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Applied Surface Science

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Enhanced photocatalytic property of Ag loaded on well-defined ferroelectricNa3VO2B6O11 crystals under visible light irradiation

Yufei Zhaia,c, Yang Zhangd, Jiao Yina, Xiaoyun Fanb,⁎

a Laboratory of Environmental Sciences and Technology, Xinjiang Technical Institute of Physics & Chemistry, and Key Laboratory of Functional Materials and Devices forSpecial Environments, Chinese Academy of Sciences, Urumqi 830011, PR Chinab School of Environment and Guangdong Key Laboratory of Environmental Pollution and Health, Jinan University, Guangzhou 510632, PR ChinacUniversity of Chinese Academy of Sciences, Beijing 100049, PR Chinad College of Chemistry and Chemical Engineering, Xinjiang Normal University, Urumqi, 830054, Xinjiang, PR China

A R T I C L E I N F O

Keywords:Ferroelectric materialsSpontaneous polarizationActive sitesKPFMVisible light photocatalyst

A B S T R A C T

How to efficiently utilize visible light and highly separate photo-induced charges is now a hot topic in the field ofphotocatalysis. Herein, various amounts (1–5%, wt%) of Ag nanoparticles were selectively deposited on thesurface of the polar material Na3VO2B6O11 (NVB) via the method of photoreduction. Due to the presence ofexternal screening effect, the Ag nanoparticles were selectively deposited on the positive domains of NVB ma-terials. Through DFT, the (110) facet with excessive electrons accumulated has a high reduction ability to depositthe Ag nanoparticles on the edge of NVB crystals. It was also found that the surface plasmon resonances effect ofAg nanoparticles led to the absorption spectrum of NVB broadened to the visible range around 400–800 nm.Under visible light irradiation, the photodegradation efficiency of NVB was significantly enhanced with the 4%Ag deposition, which reached 96% for 2-Chlorophenol (2-CP) degradation in 100min. It indicates that thescreening effect of NVB combination with the surface plasmon resonances effect from Ag nanoparticles canprovide a synergistic effect on the enhanced photocatalytic activity during the 2-CP degradation.

1. Introduction

An increasing of environmental pollution associated with rapid in-ternational economic growth has aroused widespread critical concernto environment protection. Photocatalytic technology has broad appli-cation prospects in solving environmental pollution and producing newenergy with its economic and clean advantages. Photocatalytic mate-rials, such as TiO2 [1,2], ZnO [3,4], C3N4 [5,6], have been widelystudied due to their excellent photocatalytic activity, chemical stability,low cost, high safety, non-toxicity, and no secondary pollution [7].However, these photocatalytic materials have a narrow light responserange and a low-efficiency utilization of photogenerated charges, whichlimits their practical applications. Various strategies have been widelydeveloped, such as doping, the coupling of semiconductor photo-catalysts, constructing of heterojunctions materials, surface modifica-tion, morphology engineering, depositing noble metal, and so forth toimprove their photocatalytic performance. [8–11]

Depositing with noble metal is proved to be one of the successfulmethods to improve the photocatalytic efficiency under visible light.

Due to the surface plasmon resonances effect, the light absorption rangeof the catalyst can be significantly broadened and the photogeneratedcharges also can be inhibited, leading to an enhanced photocatalyticeffect [12–14], such as Au-Ag alloy frames [15], Ag/TiO2 [16], Pt/GNS[17], Ag/Ag2WO4 [18], and so forth. It confirmed that metal depositingis an important way to improve the separation efficiency of photo-generated charges and broaden the absorption spectrum of the photo-catalyst. Compared with other noble metals, Ag has unique advantages,low cost, high work function, and can be composited with semi-conductor by a simple, fast and stable method, leading to significantlyimprove the overall photocatalytic performance of composite semi-conductor [19,20].

Recent studies have also found that spontaneous polarization insideferroelectric materials can separate photogenerated electron-hole pairs,thereby reducing their recombination probability [21], and increasingthe photocatalytic activity and photoelectric conversion efficiency[22–24]. The lifetime of the electrons in the ferroelectric materials iseven longer than TiO2 materials, for example, the decay time of pho-toluminescence of TiO2 thin film is 0.1 μs, while the ferroelectric

https://doi.org/10.1016/j.apsusc.2019.04.172Received 10 December 2018; Received in revised form 16 March 2019; Accepted 16 April 2019

⁎ Corresponding author at: School of Environment and Guangdong Key Laboratory of Environmental Pollution and Health, Jinan University, Guangzhou 510632,PR China.

E-mail address: xyfan@jun.edu.cn (X. Fan).

Applied Surface Science 484 (2019) 981–989

Available online 16 April 20190169-4332/ © 2019 Published by Elsevier B.V.

T

materials lithium niobate can reach up to 9 μs [25], which is far longerthan that of TiO2 thin film [26]. Usually, there are many small polar-ization regions inside the ferroelectric, and the electric dipole polar-izations in each small region are arranged in the same direction asdomains. The internal depolarization chemical field causes the down-ward band to bend with the reason of drives electrons to the surface ofthe C+ domain [27,28]. In the C− domain, on the contrary, electronsflow away from the surface, causing the upward band to bend. Whenthe ferroelectric material is excited by light, a photogenerated electron-hole pair will be generated, and the electron-hole pairs will be sepa-rated by its spontaneous polarization and then migrate to the catalystsurface, reducing the electron-hole pair recombination rate. The fer-roelectric materials with internal electronic field indeed perform anexcellent photocatalytic performance, however, due to their irregularly-shaped powdery form, which makes it difficult to confirm the activesites to unveil the relationship between the structure and the function.Until now, a great number of researches on photocatalysts are dedi-cated to facet-dependent photocatalysis [29–31]. Nevertheless, theunderlying synergetic effects regarding ferroelectric photocatalysis re-main largely unexplored.

Sodium vanadium borate Na3VO2B6O11 (NVB) combined with aboron oxide group and a vanadium oxide group is a non-centrosym-metric polar photocatalytic material with a d0 electronic configuration.It was proved that NVB has excellent performance in photocatalyticdegradation of 2,4-Dichlorophenol (2,4-DCP) under ultraviolet lightirradiation [32–34]. However, its absorption photon wavelengthreaches 419 nm, and the response range of NVB to sunlight is still in-sufficient. To this end, it remains a need to further extend the corre-sponding range of light and improve the utilization of solar energy byNVB.

In view of the above consideration, we deposited Ag nanoparticleson the surface of NVB by photoreduction method, which can sig-nificantly inhibit the recombination rate of photogenerated electrons(e−) and holes (h+) and enhance their photocatalytic performance. Agnanoparticles were selectively deposited on the NVB crystal where theelectrons accumulated as a result of the external screening effect. Afterthe Ag nanoparticles deposition, the absorption cutoff edge of NVBextends from the original 419 nm to the entire visible region. Thephoto-fluorescence luminescence spectrum shows a decrease in in-tensity after loading Ag, with a minimum of about a quarter of the

Scheme 1. Schematic illustration of photo-reduction deposition of Ag nanoparticles on the NVB crystal.

Fig. 1. SEM images of Ag-NVB samples (a), the inset image denotes the Ag nanoparticles deposition on NVB crystal; HRTEM images of Ag-NVB samples (b).

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original. We chose the contaminant 2-chlorophenol (2-CP) as the de-gradation experiments. It showed that Ag-NVB samples have an ex-cellent degradation efficiency under visible light irradiation. The elec-tron-hole pairs recombination rate on the polar material NVB is greatlyreduced after deposition of Ag, and the electron transport capability isgreatly improved.

2. Experimental

2.1. Preparation of Na3VO2B6O11

Na3VO2B6O11 (NVB) samples were synthesized by the solid-statereaction method according to the following reaction equation:

+ + = + ↑ + ↑3Na2CO V O 12H BO 2Na VO B O 3CO 18H O3 2 5 3 3 3 2 6 11 2 2

Powders of Na2CO3 6.07 g (AR, Tianjin HongRi reagent plant), V2O5

3.47 g (AR, Shanghai ShanPu Chemical Co., Ltd.), and H3BO3 14.17 g(AR, Tianjin chemical reagents) were mixed and ground in an agatemortar and pestle for 20min. Then put the powders into a crucible andheated in a muffle furnace for 2 h to 630 °C, which the temperature wasraised by 5 °C per minute, and heated at 630 °C for 2 h. The mixture wastaken out once every hour and ground for 20min. After naturallycooling to room temperature, collected carefully the white powders anduniformly ground again, washed 2–3 times with deionized water, thendried and stored in a vacuum desiccator.

2.2. Preparation of Ag/NVB photocatalysts

The Ag/NVB photocatalysts were prepared by a photocatalytic-in-duced deposition of Ag nanoparticles under UV-light irradiation. 0.5 gNVB and a certain amount of AgNO3 solution (the concentrations is0.025 g/L, 0.125 g/L, 0.25 g/L, 0.375 g/L and 0.5 g/L, respectively)were mixed in a 200mL beaker and dispersed in an ultrasonic cleanerfor 10min. The above suspension was stirred and exposed to UV light atambient temperature for 30min. A xenon lamp (PLS-SXE300/300UV)was used as a light source. Then, the samples were collected (namedNVB1, NVB2, NVB3, NVB4, NVB5, respectively) after irradiation, wa-shed with deionized water several times, and finally lyophilized toobtain samples and placed into the vacuum dryer for storage.

2.3. Photocatalytic activity experiment

The photocatalytic performance of the samples was evaluated byperforming degradation of 2-chlorophenol (2-CP) with the visible lightirradiation. In this experiment, 50mL of 2-CP solution (20mg/L) and50mg of the photocatalyst were dispersed in a reaction container.Before irradiation, the solution was stirred in the dark for 0.5 h con-tinuously to ensure the adsorption-desorption equilibrium establish-ment between the photocatalysts and 2-CP. In the visible light activityevaluation experiment, the light source used a xenon lamp (PLS-SXE300/300 UV) and was equipped with a 420 nm filter. In the processof photocatalytic reaction, the 2-CP solution having the photocatalystwas continuously stirred with a magnetic stirrer, and at a certain timeinterval, 1 mL of the reaction solution was sampled and filtered for

Fig. 2. EDS mapping images of NVB4 sample.

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high-performance liquid chromatography (HPLC) analysis.

2.4. Characterization of materials

Powder X-ray diffraction (XRD) patterns of the samples were col-lected on an X-ray diffractometer equipped using Cu Kα1(λ=0.15418 nm) with a scan step of 0.01° and a scan range between10° and 70° (Bruker D8). The morphologies of the samples were ex-amined by electron microscope operated on ZEISS SUPRA55VP appa-ratus and energy dispersive X-ray (EDX) spectroscopic analysis wasconducted by EDX8000. The high-resolution transmission electron mi-croscope (HRTEM) on FEI Tecnai G2 f20s-twin 200 kV field emissiontransmission electron microscope operated at an accelerating voltage of200 kV. The diffuse reflectance absorption spectra (DRS) of the sampleswere recorded in the range from 200 nm to 800 nm on a Solidspec-3700DUV spectrometer and using BaSO4 as a reference. The Fourier trans-form infrared (FTIR) spectra were obtained using a Bruker V70FTIRspectrophotometer from 4000 cm−1 to 600 cm−1.

High-performance liquid chromatography (HPLC) (Ultimate 3000,Dionex) was carried out with a C18 column (4.6 mm×250mm). Usinga mixture of methanol and water [85/15 (v/v)] as an effluent with theflow rate was 0.5 mL/min. The Brunauer-Emmett-Teller (BET) surfacearea was acquired from the N2 adsorption/desorption isotherms re-corded at 77 K (QUADRASORB IQ, Quantachrome Instrument Corp).The fluorescence emission spectrum (excited at 295 nm) were measuredon a Hitachi fluorescence spectrophotometer F-7000. Raman spectrawere conducted by using a HORIBA Jobin Yvon LabRAM HR confocalspectrometer confocal microscope Raman spectrometer employing anilluminated with a visible (532 nm) laser. Atomic force microscopy(AFM) combined with Kelvin probe force microscopy (KPFM) mea-surements were performed with an atomic force microscope (BrukerMultimode 8). In KPFM, electrostatic forces between the tip and sampleresult from a non-contact potential difference. The non-contact poten-tial difference was instantly and automatically nullified by a DC biasapplied to the probe tip. This DC bias was recorded at each position to

generate a potential image. In the experiment, the samples were firstdispersed in ethanol and then sprayed on an indium tin oxide (ITO)glass substrate to form a film. The sample was dried in a dry oven at80 °C for 6–10 h. The topography of the film surface was measured innon-contact mode and the surface potential variation was measuredusing a probe tip. The photocurrent response was carried out on a CHI660 C electrochemical workstation (Chen Hua Instrument) with a con-ventional tri-electrode system. 0.1 M Na2SO4 solution served as theelectrolyte, with a platinum foil as the counter electrode and an Ag/AgCl (saturated KCl) reference electrode. The working electrodes wereprepared by dropping a slurry of 2mg photocatalyst and 30 μL Nafion/ethanol (v/v=1:5) onto conducting indium tin oxide glass (ITO) withan area of 1 cm2 and dried in air at ambient temperature.

2.5. Theoretical methods

The first principles calculations in the framework of density func-tional theory (DFT), including structural, electronic performances, werecarried out based on the Cambridge Sequential Total Energy Packageknown as CASTEP [35]. The exchange-correlation functional under thegeneralized gradient approximation (GGA) [36] with norm-conservingpseudopotentials and Perdew–Burke–Ernzerhof functional was adoptedto describe the electron-electron interaction [37]. An energy cutoff of750 eV was used and a k-point sampling set of 7×7×1 was tested tobe converged. A force tolerance of 0.01 eV Å−1, energy tolerance of5.0× 10−7eV per atom and maximum displacement of 5.0× 10−4 Åwere considered. The surface models of NVB (001), (110), and (111)were built. The vacuum space along the z-direction is set to be 15 Å,which is enough to avoid interaction between the two neighboringimages. The bottom three atomic layers were fixed and the three atomiclayers were relaxed. The VBM and CBM were calculated with the iso-surface of 0.01 e/Bohr3.

Fig. 3. The highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of the (001), (110) and (111) facets, and the cor-responding VBM, CBM, and Fermi levels in the vacuum. The green and yellow areas represent LUMO and HOMO, respectively.

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3. Results and discussion

It needs to be pointed out that without the light irradiation, no Agnanoparticles can be successfully deposited on the NVB sample surface,which is the reason of photoreduction. For effective deposition ofAgNO3, a 300W Xenon lamp with the intensity 0.54 w/cm2 was takenin the experiment. The process of Ag/NVB samples forming can besummarized as follow, AgNO3 which act as Ag source, readily dissolveinto the water under dark conditions, and then transferred to the beakerwhich contained NVB powders, under the photoirradiation, the Ag+

ions were reduced into Ag nanoparticles. As the irradiation times in-creasing to 30min, large numbers of Ag nanoparticles were successfullydeposited on the surface of NVB. For the different amount of Ag

loading, the color of the composite Ag/NVB samples changed from lightyellow into gray, which was might be caused by the partial oxidation ofAg nanoparticles into Ag2O [38,39]. It can also be seen from Scheme 1,the morphology of NVB ferroelectrics materials changed during thephotoreduction process. A plausible mechanism was introduced to ex-plain the NVB crystals formation. When the addition of the Ag+ solu-tion to the beaker, the spontaneous nucleation of NVB sample begins totake place rapidly. During the subsequent grain growth process, anexternal screening mechanism plays a vital role in determining themorphology of the NVB grains. It has been reported that the uniqueproperties of ferroelectric surfaces have been used to achieve localizedredox reactions [40,41], on the surface of the ferroelectric NVB mate-rials, there is a spontaneous polarization causing the negative charge(C− domain) in the direction that pointing away from the surface to thebulk, under the polarization charge screening mechanism, as the timeincreased, the Ag+ ions were selectively deposited on the positive do-main of the NVB surface.

Na3VO2B6O11 crystallizes in the non-centrosymmetric structurewith space group P212121 belonging to the ferroelectric materials [34].The structure of the compound consists of two units: VO4 and B6O13

groups (Fig. S1), which share a common oxygen atom to form a three-dimensional framework. The isolated B6O13 units were originally fromthe connecting of the triangular plane BO3 and the tetrahedral BO4,then connecting to the VO4 tetrahedron to form an infinite piece, andthe Na+ cation is distributed in the extended frame.

The SEM and HRTEM were used to study detailed morphology andmicrostructure of the composite photocatalyst. It is observed that NVBsingle crystal shows a regular morphology with the maximized size upto 800 nm and the Ag nanoparticles selected deposited on the NVBsurface (Fig. 1a). The surface morphology and distribution of Ag na-noparticles on the NVB crystal surfaces was further explored byHRTEM. The d-spacing of the tiny dark parts of the composite material(Fig. 1b) is measured to be at 0.234 nm, which agrees with the (111)facet spacing of Ag nanoparticles. The EDS spectra were also recordedin parts of Fig. 1b, revealing the presence of Na, B, V, O and Ag ele-ments, which is further confirmed that the Ag nanoparticles were suc-cessfully deposited on the NVB samples (Fig. 2).

In order to determine which facet of the NVB crystal the Ag nano-particles will deposit on, the preferentially exposed (001), (110), and(111) facets of studied NVB crystal have been taken into consideration[42]. It has been reported that the largest facet occupied in the centerby (001) facet, while the (110) and (111) facets located in the edge.When the photo-deposition the Ag nanoparticles under UV irradiation,the electrons are preferentially transported to the crystal edge insteadof the crystal center of facet [43]. The calculation result based on thedensity functional theory (DFT) exhibits different band structures andband edge positions of these three facets, and a schematic diagram of

Fig. 4. XRD analysis of Ag-NVB samples (a); UV–vis diffuse reflectance spec-trum of Ag-NVB samples (b); Raman spectrum of the Ag-NVB samples (c).

Table 1FTIR and Raman and the assignment of vibrational modes of the Na3VO2B6O11

crystal.

FTIR frequencies (cm−1) Raman frequencies(cm−1)

Vibrational modes

716 cm−1 371 cm−1 VO4 asymmetric stretching420 cm−1

456 cm−1

735 cm−1 620 cm−1 BO3 bending vibration776 cm−1 BO4 breathing vibration818 cm−1

816–941 cm−1 891 cm−1 VO4 symmetric stretching923 cm−1 BO3 Symmetric stretching945 cm−1

980–1200 cm−1 BO4 asymmetric stretch1301 cm−1 BO3 antisymmetric

stretching1300–1450 cm−1 BO3 asymmetric stretch

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the simulation calculation of the surface redox ability of the (001),(110), and (111) facets are shown in Fig. 3. The green and yellow re-gions represent the lowest unoccupied molecular orbital (LUMO) andthe highest occupied molecular orbital (HOMO), respectively. It can beseen from Table S1 that the CBM of (110) facet locates at a higherposition compared with (001) and (111), which indicates the highestelectron filling probability and highest reduction ability which willfacilitate the reduction of silver ions. Under ultraviolet light, the elec-trons in the valence band of NVB are excited to transition to the con-duction band, the free silver ions in the solution preferentially obtainelectrons and are reduced to silver atoms deposited on the (110) facet.

The crystal structure of NVB and Ag/NVB samples were confirmedby XRD analysis (Fig. 4a). It can be clearly seen that the six samplesshow almost the same pattern and their diffraction peaks were all be-long to the NVB phase. Due to the low content of Ag nanoparticles, nodiffraction peak of Ag is detected. Optical response properties of the sixsamples were examined through UV–vis DRS in the range of200–800nm which is shown in Fig. 4b, where the bared NVB shows the

typical DRS in the UV range with absorption onset at about 419 nm.However, the loading of Ag nanoparticles leads to an improvement inthe intensity of light absorption and extended the light response intothe visible region. The result clearly showed that Ag/NVB compositeshave a very strong absorption band around 400–800 nm the intensityrises rapidly from 600 nm. As the amounts of Ag increasing, the ab-sorption intensity of the Ag/NVB samples increased, among the sixsamples, the NVB5 sample has the highest absorption capacity in thevisible range. It is worth noting that the absorption peak appears at425 nm [44], which is attributed to the surface plasmon resonances ofAg nanoparticles. Fig. S2 shows the FTIR spectrometer analysis curve ofthe Ag/NVB samples in the frequency range of 400 to 1600 cm−1. TheAg/NVB samples display four characteristic absorption peaks in thesection of 1300–1450 cm−1, 800–1200 cm−1, 540–790 cm−1,400–480 cm−1. This result completely coincided with the reported lit-erature [45–48], the band 1300–1450 cm−1 is related to the BeO vi-bration in the BO3 group, the band 716 cm−1 and 735 cm−1 is classifiedto bridge oxygen vibration of the BeOeB in BO3 unit. The band of980–1200 cm−1 is assigned to the BeO vibration of the BO4 group. Theband of 816–941 cm−1 is caused by the vibration of VO4 unit. No sig-nificant infrared spectral shift occurred implying that the addition of Agnanoparticles has no influence on the structure of the NVB sample. Thecrystal structure was further confirmed by Raman spectroscopy shownin Fig. 4c. For the Ag/NVB samples, the wavenumber at 1301 cm−1 isgenerated by the antisymmetric stretching vibration of the BO3 group,and the BO3 triangle is connected to the BO4 tetrahedron through thebridge oxygen atom to form boron. The oxygen ring is generally con-sidered to have a respiratory vibration of a six-membered ring having aBO4 tetrahedron in the 760–780 cm−1 band, and 776 cm−1 and818 cm−1 are classified as a breathing vibration of a boron‑oxygen ring,which involves only the vibration of an epoxy atom. The peaks at

Fig. 5. Ag-NVB samples degradation of 2-CP in aqueous solution under visible light irradiation (λ > 420 nm) (a); Photocurrent response measurements of the Ag-NVB electrodes under the irradiation of UV–visible light (b) and EIS Nyquist plots of the Ag-NVB electrodes (c); Fluorescence spectroscopy of Ag-NVB samples (d).

Table 2The BET Surface Area of the NVBX samples and the proportion (NVBX)/(NVB)(X= 1, 2, 3, 4, 5) of photocurrent response of Ag-NVB electrodes under theirradiation of UV–visible light.

Sample BET surface area (m2/g) Proportion of photocurrent

NVB 0.8 1.00NVB1 1.9 1.17NVB2 1.2 1.42NVB3 1.9 1.60NVB4 1.6 2.15NVB5 1.0 1.82

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923 cm−1 and 945 cm−1 are related to the symmetric stretching vi-bration of BO3. The peak at 620 cm−1 can be attributed to the BO3

bending vibration, and the peak at 891 cm−1 is induced by the sym-metric stretching vibration of VO4. The peaks at 371 cm−1, 420 cm−1,and 456 cm−1 are brought about by the asymmetric stretching vibra-tion of VO4. The above results are also clearly shown in Table 1. Itshould be noted that for NVB sample, the NVB1-NVB5 samples show

the obvious peaks around 776 cm−1 and 818 cm−1 and with the in-crease of Ag content, the shape of the peak becomes clear and sharp.The obvious signal enhancement of 776 cm −1-818 cm−1 can be con-cluded that the Raman signal enhancement is caused by plasma re-sonance in the presence of silver. It has been stated that the resonantexcitation of surface plasmon resonances creates an enhancement of thelocal electric field around Ag nanoparticles which can enhance the

Fig. 6. Topographic image of NVB4 sample (a); The red dash line represents the NVB crystal and the Ag nanoparticles were circled by black dash line, andcorresponding height curves denoted in panel (b); KPFM image of the NVB4 sample (c) and corresponding surface potential curves denoted in panel (d).

Scheme 2. Schematic drawing illustrating the mechanism of charge separation and photocatalytic process over Ag-NVB photocatalyst under visible light irradiation(λ > 420 nm).

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Raman scattering intensities in the visible region [49,50]. We notedthat the intensity of NVB1-NVB5 Raman spectrum around776 cm−1–818 cm−1 was enhanced while the other peaks of the Ramanspectrum were not changed. This is because the size and shape of thesilver nanoparticles in each sample are relatively uniform, and there isno significant difference. The resonant Raman spectrum at a de-termined excitation wavelength is enhanced in a certain band ratherthan the full band.

The photocatalytic activities of Ag/NVB samples were tested bydegrading 2-CP as the reaction model under visible light irradiation(λ > 420 nm). Fig. 5a shows that the degradation efficiency of NVB2,NVB3, NVB4, and NVB5 samples increase as the increasing of Ag con-tent and the degradation efficiency of NVB4 and NVB5 samples show asimilar result exceeding to 95% in 100min, which is far more higherthan that of P25 [34]. As shown in Fig. 5b and Table 2, the photo-current increases with the increase of Ag content and NVB4 sample hasa maximum photocurrent of about 2.15 times that of NVB sample,showing excellent electron generation and transportation ability.Fig. 5c shows the EIS Nyquist curve, which shows that NVB4 sample hasthe smallest radius of the curve, indicating that it has the highest carriertransport capacity. In order to further confirm the foregoing findings,we measured the photoluminescence spectra (Fig. 5d). The NVB samplehas a strong and broad peak at 295 nm wavelength excitation, while theluminescence intensity of the Ag/NVB samples decreases significantly.The NVB4 sample shows the lowest fluorescence intensity, which in-dicates that the composite photocatalyst promotes charge carriertransfer and reduces photogenerated electron and hole recombinationafter Ag deposition.

KPFM can detect the contact potential difference (CPD) between thework function of the sample and the tip of the AFM. Kelvin Probe ForceMicroscope (KPFM) can also determine to surface potential imagesthrough its nanometer-scale spatial resolution and millivolt sensitivity[51]. Considering a higher efficiency of NVB4 sample, we selectedNVB4 sample as the test model in the following experiment. The KPFMimages of NVB4 sample and the corresponding height and voltageprofiles along the straight lines showed in blue and red in the imagessections (Fig. 6). The red dash line represents the NVB crystal and theAg nanoparticles were circled by the black dash line in Fig. 6a. FromFig. 6b we can see that the height of Ag nanoparticles deposited on thesurface of the NVB is about 30 nm and this is consistent with theHRTEM result. Fig. 6c and Fig. 6d show surface potential informationfor the sample. The difference in surface potential between the placewhere Ag was deposited and not deposited was about 20mV, whichproved that the place where Ag was deposited had higher concentra-tions of electrons. This indicates that the Ag nanoparticles can promotethe photo-induced charges separation, which enhanced the electricfields in the region of Ag/NVB samples. Therefore, by adding Ag na-noparticles, the built-in electric field-induced carrier separation en-hancement can be maintained, which can continuously improve thephotocatalytic performance of the Ag/NVB samples photocatalyst.

In order to explore the mechanism of photocatalytic degradation,the reactive oxygen species trapping studies of degradation 2-CP ac-tivity with the addition of different scavengers over the sample (Fig. S3)were carried on under visible light irradiation. In Fig. S3, it can befound that the photodegradation activity of the sample after the addi-tion of hydroxyl radical scavenger (TBA) and hole scavenger (me-thanol) is slightly reduced, but in the presence of superoxide radicalscavenger (p-BQ) decreases. Since bromate ions are oxidizing, theyexhibit a better degradation effect than NVB sample when KBrO3 isadded as an electron scavenger agent. According to the above results,we can see that the photogenerated superoxide radicals are the domi-nant reactive oxygen species of Ag/NVB samples. It is further confirmedthat the amount of superoxide radicals generated is far more than thatin NVB samples in the Ag/NVB system.

The details process of degrading 2-CP under visible light irradiationcan be represented by the following steps:

+ → +− + +Ag visible light e Ag (h ) (1)

+ + →−NVB e O ˙O ̅2 2 (2)

+ →+˙O ̅ H ˙OOH2 (3)

+ + →+ −˙OOH H e H O2 2 (4)

+ → +− −H O e ˙OH OH2 2 (5)

+ − →+ + −Ag (h ), NVB, ˙OH, ˙O ̅, e , 2 CP degradation products2 (6)

The surface area of the catalyst has a significant effect on the cat-alytic efficiency of the catalyst [52]. The BET test results showed thatthey have a small surface area, the maximum BET value is only 1.9 m2/g (Table 2, Fig. S4), which limits the contact of the catalyst with con-taminants during photocatalytic degradation of contaminants. It hasbeen stated that the NVB is a ferroelectric material with the internalelectric field, which can form a space charge layer and act as a moti-vation to effect separation of the photo-induced charges. The presenceof a screening effect in the NVB materials can increase the adsorption ofcharged species, for instance, dye molecules or other contaminantmolecules under external screening [53,54]. The fact of selective de-position of Ag nanoparticles on the C+ domain of the NVB can be wellexplained by employing the strong screening effect of the polar crystal.The electrons were excited by the visible irradiation and transferred bythe screening effect to the C+ surface. The Ag+ ions in the aqueoussolution were reduced when they combined with the electrons trans-ferred from the interior of NVB crystal to its C+ surface. The electronsremoved from the C+ surface by the Ag reduction are continuously,which were confirmed to have an Ohmic contact with the NVB crystal.Electrons are not concentrated on the C− surface, in contrast, Ag+ ionsare not reduced there. It has been reported that Ag (< 30 nm) nano-particles can form the energetic charge carriers which can be trans-ferred to the surroundings or relax by locally heating the nanostructure[14,55]. When the system was under the visible light irradiation thedifference of the work function of the two-phase(Ag nanoparticles andthe NVB particles) will lead to the photo-induced charges generated bythe Ag nanoparticle transferred to the NVB surface to take place re-duction reaction, while the holes will take place oxidation reaction[56]. Furthermore, the Ag nanoparticle on the NVB enhanced thevisible light (λ > 420 nm) absorption for the reason of the surfaceplasmon resonances effect, and the photogenerated electrons trans-ported to one surface of NVB formed an electron-rich surface. At thistime, the electrons form %O2

− with O2 that in the water, and furthercombine with H+ form %OOH and H2O2 to oxidize contaminants. Onthe other side, the C− domain adsorbs a large number of contaminantmolecules under external screening and oxidizes these contaminantmolecules. A schematic diagram of the rational mechanism of chargeseparation and photocatalysis in the system is as shown in Scheme 2.

4. Conclusions

In summary, a series of visible light responsive photocatalytic ma-terials Ag/Na3VO2B6O11 were fabricated successfully. The Ag/NVBcomposites showed excellent photocatalytic performance under visiblelight irradiation. The photodegradation efficiency can reach up to 96%for NVB4 sample during 2-Chlorophenol (2-CP) degradation in 100min.The main reactive oxygen species was determined as %O2

−, radicalaccording to the trapping test. The enhanced activity is ascribed to thesynergistic effect of Ag/NVB composite materials which improved se-paration efficiency of photoexcited charges. It is demonstrated thatferroelectric spontaneous polarization electric field of NVB and thesurface plasmon resonances effect of Ag nanoparticles takes a sy-nergistic effect on the separation and transportation of the photo-generated electrons and holes. The achievements and findings in thiswork are expected to contribute to design more efficient photocatalystsystems by controlling internal fields and the noble metals. In the

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following work, we will focus on the other metals deposition on moreferroelectric materials in the visible light region.

Acknowledgments

This work was supported by the National Natural ScienceFoundation of China (51772325), and Natural Science Foundation ofXinjiang Province (2016D01A072).

Appendix A. Supplementary data

View of the structure of NVB along the a axis and structure of B6O13

units and VO4 units, FTIR spectrum of the Ag-NVB samples, reactiveoxygen species trapping studies of degradation 2-CP activity with theaddition of different scavengers under visible light irradiation over thesamples, BET analysis of NVB, Ag/NVB samples. Supplementary data tothis article can be found online at https://doi.org/10.1016/j.apsusc.2019.04.172.

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