants cannot completely treat antibiotic-like pollutants, enrooxacin is also discharged into the environment through the wastewater of the wastewater treatment plants. [9][10][11][12][13] The presence of antibiotic contaminants in the environment has a negative impact on biota and can promote and spread antibiotic resistance. [14][15][16] The photocatalysis technology has the advantages of environmental protection, high efficiency and low cost. The key to the photocatalytic technology is the development of highly efficient photocatalysts. According to previous reports, [17][18][19][20][21][22] many metal oxides, such as TiO 2 and ZnO 2 , have been developed as photocatalysts. Because single photocatalysts have some inherent characteristics, such as photo-etching and wide band gaps, their applications are limited. It is important to improve the photocatalytic performance of photocatalysts; many methods have been proven to be effective, such as ion doping methods, [23][24][25] construction of heterojunctions, [26][27][28][29][30][31] and photosensitization. [32][33][34][35] Among these, the construction of composites is an active area of research in the eld of photocatalysts. A composite photocatalyst concentrates the effects of single photocatalysts, which endows the composite system with higher light utilization efficiency, longer carrier life, higher photocatalytic performance and higher chemical stability. 36,37 Several years ago, Zou et al. 38 reported that multiple metal oxides prepared by solid-state reactions showed good photocatalytic activity under light irradiation. In recent years, many multiple metal oxides have been prepared and reported. For example, NiCo 2 O 4 was reported to have good photocatalytic activity for degrading MB under light irradiation. 39 In addition, in our previous studies, Gd 2 YbSbO 7 and In 2 YbSbO 7 with the pyrochlore-type structure were prepared by solid-state reactions and showed enhanced photocatalytic performances under light irradiation. 40 Along this line, we synthesized multiple metal oxides, such as ZnBiNbO 5 , using the solid-state reaction method. At the same time, GaOOH was selected to construct heterojunctions with ZnBiNbO 5 . Due to the low potential of the valence band of GaOOH, it has high oxidizing power for various photocatalytic reactions. 41 Additionally, twisted octahedral GaO 6 forms the unit cell of GaOOH, which can generate a strong dipole moment and improve the separation of photogenerated electrons and photogenerated holes. 42 Therefore, in this paper, pure ZnBiNbO 5 with a pyrochloretype structure was synthesized by a solid-state reaction for the rst time. Also, a GaOOH/ZnBiNbO 5 composite photocatalyst was constructed to degrade enrooxacin under illumination in this paper. Characterizations were performed to analyze the properties of the GaOOH/ZnBiNbO 5 composite, which included X-ray diffraction (XRD), scanning electron microscopy (SEM), Fourier transform infrared spectroscopy (FT-IR), X-ray photoelectron spectroscopy (XPS), and UV-Visible diffuse reectance spectroscopy (UV-Vis DRS). In the experiment of degrading enrooxacin under illumination, the reusability of the GaOOH/ ZnBiNbO 5 composite was studied and the active radicals in the degradation process were also analyzed. Finally, the superoxide radicals and hydroxyl radicals which were generated in the photocatalytic process were determined by electron paramagnetic resonance (EPR).

Synthesis and characterization
The solid-state reaction method was used to synthesize pure ZnBiNbO 5 , and the typical steps are as follows: (i) according to the theoretical atomic ratio of Zn : Bi : Nb (1 : 1 : 1), ZnO, Bi 2 O 3 and Nb 2 O 5 (Sinopharm Group Chemical Reagent Co., Ltd., Shanghai, China) were weighed accurately. The purity of the raw materials was 99.99%, and these raw materials were not subjected to further purity treatment. (ii) The three weighed raw materials were ground for 1 h for thorough mixing; then, the mixed powder was pressed into a small column and placed in an alumina crucible (Shenyang Crucible Co., Ltd., Shenyang, China). (iii) The crucible containing the mixed powder was placed in an electric furnace (KSL 1700X, Hefei Kejing Materials Technology Co., Ltd., Hefei, China). The temperature and time of calcination were set to 1100 C and 25 h, respectively. Finally, the pure ZnBiNbO 5 sample was obtained.
The solvothermal method was used to synthesize the GaOOH/ZnBiNbO 5 composite, and the synthesis process was as follows: (i) 1.1296 g Ga 2 O 3 powder (Aladdin Biochemical Technology Co., Ltd., Shanghai, China) and 2.4762 g pure ZnBiNbO 5 were weighed accurately. The purity of the Ga 2 O 3 powder was 99.99%, and it was not subjected to further purity treatment. (ii) The weighed powders were placed in a beaker, and 80 ml water and 80 ml absolute ethanol were added. (iii) The above solution was stirred for 30 min at room temperature; then, the solution was transferred into a 200 mL tetrauoro-lined reactor. (iv) The reactor was placed in an oven and heated at 200 C for 12 h. (v) Aer cooling to room temperature, the products were washed several times with water and absolute ethanol and then dried at 60 C for 12 h. In addition, as a control sample, pure GaOOH was synthesized according to the above process without adding pure ZnBiNbO 5 .
A powder X-ray diffractometer (XRD, D/MAX-RB, Rigaku Corporation, Japan) with CuKa radiation (l ¼ 1.54056) was used to analyze the crystal structure of the GaOOH/ZnBiNbO 5 composite. The scan range was set to 10 to 80 and the XRD data were obtained using a step-scan program at 295 K. Scanning electron microscopy -X-ray energy dispersion spectroscopy (SEM-EDS, LEO 1530VP, LEO Corporation, Dresden, Germany) was used to analyze the morphological features of the GaOOH/ ZnBiNbO 5 composite. X-ray photoelectron spectroscopy (XPS, ESCALABMK-2, VG Scientic Ltd., London, UK) was used to analyze the elemental contents of the GaOOH/ZnBiNbO 5 composite. A Fourier transform infrared spectrometer (Nexus, Nicolet Corporation, Madison, WI, USA) with attenuated total reectance (ATR) mode was used to analyze the main chemical vibrational species of the GaOOH/ZnBiNbO 5 composite. A UV-Vis spectrophotometer (UV-2450, Shimadzu Corporation, Kyoto, Japan) was used to analyze the band gap of the GaOOH/ ZnBiNbO 5 composite. The background material was pure BaSO 4 powder.

Photocatalytic activity tests
The degradation of enrooxacin was selected to evaluate the photocatalytic activity of the GaOOH/ZnBiNbO 5 composite. The photocatalytic experiments were performed in a photocatalytic reactor (Xujiang Machine, Nanjing, China) containing optical lters (l < 400 nm), a mercury lamp (500 W) and a magnetic stirring device. The major emission wavelength of the mercury lamp was 365 nm. Firstly, 50 ml enrooxacin solution and 0.05 g photocatalyst were placed in a test tube, and the initial concentration of enrooxacin was set to 10 mg L À1 . Then, the tube was placed in the dark for 30 min for dark adsorption. Subsequently, the mercury lamp was turned on for irradiation. During the process of irradiation, 2 mL solution was taken under ultraviolet light for 10 min, 20 min, 30 min, 40 min, and 60 min. The absorbance of enrooxacin was measured at 276 nm using the UV-visible spectrophotometer (UV-2550, Shimadzu Corporation, Kyoto, Japan). The removal rate of enro-oxacin (D%) was calculated according to the formula: D% ¼ (C 0 À C t )/C 0 Â 100%, where C 0 is the initial concentration of enrooxacin and C t is the concentration of enrooxacin at time t. The three-dimensional uorescence of enrooxacin was determined by a full-function uorescence spectrometer (Fluoromax-4, Horiba Scientic). In addition, the active free radicals which were generated during the photocatalytic process were determined by the electron paramagnetic resonance (EPR) method.

Morphology analysis
The morphologies of ZnBiNbO 5 , GaOOH and the GaOOH/ ZnBiNbO 5 composite are displayed in the SEM images shown in Fig. 1(a-c). It can be observed that the as-prepared ZnBiNbO 5 presented an irregular particle morphology. Fig. 1(b) displays that the as-prepared GaOOH presented a regular rod morphology with a cracked surface, which may provide a greater surface area to absorb contaminants. The SEM image of the as-prepared GaOOH/ZnBiNbO 5 composite is displayed in Fig. 1(c). It can be clearly seen that the shape of GaOOH in the GaOOH/ZnBiNbO 5 composite is random broken rods, which is different from the shape of pure GaOOH. In the GaOOH/ZnBiNbO 5 composite, large amounts of GaOOH were dispersed and loaded on the surface of the ZnBiNbO 5 particles. This composite structure may provide potential for transfer of photogenerated electrons and photogenerated holes. In addition, the EDS pattern of the GaOOH/ZnBiNbO 5 composite is shown in Fig. 1(d); Zn element, Bi element, Nb element, Ga element and O element were all determined successfully.

Structure analysis
The Pawley renements of ZnBiNbO 5 were performed using Materials Studio soware to analyze the structure of the asprepared ZnBiNbO 5 , and the results are displayed in Fig. 2(a). The observed XRD data of the as-prepared ZnBiNbO 5 present good agreement with the theoretical calculated data, which reveals that the as-prepared ZnBiNbO 5 was a single phase and had high crystallinity. According to the renement results, pure ZnBiNbO 5 was prepared with a cubic structure in the space group  Fig. 2(a). The XRD patterns of ZnBiNbO 5 , GaOOH and GaOOH/ ZnBiNbO 5 are displayed in Fig. 2(b). According to our previous research, pure GaOOH crystallizes with an orthorhombic structure in the space group Pbnm, and the lattice parameters of GaOOH are a ¼ 4.509526Å, b ¼ 9.771034Å and c ¼ 2.969284Å. 43 It is apparent that the main diffraction peaks of ZnBiNbO 5 and GaOOH can both be observed in the XRD pattern of GaOOH/ ZnBiNbO 5 , as shown in Fig. 2(b); this indicates that ZnBiNbO 5 was not destroyed by high temperature and high pressure in the solvothermal process and that GaOOH was successfully composited with ZnBiNbO 5 . In addition, the FT-IR spectra of ZnBiNbO 5 , GaOOH and GaOOH/ZnBiNbO 5 are displayed in Fig. 2(c). A broad peak at 2900 cm À1 and two sharp peaks at 1018 cm À1 and 948 cm À1 are present in the FT-IR spectrum of GaOOH/ZnBiNbO 5 ; all these peaks belong to GaOOH. The sharp peaks at 1018 cm À1 and 948 cm À1 were attributed to the constitutional Ga-OH bending mode and its overtones, respectively, and the broad peak at 2900 cm À1 was attributed to the vibration of O-H in GaOOH. 44,45 Moreover, the relative positions and strengths of the two characteristic peaks at 948 cm À1 and 1018 cm À1 were related to the shape of the GaOOH. 46 In order to study the elemental composition and chemical states of the GaOOH/ZnBiNbO 5 composite, X-ray photoelectron spectroscopy (XPS) measurements were performed, and the results are displayed in Fig. 3. The high-resolution XPS data of the Zn2p peaks are displayed in Fig. 3(a); the spectrum consists of two peaks at 1025.8 eV and 1048.9 eV, corresponding to the binding energies of Zn2p 3/2 and Zn2p 1/2 , respectively. 47,48 In Fig. 3(b), the two peaks at 163.0 eV and 164.3 eV can be ascribed to the binding energies of Bi4f 7/2 , 49,50 and the two peaks at 168.5 eV and 169.7 eV can be ascribed to the binding energies of Bi4f 5/2 . 51,52 Simultaneously, the XPS spectrum of Nb3d contains three peaks at 211.7 eV and 212.0 eV (corresponding to Nb3d 5/2 (ref. 53)) and 214.6 eV (corresponding to Nb3d 3/2 (ref. 54)), respectively. The XPS spectrum of Ga3d also contains three peaks, and they correspond to the binding energies of Ga3d 3/2 (19.2 eV) 55 and Ga3d 5/2 (20.9 eV and 22.5 eV), 56 respectively. There are three peaks at 529.3 eV, 531.0 eV and 532.4 eV in the XPS data of O1s (Fig. 3(e)). The binding energies at 529.3 eV and 531.0 eV belong to the oxygen in the GaOOH/ZnBiNbO 5 composite, and the binding energy at 532.4 eV belongs to surface-adsorbed hydroxides. 57

Optical properties
The UV-Vis diffuse-reectance spectra of the as-prepared ZnBiNbO 5 , GaOOH and GaOOH/ZnBiNbO 5 are shown in Fig. 4. It can be observed that the absorbance edges of ZnBiNbO 5 and GaOOH are 446 nm and 270 nm, respectively.
Aer GaOOH was loaded on the surface of ZnBiNbO 5 , a slight red shi to 450 nm was observed in the absorbance edge of GaOOH/ZnBiNbO 5 . This phenomenon indicates that the absorption spectra of GaOOH and ZnBiNbO 5 were both adjusted by dispersing GaOOH on the surface of ZnBiNbO 5 . In this paper, we used the Kubelka-Munk transformation method to analyze the band gaps of the samples. The optical absorption near the band edge of the crystalline semiconductors matches the formula ahn ¼ A(hn À E g ) n , 58,59 where A, E g , a and n are the proportional constant, band gap, absorption coefficient and light frequency, respectively. According to the above formula, the band gaps of ZnBiNbO 5 , GaOOH and GaOOH/ZnBiNbO 5 were calculated to be 2.96 eV, 4.76 eV and 2.93 eV, respectively, as shown in the inset gures of Fig. 4. A decreased band gap is benecial for improving photocatalytic activity under solar illumination.

Photocatalytic activity
The photocatalytic activity of the as-prepared GaOOH/ ZnBiNbO 5 composite was evaluated by photodegradation of enrooxacin under ultraviolet light irradiation. For comparison, the photocatalytic activities of pure ZnBiNbO 5 and pure GaOOH were also investigated. The removal rates of enro-oxacin and the degradation kinetics of enrooxacin with ZnBiNbO 5 , GaOOH or GaOOH/ZnBiNbO 5 as a photocatalyst are displayed in Fig. 5. As shown in Fig. 5(a), the as-prepared GaOOH/ZnBiNbO 5 composite presented good performance for degrading enrooxacin. Aer ultraviolet light irradiation for   60 min, the removal rate of enrooxacin with as-prepared GaOOH/ZnBiNbO 5 as a photocatalyst was 65.31%, which was 15.11% higher than that of pure ZnBiNbO 5 and 29.29% higher than that of pure GaOOH. The degradation of enrooxacin can be attributed to a pseudo-rst-order kinetics reaction with a simplied Langmuir-Hinshelwood model: ln(C/C 0 ) ¼ Àkt, where k is the pseudo-rst-order rate constant and t is the reaction time. When the initial concentration of contaminant is low, this model is generally selected to analyze the photocatalytic degradation kinetics. The degradation kinetics of enrooxacin with ZnBiNbO 5 , GaOOH or GaOOH/ZnBiNbO 5 as a photocatalyst according to the above formula are shown in Fig. 5(b). The degradation rate of enrooxacin with GaOOH/ ZnBiNbO 5 as a photocatalyst was calculated to be 0.02064 min À1 , which was 1.56 times that of pure ZnBiNbO 5 (0.01326 min À1 ) and 2.84 times that of pure GaOOH (0.00728 min À1 ). The photocatalytic activities of both pure ZnBiNbO 5 and pure GaOOH were enhanced signicantly by loading GaOOH on the surface of ZnBiNbO 5 . In addition, the three-dimensional uorescence change process of enrooxacin during photocatalytic degradation with GaOOH/ZnBiNbO 5 as a photocatalyst is displayed in Fig. 5(c). It can be directly observed that as the photocatalytic reaction progressed, the three-dimensional uorescence center point of enrooxacin slowly changed from red to light green, indicating that enro-oxacin was gradually degraded.
To evaluate the stability and reusability of the GaOOH/ ZnBiNbO 5 composite, four rounds of degradation experiments for enrooxacin under ultraviolet light irradiation with GaOOH/ ZnBiNbO 5 as a photocatalyst were performed. As shown in Fig. 6(a), as the number of cycles increased, the removal rate of enrooxacin gradually decreased. From the rst round to the fourth round, the removal rates of enrooxacin were 65.31%, 58.97%, 51.92% and 48.82%, respectively. Aer four rounds of circulation, the enrooxacin removal rate was decreased by 16.49%. In addition, the removal rates of enrooxacin with different initial pH values using GaOOH/ZnBiNbO 5 as a photocatalyst were studied to explore the adaptability of GaOOH/ ZnBiNbO 5 to the pH of the pollutant solution, and the results are displayed in Fig. 6(b). It can be seen that when the initial pH values of the enrooxacin solution were 3, 5, 9 and 11, the removal rates of enrooxacin were all lower than the removal rate at an initial pH of 7; this indicates that neither acidic nor basic conditions are conducive to the degradation of enro-oxacin. It is worth noting that the removal rate of enrooxacin under acidic conditions was lower than that of enrooxacin under alkaline conditions. A possible reason is that the abundant hydroxide ions in the solution under alkaline conditions can be converted into hydroxyl radicals to participate in the photocatalytic reaction. Moreover, the effects of different cations and anions on the degradation of enrooxacin using GaOOH/ZnBiNbO 5 as a photocatalyst were determined. As shown in Fig. 6(c) and (d), Al 3+ , Fe 3+ , Cu 2+ , CO 3 2À and SO 4 2À had almost no effect on the degradation of enrooxacin. However, NO 3 À had an inhibitory effect on the degradation of enro-oxacin; the addition of NO 3 À decreased the removal rate of enrooxacin by 10.95%. Overall, the as-prepared GaOOH/ ZnBiNbO 5 has strong potential for adaptation to actual wastewater and is expected to be further applied to the degradation of enrooxacin pollutants in actual wastewater. The removal of enrooxacin with the addition of different radical scavengers using GaOOH/ZnBiNbO 5 as a photocatalyst was studied to analyze the possible active species in the photocatalytic degradation process. The radical scavengers used herein were benzoquinone (BQ), ethylenediaminetetraacetic acid (EDTA) and tert-butanol (TBA), which are scavengers for $O 2 À , h + and $OH, respectively. The addition amounts of BQ,  EDTA and TBA were 1 mL, and the concentrations of BQ, EDTA and TBA were 0.15 mmol L À1 . It can be found in Fig. 7(a) The $O 2 À and $OH radicals are produced in the photocatalytic process, as shown in eqn (1)-(4). In alkaline solution, a large amount of OH À promotes the formation of $OH (eqn (1)) and inhibits the consumption of $O 2 À (eqn (4)). However, in acidic solution, a large amount of H + inhibits the formation of $OH (eqn (2)) and promotes the consumption of $O 2 À (eqn (4)).
This is why the removal rate of enrooxacin under acidic conditions is lower than that of enrooxacin under alkaline conditions. To further conrm the presence of $O 2 À and $OH, EPR characterization was carried out using 5,5-dimethyl-1-pyrroline N-oxide (DMPO) as a spin to trap the active radicals in the system for detection. As shown in Fig. 7(b) and (c), no signals relevant to DMPO/$O 2 À adducts or DMPO/$OH adducts were detected under dark conditions. However, aer ultraviolet light irradiation, signals were presented. It can be observed clearly in Fig. 7(b) that there was a signal of four peaks with nearly the same intensity, corresponding to the DMPO/$O 2 À adduct.
Similarly, in Fig. 7(c), a typical four-line signal with a relative intensity ratio close to 1 : 2 : 2 : 1 could be observed, which is a characteristic signal of the DMPO/$OH adduct. 60 Therefore, $O 2 À and $OH were actually produced during the photocatalytic process, which is consistent with the free radical scavenging experiment results.

Conclusions
In summary, we prepared a GaOOH/ZnBiNbO 5 composite by loading GaOOH rods on the surface of ZnBiNbO 5 particles; ZnBiNbO 5 is a novel photocatalyst that was prepared in this paper for the rst time. The morphology, structure, optical properties and photocatalytic activity of GaOOH/ZnBiNbO 5 were all studied in this paper. According to SEM images, GaOOH was successfully dispersed on the surface of ZnBiNbO 5 to construct a composite structure. Using the Pawley method, the XRD data of pure ZnBiNbO 5 were rened; the renement results showed that the novel as-prepared ZnBiNbO 5 had a cubic structure and a space group of Fd 3m, and the lattice parameters of ZnBiNbO 5 were a ¼ b ¼ c ¼ 10.498767Å. The main diffraction peaks of both ZnBiNbO 5 and GaOOH were observed in the XRD pattern of the GaOOH/ZnBiNbO 5 composite. In addition, UV-Vis DRS characterization was carried out to analyse the optical properties of the samples, and the results showed that there was a red-shi of the absorbance edge of GaOOH/ZnBiNbO 5 compared with those of pure ZnBiNbO 5 and pure GaOOH. The band gaps of ZnBiNbO 5 , GaOOH and GaOOH/ZnBiNbO 5 were calculated to be 2.96 eV, 4.76 eV and 2.93 eV, respectively. Finally, the photocatalytic activity of GaOOH/ZnBiNbO 5 was explored by degrading enrooxacin under ultraviolet light irradiation. The as-prepared GaOOH/ZnBiNbO 5 presented much higher photocatalytic performance than pure ZnBiNbO 5 or pure GaOOH. Aer ultraviolet light irradiation for 60 min, the removal rate of enrooxacin with GaOOH/ZnBiNbO 5 as a photocatalyst was 15.11% higher than that of pure ZnBiNbO 5 and was 29.29% higher than that of pure GaOOH. According to the free radical scavenging experiment results and the EPR characterization, the contribution of the free radicals in the photocatalytic process was as follows: $O 2 À > $OH > h + . Overall, the construction of the GaOOH/ZnBiNbO 5 composite enhanced the photocatalytic performance of both pure ZnBiNbO 5 and pure GaOOH, resulting in more effective removal of enrooxacin pollutants from water and purication of the water.

Conflicts of interest
There are no conicts to declare.