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Enhanced photocatalytic property of Ag loaded on well-defined ferroelectric Na3VO2B6O11 crystals under visible light irradiation
Applied Surface Science 484 (2019) 981–989
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Enhanced photocatalytic property of Ag loaded on well-defined ferroelectric Na3VO2B6O11 crystals under visible light irradiation Yufei Zhaia,c, Yang Zhangd, Jiao Yina, Xiaoyun Fanb,
T
⁎
a Laboratory of Environmental Sciences and Technology, Xinjiang Technical Institute of Physics & Chemistry, and Key Laboratory of Functional Materials and Devices for Special Environments, Chinese Academy of Sciences, Urumqi 830011, PR China b School of Environment and Guangdong Key Laboratory of Environmental Pollution and Health, Jinan University, Guangzhou 510632, PR China c University of Chinese Academy of Sciences, Beijing 100049, PR China d College of Chemistry and Chemical Engineering, Xinjiang Normal University, Urumqi, 830054, Xinjiang, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: Ferroelectric materials Spontaneous polarization Active sites KPFM Visible light photocatalyst
How to efficiently utilize visible light and highly separate photo-induced charges is now a hot topic in the field of photocatalysis. Herein, various amounts (1–5%, wt%) of Ag nanoparticles were selectively deposited on the surface of the polar material Na3VO2B6O11 (NVB) via the method of photoreduction. Due to the presence of external screening effect, the Ag nanoparticles were selectively deposited on the positive domains of NVB materials. Through DFT, the (110) facet with excessive electrons accumulated has a high reduction ability to deposit the Ag nanoparticles on the edge of NVB crystals. It was also found that the surface plasmon resonances effect of Ag 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 100 min. It indicates that the screening effect of NVB combination with the surface plasmon resonances effect from Ag nanoparticles can provide a synergistic effect on the enhanced photocatalytic activity during the 2-CP degradation.
1. Introduction An increasing of environmental pollution associated with rapid international economic growth has aroused widespread critical concern to environment protection. Photocatalytic technology has broad application prospects in solving environmental pollution and producing new energy with its economic and clean advantages. Photocatalytic materials, such as TiO2 [1,2], ZnO [3,4], C3N4 [5,6], have been widely studied 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 response range and a low-efficiency utilization of photogenerated charges, which limits their practical applications. Various strategies have been widely developed, such as doping, the coupling of semiconductor photocatalysts, constructing of heterojunctions materials, surface modification, morphology engineering, depositing noble metal, and so forth to improve their photocatalytic performance. [8–11] Depositing with noble metal is proved to be one of the successful methods to improve the photocatalytic efficiency under visible light.
Due to the surface plasmon resonances effect, the light absorption range of the catalyst can be significantly broadened and the photogenerated charges also can be inhibited, leading to an enhanced photocatalytic effect [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 depositing is an important way to improve the separation efficiency of photogenerated charges and broaden the absorption spectrum of the photocatalyst. Compared with other noble metals, Ag has unique advantages, low cost, high work function, and can be composited with semiconductor by a simple, fast and stable method, leading to significantly improve the overall photocatalytic performance of composite semiconductor [19,20]. Recent studies have also found that spontaneous polarization inside ferroelectric materials can separate photogenerated electron-hole pairs, thereby reducing their recombination probability [21], and increasing the photocatalytic activity and photoelectric conversion efficiency [22–24]. The lifetime of the electrons in the ferroelectric materials is even longer than TiO2 materials, for example, the decay time of photoluminescence of TiO2 thin film is 0.1 μs, while the ferroelectric
⁎ Corresponding author at: School of Environment and Guangdong Key Laboratory of Environmental Pollution and Health, Jinan University, Guangzhou 510632, PR China. E-mail address: [email protected] (X. Fan).
https://doi.org/10.1016/j.apsusc.2019.04.172 Received 10 December 2018; Received in revised form 16 March 2019; Accepted 16 April 2019 Available online 16 April 2019 0169-4332/ © 2019 Published by Elsevier B.V.
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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).
materials lithium niobate can reach up to 9 μs [25], which is far longer than that of TiO2 thin film [26]. Usually, there are many small polarization regions inside the ferroelectric, and the electric dipole polarizations in each small region are arranged in the same direction as domains. The internal depolarization chemical field causes the downward band to bend with the reason of drives electrons to the surface of the C+ domain [27,28]. In the C− domain, on the contrary, electrons flow away from the surface, causing the upward band to bend. When the ferroelectric material is excited by light, a photogenerated electronhole pair will be generated, and the electron-hole pairs will be separated by its spontaneous polarization and then migrate to the catalyst surface, reducing the electron-hole pair recombination rate. The ferroelectric materials with internal electronic field indeed perform an excellent photocatalytic performance, however, due to their irregularlyshaped powdery form, which makes it difficult to confirm the active sites to unveil the relationship between the structure and the function. Until now, a great number of researches on photocatalysts are dedicated to facet-dependent photocatalysis [29–31]. Nevertheless, the underlying synergetic effects regarding ferroelectric photocatalysis remain largely unexplored.
Sodium vanadium borate Na3VO2B6O11 (NVB) combined with a boron oxide group and a vanadium oxide group is a non-centrosymmetric polar photocatalytic material with a d0 electronic configuration. It was proved that NVB has excellent performance in photocatalytic degradation of 2,4-Dichlorophenol (2,4-DCP) under ultraviolet light irradiation [32–34]. However, its absorption photon wavelength reaches 419 nm, and the response range of NVB to sunlight is still insufficient. To this end, it remains a need to further extend the corresponding range of light and improve the utilization of solar energy by NVB. In view of the above consideration, we deposited Ag nanoparticles on the surface of NVB by photoreduction method, which can significantly inhibit the recombination rate of photogenerated electrons (e−) and holes (h+) and enhance their photocatalytic performance. Ag nanoparticles were selectively deposited on the NVB crystal where the electrons accumulated as a result of the external screening effect. After the Ag nanoparticles deposition, the absorption cutoff edge of NVB extends from the original 419 nm to the entire visible region. The photo-fluorescence luminescence spectrum shows a decrease in intensity after loading Ag, with a minimum of about a quarter of the 982
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Fig. 2. EDS mapping images of NVB4 sample.
2.2. Preparation of Ag/NVB photocatalysts
original. We chose the contaminant 2-chlorophenol (2-CP) as the degradation experiments. It showed that Ag-NVB samples have an excellent degradation efficiency under visible light irradiation. The electron-hole pairs recombination rate on the polar material NVB is greatly reduced after deposition of Ag, and the electron transport capability is greatly improved.
The Ag/NVB photocatalysts were prepared by a photocatalytic-induced deposition of Ag nanoparticles under UV-light irradiation. 0.5 g NVB and a certain amount of AgNO3 solution (the concentrations is 0.025 g/L, 0.125 g/L, 0.25 g/L, 0.375 g/L and 0.5 g/L, respectively) were mixed in a 200 mL beaker and dispersed in an ultrasonic cleaner for 10 min. The above suspension was stirred and exposed to UV light at ambient temperature for 30 min. A xenon lamp (PLS-SXE300/300UV) was used as a light source. Then, the samples were collected (named NVB1, NVB2, NVB3, NVB4, NVB5, respectively) after irradiation, washed with deionized water several times, and finally lyophilized to obtain samples and placed into the vacuum dryer for storage.
2. Experimental 2.1. Preparation of Na3VO2B6O11 Na3VO2B6O11 (NVB) samples were synthesized by the solid-state reaction method according to the following reaction equation:
3Na2CO3 + V2O5 + 12H3 BO3 = 2Na3VO2 B6 O11 + 3CO2 ↑ + 18H2 O↑
2.3. Photocatalytic activity experiment
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 agate mortar and pestle for 20 min. Then put the powders into a crucible and heated in a muffle furnace for 2 h to 630 °C, which the temperature was raised by 5 °C per minute, and heated at 630 °C for 2 h. The mixture was taken out once every hour and ground for 20 min. After naturally cooling to room temperature, collected carefully the white powders and uniformly ground again, washed 2–3 times with deionized water, then dried and stored in a vacuum desiccator.
The photocatalytic performance of the samples was evaluated by performing degradation of 2-chlorophenol (2-CP) with the visible light irradiation. In this experiment, 50 mL of 2-CP solution (20 mg/L) and 50 mg of the photocatalyst were dispersed in a reaction container. Before irradiation, the solution was stirred in the dark for 0.5 h continuously to ensure the adsorption-desorption equilibrium establishment between the photocatalysts and 2-CP. In the visible light activity evaluation experiment, the light source used a xenon lamp (PLSSXE300/300 UV) and was equipped with a 420 nm filter. In the process of photocatalytic reaction, the 2-CP solution having the photocatalyst was continuously stirred with a magnetic stirrer, and at a certain time interval, 1 mL of the reaction solution was sampled and filtered for 983
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Fig. 3. The highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of the (001), (110) and (111) facets, and the corresponding VBM, CBM, and Fermi levels in the vacuum. The green and yellow areas represent LUMO and HOMO, respectively.
generate a potential image. In the experiment, the samples were first dispersed 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 at 80 °C for 6–10 h. The topography of the film surface was measured in non-contact mode and the surface potential variation was measured using a probe tip. The photocurrent response was carried out on a CHI 660 C electrochemical workstation (Chen Hua Instrument) with a conventional tri-electrode system. 0.1 M Na2SO4 solution served as the electrolyte, with a platinum foil as the counter electrode and an Ag/ AgCl (saturated KCl) reference electrode. The working electrodes were prepared by dropping a slurry of 2 mg photocatalyst and 30 μL Nafion/ ethanol (v/v = 1:5) onto conducting indium tin oxide glass (ITO) with an area of 1 cm2 and dried in air at ambient temperature.
high-performance liquid chromatography (HPLC) analysis. 2.4. Characterization of materials Powder X-ray diffraction (XRD) patterns of the samples were collected on an X-ray diffractometer equipped using Cu Kα1 (λ = 0.15418 nm) with a scan step of 0.01° and a scan range between 10° and 70° (Bruker D8). The morphologies of the samples were examined by electron microscope operated on ZEISS SUPRA55VP apparatus and energy dispersive X-ray (EDX) spectroscopic analysis was conducted by EDX8000. The high-resolution transmission electron microscope (HRTEM) on FEI Tecnai G2 f20s-twin 200 kV field emission transmission electron microscope operated at an accelerating voltage of 200 kV. The diffuse reflectance absorption spectra (DRS) of the samples were recorded in the range from 200 nm to 800 nm on a Solidspec-3700 DUV spectrometer and using BaSO4 as a reference. The Fourier transform infrared (FTIR) spectra were obtained using a Bruker V70FTIR spectrophotometer 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 × 250 mm). Using a mixture of methanol and water [85/15 (v/v)] as an effluent with the flow rate was 0.5 mL/min. The Brunauer-Emmett-Teller (BET) surface area was acquired from the N2 adsorption/desorption isotherms recorded at 77 K (QUADRASORB IQ, Quantachrome Instrument Corp). The fluorescence emission spectrum (excited at 295 nm) were measured on a Hitachi fluorescence spectrophotometer F-7000. Raman spectra were conducted by using a HORIBA Jobin Yvon LabRAM HR confocal spectrometer confocal microscope Raman spectrometer employing an illuminated with a visible (532 nm) laser. Atomic force microscopy (AFM) combined with Kelvin probe force microscopy (KPFM) measurements were performed with an atomic force microscope (Bruker Multimode 8). In KPFM, electrostatic forces between the tip and sample result from a non-contact potential difference. The non-contact potential difference was instantly and automatically nullified by a DC bias applied to the probe tip. This DC bias was recorded at each position to
2.5. Theoretical methods The first principles calculations in the framework of density functional theory (DFT), including structural, electronic performances, were carried out based on the Cambridge Sequential Total Energy Package known as CASTEP [35]. The exchange-correlation functional under the generalized gradient approximation (GGA) [36] with norm-conserving pseudopotentials and Perdew–Burke–Ernzerhof functional was adopted to describe the electron-electron interaction [37]. An energy cutoff of 750 eV was used and a k-point sampling set of 7 × 7 × 1 was tested to be converged. A force tolerance of 0.01 eV Å−1, energy tolerance of 5.0 × 10−7eV per atom and maximum displacement of 5.0 × 10−4 Å were 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 neighboring images. The bottom three atomic layers were fixed and the three atomic layers were relaxed. The VBM and CBM were calculated with the isosurface of 0.01 e/Bohr3.
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Table 1 FTIR 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−1 420 cm−1 456 cm−1 620 cm−1 776 cm−1 818 cm−1 891 cm−1 923 cm−1 945 cm−1
VO4 asymmetric stretching
735 cm−1
816–941 cm−1
980–1200 cm−1 1301 cm−1 1300–1450 cm−1
BO3 bending vibration BO4 breathing vibration VO4 symmetric stretching BO3 Symmetric stretching BO4 asymmetric stretch BO3 antisymmetric stretching BO3 asymmetric stretch
loading, the color of the composite Ag/NVB samples changed from light yellow into gray, which was might be caused by the partial oxidation of Ag nanoparticles into Ag2O [38,39]. It can also be seen from Scheme 1, the morphology of NVB ferroelectrics materials changed during the photoreduction process. A plausible mechanism was introduced to explain the NVB crystals formation. When the addition of the Ag+ solution to the beaker, the spontaneous nucleation of NVB sample begins to take place rapidly. During the subsequent grain growth process, an external screening mechanism plays a vital role in determining the morphology of the NVB grains. It has been reported that the unique properties of ferroelectric surfaces have been used to achieve localized redox reactions [40,41], on the surface of the ferroelectric NVB materials, there is a spontaneous polarization causing the negative charge (C− domain) in the direction that pointing away from the surface to the bulk, under the polarization charge screening mechanism, as the time increased, the Ag+ ions were selectively deposited on the positive domain of the NVB surface. Na3VO2B6O11 crystallizes in the non-centrosymmetric structure with 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 threedimensional framework. The isolated B6O13 units were originally from the connecting of the triangular plane BO3 and the tetrahedral BO4, then connecting to the VO4 tetrahedron to form an infinite piece, and the Na+ cation is distributed in the extended frame. The SEM and HRTEM were used to study detailed morphology and microstructure of the composite photocatalyst. It is observed that NVB single crystal shows a regular morphology with the maximized size up to 800 nm and the Ag nanoparticles selected deposited on the NVB surface (Fig. 1a). The surface morphology and distribution of Ag nanoparticles on the NVB crystal surfaces was further explored by HRTEM. 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 recorded in parts of Fig. 1b, revealing the presence of Na, B, V, O and Ag elements, which is further confirmed that the Ag nanoparticles were successfully deposited on the NVB samples (Fig. 2). In order to determine which facet of the NVB crystal the Ag nanoparticles 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 center by (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 instead of the crystal center of facet [43]. The calculation result based on the density functional theory (DFT) exhibits different band structures and band edge positions of these three facets, and a schematic diagram of
Fig. 4. XRD analysis of Ag-NVB samples (a); UV–vis diffuse reflectance spectrum of Ag-NVB samples (b); Raman spectrum of the Ag-NVB samples (c).
3. Results and discussion It needs to be pointed out that without the light irradiation, no Ag nanoparticles can be successfully deposited on the NVB sample surface, which is the reason of photoreduction. For effective deposition of AgNO3, a 300 W Xenon lamp with the intensity 0.54 w/cm2 was taken in the experiment. The process of Ag/NVB samples forming can be summarized as follow, AgNO3 which act as Ag source, readily dissolve into the water under dark conditions, and then transferred to the beaker which contained NVB powders, under the photoirradiation, the Ag+ ions were reduced into Ag nanoparticles. As the irradiation times increasing to 30 min, large numbers of Ag nanoparticles were successfully deposited on the surface of NVB. For the different amount of Ag 985
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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 AgNVB 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).
typical DRS in the UV range with absorption onset at about 419 nm. However, the loading of Ag nanoparticles leads to an improvement in the intensity of light absorption and extended the light response into the visible region. The result clearly showed that Ag/NVB composites have a very strong absorption band around 400–800 nm the intensity rises rapidly from 600 nm. As the amounts of Ag increasing, the absorption intensity of the Ag/NVB samples increased, among the six samples, the NVB5 sample has the highest absorption capacity in the visible range. It is worth noting that the absorption peak appears at 425 nm [44], which is attributed to the surface plasmon resonances of Ag nanoparticles. Fig. S2 shows the FTIR spectrometer analysis curve of the Ag/NVB samples in the frequency range of 400 to 1600 cm−1. The Ag/NVB samples display four characteristic absorption peaks in the section of 1300–1450 cm−1, 800–1200 cm−1, 540–790 cm−1, 400–480 cm−1. This result completely coincided with the reported literature [45–48], the band 1300–1450 cm−1 is related to the BeO vibration in the BO3 group, the band 716 cm−1 and 735 cm−1 is classified to bridge oxygen vibration of the BeOeB in BO3 unit. The band of 980–1200 cm−1 is assigned to the BeO vibration of the BO4 group. The band of 816–941 cm−1 is caused by the vibration of VO4 unit. No significant infrared spectral shift occurred implying that the addition of Ag nanoparticles has no influence on the structure of the NVB sample. The crystal structure was further confirmed by Raman spectroscopy shown in Fig. 4c. For the Ag/NVB samples, the wavenumber at 1301 cm−1 is generated by the antisymmetric stretching vibration of the BO3 group, and the BO3 triangle is connected to the BO4 tetrahedron through the bridge oxygen atom to form boron. The oxygen ring is generally considered to have a respiratory vibration of a six-membered ring having a BO4 tetrahedron in the 760–780 cm−1 band, and 776 cm−1 and 818 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
Table 2 The 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 the irradiation of UV–visible light. Sample
BET surface area (m2/g)
Proportion of photocurrent
NVB NVB1 NVB2 NVB3 NVB4 NVB5
0.8 1.9 1.2 1.9 1.6 1.0
1.00 1.17 1.42 1.60 2.15 1.82
the simulation calculation of the surface redox ability of the (001), (110), and (111) facets are shown in Fig. 3. The green and yellow regions represent the lowest unoccupied molecular orbital (LUMO) and the highest occupied molecular orbital (HOMO), respectively. It can be seen from Table S1 that the CBM of (110) facet locates at a higher position compared with (001) and (111), which indicates the highest electron filling probability and highest reduction ability which will facilitate the reduction of silver ions. Under ultraviolet light, the electrons in the valence band of NVB are excited to transition to the conduction band, the free silver ions in the solution preferentially obtain electrons and are reduced to silver atoms deposited on the (110) facet. The crystal structure of NVB and Ag/NVB samples were confirmed by XRD analysis (Fig. 4a). It can be clearly seen that the six samples show almost the same pattern and their diffraction peaks were all belong to the NVB phase. Due to the low content of Ag nanoparticles, no diffraction peak of Ag is detected. Optical response properties of the six samples were examined through UV–vis DRS in the range of 200–800 nm which is shown in Fig. 4b, where the bared NVB shows the 986
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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, and corresponding 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).
923 cm−1 and 945 cm−1 are related to the symmetric stretching vibration 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 symmetric stretching vibration of VO4. The peaks at 371 cm−1, 420 cm−1, and 456 cm−1 are brought about by the asymmetric stretching vibration of VO4. The above results are also clearly shown in Table 1. It should 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 increase 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 concluded that the Raman signal enhancement is caused by plasma resonance in the presence of silver. It has been stated that the resonant excitation of surface plasmon resonances creates an enhancement of the local electric field around Ag nanoparticles which can enhance the 987
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Ag + visible light → e− + Ag+ (h+)
Raman scattering intensities in the visible region [49,50]. We noted that the intensity of NVB1-NVB5 Raman spectrum around 776 cm−1–818 cm−1 was enhanced while the other peaks of the Raman spectrum were not changed. This is because the size and shape of the silver nanoparticles in each sample are relatively uniform, and there is no significant difference. The resonant Raman spectrum at a determined excitation wavelength is enhanced in a certain band rather than the full band. The photocatalytic activities of Ag/NVB samples were tested by degrading 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 content and the degradation efficiency of NVB4 and NVB5 samples show a similar result exceeding to 95% in 100 min, which is far more higher than that of P25 [34]. As shown in Fig. 5b and Table 2, the photocurrent increases with the increase of Ag content and NVB4 sample has a 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 has the smallest radius of the curve, indicating that it has the highest carrier transport capacity. In order to further confirm the foregoing findings, we measured the photoluminescence spectra (Fig. 5d). The NVB sample has a strong and broad peak at 295 nm wavelength excitation, while the luminescence intensity of the Ag/NVB samples decreases significantly. The NVB4 sample shows the lowest fluorescence intensity, which indicates that the composite photocatalyst promotes charge carrier transfer and reduces photogenerated electron and hole recombination after Ag deposition. KPFM can detect the contact potential difference (CPD) between the work function of the sample and the tip of the AFM. Kelvin Probe Force Microscope (KPFM) can also determine to surface potential images through its nanometer-scale spatial resolution and millivolt sensitivity [51]. Considering a higher efficiency of NVB4 sample, we selected NVB4 sample as the test model in the following experiment. The KPFM images of NVB4 sample and the corresponding height and voltage profiles along the straight lines showed in blue and red in the images sections (Fig. 6). The red dash line represents the NVB crystal and the Ag nanoparticles were circled by the black dash line in Fig. 6a. From Fig. 6b we can see that the height of Ag nanoparticles deposited on the surface of the NVB is about 30 nm and this is consistent with the HRTEM result. Fig. 6c and Fig. 6d show surface potential information for the sample. The difference in surface potential between the place where Ag was deposited and not deposited was about 20 mV, which proved that the place where Ag was deposited had higher concentrations of electrons. This indicates that the Ag nanoparticles can promote the photo-induced charges separation, which enhanced the electric fields in the region of Ag/NVB samples. Therefore, by adding Ag nanoparticles, the built-in electric field-induced carrier separation enhancement can be maintained, which can continuously improve the photocatalytic 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 activity with the addition of different scavengers over the sample (Fig. S3) were carried on under visible light irradiation. In Fig. S3, it can be found that the photodegradation activity of the sample after the addition of hydroxyl radical scavenger (TBA) and hole scavenger (methanol) is slightly reduced, but in the presence of superoxide radical scavenger (p-BQ) decreases. Since bromate ions are oxidizing, they exhibit a better degradation effect than NVB sample when KBrO3 is added as an electron scavenger agent. According to the above results, we can see that the photogenerated superoxide radicals are the dominant reactive oxygen species of Ag/NVB samples. It is further confirmed that the amount of superoxide radicals generated is far more than that in NVB samples in the Ag/NVB system. The details process of degrading 2-CP under visible light irradiation can be represented by the following steps:
NVB +
e−
+ O2 → ˙O2 ̅
˙O2 ̅ + H+ → ˙OOH
H2 O2 +
→ ˙OH +
(2) (3)
˙OOH + H+ + e− → H2 O2 e−
(1)
OH−
Ag+ (h+), NVB, ˙OH, ˙O2 ̅ , e−, +2 − CP → degradation products
(4) (5) (6)
The surface area of the catalyst has a significant effect on the catalytic efficiency of the catalyst [52]. The BET test results showed that they 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 contaminants during photocatalytic degradation of contaminants. It has been stated that the NVB is a ferroelectric material with the internal electric field, which can form a space charge layer and act as a motivation to effect separation of the photo-induced charges. The presence of a screening effect in the NVB materials can increase the adsorption of charged species, for instance, dye molecules or other contaminant molecules under external screening [53,54]. The fact of selective deposition of Ag nanoparticles on the C+ domain of the NVB can be well explained by employing the strong screening effect of the polar crystal. The electrons were excited by the visible irradiation and transferred by the screening effect to the C+ surface. The Ag+ ions in the aqueous solution were reduced when they combined with the electrons transferred from the interior of NVB crystal to its C+ surface. The electrons removed 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+ ions are not reduced there. It has been reported that Ag (< 30 nm) nanoparticles can form the energetic charge carriers which can be transferred to the surroundings or relax by locally heating the nanostructure [14,55]. When the system was under the visible light irradiation the difference of the work function of the two-phase(Ag nanoparticles and the NVB particles) will lead to the photo-induced charges generated by the Ag nanoparticle transferred to the NVB surface to take place reduction reaction, while the holes will take place oxidation reaction [56]. Furthermore, the Ag nanoparticle on the NVB enhanced the visible light (λ > 420 nm) absorption for the reason of the surface plasmon resonances effect, and the photogenerated electrons transported to one surface of NVB formed an electron-rich surface. At this time, the electrons form %O2− with O2 that in the water, and further combine with H+ form %OOH and H2O2 to oxidize contaminants. On the other side, the C− domain adsorbs a large number of contaminant molecules under external screening and oxidizes these contaminant molecules. A schematic diagram of the rational mechanism of charge separation and photocatalysis in the system is as shown in Scheme 2. 4. Conclusions In summary, a series of visible light responsive photocatalytic materials Ag/Na3VO2B6O11 were fabricated successfully. The Ag/NVB composites showed excellent photocatalytic performance under visible light irradiation. The photodegradation efficiency can reach up to 96% for NVB4 sample during 2-Chlorophenol (2-CP) degradation in 100 min. The main reactive oxygen species was determined as %O2−, radical according to the trapping test. The enhanced activity is ascribed to the synergistic effect of Ag/NVB composite materials which improved separation efficiency of photoexcited charges. It is demonstrated that ferroelectric spontaneous polarization electric field of NVB and the surface plasmon resonances effect of Ag nanoparticles takes a synergistic effect on the separation and transportation of the photogenerated electrons and holes. The achievements and findings in this work are expected to contribute to design more efficient photocatalyst systems by controlling internal fields and the noble metals. In the 988
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following work, we will focus on the other metals deposition on more ferroelectric materials in the visible light region.
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Acknowledgments This work was supported by the National Natural Science Foundation of China (51772325), and Natural Science Foundation of Xinjiang 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, reactive oxygen species trapping studies of degradation 2-CP activity with the addition of different scavengers under visible light irradiation over the samples, BET analysis of NVB, Ag/NVB samples. Supplementary data to this article can be found online at https://doi.org/10.1016/j.apsusc. 2019.04.172. References [1] J. Schneider, M. Matsuoka, M. Takeuchi, J. Zhang, Y. Horiuchi, M. Anpo, D.W. Bahnemann, Understanding TiO2 photocatalysis: mechanisms and materials, Chem. Rev. 114 (2014) 9919–9986. [2] D.A. Erdogan, E. Ozensoy, Hierarchical synthesis of corrugated photocatalytic TiO2 microsphere architectures on natural pollen surfaces, Appl. Surf. Sci. 403 (2017) 159–167. [3] A. Mclaren, T. Valdes-Solis, G. Li, S.C. Tsang, Shape and size effects of ZnO nanocrystals on photocatalytic activity, J. Am. Chem. Soc. 131 (2009) 12540–12541. [4] C.F. Tan, A.K. Su Su Zin, Z. Chen, C.H. Liow, H.T. Phan, H.R. Tan, Q.-H. Xu, G.W. Ho, Inverse stellation of CuAu-ZnO multimetallic-semiconductor nanostartube for plasmon-enhanced photocatalysis, ACS Nano 12 (2018) 4512–4520. [5] H. Ma, J. Feng, F. Jin, M. Wei, C. Liu, Y. Ma, Where do photogenerated holes at the g-C3N4/water interface go for water splitting: H2O or OH−? Nanoscale 10 (2018) 15624–15631. [6] J. Wen, J. Xie, X. Chen, X. Li, A review on g-C3N4-based photocatalysts, Appl. Surf. Sci. 391 (2017) 72–123. [7] S. Liu, J. Yu, M. Jaroniec, Anatase TiO2 with dominant high-energy {001} facets: synthesis, properties, and applications, Chem. Mater. 23 (2011) 4085–4093. [8] J. Li, C. Zhao, K. Xia, X. Liu, D. Li, J. Han, Enhanced piezoelectric output of the PVDF-TrFE/ZnO flexible piezoelectric nanogenerator by surface modification, Appl. Surf. Sci. 463 (2019) 626–634. [9] R. Shen, C. Jiang, Q. Xiang, J. Xie, X. Li, Surface and interface engineering of hierarchical photocatalysts, Appl. Surf. Sci. 471 (2019) 43–87. [10] H. Li, Z. Bian, J. Zhu, D. Zhang, G. Li, Y. Huo, H. Li, Y. Lu, Mesoporous titania spheres with tunable chamber structure and enhanced photocatalytic activity, J. Am. Chem. Soc. 129 (2007) 8406–8407. [11] M. Nasr, S.b. Balme, C. Eid, R. Habchi, P. Miele, M. Bechelany, Enhanced visiblelight photocatalytic performance of electrospun rGO/TiO2 composite nanofibers, J. Phys. Chem. C 121 (2016) 261–269. [12] W.L. Barnes, A. Dereux, T.W. Ebbesen, Surface plasmon subwavelength optics, Nature 424 (2003) 824. [13] Y.F. Huang, M. Zhang, L.B. Zhao, J.M. Feng, D.Y. Wu, B. Ren, Z.Q. Tian, Activation of oxygen on gold and silver nanoparticles assisted by surface plasmon resonances, Angew. Chem. Int. Ed. 53 (2014) 2353–2357. [14] S. Linic, P. Christopher, D.B. Ingram, Plasmonic-metal nanostructures for efficient conversion of solar to chemical energy, Nat. Mater. 10 (2011) 911. [15] J. Ahn, D. Wang, Y. Ding, J. Zhang, D. Qin, Site-selective carving and co-deposition: transformation of Ag nanocubes into concave nanocrystals encased by Au–Ag alloy frames, ACS Nano 12 (2018) 298–307. [16] G. Yang, H. Yin, W. Liu, Y. Yang, Q. Zou, L. Luo, H. Li, Y. Huo, H. Li, Synergistic Ag/ TiO2-N photocatalytic system and its enhanced antibacterial activity towards Acinetobacter baumannii, Appl. Catal. B Environ. 224 (2018) 175–182. [17] E. Yoo, T. Okata, T. Akita, M. Kohyama, J. Nakamura, I. Honma, Enhanced electrocatalytic activity of Pt subnanoclusters on graphene nanosheet surface, Nano Lett. 9 (2009) 2255–2259. [18] J. Li, X. Zhu, F. Qiu, T. Zhang, F. Hu, X. Peng, Facile preparation of Ag/Ag2WO4/gC3N4 ternary plasmonic photocatalyst and its visible-light photocatalytic activity, Appl. Organomet. Chem. 33 (2019) e4683. [19] H. Zhang, G. Wang, D. Chen, X. Lv, J. Li, Tuning photoelectrochemical performances of Ag−TiO2 nanocomposites via reduction/oxidation of Ag, Chem. Mater. 20 (2008) 6543–6549. [20] P.V. Kamat, Photophysical, photochemical and photocatalytic aspects of metal nanoparticles, J. Phys. Chem. B 106 (2002) 7729–7744. [21] S. Tu, Y. Zhang, A.H. Reshak, S. Auluck, L. Ye, X. Han, T. Ma, H. Huang, Ferroelectric polarization promoted bulk charge separation for highly efficient CO2 photoreduction of SrBi4Ti4O15, Nano Energy 56 (2019) 840–850. [22] J.L. Giocondi, G.S. Rohrer, Spatially selective photochemical reduction of silver on the surface of ferroelectric barium titanate, Chem. Mater. 13 (2001) 241–242. [23] J. Shi, P. Zhao, X. Wang, Piezoelectric-polarization-enhanced photovoltaic performance in depleted-heterojunction quantum-dot solar cells, Adv. Mater. 25 (2013) 916–921.
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Full length article
Enhanced photocatalytic property of Ag loaded on well-defined ferroelectric Na3VO2B6O11 crystals under visible light irradiation Yufei Zhaia,c, Yang Zhangd, Jiao Yina, Xiaoyun Fanb,
T
⁎
a Laboratory of Environmental Sciences and Technology, Xinjiang Technical Institute of Physics & Chemistry, and Key Laboratory of Functional Materials and Devices for Special Environments, Chinese Academy of Sciences, Urumqi 830011, PR China b School of Environment and Guangdong Key Laboratory of Environmental Pollution and Health, Jinan University, Guangzhou 510632, PR China c University of Chinese Academy of Sciences, Beijing 100049, PR China d College of Chemistry and Chemical Engineering, Xinjiang Normal University, Urumqi, 830054, Xinjiang, PR China
A R T I C LE I N FO
A B S T R A C T
Keywords: Ferroelectric materials Spontaneous polarization Active sites KPFM Visible light photocatalyst
How to efficiently utilize visible light and highly separate photo-induced charges is now a hot topic in the field of photocatalysis. Herein, various amounts (1–5%, wt%) of Ag nanoparticles were selectively deposited on the surface of the polar material Na3VO2B6O11 (NVB) via the method of photoreduction. Due to the presence of external screening effect, the Ag nanoparticles were selectively deposited on the positive domains of NVB materials. Through DFT, the (110) facet with excessive electrons accumulated has a high reduction ability to deposit the Ag nanoparticles on the edge of NVB crystals. It was also found that the surface plasmon resonances effect of Ag 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 100 min. It indicates that the screening effect of NVB combination with the surface plasmon resonances effect from Ag nanoparticles can provide a synergistic effect on the enhanced photocatalytic activity during the 2-CP degradation.
1. Introduction An increasing of environmental pollution associated with rapid international economic growth has aroused widespread critical concern to environment protection. Photocatalytic technology has broad application prospects in solving environmental pollution and producing new energy with its economic and clean advantages. Photocatalytic materials, such as TiO2 [1,2], ZnO [3,4], C3N4 [5,6], have been widely studied 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 response range and a low-efficiency utilization of photogenerated charges, which limits their practical applications. Various strategies have been widely developed, such as doping, the coupling of semiconductor photocatalysts, constructing of heterojunctions materials, surface modification, morphology engineering, depositing noble metal, and so forth to improve their photocatalytic performance. [8–11] Depositing with noble metal is proved to be one of the successful methods to improve the photocatalytic efficiency under visible light.
Due to the surface plasmon resonances effect, the light absorption range of the catalyst can be significantly broadened and the photogenerated charges also can be inhibited, leading to an enhanced photocatalytic effect [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 depositing is an important way to improve the separation efficiency of photogenerated charges and broaden the absorption spectrum of the photocatalyst. Compared with other noble metals, Ag has unique advantages, low cost, high work function, and can be composited with semiconductor by a simple, fast and stable method, leading to significantly improve the overall photocatalytic performance of composite semiconductor [19,20]. Recent studies have also found that spontaneous polarization inside ferroelectric materials can separate photogenerated electron-hole pairs, thereby reducing their recombination probability [21], and increasing the photocatalytic activity and photoelectric conversion efficiency [22–24]. The lifetime of the electrons in the ferroelectric materials is even longer than TiO2 materials, for example, the decay time of photoluminescence of TiO2 thin film is 0.1 μs, while the ferroelectric
⁎ Corresponding author at: School of Environment and Guangdong Key Laboratory of Environmental Pollution and Health, Jinan University, Guangzhou 510632, PR China. E-mail address: [email protected] (X. Fan).
https://doi.org/10.1016/j.apsusc.2019.04.172 Received 10 December 2018; Received in revised form 16 March 2019; Accepted 16 April 2019 Available online 16 April 2019 0169-4332/ © 2019 Published by Elsevier B.V.
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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).
materials lithium niobate can reach up to 9 μs [25], which is far longer than that of TiO2 thin film [26]. Usually, there are many small polarization regions inside the ferroelectric, and the electric dipole polarizations in each small region are arranged in the same direction as domains. The internal depolarization chemical field causes the downward band to bend with the reason of drives electrons to the surface of the C+ domain [27,28]. In the C− domain, on the contrary, electrons flow away from the surface, causing the upward band to bend. When the ferroelectric material is excited by light, a photogenerated electronhole pair will be generated, and the electron-hole pairs will be separated by its spontaneous polarization and then migrate to the catalyst surface, reducing the electron-hole pair recombination rate. The ferroelectric materials with internal electronic field indeed perform an excellent photocatalytic performance, however, due to their irregularlyshaped powdery form, which makes it difficult to confirm the active sites to unveil the relationship between the structure and the function. Until now, a great number of researches on photocatalysts are dedicated to facet-dependent photocatalysis [29–31]. Nevertheless, the underlying synergetic effects regarding ferroelectric photocatalysis remain largely unexplored.
Sodium vanadium borate Na3VO2B6O11 (NVB) combined with a boron oxide group and a vanadium oxide group is a non-centrosymmetric polar photocatalytic material with a d0 electronic configuration. It was proved that NVB has excellent performance in photocatalytic degradation of 2,4-Dichlorophenol (2,4-DCP) under ultraviolet light irradiation [32–34]. However, its absorption photon wavelength reaches 419 nm, and the response range of NVB to sunlight is still insufficient. To this end, it remains a need to further extend the corresponding range of light and improve the utilization of solar energy by NVB. In view of the above consideration, we deposited Ag nanoparticles on the surface of NVB by photoreduction method, which can significantly inhibit the recombination rate of photogenerated electrons (e−) and holes (h+) and enhance their photocatalytic performance. Ag nanoparticles were selectively deposited on the NVB crystal where the electrons accumulated as a result of the external screening effect. After the Ag nanoparticles deposition, the absorption cutoff edge of NVB extends from the original 419 nm to the entire visible region. The photo-fluorescence luminescence spectrum shows a decrease in intensity after loading Ag, with a minimum of about a quarter of the 982
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Fig. 2. EDS mapping images of NVB4 sample.
2.2. Preparation of Ag/NVB photocatalysts
original. We chose the contaminant 2-chlorophenol (2-CP) as the degradation experiments. It showed that Ag-NVB samples have an excellent degradation efficiency under visible light irradiation. The electron-hole pairs recombination rate on the polar material NVB is greatly reduced after deposition of Ag, and the electron transport capability is greatly improved.
The Ag/NVB photocatalysts were prepared by a photocatalytic-induced deposition of Ag nanoparticles under UV-light irradiation. 0.5 g NVB and a certain amount of AgNO3 solution (the concentrations is 0.025 g/L, 0.125 g/L, 0.25 g/L, 0.375 g/L and 0.5 g/L, respectively) were mixed in a 200 mL beaker and dispersed in an ultrasonic cleaner for 10 min. The above suspension was stirred and exposed to UV light at ambient temperature for 30 min. A xenon lamp (PLS-SXE300/300UV) was used as a light source. Then, the samples were collected (named NVB1, NVB2, NVB3, NVB4, NVB5, respectively) after irradiation, washed with deionized water several times, and finally lyophilized to obtain samples and placed into the vacuum dryer for storage.
2. Experimental 2.1. Preparation of Na3VO2B6O11 Na3VO2B6O11 (NVB) samples were synthesized by the solid-state reaction method according to the following reaction equation:
3Na2CO3 + V2O5 + 12H3 BO3 = 2Na3VO2 B6 O11 + 3CO2 ↑ + 18H2 O↑
2.3. Photocatalytic activity experiment
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 agate mortar and pestle for 20 min. Then put the powders into a crucible and heated in a muffle furnace for 2 h to 630 °C, which the temperature was raised by 5 °C per minute, and heated at 630 °C for 2 h. The mixture was taken out once every hour and ground for 20 min. After naturally cooling to room temperature, collected carefully the white powders and uniformly ground again, washed 2–3 times with deionized water, then dried and stored in a vacuum desiccator.
The photocatalytic performance of the samples was evaluated by performing degradation of 2-chlorophenol (2-CP) with the visible light irradiation. In this experiment, 50 mL of 2-CP solution (20 mg/L) and 50 mg of the photocatalyst were dispersed in a reaction container. Before irradiation, the solution was stirred in the dark for 0.5 h continuously to ensure the adsorption-desorption equilibrium establishment between the photocatalysts and 2-CP. In the visible light activity evaluation experiment, the light source used a xenon lamp (PLSSXE300/300 UV) and was equipped with a 420 nm filter. In the process of photocatalytic reaction, the 2-CP solution having the photocatalyst was continuously stirred with a magnetic stirrer, and at a certain time interval, 1 mL of the reaction solution was sampled and filtered for 983
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Fig. 3. The highest occupied molecular orbital (HOMO) and the lowest unoccupied molecular orbital (LUMO) of the (001), (110) and (111) facets, and the corresponding VBM, CBM, and Fermi levels in the vacuum. The green and yellow areas represent LUMO and HOMO, respectively.
generate a potential image. In the experiment, the samples were first dispersed 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 at 80 °C for 6–10 h. The topography of the film surface was measured in non-contact mode and the surface potential variation was measured using a probe tip. The photocurrent response was carried out on a CHI 660 C electrochemical workstation (Chen Hua Instrument) with a conventional tri-electrode system. 0.1 M Na2SO4 solution served as the electrolyte, with a platinum foil as the counter electrode and an Ag/ AgCl (saturated KCl) reference electrode. The working electrodes were prepared by dropping a slurry of 2 mg photocatalyst and 30 μL Nafion/ ethanol (v/v = 1:5) onto conducting indium tin oxide glass (ITO) with an area of 1 cm2 and dried in air at ambient temperature.
high-performance liquid chromatography (HPLC) analysis. 2.4. Characterization of materials Powder X-ray diffraction (XRD) patterns of the samples were collected on an X-ray diffractometer equipped using Cu Kα1 (λ = 0.15418 nm) with a scan step of 0.01° and a scan range between 10° and 70° (Bruker D8). The morphologies of the samples were examined by electron microscope operated on ZEISS SUPRA55VP apparatus and energy dispersive X-ray (EDX) spectroscopic analysis was conducted by EDX8000. The high-resolution transmission electron microscope (HRTEM) on FEI Tecnai G2 f20s-twin 200 kV field emission transmission electron microscope operated at an accelerating voltage of 200 kV. The diffuse reflectance absorption spectra (DRS) of the samples were recorded in the range from 200 nm to 800 nm on a Solidspec-3700 DUV spectrometer and using BaSO4 as a reference. The Fourier transform infrared (FTIR) spectra were obtained using a Bruker V70FTIR spectrophotometer 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 × 250 mm). Using a mixture of methanol and water [85/15 (v/v)] as an effluent with the flow rate was 0.5 mL/min. The Brunauer-Emmett-Teller (BET) surface area was acquired from the N2 adsorption/desorption isotherms recorded at 77 K (QUADRASORB IQ, Quantachrome Instrument Corp). The fluorescence emission spectrum (excited at 295 nm) were measured on a Hitachi fluorescence spectrophotometer F-7000. Raman spectra were conducted by using a HORIBA Jobin Yvon LabRAM HR confocal spectrometer confocal microscope Raman spectrometer employing an illuminated with a visible (532 nm) laser. Atomic force microscopy (AFM) combined with Kelvin probe force microscopy (KPFM) measurements were performed with an atomic force microscope (Bruker Multimode 8). In KPFM, electrostatic forces between the tip and sample result from a non-contact potential difference. The non-contact potential difference was instantly and automatically nullified by a DC bias applied to the probe tip. This DC bias was recorded at each position to
2.5. Theoretical methods The first principles calculations in the framework of density functional theory (DFT), including structural, electronic performances, were carried out based on the Cambridge Sequential Total Energy Package known as CASTEP [35]. The exchange-correlation functional under the generalized gradient approximation (GGA) [36] with norm-conserving pseudopotentials and Perdew–Burke–Ernzerhof functional was adopted to describe the electron-electron interaction [37]. An energy cutoff of 750 eV was used and a k-point sampling set of 7 × 7 × 1 was tested to be converged. A force tolerance of 0.01 eV Å−1, energy tolerance of 5.0 × 10−7eV per atom and maximum displacement of 5.0 × 10−4 Å were 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 neighboring images. The bottom three atomic layers were fixed and the three atomic layers were relaxed. The VBM and CBM were calculated with the isosurface of 0.01 e/Bohr3.
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Table 1 FTIR 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−1 420 cm−1 456 cm−1 620 cm−1 776 cm−1 818 cm−1 891 cm−1 923 cm−1 945 cm−1
VO4 asymmetric stretching
735 cm−1
816–941 cm−1
980–1200 cm−1 1301 cm−1 1300–1450 cm−1
BO3 bending vibration BO4 breathing vibration VO4 symmetric stretching BO3 Symmetric stretching BO4 asymmetric stretch BO3 antisymmetric stretching BO3 asymmetric stretch
loading, the color of the composite Ag/NVB samples changed from light yellow into gray, which was might be caused by the partial oxidation of Ag nanoparticles into Ag2O [38,39]. It can also be seen from Scheme 1, the morphology of NVB ferroelectrics materials changed during the photoreduction process. A plausible mechanism was introduced to explain the NVB crystals formation. When the addition of the Ag+ solution to the beaker, the spontaneous nucleation of NVB sample begins to take place rapidly. During the subsequent grain growth process, an external screening mechanism plays a vital role in determining the morphology of the NVB grains. It has been reported that the unique properties of ferroelectric surfaces have been used to achieve localized redox reactions [40,41], on the surface of the ferroelectric NVB materials, there is a spontaneous polarization causing the negative charge (C− domain) in the direction that pointing away from the surface to the bulk, under the polarization charge screening mechanism, as the time increased, the Ag+ ions were selectively deposited on the positive domain of the NVB surface. Na3VO2B6O11 crystallizes in the non-centrosymmetric structure with 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 threedimensional framework. The isolated B6O13 units were originally from the connecting of the triangular plane BO3 and the tetrahedral BO4, then connecting to the VO4 tetrahedron to form an infinite piece, and the Na+ cation is distributed in the extended frame. The SEM and HRTEM were used to study detailed morphology and microstructure of the composite photocatalyst. It is observed that NVB single crystal shows a regular morphology with the maximized size up to 800 nm and the Ag nanoparticles selected deposited on the NVB surface (Fig. 1a). The surface morphology and distribution of Ag nanoparticles on the NVB crystal surfaces was further explored by HRTEM. 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 recorded in parts of Fig. 1b, revealing the presence of Na, B, V, O and Ag elements, which is further confirmed that the Ag nanoparticles were successfully deposited on the NVB samples (Fig. 2). In order to determine which facet of the NVB crystal the Ag nanoparticles 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 center by (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 instead of the crystal center of facet [43]. The calculation result based on the density functional theory (DFT) exhibits different band structures and band edge positions of these three facets, and a schematic diagram of
Fig. 4. XRD analysis of Ag-NVB samples (a); UV–vis diffuse reflectance spectrum of Ag-NVB samples (b); Raman spectrum of the Ag-NVB samples (c).
3. Results and discussion It needs to be pointed out that without the light irradiation, no Ag nanoparticles can be successfully deposited on the NVB sample surface, which is the reason of photoreduction. For effective deposition of AgNO3, a 300 W Xenon lamp with the intensity 0.54 w/cm2 was taken in the experiment. The process of Ag/NVB samples forming can be summarized as follow, AgNO3 which act as Ag source, readily dissolve into the water under dark conditions, and then transferred to the beaker which contained NVB powders, under the photoirradiation, the Ag+ ions were reduced into Ag nanoparticles. As the irradiation times increasing to 30 min, large numbers of Ag nanoparticles were successfully deposited on the surface of NVB. For the different amount of Ag 985
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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 AgNVB 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).
typical DRS in the UV range with absorption onset at about 419 nm. However, the loading of Ag nanoparticles leads to an improvement in the intensity of light absorption and extended the light response into the visible region. The result clearly showed that Ag/NVB composites have a very strong absorption band around 400–800 nm the intensity rises rapidly from 600 nm. As the amounts of Ag increasing, the absorption intensity of the Ag/NVB samples increased, among the six samples, the NVB5 sample has the highest absorption capacity in the visible range. It is worth noting that the absorption peak appears at 425 nm [44], which is attributed to the surface plasmon resonances of Ag nanoparticles. Fig. S2 shows the FTIR spectrometer analysis curve of the Ag/NVB samples in the frequency range of 400 to 1600 cm−1. The Ag/NVB samples display four characteristic absorption peaks in the section of 1300–1450 cm−1, 800–1200 cm−1, 540–790 cm−1, 400–480 cm−1. This result completely coincided with the reported literature [45–48], the band 1300–1450 cm−1 is related to the BeO vibration in the BO3 group, the band 716 cm−1 and 735 cm−1 is classified to bridge oxygen vibration of the BeOeB in BO3 unit. The band of 980–1200 cm−1 is assigned to the BeO vibration of the BO4 group. The band of 816–941 cm−1 is caused by the vibration of VO4 unit. No significant infrared spectral shift occurred implying that the addition of Ag nanoparticles has no influence on the structure of the NVB sample. The crystal structure was further confirmed by Raman spectroscopy shown in Fig. 4c. For the Ag/NVB samples, the wavenumber at 1301 cm−1 is generated by the antisymmetric stretching vibration of the BO3 group, and the BO3 triangle is connected to the BO4 tetrahedron through the bridge oxygen atom to form boron. The oxygen ring is generally considered to have a respiratory vibration of a six-membered ring having a BO4 tetrahedron in the 760–780 cm−1 band, and 776 cm−1 and 818 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
Table 2 The 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 the irradiation of UV–visible light. Sample
BET surface area (m2/g)
Proportion of photocurrent
NVB NVB1 NVB2 NVB3 NVB4 NVB5
0.8 1.9 1.2 1.9 1.6 1.0
1.00 1.17 1.42 1.60 2.15 1.82
the simulation calculation of the surface redox ability of the (001), (110), and (111) facets are shown in Fig. 3. The green and yellow regions represent the lowest unoccupied molecular orbital (LUMO) and the highest occupied molecular orbital (HOMO), respectively. It can be seen from Table S1 that the CBM of (110) facet locates at a higher position compared with (001) and (111), which indicates the highest electron filling probability and highest reduction ability which will facilitate the reduction of silver ions. Under ultraviolet light, the electrons in the valence band of NVB are excited to transition to the conduction band, the free silver ions in the solution preferentially obtain electrons and are reduced to silver atoms deposited on the (110) facet. The crystal structure of NVB and Ag/NVB samples were confirmed by XRD analysis (Fig. 4a). It can be clearly seen that the six samples show almost the same pattern and their diffraction peaks were all belong to the NVB phase. Due to the low content of Ag nanoparticles, no diffraction peak of Ag is detected. Optical response properties of the six samples were examined through UV–vis DRS in the range of 200–800 nm which is shown in Fig. 4b, where the bared NVB shows the 986
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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, and corresponding 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).
923 cm−1 and 945 cm−1 are related to the symmetric stretching vibration 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 symmetric stretching vibration of VO4. The peaks at 371 cm−1, 420 cm−1, and 456 cm−1 are brought about by the asymmetric stretching vibration of VO4. The above results are also clearly shown in Table 1. It should 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 increase 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 concluded that the Raman signal enhancement is caused by plasma resonance in the presence of silver. It has been stated that the resonant excitation of surface plasmon resonances creates an enhancement of the local electric field around Ag nanoparticles which can enhance the 987
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Ag + visible light → e− + Ag+ (h+)
Raman scattering intensities in the visible region [49,50]. We noted that the intensity of NVB1-NVB5 Raman spectrum around 776 cm−1–818 cm−1 was enhanced while the other peaks of the Raman spectrum were not changed. This is because the size and shape of the silver nanoparticles in each sample are relatively uniform, and there is no significant difference. The resonant Raman spectrum at a determined excitation wavelength is enhanced in a certain band rather than the full band. The photocatalytic activities of Ag/NVB samples were tested by degrading 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 content and the degradation efficiency of NVB4 and NVB5 samples show a similar result exceeding to 95% in 100 min, which is far more higher than that of P25 [34]. As shown in Fig. 5b and Table 2, the photocurrent increases with the increase of Ag content and NVB4 sample has a 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 has the smallest radius of the curve, indicating that it has the highest carrier transport capacity. In order to further confirm the foregoing findings, we measured the photoluminescence spectra (Fig. 5d). The NVB sample has a strong and broad peak at 295 nm wavelength excitation, while the luminescence intensity of the Ag/NVB samples decreases significantly. The NVB4 sample shows the lowest fluorescence intensity, which indicates that the composite photocatalyst promotes charge carrier transfer and reduces photogenerated electron and hole recombination after Ag deposition. KPFM can detect the contact potential difference (CPD) between the work function of the sample and the tip of the AFM. Kelvin Probe Force Microscope (KPFM) can also determine to surface potential images through its nanometer-scale spatial resolution and millivolt sensitivity [51]. Considering a higher efficiency of NVB4 sample, we selected NVB4 sample as the test model in the following experiment. The KPFM images of NVB4 sample and the corresponding height and voltage profiles along the straight lines showed in blue and red in the images sections (Fig. 6). The red dash line represents the NVB crystal and the Ag nanoparticles were circled by the black dash line in Fig. 6a. From Fig. 6b we can see that the height of Ag nanoparticles deposited on the surface of the NVB is about 30 nm and this is consistent with the HRTEM result. Fig. 6c and Fig. 6d show surface potential information for the sample. The difference in surface potential between the place where Ag was deposited and not deposited was about 20 mV, which proved that the place where Ag was deposited had higher concentrations of electrons. This indicates that the Ag nanoparticles can promote the photo-induced charges separation, which enhanced the electric fields in the region of Ag/NVB samples. Therefore, by adding Ag nanoparticles, the built-in electric field-induced carrier separation enhancement can be maintained, which can continuously improve the photocatalytic 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 activity with the addition of different scavengers over the sample (Fig. S3) were carried on under visible light irradiation. In Fig. S3, it can be found that the photodegradation activity of the sample after the addition of hydroxyl radical scavenger (TBA) and hole scavenger (methanol) is slightly reduced, but in the presence of superoxide radical scavenger (p-BQ) decreases. Since bromate ions are oxidizing, they exhibit a better degradation effect than NVB sample when KBrO3 is added as an electron scavenger agent. According to the above results, we can see that the photogenerated superoxide radicals are the dominant reactive oxygen species of Ag/NVB samples. It is further confirmed that the amount of superoxide radicals generated is far more than that in NVB samples in the Ag/NVB system. The details process of degrading 2-CP under visible light irradiation can be represented by the following steps:
NVB +
e−
+ O2 → ˙O2 ̅
˙O2 ̅ + H+ → ˙OOH
H2 O2 +
→ ˙OH +
(2) (3)
˙OOH + H+ + e− → H2 O2 e−
(1)
OH−
Ag+ (h+), NVB, ˙OH, ˙O2 ̅ , e−, +2 − CP → degradation products
(4) (5) (6)
The surface area of the catalyst has a significant effect on the catalytic efficiency of the catalyst [52]. The BET test results showed that they 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 contaminants during photocatalytic degradation of contaminants. It has been stated that the NVB is a ferroelectric material with the internal electric field, which can form a space charge layer and act as a motivation to effect separation of the photo-induced charges. The presence of a screening effect in the NVB materials can increase the adsorption of charged species, for instance, dye molecules or other contaminant molecules under external screening [53,54]. The fact of selective deposition of Ag nanoparticles on the C+ domain of the NVB can be well explained by employing the strong screening effect of the polar crystal. The electrons were excited by the visible irradiation and transferred by the screening effect to the C+ surface. The Ag+ ions in the aqueous solution were reduced when they combined with the electrons transferred from the interior of NVB crystal to its C+ surface. The electrons removed 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+ ions are not reduced there. It has been reported that Ag (< 30 nm) nanoparticles can form the energetic charge carriers which can be transferred to the surroundings or relax by locally heating the nanostructure [14,55]. When the system was under the visible light irradiation the difference of the work function of the two-phase(Ag nanoparticles and the NVB particles) will lead to the photo-induced charges generated by the Ag nanoparticle transferred to the NVB surface to take place reduction reaction, while the holes will take place oxidation reaction [56]. Furthermore, the Ag nanoparticle on the NVB enhanced the visible light (λ > 420 nm) absorption for the reason of the surface plasmon resonances effect, and the photogenerated electrons transported to one surface of NVB formed an electron-rich surface. At this time, the electrons form %O2− with O2 that in the water, and further combine with H+ form %OOH and H2O2 to oxidize contaminants. On the other side, the C− domain adsorbs a large number of contaminant molecules under external screening and oxidizes these contaminant molecules. A schematic diagram of the rational mechanism of charge separation and photocatalysis in the system is as shown in Scheme 2. 4. Conclusions In summary, a series of visible light responsive photocatalytic materials Ag/Na3VO2B6O11 were fabricated successfully. The Ag/NVB composites showed excellent photocatalytic performance under visible light irradiation. The photodegradation efficiency can reach up to 96% for NVB4 sample during 2-Chlorophenol (2-CP) degradation in 100 min. The main reactive oxygen species was determined as %O2−, radical according to the trapping test. The enhanced activity is ascribed to the synergistic effect of Ag/NVB composite materials which improved separation efficiency of photoexcited charges. It is demonstrated that ferroelectric spontaneous polarization electric field of NVB and the surface plasmon resonances effect of Ag nanoparticles takes a synergistic effect on the separation and transportation of the photogenerated electrons and holes. The achievements and findings in this work are expected to contribute to design more efficient photocatalyst systems by controlling internal fields and the noble metals. In the 988
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following work, we will focus on the other metals deposition on more ferroelectric materials in the visible light region.
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