Light-Induced Dynamic Stability of Oxygen Vacancies in BiSbO4for Efficient Photocatalytic Formaldehyde Degradation

Maoxi Ran,Wen Cui,Kanglu Li,Lvcun Chen,Yuxin Zhang ,Fan Dong,and Yanjuan Sun*

Defect engineering has been regarded as a versatile strategy to maneuver the photocatalytic activity.However,there are a few studies concerning how to maintain the stability of defects,which is important to ensure sustainable photocatalytic performance.Here,a novel strategy to modulate the structural properties of BiSbO4using light-induced dynamic oxygen vacancies is reported by us for efficient and stable photocatalytic oxidation of formaldehyde.Interestingly,the continuous consumption and replenishment of vacancies(namely dynamic vacancies)ensure the dynamic stability of oxygen vacancies,thus guaranteeing the excellent photocatalytic stability.The oxygen vacancies could also accelerate the electron migration,inhibit the photogenerated electron/hole recombination,widen the light absorption spectra,and thus improve the photocatalytic formaldehyde removal performance.Combined with the results of in situ DRIFTS,the reaction mechanism for each step of formaldehyde oxidation is revealed.As supported by DFT calculation of Gibbs free energy,the introduction of oxygen vacancies into BiSbO4can promote spontaneous process of formaldehyde oxidation.Our work highlights a promising approach for stabilizing the defects and proposes the photocatalytic reaction mechanism in combination with the thermodynamic functions.

Keywords

dynamic stability,formaldehyde degradation,oxygen vacancy,photocatalysis,reaction mechanism

1.Introduction

Ascribing to the deterioration of atmospheric environment,the development of efficient technology for high air quality is regarded as high priority.[1-4]Compared with traditional technologies for air purification,such as adsorption,[5,6]thermal catalytic oxidation,[7]plasma technology,[8]photocatalytic oxidation has been widely applied in purification of typical air pollutants because of its advantages of mild reaction conditions,no extra fuel required and lower energy consumption.[9-15]Until now,many metal oxides or ternary compounds materials,such as TiO2,[16-19]Co3O4,[20-22]BiVO4[23]and BiOX(X=Cl,Br,and I),have been synthesized and applied in air purification.[24-27]However,these photocatalysts are still under the requirements for practical applications on account of their low efficiency and unsatisfactory stability owing to the high recombination of charge carriers,narrow photoabsorption range,and sluggish surface reaction.Hence,it is urgent to design and fabricate more efficient photocatalysts to upgrade the performance.

Many strategies are devoted to optimizing these photocatalysts,including defect engineering,construction of heterojunction,heteroatom modification,and morphology modulation.[28-31]Among them,defect engineering is a valid strategy to modulate electronic structure and tune surface properties to improve the photocatalytic activity.However,for typical artificial defects under working conditions,they are easily filled by water/oxygen in the air or blocked by intermediate products,leading to the gradual decline in activity until the inactivation of catalyst.[32]Therefore,to ensure the stability of defects turns out to be the necessity for efficient and stable photocatalysts.Recently,Chen et al.prevented the deactivation of oxygen vacancies by loading Bi metal onto Bi2O2CO3surface so as to obtain stable and efficient photocatalytic NO performance.[32]To the best of our knowledge,reports on the stability of defects in semiconductor photocatalysts,an important issue for sustainable application,are still limited.Especially,it is still a great challenge for the maintenance of defects under working conditions.

In this work,BiSbO4is first prepared by hydrothermal measure,and then oxygen vacancies are directly introduced into BiSbO4via an in situ light-induced method.The vacancies are consumed but compensated simultaneously,that is,reaching the dynamic equilibrium of vacancies,which guarantees the high efficiency and stability of photocatalyst.Combining the thermodynamic functions via the calculation of Gibbs free energy and the results of in situ DRIFTS as experimental basis,the photocatalytic formaldehyde degradation mechanism has been clearly revealed.The in situ dynamic vacancies strategy can be also extended for development of other oxide defective photocatalysts.

2.Results and Discussion

2.1.Phase and Morphology Structure

As can be seen from Figure 1,samples prepared at different hydrothermal temperatures(except for samples prepared at 120°C)reveal obvious characteristic diffraction peaks at 27°,31°,33°,37°,50°,and 51°,which are attributed to(-112),(112),(200),(020),(220),and(204)crystal planes,respectively.This is in full accordance with the XRD patterns of BiSbO4(ICDD,76-1362).Concomitantly,the catalyst after the reaction test(BiSbO4-200-used)is also subjected to XRD scanning,and the characteristic diffraction peaks are still in one-to-one correspondence with the standard card,indicating that the catalyst after the reaction has no change in crystal structure.

Figure 1.a)XRD patterns of BiSbO4samples prepared with different hydrothermal temperatures;b,c)SEM,HR-TEM images of the BiSbO4-200 d,e)and BiSbO4-200-used f-i),respectively;EDX mapping patterns of the BiSbO4-200-used.

Next,the low-magnification SEM images(Figure 1b,d)show that the BiSbO4before and after the reaction display a similar walnut-like morphology.From the high-resolution TEM images(Figure 1c,e),it is revealed that the identical lattice fringe with an interplanar spacing of 0.32 nm for BiSbO4-200 and BiSbO4-200-used can be indexed to the(-112)plane.The lattice spacing of 0.16 nm(indexed to(224)plane)can also be observed for BiSbO4-200-used.To sum up,we can roughly determine that the BiSbO4-200 and BiSbO4-200-used are dominated by exposed(-112)crystal plane.The energy-dispersive X-ray spectrometry(EDS)elemental mappings of the BiSbO4-200-used(Figure 1f-i)present an obvious result that the elements Bi,O,Sb are distributed uniformly across the sample.The N2adsorption--desorption isotherms curves(Figure S1)of BiSbO4-200 and BiSbO4-200-used samples are of type IV and H3 hysteresis loops.This result indicates the formation of mesoporous structure,which is further confirmed by the Bar-rett-Joyner-Halenda(BJH)pore-size distribution curves(Figure S1,inset).

2.2.Photocatalytic Activity for HCHO Removal

The photogenic spectrometer was employed to monitor the concentration of HCHO to assess the photocatalytic activity and stability.Figure 2a displays an obvious trend that BiSbO4materials prepared at different hydrothermal temperatures exhibit different performance on HCHO degradation with the BiSbO4-200 sample showing the highest removal ratio up to 92.0% .Among them,the degradation performance of formaldehyde in BiSbO4-180 sample decreases slightly after 20 minutes of reaction,probably because the intermediate products produced during the degradation of HCHO could block some active sites.Although the photocatalytic activity of BiSbO4-200 is not the best compared with traditional catalysts(Pt/TiO2,Pt load with defective NaInO2,etc),[33-35]BiSbO4containing defects has its own unique advantages,such as the simple preparation and low cost of the catalyst,and provides reference for the subsequent studies on the stability of defects.Additionally,the crucial problem for photocatalysis is its photochemical stability.The recycle experiments of BiSbO4-200 present no distinct decline of activity in the oxidation of HCHO,showing the excellent photocatalytic stability(Figure 2b).Most importantly,the result of the control photocatalysis experiment without catalyst under light irradiation shows that the catalyst is a necessary condition in formaldehyde oxidation,and only light irradiation cannot degrade formaldehyde(Figure S2).

Figure 2.a)Photocatalytic HCHO removal performance of BiSbO4-160,BiSbO4-180 and BiSbO4-200,b)and five consecutive cycle tests over BiSbO4-200.

It is noted that the color of the catalyst after the reaction changes from white to black gray(Figure 3a,b),which indicates that the light irradiation exerts significant influence on the structure of BiSbO4.To reveal the essential reason for this observation,the room temperature solid-state electron paramagnetic resonance(EPR)technology was employed to detect the vacancies in the samples.Surprisingly,a nearly sixfold enhancement in the oxygen vacancies (OVs) signal(g=2.002)intensity under the dark can be observed for BiSbO4-200-used compared with pristine BiSbO4-200,which directly demonstrates the increased concentration of OVs after the light irradiation.It is reported that the light energy of 280 nm can break the Bi-O and Sb-O bond,thereby forming oxygen defects,which also explains the reason for the formation of defects in the material under the irradiation of 300W mercury lamp(including 280nm wavelength)in this experiment.[36]The pristine BiSbO4-200 shows a weak signal implying that the intrinsic OVs exist in the sample(Figure 3c).It should be noted that the oxygen vacancies would be inclined to be filled by oxygen in the air according to previously published articles.[36]However,from the results shown in Figure 3c,the BiSbO4-200-used still appears a strong vacancy signal in dark,suggesting that oxygen vacancies are first consumed and then compensated simultaneously under light irradiation,reaching the equilibrium state of dynamic stability of vacancies.Hence,we conclude that the reason for the color change of BiSbO4-200 is associated with the generation of dynamic vacancies.Furthermore,the mobility of electrons is significantly elevated under the light on because of the dynamic vacancies.[37]

Figure 3.a,b)The color of BiSbO4-200 and BiSbO4-200-used samples;c)room temperature solid-state EPR spectra for the samples.

2.3.Mechanism of Photocatalytic Formaldehyde Oxidation

To investigate the photocatalytic oxidation mechanism of HCHO over BiSbO4-OV,the in situ DRIFTS be carried out to online record the process of adsorption and reaction with exposing time in a flow of HCHO+O2at room temperature(Figure 4a).The adsorption peak of HCHO molecule at 1722 cm-1[38]could not be directly found,but there are many other complex peaks being monitored associated with HCHO.The weak absorption peaks at 2790,2863,2943,and 1408 cm-1are ascribed to the stretching mode υ(CH)or δ(CH)and υ(COO-)of formate species,[38-41]respectively.The peaks at 3521 and 1652 cm-1could be ascribed to the hydroxyl groups of adsorbed water on the catalyst,[42,43]which originates from the water adsorbed on the catalyst surface.The peak at 1230 cm-1is attributed to carbonate species.[38]The formate and carbonate species observed in the adsorption process suggests that HCHO adsorbed on the surface of photocatalyst has a chemical reaction with O2at the defect sites on the surface.

After 30 minutes of the adsorption,the light was turned on immediately to initiate the photocatalysis reaction.Two strong new peaks at 1367 and 1566 cm-1[40,44]are observed accompanying with the decrease of peak intensity at 3521 and 1652 cm-1.At the same time,the new peak attributed to carbonate species at 1260 cm-1[45]could also be found.The peaks associated with C-H bond from formate species are observed with the in situ DRIFT,which is easily generated by the reaction of HCHO with hydroxyl radicals under UV light.The band appeared at 2155 and 2229 cm-1in the spectra could be ascribed to the adsorbed CO and CO2,[40,46]deriving from the hydrolysis of formate and carbonate species on the surface of photocatalyst,respectively.The peak(1230 cm-1)intensity trend is raising first then falling.The gradually decreased peak(1408 cm-1) intensity and gradually increased peak(3200 cm-1)[39,47]intensity indicate that the formate species could be further converted into carbonate species over time and then the carbonate is hydrolyzed to CO2and H2O.The normalized absorbance curves clearly show the change trend of the species over time(Figure S3).The formate species accumulated on the surface of the catalyst increase gradually after the light is turned on,the carbonate species increase first and then decrease gradually,and both CO and CO2increase significantly.After turning off the light for 10 minutes,the attribution and relative strength of all the peaks are consistent with those before turning off the light,indicating that the catalyst does not react with pollutant under dark conditions. All assignments of the FT-IR bands observed during HCHO adsorption over photocatalysts are summarized in Table S1.

Inspired by the above analysis results,we could propose the reaction paths in the following:HCHO+·OH→HCOOH+H(1);HCOOH→CO+H2O (2)or HCOOH+·OH→H2CO3+H(3);H2CO3→CO2+H2O(3).The free energy of each step and structures corresponding to the optimal reaction paths is calculated and shown in Figure 4b,Table S2,and Table S3,respectively.For the BiSbO4-OV,the ΔG for HCHO→HCOOH formation is-0.16 eV,which is less than zero and thus could occur spontaneously.However,the ΔG for HCHO oxidization to HCOOH on BiSbO4is an uphill by 0.2 eV,which is more difficult than that of the first step reaction on BiSbO4-OV.Additionally,from the results of the HCHO adsorption energy(Eads)(-0.13 eV for pure BiSbO4and-0.22 eV for BiSbO4-OV),the BiSbO4-OV is more capable of oxidizing HCHO(Figure 5 and Table 1)into HCOOH.The corresponding adsorption structure is presented in the supporting information(Figure 5).

Figure 4.a)In situ FT-IR spectra of the reaction process of photocatalytic HCHO degradation on BiSbO4-OV;b)calculated free energy diagram corresponding to the reaction pathway followed by the HCHO conversion on BiSbO4and BiSbO4-OV,respectively.

Figure 5.The adsorption energy for HCHO,HCOOH,and H2CO3over BiSbO4and BiSbO4-OV,respectively.Eadsstands for the adsorption energy,negative values mean heat release;red,purple,golden brown,and pink represent O,Bi,Sb,and H atoms,respectively.

Table 1.The calculated energies of various intermediates with BiSbO4and BiSbO4-OV

Next,the HCOOH adsorption energy on BiSbO4-OV and BiSbO4is-0.48 and-0.23 eV(Figure 5 and Table 1),respectively,suggesting that HCOOH could be easier to be adsorbed on the active site of BiSbO4-OV and further being oxidized.The HCOOH transformation process could proceed in two possible paths:HCOOH→CO+H2O or HCOOH+·OH→H2CO3(CO2+H2O)+H.HCOOH hydrolysis to CO is not feasible due to the high ΔG(0.66 eV)for BiSbO4-OV.While the ΔG of HCOOH → H2CO3(-0.25 eV)is much lower than that of the reaction HCOOH→CO,indicating that the process of HCOOH oxidization is easy to proceed.Similarly,the oxidation process of HCOOH(ΔG=-0.04 eV)over BiSbO4is more favorable than that the hydrolysis of HCOOH (ΔG=0.25 eV).But the ΔG of HCOOH→H2CO3(-0.25 eV)for BiSbO4-OV is much larger than that of BiSbO4(ΔG=-0.04 eV),implying that the presence of oxygen vacancies could promote the oxidation of formic acid.Correspondingly,the adsorption energy of H2CO3(Figure 5 and Table 1)is noticeably increased from-0.13 eV for pure BiSbO4to-0.22 eV for BiSbO4-OV,manifesting that the vacancies could facilitate the adsorption and activation of carbonic acid,and finally hydrolyzed it into CO2and H2O.

2.4.Optical Properties and Charge Transformation

The quenching of PL peaks of BiSbO4-200-used(Figure 6a)and the light absorption property(Figure 6b)verify that the recombination of electron-hole pairs is greatly inhibited and the light absorption capacity is highly enhanced,demonstrating that the formation of OVs could boost the separation of carriers effectively and broaden the light absorption range,thus guaranteeing the excellent photocatalytic activity and stability.According to the XPS spectra(Figure S4),the binding energies for Bi 4f and Sb 3d over BiSbO4-200 and BiSbO4-200-used are shifted to the lower energy in comparison with the pristine BiSbO4-200,suggesting that the construction of oxygen vacancies promotes the enrichment of more electrons in Bi and Sb atoms.In addition,the active oxygen species(ROS)play a decisive role in the HCHO oxidation.Therefore,using the DMPO spin-trapping ESR experimental technology,the DMPO-·O2-,DMPO-·OH,and trapped electrons(e-)signals for BiSbO4-200 and BiSbO4-200-used are investigated.As can be seen from Figure 6c,there is no distinct signal to be observed for BiSbO4-200 and BiSbO4-200-used in dark conditions.While excited by UV light,the Bi-O and Sb-O bond are brokened and thus abundant oxygen vacancies are generated,which promotes the charge transfer between the O2and the BiSbO4-200 surface,resulting in the O2adsorbed on BiSbO4-200 being activated easily and inclined to receive more electron for the production of superoxide radicals(·O2-).So,strong ·O2-signals could be recorded on BiSbO4-200,which is even the same as the BiSbO4-200-used.Since both samples have been irradiated under ultraviolet light,oxygen defects will be formed.There is no obvious difference in the·O2-signals.Water adsorbed on the surface of photocatalyst is oxidized by the hole to generate hydroxyl radical(·OH)participating in the HCHO removal,which also matches very well with the results of Figure 6d.It can be revealed from Figure 6e that electrons are consumed in large quantities under light,which also confirmed the fact described in Figure 6a that electrons react with O2to generate superoxide radicals.In brief,abundant active oxygen species are generated due to the formation of dynamic vacancies,which would contribute to the high and stable photocatalytic activity of BiSbO4-200.

Figure 6.a)Photoluminescence spectra b)and the UV-vis spectra c)of BiSbO4-200 and BiSbO4-200-used,respectively.DMPO ESR spectra in methanol dispersion for ·O,d)aqueous dispersion for ·OH e)and aqueous dispersion for e-were carried out in both dark and ultraviolet irradiation for 15 min,respectively.

3.Conclusions

In conclusion,we have developed a novel strategy to stabilize the defects in semiconductor photocatalysts for sustainable air pollution purification.The oxygen vacancies were introduced into BiSbO4using an in situ light-induced method,which allowed the defects to be continuously consumed while being regenerated to achieve the dynamic stability of oxygen vacancies.The dynamic oxygen vacancies could expand the light absorption range of BiSbO4,and promote the charge separation/transfer as confirmed by the results of UV and PL.Their favorable factors contributed to the high efficiency and stability for photocatalytic formaldehyde oxidation on the defective BiSbO4.In addition,compared with the mechanism reported in the previous literature,the in situ DRIFTS and thermodynamic functions were highly combined to determine and verify the dynamic process of each step for formaldehyde oxidation,and confirm that oxygen vacancies in BiSbO4can promote spontaneous process of formaldehyde oxidation.Our work could provide a general light-induced strategy for stabilizing the defects via dynamic vacancies,and new insights on the reaction mechanism of formaldehyde oxidation.

4.Experimental Section

General information:The chemicals we used in this case were analytical and were used without further treatment.All chemicals include antimony(Ш)trioxide(Sb2O3,),bismuth nitrate pentahydrate(Bi(NO3)3·5H2O,AR),sodium hydroxide(NaOH,AR),nitric acid(HNO3,AR),and ethanol(C2H5OH,AR).

Preparation of catalysts:The walnut-like BiSbO4material was synthesized following the hydrothermal method.First,2.5 mmol Sb2O3was dissolved in 0.5 mM Bi(NO3)3·5H2O(15 ml)under continuous stirring,and then adjust the pH to 1 with the nitric acid solution or sodium hydrate.The above solution was transferred into 50-mL Teflon lined autoclave,which was sealed and heated at 120,160,180,and 200°C for 48 h,respectively.After cooling at room temperature,the resulting mixture was washed with deionized water and ethanol several times,then drying in an oven at 60°C.All samples were signed as BiSbO4-X(X=120,160,180,and200).

Characterization:The crystalline structure of the prepared samples was detected by XRD with Cu Kα radiation(model D/max RA,Rigaku Co,Japan).The microstructure and structure were characterized by SEM(model JSM-6490,Japan)and TEM(model JEM-2010,Japan).Nitrogen adsorption-desorption isotherms were obtained on a nitrogen adsorption apparatus(ASAP 2020,USA)with all samples degassed at 110 °C for 4 h before measurements.The DMPO-·O2-,DMPO-·OH,and trapped electrons(e-)signals for samples were investigated using the DMPO spin-trapping ESR experimental technology(JES FA200 spectrometer).EPR measurements equipped with Bruker ESP 500 spectrometer were carried out to record the vacancy signal.Photoluminescence(PL,F-7000,HITACHI,Japan,Measurement type:Wavelength scan,Scan mode:Emission,Excitation Wavelength:360nm,PMT Volt:700V,EX Slit:5nm,EM Slit:5nm),and UV-vis diffuse reflectance spectra(UV-vis DRS,UV-2450,Shimadzu,Japan)were used to investigate the optical properties of the materials.

Photocatalytic formaldehyde removal:The gas-solid phase photocatalytic formaldehyde removal was conducted in a continuous reactor(Scheme S1).First,0.4 mg samples were averagely dispersed on four frosted glasses(40 mm×100 mm)with ethanol,and then the above glasses were dried in an oven in 60°C.After that,the dried frosted glasses were moved to the aluminum rectangular reactor(340 ml volume with a quartz cover,200 mm×100 mm×17 mm).Subsequently,formaldehyde was introduced into the reactor in 1.0 L/min flow.Under the light irradiation(the 300 W mercury lamp was used as light source),the concentration of formaldehyde was monitored by a photogenic spectrometer(GASERA-ONE).For detection of formaldehyde,the GASERA-ONE showed a linear relationship in the range of 0.1 to 10 000 ppm.The linear relationship was calibrated,and it could also be self-calibrated by the instrument after purging N2.The detection limit for these gases was 0.1 ppm,and the detection accuracy was plus or minus 1% .Finally,the formula(η=(1-C/C0)× 100% ,C and C0represent formaldehyde concentration of the export and import,respectively)to calculate the removal ratio(η)of formaldehyde,respectively.

Theoretical calculations:The Vienna ab initio simulation package(VASP 5.4.1)was based on the density functional theory(DFT)of the Generalized Gradient Approximation(GGA),using the Projector Augmented Wave(PAW)and the plane wave as the basis function to process the interaction between the nucleus and inner electrons and the valence electrons.[48]A functional based on the Perdew-Berke-Ernzerhof(PBE)method was applied.In the framework of projector augmented wave approach,a periodic plane-wave basis with cutoff energy of 400 eV was used.[49]3×3×1 K point was set.The whole atoms were converged to 0.01 eV/?A.The supercell of BiSbO4was set to 2×2×1,including 64 oxygen atoms,16 bismuth atoms,and 16 antimony atoms.The computational model of BiSbO4containing two cycles and the corresponding three-view structure is presented in the supporting information(Figure 7).In the process of geometry optimizations,the whole atoms were fully relaxed.All layers were fixed except for the surface layer while small molecules were relaxed during the adsorption calculations.In free energies calculations,the entropy corrections and zeropoint energy(ZPE)have been included.

Figure 7.a)The computational model of BiSbO4containing two cycles b)and the corresponding threeview structure.Red,purple,and golden brown represent O,Bi,and Sb atoms,respectively.

The free energy is calculated using the following chemical reaction as an example:

The free energy change of the above example is

The free energy of a substance could be calculated by the standard formula

where EDFT,ZPE,and TS are the total energy from DFT calculations.So according to the above formula,the free energy change can be rewritten as

the relevant data of gas-phase species,adsorbates,substrates,and free energies(ΔG)are given in Tables S1 and S2.Free energy diagrams are displayed in Figure 4b.

The adsorption energy,Eadsis defined as

where Etot,Eslab,and Emolstand for the total energy of the adsorption structure,BiSbO4structure,and isolated molecule,respectively.

In situ DRIFTS investigation:As shown in Scheme S2,combined in situ diffuse reflectance unit with high-temperature reaction chamber,the in situ DRIFTS measurements were performed.Additionally,the reaction chamber was provided with two coolant ports and three gas ports for Helium,Oxygen,and pollutant to go in.The total gas flow was set as 100 ml/min(pollutant:oxygen=1:1).The chamber was enclosed in a dome with three windows:two for entrance and detection of IR light,and one for the illumination of photocatalyst.The ultraviolet point light source(MVL-165,JPN)was used as the light source of photocatalytic reaction.Before detection,the samples were preheated at 110°C for 20 min.After feeding polluted gas and oxygen for 30 minutes,the light was turned and changes in the adsorbed material on the photocatalyst surface were immediately recorded online.

Acknowledgments

This work was supported by the National Natural Science Foundation of China(21822601,21777011,and 21501016),the Innovative Research Team of Chongqing(CXQT19023).The authors also acknowledge the AM-HPC in Suzhou,China for computational support.

Conflict of Interest

The authors declare no conflict of interest.

Supporting Information

Supporting Information is available from the Wiley Online Library or from the author.