Ultrafine Bi3TaO7 Nanodot-Decorated V, N Codoped TiO2Nanoblocks for Visible-Light Photocatalytic Activity: InterfacialEffect and Mechanism InsightChengzhang Zhu,† Yuting Wang,† Zhifeng Jiang,‡,§ Annai Liu,∥ Yu Pu,∥ Qiming Xian,*,†

Weixin Zou,*,∥ and Cheng Sun†

†State Key Laboratory of Pollution Control and Resource Reuse, School of the Environment and ∥Key Laboratory of MesoscopicChemistry of MOE, School of Chemistry and Chemical Engineering, Nanjing University, Nanjing 210023, PR China‡School of Life Sciences, The Chinese University of Hong Kong, Shatin, New Territories 999077, PR China§Institute for Energy Research, Jiangsu University, Zhenjiang 212013, PR China

*S Supporting Information

ABSTRACT: Bi3TaO7 is a potential photocatalyst because of its high chemical stability, defective fluorite-type structure, andsuperior mobility of photoinduced holes. However, few studies have focused on the interfacial effects of Bi3TaO7-basedphotocatalysts. In this work, 0D Bi3TaO7 nanodot-hybridized 3D V and N codoped TiO2 nanoblock (B/VNT) compositeswere first synthesized for the photocatalytic removal of oxytetracycline hydrochloride, 2,4,6-trichlorophenol, andtetrabromobisphenol A. The fabricated B/VNT had a photocatalytic performance superior to that of pristine components,and probable degradation pathways were proposed according to the primary intermediates identified by a gas chromatography−mass spectrometer. Interestingly, on B/VNT, the transfer of interfacial electrons was observed from V/N−TiO2 to Bi3TaO7,and the formed built-in electronic field led to a direct Z-scheme structure, rather than type II, as confirmed by the generated•OH and •O2

− radicals and band structures from the density functional theory calculation. Therefore, the strong interfacialelectronic interaction on the B/VNT was significant, which drove faster photogenerated charge transfer, more visible-lightadsorption, and active •OH and •O2

− generation, thus improving the photocatalytic activity.

KEYWORDS: Bi3TaO7 nanodots, V/N−TiO2 nanoblocks, electronic interaction, Z-scheme, degradation pathways

■ INTRODUCTION

Over the last few decades, the environmental problems(especially antibiotics, disinfection byproducts and brominatedflame retardants) caused by the worldwide burgeoningindustrialization have seriously threatened human health.1−4

Hence, the elimination of toxic pollutants becomes one of themost important emerging research fields, arousing tremendousattention. To date, various kinds of technologies, such asflocculation,5 membrane filtration,6 and biological technology,7

have been explored to address water pollution. Among theavailable methods, semiconductor photocatalysis is regarded asan attractive way for solving these issues because of theeconomical use of photocatalytic materials and solarenergy.8−10 Because visible-light energy takes up most of the

solar radiation, developing efficient visible-light-driven photo-catalysts is highly required.11

With the advantages of suitable band gap (2.74−2.95 eV),excellent chemical stability and defect fluorite-type structure,bismuth tantalate (Bi3TaO7) has been demonstrated to be apromising photocatalyst for pollutant degradation.12,13 Thehybridized valence band (VB) is primarily contributed by theBi 6s and O 2p orbits, which not only favors an efficientutilization of visible light but also promotes the mobility ofphotogenerated holes and the oxidation reaction. However, the

Received: January 15, 2019Accepted: March 15, 2019Published: March 15, 2019

Research Article

www.acsami.orgCite This: ACS Appl. Mater. Interfaces 2019, 11, 13011−13021

© 2019 American Chemical Society 13011 DOI: 10.1021/acsami.9b00903ACS Appl. Mater. Interfaces 2019, 11, 13011−13021

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widespread use of Bi3TaO7 is severely limited owing to therapid electron−hole recombination. To this end, massive efforthas been dedicated to mitigate the above challenge. CouplingBi3TaO7 with other semiconductor to form a heterostructuralphotocatalytic system is an effective one, and with the aid ofthe unique heterostructure, the charge separation can beeffectively achieved.14,15 In fact, for a type-II heterojunction,the photoinduction on the conduction band (CB) of onephotocatalyst would transfer to the VB of another photo-catalyst, during which the redox activity of electrons and holeswould be weakened, implying that the charge carriers may notpossess sufficient redox ability to be practically applied inwastewater treatment.16 Previous reports have indicated thatthe direct Z-scheme heterojunction is a more valid alternativeto the traditional type-II heterojunction.17,18 In a direct Z-scheme system, the reductive electrons in the higher CB andoxidative holes in the lower VB can be retained, leading toboth high redox capacity and charge separation efficiency.19

Moreover, with the strong electrostatic force between electronsand holes, the Z-scheme charge-transfer behavior is morefeasible in physical relative to that of the typical hetero-junctions. Therefore, it is imperative to exploit a facialsynthesis of Bi3TaO7-based Z-scheme heterojunctions withsuperior photocatalytic performance.As an industrially significant n-type semiconductor, titanium

oxide (TiO2), with the merits of earth abundance, highrefractive index, strong oxidizing power, and resistance tophotocorrosion, has been the most extensively investigatedphotocatalyst.20−25 Intensive studies focused on band gapengineering, with the purpose of overcoming the defectsinherent to TiO2.

26,27 Doping with N or V can optimize theelectronic structure of TiO2 to a narrowed band gap, whichremarkably improves the solar light harvesting of TiO2 andshallowly traps the initially photogenerated carriers to inhibittheir rapid recombination.28−30 More importantly, consideringthe well-matched band structure between the two, it can bereasonably predicted that the doped TiO2 may be a goodcandidate for fabricating an interfacial Z-scheme hetero-junction toward Bi3TaO7. Although researches on thephotocatalytic activity of the Bi3TaO7-based Z-schemecomposite have been reported, there is still some space tofurther study on optimizing morphology and interfacialinteraction of photocatalytic composites, which are vital factorsfor enhanced photocatalytic performance.Herein, three-dimensional (3D) V/N−TiO2 nanoblocks

(NBs) modified with zero-dimensional (0D) Bi3TaO7 nano-dots (NDs) were prepared for the first time to form a 3D/0DZ-scheme structure for the use in toxic pollutants photo-degradation. It was found that B/VNT heterojunctions withbroad-spectrum photocatalytic capacities showed superiorphotocatalytic behavior to V/N−TiO2 and Bi3TaO7, mainlyowing to the synergistic effect of the tiny size effect of theBi3TaO7 NDs, incorporation of V and N species, andconstructed direct Z-scheme heterostructure. The results ofelectron spin resonance (ESR) and trapping experimentsrevealed that superoxide (•O2

−) and hydroxyl (•OH) radicalswere the main reactive species in the photocatalytic oxidationof oxytetracycline. Combined with these produced oxidativespecies, the energy-level positions of V/N−TiO2 and Bi3TaO7,DFT, and X-ray photoelectron spectroscopy (XPS) resultsclearly demonstrated strong interfacial effects and theinterfacial charge-transfer path of 10% B/VNT, verifying thatsuch a system works in the Z-scheme way. Furthermore, the

diagram of electron−hole pair separation and reactionmechanism over the direct Bi3TaO7@V/N−TiO2 Z-schemephotocatalysts were also discussed in detail. This study mightprovide new insights into the rational design of Z-schemeheterojunction catalysts with strong interfacial interactions forenvironmental remediation.

■ EXPERIMENTAL SECTIONMaterials. Ammonium metavanadate (NH4VO3), ammonia

(NH3), tantalum (V) chloride (TaCl5), bismuth nitrate pentahydrate(Bi(NO3)3·5H2O), tetrabutyl titanate (TBT), potassium hydroxide(KOH), isopropyl alcohol ((CH3)2CHOH), and absolute ethanol(C2H5OH) purchased from Sinopharm Chemical Reagent Co., Ltdare of analytical grade. Oxytetracycline hydrochloride (OTTCH),2,4,6-trichlorophenol (2,4,6-TCP), and tetrabromobisphenol A(TBBPA) were obtained from Aladdin Reagent Co., Ltd. Ultrapurewater (18.2 MΩ cm) was achieved by a Simplicity UV ultrapure watersystem (Merck Millipore).

Synthesis of V and N Codoped TiO2 NBs. TBT was addeddropwise into isopropyl alcohol and stirred for 30 min, in which acertain amount of NH4VO3 dispersed in the mixture containingultrapure water (0.1 mL) and absolute ethanol (70 mL) was injectedunder a rate of 1.0 mL min−1. After refluxing at 80 °C for 5 h, thesynthesized V-doped TiO2 were repeatedly washed with ethanol andfully dried in a vacuum oven at 60 °C, followed by thermal treatmentin flowing NH3 at 600 °C. Upon cooling naturally to an ambienttemperature, V and N codoped TiO2 (denoted as V/N−TiO2) wereground to fine powder.

Loading Bi3TaO7 NDs onto V/N−TiO2 NBs. The B/VNTphotocatalysts were fabricated using a facile one-step solvothermalroute. Typically, 0.15 g of newly prepared V/N−TiO2 NBs wasultrasonically dispersed in absolute ethanol (35 mL). Then,appropriate amounts of Bi(NO3)3·5H2O and TaCl5 were graduallyadded into the suspension under vigorous stirring. Subsequently, thepH value of the above homogeneous solution was adjusted to 10.0with the addition of KOH solution (7 M). After that, the mixture wastransferred to a Teflon autoclave and heated at 230 °C for 24 h.Finally, the final products were thoroughly washed with ultrapurewater and ethanol in ultrasound several times before being dried at 60°C for 6 h. According to this method, different molar ratios of theBi3TaO7 to V/N−TiO2 samples (i.e., 5, 10, 15, and 20% Bi3TaO7/V/N−TiO2) were obtained and defined as 5% B/VNT, 10% B/VNT,15% B/VNT, and 20% B/VNT, respectively.

Characterization. X-ray diffraction (XRD) was characterized on aPhilips X’Pert Pro diffractometer equipped with Ni-filtered Cu Kαradiation (λ = 1.5418 nm). The morphologies and particle size of thesynthesized catalysts were examined with transmission electronmicroscopy (TEM, JEM-200CX) at 200 kV. Atomic force microscopy(AFM, MFP-30) was utilized to determine the thickness of theproduct. Brunauer−Emmett−Teller (BET) surface areas weremeasured by nitrogen adsorption at 77 K on a Micromeritics ASAP2010 adsorption apparatus. The optical properties of samples in therange of 200−800 nm were analyzed by UV−vis spectrophotometerdiffuse reflectance spectroscopy (DRS, UV-3600Plus) with BaSO4 asreference. XPS (ESCA PHI500) was employed to examine surfacechemical composition and VB. Photoluminescence (PL) and time-resolved PL decay spectroscopy were obtained by a Horiba Fluorolog3-22 type fluorescence spectrophotometer. Total organic carbon(TOC) analyses were carried out on a multi N/C 2100 (Analytik JenaAG) TOC analyzer. ESR signals of paramagnetic radicals (•OH and•O2

−) spin-trapped by 5,5-dimethyl-1-pyrroline N-oxide (DMPO) inwater or methanol were detected by a Bruker model ESR JES-FA200spectrometer.

The photoelectrochemical measurements, including electrochem-ical impedance spectroscopy (EIS) and photocurrent, were performedon an electrochemical workstation (CHI760E) with a standard three-electrode system in 0.2 M Na2SO4. The as-prepared photocatalystswere spin-coated onto the indium tin oxide (ITO, 1 × 2 cm2) thatacted as the working electrode, while the saturated Ag/AgCl and

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platinum wire served as the reference and counter electrodes,respectively. A 500 W Xe arc lamp was used for irradiation in theelectrochemical experiments.Photocatalytic Text. The photocatalytic experiments were

performed in an open homemade thermostatic photoreactor (XPA-V, Xujiang, China, Nanjing) under an irradiation of a 1000 W Xenonlamp with a 420 nm cutoff filter. Prior to light illumination, asuspension containing 50 mg of catalyst and 50 mL of the modelpollutants with the concentrations of OTTCH (20 mg L−1), TBBPA(10 mg L−1), and 2,4,6-TCP (10 mg L−1) were magnetically stirred indarkness for 1 h to achieve adsorption−desorption equilibriumbetween the two. During the irradiation, approximately 5 mL of thesuspension was withdrawn at 20 min intervals and filtered through0.22 μm Millipore filter to remove the photocatalyst, with the purposeof detecting the changes of the pollutant concentrations.Analytical Methods. The concentration of OTTCH was

analyzed at its maximum absorption wavelength (356 nm) byLambda 750 UV−vis spectroscopy (PerkinElmer) with deionizedwater as the reference. The photodegradation rate (DR) wascalculated by the following formula:

− ≈C C k tln( / )0 app (1)

Here, kapp is the apparent rate constant (min−1) and C0 and C are theinitial concentration and instant concentration of the target pollutantsat time t, respectively.The concentrations of 2,4,6-TCP and TBBPA were determined by

an Agilent 1200 high-performance liquid chromatography instrumentequipped with a C18 reversed-phase column (4.6 mm × 250 mm × 5μm) and UV detector set as 290 nm for 2,4,6-TCP and 220 nm forTBBPA. The mobile phase was the mixture of methanol−ultrapurewater (85:15, v/v) with a flow rate of 1 mL min−1, and the columntemperature was maintained at 30 °C during the sample analysis. Theintermediate products of OTTCH were identified by a gaschromatography−mass spectrometer (GC−MS) equipped with aTG-5SILMS column (30 m × 0.25 mm × 0.25 μm). Before the GC−MS analysis, the reaction solution was filtered with a 0.22 μmMillipore filter and was subsequently extracted with a dichloro-methane (5 mL) three times. Afterward, the extracts were dried underflowing nitrogen and then dispersed in 1 mL acetonitrile. After that,trimethylsilylation was performed at 60 °C for 1 h using 100 μL ofbis(trimethylsilyl) trifluoroacetamide. The column temperatureprogram was set as follows: the initial temperature of GC was 50°C for 3 min, then increased up to 300 °C with a heating rate of 5 °Cmin−1, and remained at this temperature for 2 min. The injectortemperature was 250 °C. Helium was used as the carrier gas. Massspectrometric detection was operated with 70 eV electron impact (EI)mode.Theoretical Calculation. Density functional theory (DFT)

calculations were conducted using the Vienna Ab-initio SimulationPackage (VASP). All-electron plane-wave basis sets with an energycutoff of 400 eV and a projector augmented wave method wereadopted. The samples were simulated using a surface model of p(2 ×2) unit cell periodicity. A (3 × 3 × 1) Monkhorst−Pack mesh wasused for the Brillouin-zone integrations to be sampled. Electronicdensity-of-states (DOS) of (2 × 2) supercells were calculated using ahigher 9 × 9 × 1 K-point mesh. The conjugate gradient algorithm wasused in the optimization. The convergence threshold was 1 × 10−4 eVin total energy and 0.05 eV/Å in force on each atom.

■ RESULTS AND DISCUSSIONStructural, Compositional, and Morphological In-

formation. The phase structures of Bi3TaO7, TiO2, V/N−TiO2, and B/VNT composites were investigated by XRDanalysis. As shown in Figure 1a, bare TiO2 exhibited severaldiffraction peaks at ca. 25.3°, 37.8°, 47.9°, 54.0°, and 62.7°,perfectly indexed as the (101), (004), (200), (105), and (204)planes (JCPDS no. 21-1272), respectively.31,32 Notably, theanatase-to-rutile (110) phase transformation appeared, and the

prominent (101) peak of both V/N−TiO2 and B/VNTcomposites slightly shifted to a lower diffraction angle relativeto TiO2 (Figure 1b), which all suggested the incorporation ofV and N into the TiO2 matrix. The results were consistent withthose of XPS, in which the V 2p core level revealed that V4+

2p2/3 (515.7 eV) and V5+ 2p2/3 (516.8 eV) ions wereincorporated into the crystal lattice of TiO2 and formed aTi−O−V bond (Figure S1a). Meanwhile, the occurrence ofthe N−Ti−O linkage (400.1 eV), Ti−O−N, and Ti−O−N−O(401.4 eV) in the N 1s region further confirmed the successfuldoping of N species (Figure S1b).33 After loading Bi3TaO7NDs onto the surface of V/N−TiO2, a series of crystal peakscorresponding to Bi3TaO7 (JCPDS no. 87-1256) phasesoccurred and gradually intensified as the Bi3TaO7 contentsincreased, indicating that Bi3TaO7 NDs had been coupled withV/N−TiO2 NBs. Moreover, all signals of V, N, Ta, Bi, Ti, andO appeared in the survey XPS spectrum of 10% B/VNT(Figure S2), further demonstrating its hybrid structure.The general morphology and microstructure of the samples

were characterized by TEM and high-resolution TEM(HRTEM). It is apparent in Figure 2c that most of the V/N−TiO2 presented a relatively regular NB morphology. Thethickness of the NBs is approximately 8.4 and 9.6 nm asevidenced by the AFM image and height profile (Figure 2a,b).In addition, the typical diffused halo ring and dot patternsexisted in the selected-area electron-diffraction (SAED)pattern (inset in Figure 2c), demonstrating the polycrystallineproperty of V/N−TiO2 NBs. Figure 2d exhibits the TEMimage of the resulting 10% B/VNT composite, in which V/N−TiO2 NBs are employed as an excellent platform that wasevenly and tightly decorated with Bi3TaO7 NDs. Well-dispersed ultrafine Bi3TaO7 NDs with an average size ofabout 4.92 nm (Figure 2e) could ensure sufficient contact withV/N−TiO2 NBs and induce an interfacial interaction becauseof the quantum size effect, thereby facilitating the fastinterfacial charge separation.34,35 From the HRTEM image(Figure 2f), a clear d-spacing of 0.351 nm obtained from V/N−TiO2 was consistent with the (101) planes, while theinterplanar spacing of ca. 0.273 and 0.312 nm matched wellwith that of the (200) and (111) crystal facet of Bi3TaO7,

Figure 1. (a) Normal and (b) partially enlarged XRD patterns of B/VNT in varying proportions, together with those of TiO2, V/N−TiO2, and Bi3TaO7.

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which proved that the nanojunction was indeed constructed inthe 10% B/VNT composite. Additionally, elemental mapping(Figure S3) of 10% B/VNT clearly displayed that the elementsof Bi, Ta, Ti, O, N, and V were homogeneously distributed inthe heterojunction, further suggesting the formation ofintimate interfaces between the two, rather than simply mixing.Nitrogen adsorption−desorption isotherms and the corre-

sponding pore size distribution of Bi3TaO7, TiO2, V/N−TiO2,and 10% B/VNT are depicted in Figure S4. All of the samplesexhibited similar type-IV isotherms with hysteresis loops, andthe BET surface area trend followed the order of 10% B/VNT(62.9 m2/g) > V/N−TiO2 (46.3 m2/g) > TiO2 (38.5 m2/g) >Bi3TaO7 (23.7 m2/g), which revealed that V and Nimplantation suppressed crystalline growth, eventually increas-ing the surface area of TiO2. Simultaneously, the addition ofBi3TaO7 NDs also had a positive effect on the specific surfacearea, primarily because of the uniform dispersion of Bi3TaO7NDs on the surface of V/N−TiO2 NBs. Therefore, anabundant interaction area between the above two moietieswas achieved, which could shorten the distance of masstransfer and provide more surface-reactive sites for thedegradation of toxic pollutants.36

Band Gap Structure. The DFT calculation was rationallyemployed to theoretically investigate the electronic structuresof Bi3TaO7 and V/N−TiO2 theoretically. The schematicillustrations of Bi3TaO7 and V/N−TiO2 supercell modelsutilized in the calculations are presented in Figure 3a,b. It was

observed that the VB maximum (VBM) and CB minimum(CBM) of Bi3TaO7 were O 2p and Ta 4d orbitals, respectively.For V/N−TiO2, the VBM was the orbital mixture of O 2p andN 2p, whereas the CBM was the orbital mixture of V 3d and N2p. Furthermore, the band energies of Bi3TaO7 and V/N−TiO2 were separately calculated to be 2.79 and 2.36 eV usingthe DFT method (Figure 3c,d), which were closed to theexperimental results of Bi3TaO7 and V/N−TiO2 (Figure S5b),that is, 2.84 and 2.52 eV, respectively. However, the theoreticalband gaps were moderately lower than the experimental values,which could be ascribed to the well-known restriction of theDFT calculation, that is, the unknown exchange−correlationenergy.37 Besides, the VB maximum (VBM) of samples werealso measured by the VB X-ray photoelectron spectra (Figure3e). As for single Bi3TaO7, the VBM was estimated to be 3.11eV, while a blue shift occurred from 2.34 for TiO2 to 2.07 eVfor V/N−TiO2, indicating the existence of mid-gap band statesbetween the VB and CB in V/N−TiO2. According to theformula of ECB = EVB − Eg, the corresponding CB minimum(CBM) edge position of Bi3TaO7, TiO2, and V/N−TiO2would be located at ca. 0.27, −0.76, and −0.45 eV,respectively. Combined with the above results, the well-matched band structure of Bi3TaO7 and V/N−TiO2 wasillustrated in Figure 3f, which might promote the separation ofphotogenerated carriers (electron and hole).

Optical and Photoelectrical Properties. Consideringthat the light-harvesting capacity played a crucial role in

Figure 2. (a) AFM image, (b) height profile, (c) TEM image and corresponding SAED pattern (inset of (c)) of V/N−TiO2 NBs. (e) Sizestatistical analysis of Bi3TaO7 NDs. (d) TEM and (f) HRTEM images of 10% B/VNT heterojunction.

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photocatalysis, the UV−vis DRS test was carried out (FigureS5a). The data showed that the absorption edge of Bi3TaO7was 440 nm with a band gap of approximately 2.8 eV, whereasTiO2 only suffered photoabsorption in the ultraviolet region. Itwas noteworthy that the introduction of V and N extended thelight absorption spectral range of TiO2 to the visible region,which might be ascribed to the introduced mid-gap states thatresult from the doping species.38 Moreover, the absorptionedges of the B/VNT heterojunctions displayed an obvious redshift compared with V/N−TiO2 and Bi3TaO7, leading to moreavailable visible-light adsorption. This result implied that B/VNT could absorb more photons in the photocatalyticreaction.Generally, the superior charge separation property is of great

significance to the photocatalytic efficiency of catalysts.39,40

Hence, steady-state photoluminescence (PL) of B/VNTcomposites excitated at 256 nm were employed to evaluatethe charge separation and recombination (Figure S6a).Compared with TiO2, the higher photocurrent response ofV/N−TiO2 could be ascribed to the improved light absorptionthat produces more electron−hole pairs and the codoped Vand N that shallowly trap the charge carriers to prevent themfrom rapid recombination.21,41 Moreover, the fluorescencequenching via coupling Bi3TaO7 NDs mainly results fromimproved interfacial charge transfer between Bi3TaO7 and V/N−TiO2, which could improve the separation of electron−hole pairs in the constructed heterojunction. Thereinto, 10%B/VNT exhibited the weakest emission intensity, matchingwell with the degradation results. To investigate the behaviorof the photoinduced electrons and holes in depth, the time-resolved PL decay measurement was performed, as shown in

Figure S6b. The results suggested that 10% B/VNTheterojunction showed the longest lifetime of free carrierswith respect to that of Bi3TaO7 and V/N−TiO2, therebyleading to enhanced photocatalytic activity.The photocurrent and EIS were further performed to study

both separation of electron−hole pairs and interfacial electrontransfer. As depicted in Figure S7a, the sensitive photocurrentresponse occurred once the light source was switched on,suggesting the rapid charge transport. When 10% B/VNT wasspin-coated onto the pretreated ITO, the correspondingphotocurrent density increased dramatically, rising up to 0.73μA, which was approximately 2.9, 2.3, and 1.6-fold greater thanTiO2, Bi3TaO7, and V/N−TiO2, respectively. This resultmanifested that the fabricated 10% B/VNT heterostructureswith strong interfacial interactions could efficiently hinder thechance of electron−hole recombination and accelerate thecharge transfer.42 As excepted, the EIS performances ofBi3TaO7, TiO2, V/N−TiO2, and 10% B/VNT exhibitedsimilar variation trends (Figure S7b). Among them, 10% B/VNT displayed the shortest arc radius of the Nyquist circle,which reflected that the minimum charge-transfer resistancewas obtained, again verifying that there was a higher efficiencyof interfacial charge transport in the 10% B/VNT hetero-structure.

Photocatalytic Activity and Degradation Pathways.The photocatalytic activity of different samples was firstevaluated by the photodegradation of OTTCH (20 mg/L)under visible-light illumination (λ > 420 nm). It could beobserved in Figure 4a that the adsorption−desorptionequilibrium between the catalysts and OTTCH moleculeswas reached within 60 min in darkness, and thus self-photolysiscould be neglected. For bare TiO2 and Bi3TaO7, only 39.5 and45.2% of OTTCH were decomposed after irradiation for 80min because of the high recombination rate of photogeneratedcarriers. However, once the V and N dopants wereincorporated into TiO2, the resulting V/N−TiO2 achievedmore superior performance for OTTCH removal. All B/VNTphotocatalysts realized an improved DR compared with theothers, particularly the 10% B/VNT heterojunction. Never-theless, the photocatalytic behavior started to decline with thefurther increase in Bi3TaO7 content, because excessivedeposition of Bi3TaO7 NDs might reduce the light absorptionand surface adsorption ability of catalysts and occupy parts ofavailable active sites on V/N−TiO2 NBs. Furthermore, thephotocatalytic activity of physical mixture of Bi3TaO7 and V/N−TiO2 (B/VNT-PM) was substantially lower than 10% B/VNT, which implied that the strong heterojunction effect wasformed on the interface between two moieties, rather than asimple mechanical mixture. As depicted in Figure 4b, the UV−vis characteristic absorption peak at 354 nm quicklydiminished, accompanied by a gradual blue shift as theirradiation proceeded, suggesting that the organic group ofOTTCH was destroyed by 10% B/VNT.43 Moreover, thephotocatalytic degradation followed the pseudo first-orderkinetics, and the reaction rate constants calculated from Figure4c were presented in Table S1. To reduce the effect of specificsurface area on the photoactivity, the DRs of the catalysts werenormalized with their surface areas according to previousreport,44 and the normalized rate constants (k′) were displayedin Table S2. It can be found that after being normalized withrespect to the specific surface area, the 10% B/VNT stillshowed the best degradation efficiency in all catalysts, followedthe order of 10% B/VNT > Bi3TaO7 > V/N−TiO2 > TiO2. In

Figure 3. Crystal structures and calculated DOS of (a,c) Bi3TaO7 and(b,d) V/N−TiO2. Valence-band XPS spectra (e) and schematic of theelectronic band structures (f) of Bi3TaO7, TiO2, and V/N−TiO2.

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addition, the k′ value of Bi3TaO7 was higher than that of V/N−TiO2, opposite to the order of reaction rate constant, whichsuggested that the specific surface area is also important toOTTCH photodegradation. Notably, the DR of OTTCHusing 10% B/VNT is preferable to most of the reportedBi3TaO7-based or TiO2-based catalysts. Comparison of theresults is listed (Table S3).To gain deeper insights into the photocatalytic properties of

the as-prepared heterojunctions, the TOC measurement wasconducted. Typically, the mineralization rate of TOC (70.4%)was lower than that of DR (93.7%) toward the removal ofOTTCH (Figure 4d), suggesting that the structure of OTTCHcould be effectively decomposed, however, the OTTCHmolecules were more difficult to be mineralized into CO2and H2O. Hence, some small intermediate products (this isproven later) were generated during the photodegradationprocess.45 The TOC removal curve displayed a similarvariation trend to the DR curve of 10% B/VNT, whichindicated the accurate evaluation of the photocatalytic activity.Simultaneously, the high TOC removal rate revealed that 10%B/VNT could be regarded as a potential candidate forenvironmental applications.TBBPA, a common brominated flame retardant that has

been extensively used in textiles, circuit boards, and thermo-plastic plastics, may induce the disruption of cytotoxicity,

immunotoxicity, and neurotoxicity.46 In addition, 2,4,6-TCP, achlorinated phenol that has been utilized as a fungicide,herbicide, and glue preservative in aquatic environments, canseriously threat the human health.47 Thus, efficient removal of2,4,6-TCP and TBBPA using photocatalysis technology is ofvital significance. Analogously, 10% B/VNT still showedoptimal photocatalytic performance toward the degradationof TBBPA (DR, 69.9%) and 2,4,6-TCP (DR, 89.5%) within 80min (Figure 4e), which suggested the broad-spectrumphotodegradation capacities of the heterojunction material.Although the superior photocatalytic behavior of 10% B/VNTwas verified, the stability with regard to practical applications isunclear. Therefore, cycle experiments for OTTCH degradationover Bi3TaO7 NDs, V/N−TiO2 NBs, and 10% B/VNT wererepeatedly performed four times under identical reactionconditions (Figure 4f). No notable decrease of the degradationefficiency could be found after four recycles. Moreover, theXRD results (Figure S8) also exhibited that no noticeablechange of the phase could be observed in the fresh and reusedcomposites. These results indicated that 10% B/VNT was ahighly efficient photocatalyst with broad photocatalyticspectrum and impressive stability.As the structure of OTTCH molecules was decomposed,

some intermediate products were generated. The intermediateproducts were identified by GC−MS analysis with EI full-scan

Figure 4. (a) Time-dependent concentration of OTTCH solution on exposure to visible light (λ > 420 nm) using the prepared photocatalysts, (b)temporal UV−vis absorption spectral changes during the OTTCH degradation in the presence of 10% B/VNT, (c) linear relationship of ln(C/C0)versus time, (d) TOC removal of OTTCH for 10% B/VNT by visible-light irradiation for 80 min, (e) photocatalytic activity of TBBPA and 2,4,6-TCP with 10% B/VNT, and (f) recycling photocatalytic tests for OTTCH degradation over pure Bi3TaO7, V/N−TiO2, and 10% B/VNTheterojunction.

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patterns and the standard mass spectrum data of the U.S.National Institute of Standards and Technology.48 Theretention time (min), molecular ion (m/z), and structureformula of the 13 identified products are listed in Table 1. In

particular, open-ring reactions and cleavages of the centralcarbon might be the two main reaction routes for OTTCHdegradation. The intermediates (the second row in Figure 5)were primarily derived from the open-ring reactions, followedby the cleavage of the central carbon to form smaller organicmolecules (the third row).49 With the introduction of reactiveradicals caused by photogenerated electron−hole pairs, thering was destroyed, after which CO2, H2O, and NH4

+ wereeventually involved in the photocatalytic oxidation reaction.50

Combined with these results, the proposed degradationpathways of OTTCH over 10% B/VNT are shown in Figure 5.Possible Photocatalytic Mechanism. In general, the

optical/photoelectrical properties and photocatalytic perform-

ance were closely related to the interfacial electronicinteraction of photocatalyst.51 Hence, XPS characterizationwas employed to investigate the interfacial effect. It is wellknown that an increase in the binding energies (BE) impliesthe weakened electron screening effect due to the decreasedelectron density, whereas the increment of electron densityresults in a lower BE value.52,53 As shown in Figure 6a−c, onceBi3TaO7 NDs were hybridized with V/N−TiO2 NBs, a notableBE shift was observed from the Bi 4f, Ta 4f, and Ti 2pspectrum. In the XPS spectra of Bi 4f and Ta 4f, it was foundthat the BE were shifted to lower values on 10% B/VNTrelative to Bi3TaO7, which suggested that the electron densitywas increased in the Bi and Ta elements of B/VNT.Additionally, for the spectra of Ti 2p, it was observed that10% B/VNT had a higher BE than that of V/N−TiO2 NBs,indicating that the electrons on Ti were reduced on B/VNT.Accordingly, it was deduced that on the B/VNT interface, theelectrons transferred from V/N−TiO2 to Bi3TaO7, leading tothe formation of the build-in electric field. In this case, thestronger electronic interaction on B/VNT interface drovephotogenerated electrons transfer from CB (Bi3TaO7) to VB(V/N−TiO2). Therefore, a direct Z-scheme photocatalyticsystem was constructed in the B/VNT photocatalyst, insteadof type II.Moreover, the main active species involved in the photo-

catalytic oxidation of OTTCH were explored to validate theabove Z-scheme photocatalytic structure. Different scavengersincluding 1 mM 4-hydroxy-2,2,6,6-tetramethylpiperidinyloxy(TEMPOL), isopropanol (IPA), and ammonium oxalate (AO)were utilized as the scavengers of superoxide (•O2

−), hydroxyl(•OH), and holes (h+), respectively. As illustrated in Figure7a,b, the photodegradation efficiency of OTTCH was slightlydepressed with the addition of AO, whereas an obvious changewas observed when the IPA or TEMPOL was introduced.Consequently, it could be preliminarily inferred that theproduced •O2

− and •OH had positive effects on OTTCHdegradation. In addition, the generation of •O2

− and •OH inthe reaction system was further validated by the ESR spin-traptechnology (Figure 7c,d). The typical peaks corresponding to

Table 1. Identification of the PhotodegradationIntermediates of OTTCH by GC−MS

Figure 5. Proposed photodegradation pathways of OTTCHirradiated by visible light.

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DMPO−•O2− and DMPO−•OH adducts were obviously

observed under visible-light illumination, and the peakintensity increased as irradiation time was prolonged. TheESR results indicated that both •O2

− and •OH radicals werethe dominant oxidative species in the photocatalytic process,which was in consistent with the results of trappingexperiments.However, if the type II structure was present in the B/VNT

photocatalyst (Figure 7e), OH− could not be oxidized togenerate •OH by h+ leaving in the VB of V/N−TiO2 becausethe EVB edge potential of V/N−TiO2 (2.07 V vs NHE) wasmore negative than E(•OH/OH−) (2.40 V vs NHE).54

Similarly, the ECB edge potential of Bi3TaO7 (0.27 V vsNHE) was more positive than E(O2/

•O2−) (−0.046 V vs

NHE), which suggested that the photoinduced electrons in theCB of Bi3TaO7 could not combine with dissolved O2 to form•O2

−.15 On the basis of the generation of •OH and •O2−, we

reasonably concluded that, instead of a type-II structure, adirect Z-scheme photocatalytic system was constructed in B/VNT. Because of the built-in electronic field, the V/N−TiO2acted as an electron reservoir to trap photogenerated electronsthat were emitted from Bi3TaO7, preventing the electron−holepair recombination. The h+ in the VB of Bi3TaO7 oxidizedOH− to •OH and the e− in the CB of V/N−TiO2 reduced O2to •O2

−, which were consistent with the in situ ESR results.Moreover, the direct Z-scheme reaction mechanism for theenhanced photocatalytic activity over 10% B/VNT hetero-junction (Figure 8) was proposed as follows

ν− +

→ − + +− + − +

hV/N TiO /Bi TaO

V/N TiO (e h )/Bi TaO (e h )2 3 7

2 3 7 (2)

ν− + → +− + hV/N TiO (e ) Bi TaO (h ) heat2 3 7 (3)

Figure 6. High-resolution XPS spectra of (a) Bi 4f, (b) Ta 4f, and (c) Ti 2p region for Bi3TaO7, V/N−TiO2, and 10% B/VNT composite.

Figure 7. Photocatalytic performance (a) and DR (b) of OTTCH with the addition of AO, TEMPOL, and IPA. DMPO spin-trapping ESR spectrawith 0.1 mg/mL 10% B/VNT in the dark and under visible-light irradiation (3 and 6 min) for DMPO−•OH in aqueous dispersion (c), andDMPO−•O2

− in methanol dispersion (d). Illustration of the charge-transfer mechanisms for type-II heterojunction (e).

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− + →− • −V/N TiO (e ) O O2 2 2 (4)

+ → ++ • −Bi TaO (h ) H O OH OH3 7 2 (5)

+ →• − •O (or OH) organic pollutants degraded products2(6)

As a result, the superior photocatalytic behavior of 10% B/VNT could be ascribed to the following factors:

(i) The incorporation of V and N species into the TiO2matrix could be responsible for the enhanced visible-light absorption because of the mid-gap impurity levelsformed between the CBM and VBM of V/N−TiO2,which not only narrowed the band gap (2.52 eV) butalso increased the amount of the photoexcited electron−hole pairs, based on the UV−vis DRS and DFTcalculation. Specifically, it was easier for electron transferand reducing the recombination of electron−hole pairs,as confirmed by the results of transient photocurrentresponse and ESI.

(ii) Compared with pristine components, the 10% B/VNTheterojunction with higher surface areas was capable ofshortening the distance of mass transfer and facilitatingthe adsorption of pollutant molecules for surfacereactions.21 On the other hand, in addition to thematched energy-level positions (Bi3TaO7 and V/N−TiO2), a novel well-designed direct Z-scheme hetero-structure with the strong interaction of built-in electricfield was constructed, rather than the traditional type-IIheterojunction, thereby broadening visible-light adsorp-tion, boosting the charge separation efficiency atintimate 3D/0D interfaces, and enabling the formationof more reactive oxygen species for the effective removalof organic pollutants (Figure 8).

■ CONCLUSIONSIn summary, 0D/3D heterostructures of B/VNT weresuccessfully synthesized for the degradation of OTTCH,TBBPA, and 2,4,6-TCP under visible-light irradiation. The10% B/VNT composite showed superior photodegradationactivity (93.7%) and TOC removal efficiency (70.4%) towardOTTCH degradation, and possible degradation pathways wereproposed. Furthermore, at the interface of Bi3TaO7 and V/N−TiO2, a stronger electronic interaction, that is, built-in electricfield, resulted in the construction of Z-scheme heterojunction,which was beneficial for faster photogenerated charge transfer,the generation of more active radicals, and the enhanceddegradation of toxic pollutants.

■ ASSOCIATED CONTENT*S Supporting InformationThe Supporting Information is available free of charge on theACS Publications website at DOI: 10.1021/acsami.9b00903.

High-resolution XPS spectra of V 2p, N 1s region; XPSsurvey spectra; SEM images of 10% B/VNT (element-sensitive mapping images); BET; UV−vis DRS; PL;time-resolved fluorescence decay; transient photocurrentresponse and ESI Nyquist plots of samples; XRD spectraof 10% B/VNT heterojunction after photocatalysis;pseudo-first-order constants (kapp) of different photo-catalysts; comparison of the photodegradation activity;and the results of surface area, kapp, and rate constantnormalized with the surface areas (k′) (PDF)

■ AUTHOR INFORMATIONCorresponding Authors*E-mail: xianqm@nju.edu.cn. Phone/Fax: +86 25 89680259(Q.X.).*E-mail: wxzou2016@nju.edu.cn (W.Z.).ORCIDZhifeng Jiang: 0000-0002-6589-6122Qiming Xian: 0000-0002-6965-6425NotesThe authors declare no competing financial interest.

■ ACKNOWLEDGMENTSThe authors gratefully acknowledge the financial support fromthe Major Science and Technology Program for WaterPollution Control and Treatment (2017ZX07204004), Na-tional Natural Science Foundation of China (grant 21876078,51878331, 21707066), Jiangsu Key R&D Plan (BE2017711)and the Scientific Research Foundation of Graduate School ofNanjing University (2018CW08).

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