Hydrothermal preparation of Nb-doped NaTaO3 with enhanced photocatalytic activity for removal of organic dye

Hao Zhou*,Qi Yin

State Key Laboratory of Clean Energy Utilization,Zhejiang University,Hangzhou 310027,China

Keywords: Photocatalytic degradation Methylene blue NaTaO3

ABSTRACT The construction of efficient photocatalysts has received much attention in wastewater treatment.In this study,Nb-doped NaTaO3 was prepared with different doping ratio by a facile hydrothermal method.The prepared catalysts were analyzed by X-ray diffraction,scanning electron microscopy,Brunauer-Emmett-Teller (BET) theory,ultraviolet–visible diffuse reflectance spectroscopy.Then the synthesized catalysts were employed to degrade a model pollutant,methylene blue (MB),under a 250 W mercury lamp.The characterization tests confirm that Nb was successfully doped into the crystal structure of NaTaO3 and the modified NaNbxTa1-xO3 (x is the doping ratio; x=0.25,0.5,0.75) samples were formed,which were in regular cubic shapes just like pure NaTaO3.The synthesized NaNbxTa1-xO3 (x=0.25,0.5,0.75)had improved specific surface area and narrowed band gaps compared with pure NaTaO3.In photocatalytic degradation experiments,NaNb0.75Ta0.25O3 presented the best photocatalytic activity ascribed to the narrow band gap and the high surface area,degradating 95.7% of MB after 180 min of reaction,which was even twice the ability of pure NaTaO3.Besides,the effects of possible inorganic anions and cations in wastewater on photocatalytic degradation were investigated.Electron spin resonance (ESR) tests and capture experiments were also conducted and a possible photocatalytic mechanism was proposed.This work provides a direction for constructing superior NaTaO3-based photocatalysts to be widely utilized in environmental protection.

1.Introduction

Large amounts of industrial wastewater and living sewage have been poured into water with accelerated industrialization and urbanization,resulting in serious damge to water resources.Dyes are one of the organic pollutants in water that cannot be ignored.Photocatalytic degradation technology has been developed under this background,which means decompose pollutants by adding photocatalyst to the pollutant system in the sunlight.This technology can completely oxidize various organic and inorganic pollutants with low operating cost and high degradation efficiency in comparison to other methods,which gives it broad prospect for application.Therefore,it is a long-term research project for worldwide scholars to find stable and efficient catalysts for photocatalytic degradation of pollutants like dyes in wastewater.

In recent years,a new kind of semiconductor material named perovskite has drawn great attention.Perovskite is a compound with the general molecular formula ABX3in which A and B refer to cations and X to anions.It was first discovered as calcium titanate(CaTiO3)in perovskite,and was named after Russian geologist Lev Perovski in 1839 [1].Later,perovskite-formed composite oxides have been considered as promising photocatalysts since Voorhoeveet al.[2] firstly discovered their high photocatalytic activity in the 1970s.Among various perovskites,NaTaO3is expected to be a promising photocatalyst due to its good structure and high charge separation efficiency.However,the high band-gap energy of NaTaO3hinders efficient utilization of solar light.Therefore,scholars across the world have done large amounts of modification on NaTaO3to further improve its photocatalytic activity,such as ion doping[3–7],heterojunctions[7–12],noble metal loading [12–15] and different crystal facets exposing [16].It is worth mentioning that Wanget al.[5]prepared NaNbxTa1-xO3perovskite photocatalyst and applied it to photocatalytic degradation of pollutants for the first time.However,the molten salt method they used to prepare photocatalyst required a high reaction temperature,and the photocatalyst still had room for improvement in the light response.

Herein,we firstly reported a facile hydrothermal method to dope Nb into the crystal structrue of NaTaO3.The synthesized NaNbxTa1-xO3(x=0.25,0.5,0.75),especially NaNb0.75Ta0.25O3,exhibited superior photocatalytic activities compared with pure NaTaO3for degradation of a model pollutant,methylene blue(MB),in ultraviolet light.This should be attributed to improved specific surface area and narrowed band gaps,which is verified by a series of characterization tests.Besides,we investigated the effects of inorganic anions and cations that may exist in wastewater on photocatalytic degradation and proposed a possible photocatalytic mechanism over the catalysts we prepared.

2.Experimental

2.1.Chemicals

Sodium hydroxide (NaOH,AR),sodium chloride (NaCl,AR),sodium sulfate (Na2SO4,AR),sodium nitrate (NaNO3,AR),calcium nitrate tetrahydrate (Ca(NO3)2?4H2O,98.5%) were purchased from Sinopharm Chemical Regent Co.Ltd (Shanghai,China).Niobium oxide (Nb2O5,99.9%),methylene blue (MB,biotechnology level),ferric sulfate nonahydrate(Fe(NO3)3?9H2O,98.5%)were purchased from Shanghai Macklin Biochemical Co.,Ltd.Tantalum oxide(Ta2O5,99.99%) was purchased from Shanghai Adamas Reagent Co.LTD.L-ascorbic acid (99%) was purchased from Acros Organics USA Company.Isopropanol(IPA,99.8%)was purchased from Aladdin Reagent (Shanghai) Co.,LTD.Ethylenediaminetetraacetic acid disodium salt (EDTA-2Na,AR) was purchased from Beijing Bioss Biotechnology Co.LTD.All of the reagents were used without further purification.

2.2.Preparation

NaNbxTa1-xO3(x=0,0.25,0.5,0.75) photocatalysts were synthesized by a hydrothermal method based on the previous study[17].In a typical procedure,0.442 g Ta2O5and 40 ml 1.5 mol?L-1NaOH solution were added into a 50 ml Teflon-lined stainlesssteel autoclave.The autoclave was sealed and kept at 140 °C for 12 h and then cooled to room temperature in air.The product was first washed with deionized water for several times,and then dried overnight at 80 °C to obtain NaNbxTa1-xO3(x=0),namely pure NaTaO3.For Nb-doped samples,the mass of Ta2O5in the raw material was reduced while the mass of Nb2O5was increased according to the molar ratio with remaining processes unchanged.The obtained catalysts were referred to as NaNbxTa1-xO3(x=0.25,0.5,0.75),respectively.

2.3.Characterization

The crystal structures of prepared samples were characterized by X-ray diffraction (XRD) (Cu Kα1 radiation,λ=0.154 nm) for a test range of 2θ=15°-90°.The microscopic morphologies were observed by scanning electron microscopy (SEM,Hitachi Su-8010),with an acceleration voltage of 5.0 kV.The Brunauer-Emmett-Teller(BET)theory was employed to determine the specific surface areas of the samples using nitrogen as the analysis gas.The degassing temperature was set at 423 K and the degassing time was 20 h.The ultraviolet-visible (UV-vis) diffuse reflectance spectra (DRS) were obtained by a UV-visible spectrophotometer(Shimazu UV-3600) with a scanning range of 200–600 nm.

2.4.Photocatalytic degradation experiment

The photocatalytic activities of prepared samples were evaluated by the degradation of MB using a 250 W mercury lamp.First,50 mg catalyst was dispersed in 50 ml MB solution (10 mg?L-1).The suspension was stirred in the dark for 30 min to attain the adsorption–desorption equilibrium.Then it was exposed to the UV light and the reaction started.Stirring and water cooling were maintained throughout the reaction.Liquid samples were taken and separated with a syringe filter every 30 min.And a visible spectrophotometer (INESA 7230G) was employed to analyze their absorbance at 664 nm.

The absorbanceAof the solution is proportional to the dye concentrationCaccording to the Beer-Lambert law,so degradation efficiency η of the dye can be expressed as

C0andA0are the initial concentration and absorbance of the solution.CandAare the concentration and absorbance after irradiation forttime.

The kinetic process of photocatalytic reaction can be characterized by the Langmuir-Hinshelwood kinetic equation with the rate of degradation of reactantsRexpressed as

k1is the limiting rate constant of photocatalytic reaction;k2is the equilibrium constant of adsorption which is usually far less than 1.

After calculating the integration of the above formula,it can be expressed as

This is the pseudo-first order correlation wherekappis the apparent rate constant of the photocatalytic reaction.

The slope can be obtained by linear fitting of experimental results according to the above formula.The value of the slope is the apparent rate constantkapp,which can characterize the activity of the catalyst quantitatively.

In order to explore the influence of anions (Cl-,) in wastewater on photocatalytic activity,the corresponding sodium salts (0.1 mol?L-1) were added into MB solution,respectively.And the corresponding nitrates (0.1 mol?L-1) were employed to explore the influence of cations (Na+,Ca2+,Fe3+).

Electron spin resonance (ESR) spectroscopic experiments were conducted on a Bruker EMXPLUS spectrometer to investigate the generated active free radicals over the catalyst under irradiation.The spin-trap reagents were 2,2,6,6-tetramethyl-1-piperidinyloxy(TEMPO) or 5,5-dimethyl-1-pyrrolineN-oxide (DMPO) in water or methanol.Capture experiments were carried out in combination with ESR test results to identify the active free radicals involved in the reaction process.L-ascorbic acid,IPA and EDTA-2Na(1 mmol?L-1)were added to MB solution as scavengers of superoxide radicals (),hydroxyl radicals (·OH) and holes (h+),respectively.

3.Results and Discussion

3.1.Characterization of as-prepared catalysts

The crystal structures of synthesized NaNbxTa1-xO3(x=0,0.25,0.5,0.75) were characterized by XRD,as shown in Fig.1.Whenx=0,the diffraction peaks at 22.8°,32.4°,46.6° and 52.5° were clearly observed,corresponding to (0 2 0),(1 2 1),(2 0 2),(2 2 2)planes of NaTaO3crystal,respectively.This was consistent with the standard card of orthorhombic NaTaO3crystal (PDF#73-0878,a=5.521,b=7.795,c=5.484,α=β=γ=90°)[5].To our knowledge,the characteristic diffraction peaks of cubic NaNbO3crystal(PDF#75-2102,a=b=c=3.906,α=β=γ=90°) are located at 22.7°,32.4°,46.5° and 52.3°,indexed to (1 0 0),(1 1 0),(2 0 0),(2 1 0)planes of NaNbO3crystal,respectively[18].As shown in the enlarged part of the XRD pattern,the diffraction peak of Nb-doped NaTaO3was disorted at 22.8° and approached the characteristic diffraction peak of NaNbO3at 22.7°.Similar changes also occurred at other characteristic diffraction peaks.It can be inferred that Nb was successfully doped into the crystal structure of NaTaO3forming the modified NaNbxTa1-xO3crystal by adding a certain amount of Nb2O5in the hydrothermal process.

Fig.1.XRD pattern of as-prepared NaNbxTa1-xO3 (x=0,0.25,0.5,0.75).

The microscopic morphologies of the synthesized samples were observed by SEM (Fig.2).It turns out that all samples were wellcrystallized.Fig.2(a)shows that the pure NaTaO3sample had regular cubic morphology with a particle size of about 1 μm.As shown in Fig.2(b)-(d),the doped samples (NaNbxTa1-xO3,x=0.25,0.5,0.75) also had regular cubic morphologies similar to pure NaTaO3sample.However,the particle size of the doped samples was reduced to a certain extent,falling in the range of less than 1.The results of BET tests (Table 1) agree well with the phenomena observed by SEM.The specific surface area of NaNbxTa1-xO3was 0.8063,0.9813,1.737,2.502 m2?g-1corresponding tox=0,0.25,0.5,0.75,respectively.It is obvious that the specific surface area increased gradually with the increase of Nb-doping ratio.NaNb0.75-Ta0.25O3had the largest specific surface area,which was more than three times of pure NaTaO3.As is known that the specific surface area of catalysts has a significant influence on their catalytic activities.The Nb-doped samples were expected to perform well in the photocatalytic reaction from the results of SEM and BET tests.

Table 1 Specific surface area of NaNbxTa1-xO3 (x=0,0.25,0.5,0.75)

The optical properties of synthesized NaNbxTa1-xO3(x=0,0.25,0.5,0.75) were characterized through a DRS pattern as shown in Fig.3.It is obvious that pure NaTaO3only had strong absorption in the ultraviolet region with an absorption threshold (λg) of around 325 nm.Compared with the pure NaTaO3sample,the absorption regions of Nb-doped samples (NaNbxTa1-xO3,x=0.25,0.5,0.75) presented a significant phenomenon of redshift,with the absorption thresholds extending to about 366,382,416 nm,respectively.It cannot be neglected that the redshift became more obvious with the increase of Nb-doping ratio,and NaNb0.75Ta0.25O3had the largest absorption region.This may be attributed to the substitution of Ta5+by Nb5+in the NaTaO3photocatalysts [5].The values of band gap energy (Eg,eV) for NaNbxTa1-xO3(x=0,0.25,0.5,0.75)were calculated as 3.82,3.39,3.25,2.98 eV,respectively,according to the equation [4,9]

Fig.2.SEM images of NaNbxTa1-xO3 (a) x=0,(b) x=0.25,(c) x=0.5,(d) x=0.75.

Fig.3.UV–vis diffuse reflectance spectra of NaNbxTa1-xO3 (x=0,0.25,0.5,0.75).

The decrease in the band gap energy can be explained by the locations of the valence and conduction bands of the prepared catalysts.The valence band potential of NaNbxTa1-xO3(x=0,0.25,0.5,0.75)was meant to be similar,due to the valence bands all consisting of O2porbitals.However,the conduction band potential of NaNbxTa1-xO3growed less negative with more accumulation of Nb4dstates,because the potential energy of Ta5dorbital is higher than the Nb4dorbital.Therefore,the band gap energy of NaNbx-Ta1-xO3(x=0,0.25,0.5,0.75) decreased in the following order:NaTaO3>NaNb0.25Ta0.75O3>NaNb0.5Ta0.5O3>NaNb0.75Ta0.25O3[19].In addition to the redshift,the absorption intensity of doped samples in the ultraviolet region was also enhanced to different degrees as shown in the pattern.The absorption enhancement of NaNb0.25Ta0.75O3and NaNb0.5Ta0.5O3was particularyly obvious.The Nb-doped samples were expected to perform better photocatalytic activities due to the enhanced light absorption in the entire ultraviolet and visible region.

3.2.Photocatalytic performance

3.2.1.Optimal ratio of Nb-doping

The photocatalytic degradation of MB on NaNbxTa1-xO3(x=0,0.25,0.5,0.75) under the 250 W mercury lamp is shown in Fig.4(a).It can be seen that the concentrations of MB all decreased over different samples during– 30–0 min.It was due to the dye molecules adsorbed by the catalyst.The ability of different catalysts to adsorb dyes was in the following order from small to large:NaTaO3

However,this paper is meant to focus on the photocatalytic activities of the prepared samples as photocatalysts in the process of photocatalytic degradation.It is obvious that the concentrations of MB continuously declined since the light was turned on at 0 min as what Fig.4(a) shows.After 180 min of reaction,the concentration of MB decreased to 53.0% of the initial value using NaTaO3as the photocatalyst,meaning that the degradation proportion was 47.0% .With more Nb doped in the prepared samples,they performed better photocatalytic activities.The degradation efficiency of MB was 87.4% and 93.1% over NaNb0.25Ta0.75O3and NaNb0.5-Ta0.5O3,respectively.NaNb0.75Ta0.25O3was the best photocatalyst in this paper,degradating 95.7% of MB after 180 min of reaction,which was even twice the ability of pure NaTaO3.This was in great agreement with the results of BET tests and the UV–vis diffuse reflectance spectra.According to the above discussion,the following factors account for the high photocatalytic activity of NaNb0.75-Ta0.25O3.First,its narrowed band gap can improve the utilization of ultraviolet and visble light.Second,the high surface area can facilitate transport of substrate to the active sites of catalyst,hence enhancing the photocatalytic efficiency.Fig.4(b) shows the reaction kinetic curves of MB degradated over the prepared samples.It can be concluded from the figure that the degradation process follows the pseudo-first-order kinetics.The apparent rate constants were 0.00313,0.00924,0.01090,0.01241 min-1,for NaTaO3,NaNb0.25Ta0.75O3,NaNb0.5Ta0.5O3,NaNb0.75Ta0.25O3,respectively.The constants can intuitively reflect the photocatalytic activities of prepared samples fitting well with Fig.4(a).

It should be noted that the concentration of MB maintained at a high level (79.8% of the initial concentration after 180 min of irradiation) without any catalyst added in the solution.This control experiment indicates that the degradation of MB as shown in Fig.4 should be attributed to the catalysts we prepared.

3.2.2.Influence of inorganic salt ions in wastewater

Inorganic salts in wastewater may have impacts on the removal process of pollutants [12,23].In order to explore the influence of anions(Cl-,)in wastewater on NaNb0.75Ta0.25O3,the corresponding sodium salts (0.1 mol?L-1) were added into MB solution,respectively (Fig.5).It can be concluded easily from Fig.5(a)that all the inorganic salt ions in the wastewater had an obvious inhibitory effect on the absorption process.This may be because that large amounts of ions were adsorbed on the surface of samples which affected the surface charge and the electrostatic interaction with dye molecule [24].It is obvious in Fig.5(a) thatand Clinhibited the photocatalytic degradation of MB on the catalyst.However,the rate of photocatalytic degradation was slightly increased after the addition of.The dynamic analysis in Fig.5(b) supports the above conclusions.The apparent rate constants were 0.00278,0.00389,0.01616 min-1after the addition of,Cl-,,respectively,while the constant was 0.01241 min-1for the blank experiment.These phenomena can be explained by Eqs.(5)–(11) [25,26].Free radicals like h+and·OH are active substances that takes effect in photocatalytic degradation,which was verified in Section 3.3.To our knowledge,can act as a scavenger reacting with h+and·OH and producing the sulfate radical,.Thoughis a very strong oxidant (redox potential=+2.6 eV) [25],its oxidation capacity is inferior to the photo-generated h+over NaNb0.75Ta0.25O3.The estimated valence band potential of pure NaTaO3prepared by hydrothermal method is +3.0 eV according to previous studies [10,12].The valence band potential of NaNb0.75Ta0.25O3in this paper can be regarded as around +3.0 eV,which was more positive than the redox potential of,as the doping of Nb had no effect on the valence band potential as analyzed in Section 3.1.And this leads to the superior oxidation capacity of photo-generated h+over NaNb0.75Ta0.25O3compared with.Therefore,the decrease of degradation efficiency after the addition ofcan be attributed to the photo-generated h+over NaNb0.75Ta0.25O3replaced by.The reason for Cl-inhibiting the photocatalytic degradation is similar to that of[26,27].In contrast toand Cl-,can support the direct or indirect formation of·OH in the presence of photons,resulting in the slight increase of the degradation rate.

Fig.4.(a) Photocatalytic degradation curves of MB over NaNbxTa1-xO3 (x=0,0.25,0.5,0.75) under 250 W Hg lamp and (b) corresponding reaction kinetic curves.

Fig.5.(a) Effects of anions on the photocatalytic degradation of MB over NaNb0.75Ta0.25O3 under 250 W Hg lamp and (b) corresponding reaction kinetic curves.

In order to explore the effects of cations (Na+,Ca2+,Fe3+) in the wastewater on photocatalytic degradation,the corresponding nitrate (0.1 mol?L-1) was added to the MB solution (Fig.6).The results in Fig.6(a) reflect that the reaction rate slightly increased with the addition of Na+or Ca2+.The apparent rate constants were 0.01616,0.01855 min-1after the addition of Na+,Ca2+,respectively,while the constant was 0.01241 min-1for the blank experiment as in Fig.6(b).Considering the effect of nitrate ion,it can be concluded that Na+and Ca2+have little influence on photocatalytic degradation process.However,the degradation efficiency reached almost 100% within 10 min after Fe3+was added,which is not shown in Fig.6.It is obvious that Fe3+can remarkably enhance the degradation rate of MB,which can be explained in two ways.For one thing,Fe3+can unite with photogenerated electrons and holes forming Fe2+and Fe4+ions,which promotes the separation of electrons and holes.For another,Fe3+,as an effective photosensitive substance,can produce·OH under weak acidic conditions after adsorbing light.This accelerates the degradation process resembling the photo-Fenton reactions [12].

3.3.Possible photocatalytic mechanism

ESR tests were conducted to directly investigate the generated reactive oxygen species and radicals over NaNb0.75Ta0.25O3under irradiation.Fig.7(a)-(c) present the ESR spectra for the detection of h+,·OH,and,respectively.Under the dark condition,the signal of the spin-trap reagent,TEMPO,was observed as in Fig.7(a).The signal decreased when the UV light was on,which was due to the combination of TEMPO and h+to form TEMPO+.This indicates that the electrons transit from the valence band to the conduction band after the catalyst absorbing photons,leaving active h+in the valence band.The characteristic signals of the DMPO-·OH [27,28] appeared after the UV light was on as Fig.7(b)shows,which confirms that·OH was formed over the catalyst under irradiation.Fig.7(c) shows that the characteristic signals of[11,28] were detected under the UV light,indicating thatwas also generated.

Fig.6.(a) Effects of cations on the photocatalytic degradation of MB over NaNb0.75Ta0.25O3 under 250 W Hg lamp and (b) corresponding reaction kinetic curves.

Fig.7.ESR tests of(a)h+,(b) ·OH,(c) and(d)Photocatalytic degradation efficiency of MB under 250 W Hg lamp in the absence or presence of scavengers (EDTA-2Na,Lascorbic acid,IPA) over NaNb0.75Ta0.25O3.

Fig.8.Schematic model of a possible photocatalytic mechanism over NaNb0.75Ta0.25O3.

However,the ESR tests can only qualitatively verify the active substances produced on the catalyst.It needs to be combined with the capture experiments to determine the dominant active free radicals in the reaction process,as Fig.7(d)shows.L-ascorbic acid,IPA and EDTA-2Na (1 mmol?L-1) were added to MB solution as scavengers of,·OH,and h+,respectively.The results reflect that the degradation efficiency decreased the most after adding EDTA-2Na,followed by IPA.This indicates that h+mainly plays a driving role in the degradation process.And·OH also takes effect to a certain extent during the reation process.The degradation efficiency did not decrease,but even increased slightly in the presence of Lascorbic acid.This indicates that only a trace amount ofwas generated on the catalyst,which has little effect on the degradation process.The slight increase of degradation efficiency may be due to the capturing effect of L-ascorbic acid on,which is positive for Eq.(12)[12].This causes more electrons to be consumed,promoting the separation of electron-hole pairs and making more h+participate in the reaction.Hence,it can be summarized that h+and·OH are the main active substances in the photocatalytic degradation process of MB over NaNb0.75Ta0.25O3.

Fig.8 shows the proposed mechanism how Nb-doping improves the photocatalytic activity of NaTaO3.Substitution of Ta within the NaTaO3lattice by Nb creates an additional energy level just below the Ta5dorbital as the new hybrid Ta5dand Nb4dorbitals.This makes the conduction band potential of NaNb0.75Ta0.25O3less negative than pure NaTaO3.Therefore,the band gap energy of NaNb0.75Ta0.25O3decreases to 2.98 eV from 3.82 eV of pure NaTaO3,and this improves the utilization of light.When the light is incident on the surface of NaNb0.75Ta0.25O3,the electrons in the valence band transit easily to the conduction band,leaving active h+in the valence band.The h+can not only directly oxidize MB,but also react with H2O to generate·OH,which can also degradate MB.Besides,the higher surface area of NaNb0.75Ta0.25O3facilitates the degradation of MB and the conversion of H2O to·OH by h+.Thus,the superior photocatalytic activity of NaNb0.75Ta0.25O3can be attributed to the narrow band gap and the high surface area.

4.Conclusions

In this study,Nb was successfully doped into the NaTaO3lattice to form NaNbxTa1-xO3(x=0,0.25,0.5,0.75) photocatalysts with regular cubic shapes through hydrothermal method.Among the synthesized samples in this paper,NaNb0.75Ta0.25O3exhibited the best photocatalyst acitivity,degradating 95.7% of MB after 180 minutes of irradiation under a 250 W mercury lamp,which was even twice the ability of pure NaTaO3.The excellent photocatalytic activity of NaNb0.75Ta0.25O3is ascribed to the narrowed band gap(2.98 eV) and the increased surface area (2.502 m2?g-1) compared with pure NaTaO3(3.82 eV,0.8063 m2?g-1)according to the results of various characterization tests.Influences of various anions and cations in wastewater on the photocatalytic activity of NaNb0.75-Ta0.25O3were explored.and Cl-inhibited the photocatalytic degradation of MB on NaNb0.75Ta0.25O3due to the scavenging effect on h+and·OH,whileincreased the rate of photocatalytic degradation slightly because of the promotion for the direct or indirect formation of·OH under irradiation.Fe3+remarkably enhanced the degradation rate of MB,because it can promote the separation of electrons and holes and produce·OH under weak acidic conditions after adsorbing light.This study provides a direction for constructing novel efficient photocatalysts,making it possible for superior NaTaO3-based photocatalysts to be widely utilized in environmental protection.

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

This work was supported by the Innovative Research Groups of the National Natural Science Foundation of China (51621005).