Ternary Ni2P/Bi2MoO6/g-C3N4 composite with Z-scheme electrontransfer path for enhanced removal broad-spectrum antibiotics by the synergistic effect of adsorption and photocatalysis

Feng Guo,Zhihao Chen,Xiliu Huang,Longwen Cao,Xiaofang Cheng,Weilong Shi,Lizhuang Chen,*

1 School of Energy and Power,Jiangsu University of Science and Technology,Zhenjiang 212003,China

2 School of Environmental and Chemical Engineering,Jiangsu University of Science and Technology,Zhenjiang 212003,China

3 School of Material Science and Engineering,Jiangsu University of Science and Technology,Zhenjiang 212003,China

4 College of Chemistry,Zhengzhou University,Zhengzhou 450001,China

Keywords:Ni2P Bi2MoO6/g-C3N4 Z-scheme Photocatalysis Adsorption

ABSTRACT Constructing the stable,low-cost,efficient,and highly adaptable visible light-driven photocatalyst to implement the synergistic effect of photocatalysis and adsorption has been excavated a promising strategy to deal with antibiotic pollution in water bodies.Herein,a novel 3D ternary Z-scheme heterojunction photocatalyst Ni2P/Bi2MoO6/g-C3N4 (Ni2P/BMO/CN) was fabricated by a simple solvothermal method in which the broad spectrum antibiotics(mainly tetracyclines and supplemented by quinolones)were used as target pollution sources to evaluate its adsorption and photocatalytic performance.Notably,the Zscheme composite significantly exhibit the enhancement for degradation efficiency of tetracycline and other antibiotic by using Ni2P nanoparticles as electron conductor.Active species capture experiment and electron spin resonance(ESR)technology reveal the mechanism of Z-scheme Ni2P/BMO/CN photocatalytic reaction in detail.In addition,based on the identification of intermediates by liquid chromatography–mass spectroscopy(LC–MS),the possible photocatalytic degradation pathways of TC were proposed.

1.Introduction

Over past decades,tetracycline(TC)antibiotics are widely used in the treatment of various bacterial infections due to their wide applicability and low price [1–3].Worryingly,misuse of TC not only causes serious environmental pollution,but also affects the health of organisms[4,5].Among the various methods to deal with antibiotic pollution,adsorption and photocatalytic degradation technology have attracted great interest because of their advantages of convenience,efficiency and economy[6,7].However,poor adsorption and weak photocatalytic activity cannot effectively degrade antibiotic pollutants,resulting in secondary pollution and other problems [8–11].Consequently,designing photocatalysts with remarkable adsorption and photocatalytic activity still faces great challenges in practical applications.

In recent years,bismuth-based oxides with stable crystal structure,excellent photoelectric properties and abundant earth resources have attracted wide attention in photocatalytic applications and energy transduction fields [12–17].Moreover,previous researchers have found that the valence band of bismuth-based semiconductors construct a continuous valence band through the hybridization of Bi 6s and O 2p orbitals which be situated above the original valence band and further conducive to improving the mobility of photogenerated carriers [18,19].Bismuth molybdate(Bi2MoO6) acted as the typical of Aurivillius oxide exhibits a layered structure,in which the [Bi2O2]2+layers are composed sited between MoO42-slabs,which makes it possess narrow band gap(2.5–2.8 eV)and act as a visible light-driven semiconductor photocatalyst [20–22].Generally,the traditional method to construct heterojunction composite is considered as a promising strategy to improve the photocatalytic activity of the single semiconductor.Owing to the unique electronic structure and stable chemical properties of Z-scheme heterojunction photocatalyst,it has become one of the most effective methods to solve above problems,and shown better photocatalytic performance compared that the traditional type II heterojunction photocatalyst [23–25].Therefore,to fabricate the Z-scheme Bi2MoO6-based heterostructures with proper materials may be a great potential to separate the electron and hole pairs of pure Bi2MoO6,thus improving its photocatalytic activity.It is fortunately reported that graphite phase carbon nitride(g-C3N4) with proper redox potential,matching well with the Bi2-MoO6,can form a stable binary Z-scheme Bi2MoO6/g-C3N4heterostructure [26].Moreover,several positive results of Bi2-MoO6/g-C3N4combined with solid electronic medium have been reported to further improve its photocatalytic activity,such as Bi2-MoO6/CNTs/g-C3N4[27],Bi2MoO6/Ru/g-C3N4[28],Bi2MoO6/C/g-C3N4[29].Nevertheless,for the above ternary composites,much attention has been paid to only revolving around the design of one dimension (1D) or two dimension (2D) and the investigation of the carbon-based materials or noble metals as solid electronic media.Concerning the other dimensions,such as three dimension(3D),and the introduction of novel non-noble metal electronic media,which remains critical and highly desirable for efficient use of solar energy for water pollution treatment and further practical applications.

As a typical transition metal phosphide,Ni2P has the characteristics of high stability and excellent electrical conductivity,which provides a possible candidate material for electrocatalysis and photocatalysis[30,31].Additionally,Ni2P is widely used in the field of photocatalysis as a co-catalyst to modify semiconductor materials to enhance photocatalytic activity[32].For example,Zhaoet al.synthesized graphitic g-C3N4with the Ni2P as co-catalyst to significantly improve the efficiency of photocatalytic hydrogen production [33].To act Ni2P as an electrical mediums introduce Bi2MoO6/g-C3N4composite is still a challenge in photocatalysis.It is worth mentioning that owing to the proper Fermi energy level(Ef) between Bi2MoO6and g-C3N4,it indicates that Ni2P may possess a potentially excellent electron conductor to couple with Bi2-MoO6/g-C3N4heterostructure to form a Z-scheme ternary composite.

In this work,noble-metal-free Ni2P was used as electron carrier to form a novel 3D ternary all-solid-state Z-scheme Ni2P/Bi2MoO6/g-C3N4photocatalyst was successfully synthesized by a simple solvothermal method for degradation of TC.The adsorption and photocatalytic properties of Ni2P/Bi2MoO6/g-C3N4in different pH values,antibiotic species and concentration water environment were investigated by simulating the actual environment.In addition,the enhancement of photocatalytic degradation performance and electron transfer path of Ni2P/Bi2MoO6/g-C3N4Z-scheme heterojunction photocatalyst were studied in detail,and the intermediate products of photocatalytic degradation were detected by liquid chromatography–mass spectrometry (LC–MS).

2.Experimental

2.1.Preparation of g-C3N4 nanosheets

10 g urea was put into the muffle furnace,heated to 550 °C at the heating rate of 5 °C?min-1and kept for 3 h.After cooling to room temperature,the bulk g-C3N4cake was grounded into powder and heated to 550 °C with a ramping rate of 5 °C?min-1and kept for 2 h to obtain the twice calcined g-C3N4nanosheet which was named as CN.

2.2.Preparation of Ni2P nanoparticles

Initially,1 g Ni (NO3)2?6H2O and 1 g NaOH was dissolved in 200 ml deionized water with the magnetic stirring for 1 h.The mixed solution was centrifuged to separate out the green precipitate after standing for 2 h and dried at 60 °C to obtain the precursor.Next,the dried precursor was mixed with 0.5 g NaH2PO2and ground thoroughly.Then,the light green powder was calcined in a tube furnace at 300 °C,1 °C?min-1for 1 h.The calcined sample was further fully ground and washed with water and ethanol 3 times,and finally dried in vacuum at 60 °C to obtain the Ni2P nanoparticles.

2.3.Preparation of Bi2MoO6 microspheres

Typically,0.477 g Bi(NO3)3?5H2O and 0.119 g Na2MoO4?2H2O were dissolved in 10 ml ethylene glycol,respectively,and mixed after ultrasonic treatment for 30 min.Then,60 ml of absolute ethanol was added in the above solution under magic stirring for 30 min.The obtained mixture was transferred to Teflon-lined stainless autoclave with a capacity of 100 ml and heated at 160°C for 24 h.Finally,the as-prepared Bi2MoO6microspheres was washed with water and ethanol by three times,and dried in a vacuum at 80 °C and denoted as BMO.

2.4.Preparation of Ni2P/BMO/CN composite

The synthetic route of Ni2P/BMO/CN composites was shown in Scheme 1.Typically,0.477 g Bi(NO3)3?5H2O and 0.119 g Na2MoO4-?2H2O were dissolved in 10 ml ethylene glycol,respectively,and mixed after ultrasonic treatment for 30 min.Then,60 ml of absolute ethanol and 5%,7%,9%,11% and 13% (mass) of Ni2P nanoparticles were added to the above solution and continued to stir for 1 h.Then,0.3 g CN (30% (mass) weight ratio to the output of Bi2-MoO6) was added in the above solution under magic stirring for 30 min.The resulting mixture was transferred to a 100 ml Teflon-lined stainless autoclave and heated at 160 °C for 24 h,and finally the as-prepared composite was washed with water and ethanol by three times,and dried in a vacuum at 80 °C and denoted as 5% Ni2P/BMO/CN,7% Ni2P/BMO/CN,9% Ni2P/BMO/CN,11% Ni2P/BMO/CN and 13% Ni2P/BMO/CN,respectively.

Scheme 1.Preparation of 3D Ni2P/BMO/CN composite photocatalyst by one-step solvothermal route.

The various characterization technology parameters and photocatalytic experimental process can be referred by Supplementary Material.

3.Results and Discussions

As shown in Fig.1(a),the detailed crystal structure and crystallinity of as-prepared samples were supported by X-ray diffraction (XRD).It can be seen that the diffraction peaks of pure BMO at 28.1°,32.2°,46.6°,55.3° and 58.1° were correspond to the(131),(200),(062),(331) and (191) crystal planes,which are consistent with the orthorhombic crystal phase of BMO (JCPDS-72-1524) without any impurity peaks [34–36].And the diffraction peaks at 40.6°,44.6°,47.4° and 54.1° were indexed to the diffraction planes of (111),(201),(210) and (002) of orthorhombic Ni2P(JCPDS-89-2742) [37].In addition,the crystal structures of BMO/CN and 9% Ni2P/BMO/CN composite are almost consistent with the pure BMO,but the diffraction peaks of CN and Ni2P are hardly not found in the composites,which may be due to the less loading amounts of CN and Ni2P.In order to further prove the substance in the synthesized material,the Fourier transform infrared spectroscopy (FT-IR) was carried out and exhibited in Fig.1(b).For the pure BMO (cyan line),the obvious broad absorption arrange from 600 to 1000 cm-1is ascribed to the typical vibration peaks of Bi-O and Mo-O stretching modes and the bridging stretching of Mo-O-Mo [19].For the pristine CN (orange line),it can be clearly seen that the remarkable absorption peak at 805.7 cm-1which is ascribed to the characteristic out-of-plane bending vibration modes of thetri-s-triazine units [38–40],and several prominent peaks centered at 1200–1700 cm-1are derived to the typical stretching vibrations of heptazine ring [41–43].Furthermore,the wide band between 3077 and 3384 cm-1can attribute to the vibration modes of NH/NH2and at the wide band of 3400–3600 cm-1are originated from hydroxyl group/adsorbed water[44–48].For the BMO/CN composite (blue line),the characteristic peaks of pure BMO and CN can also be obviously found,which proves that BMO/CN has been successfully synthesized.The FT-IR curve of 9%Ni2P/BMO/CN(red line)remained basically unchanged compared with BMO/CN,and the peak intensity decreased slightly at 3315–3608 cm-1,which may be due to the Ni2P nanoparticles loading on the surface of the composite[49].In addition,compared with BMO/CN,the main characteristic peaks of CN in 9%Ni2P/BMO/CN ternary composite shift to a lower wavenumber,suggesting that the bond strengths of C=N and C-N are weakened due to the conjugate system stretching of CN,which may lead to synergies to improve photocatalytic performance [50–53].

Fig.1.(a) XRD patterns and (b) FT-IR spectra of BMO,CN,Ni2P,BMO/CN and 9% Ni2P/BMO/CN samples.

The scanning electron microscope (SEM) was used to further inspect the morphology and microstructure of each as-fabricated sample and exhibited in Fig.2.It can be seen that the pure CN in Fig.2(a) shows the shape of the layered nanosheets,and the pure BMO presents a 3D spherical stacking structure in Fig.2(b).In Fig.S1,the microstructure of BMO surface is analyzed by using higher magnification SEM which can be clearly seen that the 3D spherical BMO surface is composed of dense 2D nanosheets with thickness of about 20 nm.After combining BMO with CN in Fig.2(c),the fine nanosheets on the 3D spherical surface gradually disappeared which attributes to the CN nanosheets wrapping on the surface of BMO.The ternary composite material of 9% Ni2P/BMO/CN still presents the even size microsphere structure in Fig.2(d).All the elements in the 9% Ni2P/BMO/CN composite sample are given by energy dispersive spectrum(EDS)in Fig.S2,the result further proves the successful preparation of the ternary sample.

Fig.2.SEM images of (a) CN,(b) BMO,(c) BMO/CN and (d) 9% Ni2P/BMO/CN.

Fig.3.(a–c) TEM,(d) HRTEM,(e) HAADF-STEM and (f–l) elemental mapping images of 9% Ni2P/BMO/CN.

The fine microstructure of 9% Ni2P/BMO/CN ternary hybrid composite was further observed by transmission electron microscope (TEM).It can be clearly seen from Fig.3(a) that 9% Ni2P/BMO/CN presents uniform 3D spherical structure with assembled from a huge number of nanosheets on the surface.The enlarged image of 9% Ni2P/BMO/CN in Fig.3(b) exhibits various nanosheets of BMO and CN interweave together.Further amplification of nanosheets (Fig.3(c)),it can be observed that Ni2P nanoparticles are uniformly dispersed on the surface.The observed HRTEM image (Fig.3(d)) presents that the interplanar spacing of 0.32 nm and 0.22 nm corresponds to the (131) and (111) crystal planes of BMO and Ni2P,respectively[19,54].In addition,the HRTEM image of 9%Ni2P/BMO/CN exhibits that three kinds of Ni2P,BMO and CN are closely connected with the strong interface interaction (green dotted line),which is conducive to the formation of heterojunction.After determining the range of high-angle annular dark-field scanning transmission electron microscope(HAADF-STEM)in Fig.3(e),the corresponding element mapping images of 9% Ni2P/BMO/CN were carried out in Fig.3(f)-(l).The result reveals that the homogeneous distribution of Mo,Bi,O,C,N,Ni and P elements all existed in nanocomposite.

The surface chemical composition and valence states of 9%Ni2P/BMO/CN were further investigated by X-ray photoelectron spectroscopy (XPS) and shown in Fig.4.The survey spectrum of 9%Ni2P/BMO/CN in Fig.4(a) showed the composite is comprised of the Bi,Mo,O,Ni,P,C and N elements.For the Bi 4f spectrum(Fig.4(b)),the peaks of Bi 4f sited at 158.9 and 164.2 eV are corresponded to Bi 4f7/2and Bi 4f5/2,respectively [55].As depicted in Fig.4(c) of Mo 3d,the two peaks located at 232.2 and 235.3 eV were attributed to Mo 3d5/2and Mo 3d3/2,which confirm the existence of Mo6+in BMO[56].For the O 1s spectrum in Fig.4(d),three peaks at 529.5,530.2 and 531.4 eV were originated from Bi-O,Mo-O,and O-H,respectively [57].As exhibited in Fig.4(e),the Ni 2p of XPS can be divided into six peaks,in which the peaks depended on 853.4,855.8 and 861.8 eV corresponded to Ni 2p1/2,Ni 2p3/2and surface oxidation states [30,33,54].In addition,the peaks at 869.9,874.1 and 880.4 eV were assigned to Niδ+oxidation state in Ni2P,while Ni2P exposed to the air generated surface oxidation and the satellite peaks,respectively[58].For the P 2p spectrum in Fig.4(f),the peaks at 129.6 and 132.9 eV were ascribed to Pδ+on the metal phosphides and the oxidized P species,respectively [59].The C 1s in Fig.4(g) shows two peaks at 284.7 and 288.2 eV which originated from C-C coordination of graphitic carbon atoms and C-N or C-(N)3of the aromatic lattice,respectively[42].As shown in Fig.4(h),the N 1s spectra decomposed into four peaks which sited at 398.5,399.1,400.6 and 404.8 eV were corresponded to C-N=C,N-(C)3,C-N-H and C=N conjugated structures by π excitations,respectively [41].By contrast with previous reports for pure Ni2P,BMO and CN [60–62],the binding energies of all high-resolution XPS spectra for 9% Ni2P/BMO/CN have been shifted,which also indicates the successful formation of heterojunction due to the offset of the electron cloud density.

Fig.4.(a) XPS survey spectrum of 9% Ni2P/BMO/CN composite and high-resolution spectra of (b) Bi 4f,(c) Mo 3d,(d) O 1s,(e) Ni 2p,(f) P 2p,(g) C 1s and (h) N 1s.

UV–vis diffuse reflectance spectrometer (DRS) was used to investigate the light absorption properties of the as-prepared and shown in Fig.5(a).It can be seen that the light absorption edges of pure CN and BMO are approximately at the wavelength 450 and 550 nm,respectively,which are agreement with previous researches [42,56].Compared with pure BMO and CN,the absorption edge of binary BMO/CN composite is between the two materials,indicating the formation of heterojunction.After the introduction of Ni2P,the visible light absorption range (400–800 nm) of ternary of Ni2P/BMO/CN has been obviously enhanced as the amount increases gradually,which may be attribute to the interaction between Ni2P,BMO and CN.Furthermore,the band gap energy of pure BMO and CN samples are calculated by the equation αhv=A(hv -Eg)(1/2)established by Tauc model,where the letters ofh,v,α,AandEgare denoted as Plank’s constant,light frequency,absorption coefficient,absorbance and band gap energy[63].Consequently,the values could be estimated to be 2.88 and 2.69 eV for CN and BMO,respectively Fig.5(b).The color changes of photocatalysts with different mass ratios can be seen in Fig.S3.In the case of measuringEgvalues of pure BMO and CN,it is necessary to test the conduction band potential(CB)or valence band potential(VB)values to determine their detailed energy band positions.As shown in Fig.5(c)and (d),through the teste of Mott-Schottky plots,the flat band potentials of pure BMO and CN were defined at -0.09 and -1.17 eV,respectively.And the both slope of plotsC-2potential curves were positive,which can prove then-type semiconductors of BMO and CN [62].Furthermore,the flat band potential ofn-type semiconductor is more positive about 0.1 eV than that of CB,and this can be determined that the flat band potential values of BMO and CN is -0.19 and -1.27 eV,respectively[64].Consequently,based on the relation of the equation (EVB=ECB+Eg),theEVBvalues of pure BMO and CN are calculated in 2.5 and 1.61 eV,respectively.

Fig.5.(a)The UV–vis DRS spectra of as-prepared samples and(b)the corresponding Eg presented by Kubelka-Munk transformed reflectance spectra.The Mott-Schottky plots for (c) pure BMO and (d) CN.

In order to explore the photocatalytic activity of the asprepared samples,25 mg catalysts were added into the solution with 20 mg?L-1TC as pollution source.Then,the solution was placed in a photoreactor connected with circulating condensate water and stirred for 30 min in the dark to achieve adsorption desorption equilibrium (Fig.6(a)).Meanwhile,control reaction was carried out in TC solution without catalyst,and the results showed that there was no self-degradation in the process of photocatalysis(Fig.6(b)).During the process of photocatalytic reaction (A 500 W xenon lamp with a 420 nm cut-off filter),it can be seen that the photocatalytic activity of pure BMO is poor and that of pure CN can be neglected,while the photocatalytic performance of BMO/CN has been enhanced duo to the formation of heterojunction.Moreover,with the introduction the contents of Ni2P nanoparticles,the photocatalytic degradation ability of the ternary composite gradually increased,in which 9% Ni2P/BMO/CN showed the strongest photocatalytic degradation rate (80.1% within 120 min visible light irradiation).The enhancement of photocatalytic activity is due to the strong electron conduction ability of Ni2P,which accelerates the electron transfer [32].The photocatalytic degradation performance of the composite sample decreased gradually after further increasing the Ni2P content,which was due to the excessive Ni2P on the surface of composite photocatalyst,further occupying part of the reaction center and resulting in poor absorption of visible light [65].The corresponding pseudo-first-order dynamics were derived from the Langmuir-Hinshelwood model(Fig.6(c)),which the apparent rate constants of 9% Ni2P/BMO/CN(0.01007 min-1) are 32.5 and 3.0 times higher than that of pure CN(0.00031 min-1)and BMO(0.00336 min-1),respectively.Moreover,the specific analysis of the adsorption process and the photocatalytic process showed that the degradation rates of CN,BMO,BMO/CN and 9% Ni2P/BMO/CN in the 20 mg?L-1TC solution were 3.2%,41.7%,48.0% and 80.1%,respectively,of which 9% Ni2P/BMO/CN possesses excellent adsorption and photocatalytic degradation effect at a high concentration of TC solution(Fig.6(d)).Significantly,the as-prepared Ni2P/BMO/CN composite material showed outstanding photocatalytic performance among the composite photocatalysts reported in recent years for degradation of TC (see Table S1).In order to further understand the practical application ability of the as-prepared ternary Ni2P/BMO/CN composite,different experimental conditions such as initial concentration,pH liquid environment and different kinds of antibiotics were investigated.As exhibited in Fig.6(e)and(f),it can be clearly seen that with the increase of TC concentration,the adsorption performance of 9% Ni2P/BMO/CN for TC in dark reaction and photocatalytic degradation performance for TC in light reaction are gradually decreased,which is mainly due to the scarcity of adsorption sites relative to high concentrations[66].Moreover,the apparent rate constant corresponding to the increase of TC concentration also gradually decreases(Fig.6(g)).In the whole reaction process of adsorption and photocatalytic reaction,the total degradation rates of 9% Ni2P/BMO/CN in 20 mg?L-1,30 mg?L-1,40 mg?L-1and 50 mg?L-1TC solutions were 80.1%,48.0%,34.5%and 30.6%,respectively (Fig.6(h)).Furthermore,the effect of in different pH water environment was test over 9%Ni2P/BMO/CN composite photocatalyst by keeping the original reaction state unchanged and only changing the pH value of the solution.The results showed that the adsorption performance of 9% Ni2P/BMO/CN decreased in too acidic or too alkaline water environment,which was not conducive to the dark adsorption reaction (Fig.6(i)).Notably,with the increase of solution alkalinity,the photocatalytic performance of 9% Ni2P/BMO/CN has been increased,reaching the optimum at pH=9,and then decreasing under strong alkaline condition(Fig.6(j)and(k)).In the whole process of dark adsorption and photocatalytic reaction,the degradation rates of 9% Ni2P/BMO/CN at pH=3,pH=5,pH=7,pH=9 and pH=11 of TC solution(20 mg?L-1)were 38.6%,59.4%,80.1%,86.1% and 79.7%,respectively (Fig.6(l)).Under visible light irradiation,the variation of total organic carbon(TOC)reflects the mineralization degree of TC in the photocatalytic process.In Fig.S4,the removal rate of TOC without photocatalyst is 0.4%,while 9%Ni2P/BMO/CN composite reduces TOC by 51.4%.The results show that 9%Ni2P/BMO/CN photocatalyst exhibit well photocatalytic activity and high TC mineralization efficiency.Meanwhile,in order to explore the universality of as-prepared 9%Ni2P/BMO/CN composite photocatalyst has the same photocatalytic degradation performance to other antibiotics,three different kinds of antibiotics,including oxytetracycline (OTC),chlortetracycline (CTC) and ciprofloxacin (CIP),were introduced under the same experimental reaction conditions.The results presented that 9% Ni2P/BMO/CN also exhibits a strong adsorption and photocatalytic degradation performance on CTC under dark and visible light irradiation in Fig.6(m) and (n),and the corresponding apparent rate constants were provided in Fig.6(o).Moreover,the degradation rates of 9% Ni2P/BMO/CN over OTC,CTC and CIP were 38.5%,73.9%and 36.0%,respectively(Fig.6(p)).Additionally,it is common know that the inorganic salts in actual water bodies are ubiquitous,therefore it is necessary to detect their effects on the photocatalytic degradation process of TC [67,68].During the adsorption and photocatalytic reaction,the Ca2+and(CaCl2and NaNO3were chosen as the providers)were the added and the results were given in Fig.S5.With the increase of Ca2+andion concentration,the degradation efficiencies of TC were reduced from 80.1%to 67.5% and 67.0%,respectively (the ion concentration is 0.01 mol?L-1),and the obtained results still remained above the degradation rate of 50%,which can also prove that the ternary Ni2P/BMO/CN composite has good adaptabilities in actual water bodies.

Fig.6.Adsorption equilibrium tests in the dark reaction process of different photocatalysts (a),initial concentration (e),pH values (i) and antibiotics (m).Photocatalytic degradation under visible light irradiation of different photocatalysts (b),initial concentration (f),pH values (j) and antibiotics (n) and the corresponding reaction kinetics diagrams of (c),(g),(k) and (o) and comparison of photocatalytic and adsorption degradation efficiencies of (d),(h),(l) and (p).

In the following part of Fig.7(a),for the purpose of further exploring the performance of photo-generated electron migration,the transient photocurrent responses in four intermittent switching cycles were recorded.It can be clearly seen that pure BMO and CN show faintish photocurrent densities,which is caused by the rapid recombination of photogenerated electron-hole pairs,while BMO/CN photocurrent values increased significantly,indicating that the formation of CN and BMO heterojunction structures is beneficial to visible-light absorption.After the introduction of Ni2P nanoparticles,the photocurrent density of 9% Ni2P/BMO/CN reached the optimum value which indicates that ternary heterojunction can effectively reduce the recombination of photogenerated electron hole pairs.In addition,the analysis of the electrochemical impedance spectra (EIS)-Nyquist diagram (Fig.7(b))reveals that the semicircle diameter of 9% Ni2P/BMO/CN is the smallest among the as-prepared samples,which means that its corresponding electrochemical impedance is smallest,further proving that ternary composite has a faster electron transfer rate with the addition of Ni2P nanoparticles act as electron conductor.

The separation efficiency of photo-induced charges in asprepared samples was measured by photoluminescence(PL)spectrum with the excitation wavelength at 325 nm and shown in Fig.8(a).The pristine CN shows obvious PL emission peak at around 460 nm,which is due to its excitation of intrinsic band gap [69–71].After coupled with BMO,the PL emission peak intensity has been decreased in the formation of BMO-CN heterojunction due to promoting the effective separation of photogenerated charges.Significantly,it can be seen that the peak intensity of 9% Ni2P/BMO/CN is the lowest among the samples,which indicates that the recombination of photogenerated electron hole pairs is effectively suppressed in the ternary composite photocatalyst.Fig.8(b) shows the time-resolved transient PL decay spectra of BMO/CN and 9% Ni2P/BMO/CN for obtaining the corresponding average fluorescent lifetime(τav),which can be calculated by the following equation:τav=τ1×A1+τ2×A2.The τ1(fast component)and τ2(slow component) and corresponding to theA1andA2of the proportion of τ1and τ2in fluorescence lifetime are presented in Table S2.As a result,the τavvalues of CN,BMO/CN and 9%Ni2P/BMO/CN are 5.94,5.11 and 4.46 ns,respectively.Generally,shorter fluorescence lifetime implies more efficient separation efficiency of photogenerated electron-hole pairs [72–74],enabling ternary Ni2P-BMO-CN to exhibit excellent photocatalytic activity.

The stability of the sample is also one of the decisive factors to evaluate whether the sample can be used in practical application.As displayed in Fig.9(a),the cyclical stability of photocatalytic degradation performance over 9% Ni2P/BMO/CN was evaluated via four times degradation experiments of TC (20 mg?L-1),revealing that there was no obvious deactivation of the as-prepared ternary composite after quartic degradation experiments.There is no denying that the production of active species is an important factor to further understand the mechanism of photocatalytic reaction.From Fig.9(b),the ethylenediaminetetraacetic acid disodium salt(EDTA-2Na),isopropanol(IPA)and vitamin C(VC)are used as scavengers for capturing holes (h+),hydroxyl radicals (·OH) and superoxide radicalsactive species in the process of photocatalytic reaction.Compared with control group of without adding scavengers,the degradation efficiency has slightly decreased to 46.3%after adding IPA.While after adding EDTA-2NA and VC to the solution,the photocatalytic degradation efficiency of TC dropped sharply to 8.2% and 12.6%,respectively,confirming that h+andare the main active species in the process of photocatalytic degradation of TC,while the role of ·OH contributes a little.In addition,the electron spin resonance (ESR) signal spin capture technology is used to further detect the generation of the main reactive speciesand ·OH) and shown in Fig.9(c) and (d).Obviously,it can be clearly seen that there is noand ·OH signal peaks in dark,but with the extension of visible light illumination time,the signal peaks have been enhanced signal peak ofand ·OH.On the basis of the above results,and ·OH can be produced during the photocatalytic degradation of TC and thereact as the main reactive species.

Fig.7.(a) Transient photocurrent density responses and (b) EIS Nyquist plots of CN,BMO,BMO/CN and 9% Ni2P/BMO/CN.

Fig.8.(a) PL curves and (b) time-resolved transient PL decay spectra of CN,BMO/CN and 9% Ni2P/BMO/CN.

Fig.9.(a) Cyclic stability experiment of 9% Ni2P/BMO/CN photocatalytic degradation of TC.(b) Photocatalytic activity test of 9% Ni2P/BMO/CN in the presence of different scavengers.(c,d) Determination of and ·OH during the photocatalytic reaction over 9% Ni2P/BMO/CN by ESR spectra.

For the observed results,the probable Z-scheme reaction mechanism over Ni2P/BMO/CN composite photocatalyst was illustrated in Fig.10.Based on the previous study,BMO and CN have the matching energy levels to form Z-scheme [75].According to band theory,electrons can move from semiconductors with high fermi energy (Ef) to that of with lowEfuntil the Efreaches equilibrium.TheEfof the Ni2P is (-0.244 eV) [76],which is between BMO(0.76 eV)[27]and CN(-0.35 eV)[77],indicating that photogenerated electrons will quickly transfer from the conduction band(CB)of the BMO to the Ni2P,and reach the valence band(VB)of the CN.Therefore,with the excitation of visible light,the transfer path of photogenerated electrons remains Z-scheme mechanism and the Ni2P nanoparticles as the electronic transfer media for further accelerating the separation efficiency of photogenerated carriers.In addition,since Ni2P nanoparticles function as electron acceptors,the photoinduced electrons in the CB of BMO and CN could rapidly transfer to the surface of Ni2P nanoparticles,further enhancing the separation efficiency of photo-induced charges.The free OH-in water can be trapped by holes on the VB of BMO and further be oxidized to ·OH (2.4 eVvsNHE) [78].Moreover,the accumulated electrons on the CB of CN can remain stable under visible light irradiation,which improves the ability of continuous reduction of O2to(-0.046 eVvsNHE)[79].Ultimately,these major active species could oxide TC into small molecular,H2O and CO2,etc.The corresponding Z-scheme reaction mechanism of photocatalytic TC degradation over Ni2P/BMO/CN can be summarized as the following equations:

Fig.10.Possible photocatalytic mechanism of Z-scheme Ni2P/BMO/CN composite for photocatalytic degradation of TC under visible light irradiation.

In order to further understand the intermediate products produced during the photocatalytic degradation of TC,the solution was centrifuged several times after the photoreaction and the collected supernatant was analyzed by liquid chromatography-mass spectrometry (LC/MS) technique.The possible reaction paths and the corresponding LC/MS spectra of intermediates in the process of photodegradation are displayed into Fig.11 and Fig.S6,respectively.In the process of photocatalytic degradation of TC,the reactive species attack TC and make the macromolecular organic compounds transform into small molecules by dehydrogenation,breaking of functional group bonds and ring opening reaction[80].Based on the existing data and previous studies,the photocatalytic degradation of TC by 9% Ni2P/BMO/CN can be divided into two possible paths.For the first pathway (I),the formation of TC2 is caused by the breaking of theN-methyl bond in TC and the removal of the hydroxyl group[81].The ring-opening reaction results in the removal of amino groups and the formation of TC9 when TC2 is further degraded [82].Next,TC9 is decomposed by the cleavage of the ring and the replacement of hydroxyl group for the C-C single bond,and the generated TC3 forms carboxyl groups and dissociates hydroxyl groups to obtain TC4 [4] and finally degraded to H2O,CO2andetc.For another pathway(II),due to the breaking of the N-C bond,TC can be converted to TC5 through demethylation [83].Subsequently,the amino group in TC5 was removed to form TC6,which was further converted into T7 by deamidation and then formed to TC8 by ring opening reaction [2].Next,TC9 can be formed from TC8 molecule due to the process of demethylation,and further dehydration and decarbonylation reactions occur to transform into TC10[4].With further ring opening reaction and oxidative decomposition,TC10 can be gradually transformed into TC11 and TC12,and finally degraded into H2O,CO2andetc[84].

Fig.11.Proposed degradation intermediates and possible pathways of photocatalytic TC degradation in the presence of 9% Ni2P/BMO/CN.

4.Conclusions

In summary,for the purpose to further enhance the separation efficiency of photogenerated electron-hole pairs in binary Zscheme heterojunction composite,noble-metal-free Ni2P nanoparticles were acted as the electron carrier to form 3D ternary allsolid-state Ni2P/BMO/CN composite photocatalyst by a simple solvothermal method.The microscopic morphology,structural composition,element species,reactive species,and degradation intermediates were analyzed by various characterization techniques.The adsorption and photocatalytic performance of Ni2P/BMO/CN in different pH values,antibiotic species and concentration water environment were investigated for simulating the actual environment.The results showed that the ternary Ni2P/BMO/CN composite exhibits outstanding adsorption and photocatalytic activity for degradation of TC compared with binary BMO/CN,single BMO and CN.The enhancement of degradation activity was mainly attributed to the introduction of Ni2P,as electron conductor,which provided more reactive active sites and accelerated the rapid separation of photogenerated carriers.This study provides a reference and solution for improving the practical application of composite photocatalytic materials.

Declaration of Competing Interest

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.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (No.21906072,22006057,21671084 and 51902140),the Natural Science Foundation of Jiangsu Province (BK20190982),Henan Postdoctoral Foundation(202003013),‘‘Doctor of Mass entrepreneurship and innovation”Project in Jiangsu Province,Jiangsu 333 talents project funding(BRA2018342),Jiangsu provincial government scholarship for overseas studies,the Doctoral Scientific Research Foundation of Jiangsu University of Science and Technology (China)(1062931806 and 1142931803).

Supplementary Material

Supplementary data to this article can be found online at https://doi.org/10.1016/j.cjche.2021.08.024.