Constructing and Photocatalytic Performance of Flower?like CeO2/TiO2 Heterostructures

WANG Hong?Xia LI Xin?Xing ZHOU Yu

(1Department of Information and Engineering,Suqian University,Suqian,Jiangsu 223800,China)

(2Suqian Key Laboratory for Functional Materials,Suqian University,Suqian,Jiangsu 223800,China)

Abstract:A kind of three?dimensional flower?like CeO2/TiO2 heterojunction as photocatalysts was designed by the solvothermal method.The photocatalytic activity was evaluated by the decomposition of methyl orange(MO)under xenon lamp irradiation.The results showed that the flower?like structure was composed of thin nanosheets,on which many CeO2particles were uniformly attached.The molar ratio of Ce to Ti(nCe/nTi)and the solvothermal time influ?enced on the photocatalytic performance.When nCe/nTi=0.1 and the solvothermal time was 6 h,the photocatalytic activity of CeO2/TiO2reached the best,and the degradation rate reached 95% under xenon lamp irradiation for 50 min.The results suggested that the photocatalytic activity of CeO2/TiO2 heterojunction was greatly improved,compared to TiO2,which was mainly the function of heterojunction formed by CeO2and TiO2,and was conducive to the separation of photogenerated electrons and holes.

Keywords:heterostructure;photocatalysis;photodegradation;micro/nano?materials;semiconductor

0 Introduction

Photocatalytic technology can be used to simulate natural photosynthesis,which can change solar energy into chemical energy,and degrade organic pollutants in sewage into harmless substances such as CO2and H2O under normal temperature and pressure[1?3],thus avoiding the secondary pollution problem with tradi?tional methods.TiO2is an n?type semiconductor cata?lyst that is non?toxic,highly active,chemically stable,cheap,environmentally friendly,and it has been widely studied as an ideal photocatalyst[4?7].However,in the process of photocatalysis,TiO2has some defects,such as low quantum efficiency,easy recombination of elec?tron?hole pairs,and low utilization of sunlight,which greatly restricts its extensive industrial application.The solution to these problems depends on in?depth and systematic basic research.

To improve the photocatalytic activity of TiO2,the researchers used a variety of methods,such as control?ling the morphology[8?11],doping transition metal ions and non ?metallic ions[12?16],surface sensitization[17?18],semiconductor composite[19?20].Recent studies show that the selection of semiconductors with appropriate energy bands to couple with TiO,such as BiWO[21?22],226g?C3N4[23?25],CdS[26?27],CeO2[28?29],is conducive to separat?ing electrons and holes,and improving the visible light catalysis of TiO2.CeO2has high conductivity,thermal stability,oxygen storage capacity,and has a narrow energy gap(2.92 eV).Moreover,Ce4+and Ce3+ions are easy to reciprocal transformation,which makes CeO2have good electron transfer ability and light absorption ability.The bandgap difference between TiO2and CeO2can promote the separation of photogenerated electron?hole pairs and improve catalysis activity[30].Although TiO2and CeO2composite materials have received extensive attention,the research of CeO2/TiO2as prom?ising photocatalytic materials is not deep enough.In particular,the photocatalytic efficiency of CeO2/TiO2is far from practical application.Therefore,it is necessary to further improve the photocatalytic performance of CeO2/TiO2by optimizing the experiment.In this work,we prepared CeO2/TiO2photocatalyst materials with a three?dimensional flower structure by solvothermal method.Under xenon lamp irradiation,flower?like CeO2/TiO2photocatalyst had high activity for methyl orange degradation.

1 Experimental

1.1 Preparation of the samples

Preparation of CeO2：All the chemical reagents were chemically pure and were used directly without further processing.The water used was distilled water.Under strong stirring,0.26 g cerium nitrate was dissolved in 100 mL water.After stirring frequently for 30 min,NaOH was added to the solution to control the pH to 9?10,followed by hydrothermal treatment at 180 ℃ in a Teflon?lined autoclave for 24 h.The prod?uct was centrifugally separated,washed with ethanol and distilled water,then dried.The sample was collect?ed and then put into the annealing furnace at 500℃for 2 h to obtain CeO2.

Preparation of CeO2/TiO2：polyethylene glycol,cetyltrimethyl ammonium bromide,and carboxamide were immersed into 70 mL acetic acid solution,and after vigorous stirring to dissolve them,CeO2was added into the above?mixed solution,finally added 2 mL butyl titanate by dropping and stirring for 20 min,and then moved the solution to 100 mL stainless steel autoclave lined with polytetrafluoroethylene.The reaction time was different at 150℃,and cooling with the furnace to room temperature.The precipitates were washed with ethanol and water thoroughly three times,drying at 80℃and calcining at 450℃for 1 h.According to the above preparation method,the samples prepared with Ce/Ti molar ratiosnCe/nTiof 0.05,0.1 and 0.2 in the reaction system were marked as 0.05CeO2/TiO2,0.1CeO2/TiO2,0.2CeO2/TiO2respectively.

1.2 Characterization

Under the conditions of Cu target,40 kV and 40 mA with CuKαX?ray radiation source(λ=0.154 nm)and 2θrange of 20°?80°,the samples were recorded by X?ray diffractometer of Dandong Haoyuan instrument company;the morphologies of the synthetic samples were used by scanning electron microscope(SEM,Zeiss Merlin field emission)at the acceleration voltage of 5 kV;the specific surface area was measured using the measurement instrument(ASAP2460).The U?3900 ultraviolet?visible spectrophotometer with integrating sphere in Japan was used to measure the absorbance of powder.X?ray photoelectron spectroscopy(XPS)mea?surements were measured on an Escalab 250 Xi spec?trometer.Photoluminescence(PL)spectra were mea?sured using FLS 980 fluorescence spectrophotometer.The photocurrent response and electrochemical imped?ance spectroscopy(EIS)were carried by an electro?chemical workstation(CHI660E).

1.3 Photocatalytic activity measurement

CeO2/TiO2was added to methyl orange(MO)solution,then the MO solution was illuminated.The photocatalytic performance of the sample was tested by measuring the degradation rate of MO.The specific processes were listed as follows：0.02 g of catalyst sample was added to 80 mL MO solution(10 mg·L-1),and ultrasonic agitation was performed for 30 min to achieve adsorption?desorption equilibrium in the dark.A 300 W xenon lamp was used to simulate and irradi?ate from the top of the MO solution.The xenon lamp was 10 cm away from the liquid surface.A small portion of the solution was taken every 10 min to be centrifuged and separated.The absorbance of the resid?ual MO was analyzed by an ultraviolet?visible spectro?photometer.

2 Results and discussion

2.1 Characterization of the samples

Fig.1 shows the XRD patterns of CeO2/TiO2heterojunction prepared by adding different amounts of CeO2.There were several different diffraction peaks of CeO2/TiO2heterojunction nanoflowers at 2θ=25.3°,37.9°,48.1°,54.1°,55.2°,62.6°,and 70.3°respective?ly,corresponding to anatase TiO2(PDF No.21?1272).The diffraction peaks with 2θ=28.6°,33.2°,56.6°,and 59.5°belong to the characteristic diffraction peaks of CeO2(PDF No.34 ?0394),indicating that the hetero?structure nanocomposite composed of TiO2and CeO2.It can be seen from the figure that the intensity of the diffraction peak of CeO2increased gradually with the increase of CeO2content.

Fig.1 XRD patterns of CeO2/TiO2

Fig.2 showed that the prepared CeO2/TiO2hetero?junction had a three?dimensional flower?like structure,and nano?CeO2particles adhered to the petals of TiO2.With the increase of CeO2content,the number of CeO2nanoparticles on the petals of TiO2increased gradually.

Fig.2 SEM and TEM images of(a,b)0.05CeO2/TiO2,(c,d)0.1CeO2/TiO2,and(e,f)0.2CeO2/TiO2

Solvothermal time can affect the morphology and properties of the samples.When the molar ratio of Ce and Ti was 0.1,and the samples were labeled as CeO2/TiO2?t,wheretmin was the reaction time.Fig.3 shows that the diffraction peaks correspond to the characteris?tic diffraction peaks of TiO2and CeO2respectively.

Fig.3 XRD patterns of(a)CeO2/TiO2?4,(b)CeO2/TiO2?6 and(c)CeO2/TiO2?12

Fig.4 shows the SEM images of CeO2/TiO2.It can be seen that under solvothermal conditions for 4 h,the CeO2/TiO2heterojunction was a three?dimensional flow?er?like microsphere structure.The diameter of the mi?crospheres was between 0.61 and 0.96 μm.The aver?age diameter was 0.77 μm.The flower structure was formed by the directional aggregation of nanoparticles.When the reaction time increased up to 6 h,the diame?ter of the flower?like microspheres ranged from 0.58 to 1.29 μm,with an average diameter of 0.59 μm.When the solvothermal time was 12 h,the diameter of the three?dimensional flower?like structure was 0.88?1.89 μm,with an average diameter of 1.36 μm.CeO2parti?cles were oriented and integrated into a shuttle shape embedded between thin plates.With the increase of solvothermal time,the diameter of flower?like TiO2became smaller at the beginning and larger at the next stage,and CeO2gradually aggregated from nanoparti?cles to shuttle shape.

Fig.4 SEM images of(a,b)CeO2/TiO2?4,(c,d)CeO2/TiO2?6 and(e,f)CeO2/TiO2?12

Fig.5 shows the N2adsorption ?desorption iso?therms and BJH(Barrette?Joyner?Halenda)pore size distribution curves of samples.The Brunauer?Emmett?Teller specific surface area(SBET),pore volume(VP),and average pore size of the samples are shown in Table 1.The results showed the prepared samples had highSBETand largeVP,providing more active sites and light?harvesting capacity,and improving the utilization efficiency of light,thereby contributing to the degrada?tion of organic pollutants.

Fig.5 (a)N2adsorption?desorption isotherms and(b)pore size distribution curves for CeO2/TiO2?t

Fig.6 shows the full spectrum of CeO2/TiO2?6 and the high?resolution XPS spectra of Ti2p,O1s,and Ce3d.It can be seen from Fig.6a that the sample only contained C,O,Ti,and Ce elements.C was mainly derived from the residual carbon of some organic pre?cursors during heat treatment and the oily carbon from the XPS instrument itself.The binding energies of 458.78 and 464.48 eV in Fig.6b correspond to the char?acteristic peaks of Ti2p2/3and Ti2p1/2orbits respective?ly,which are the standard bond energies of Ti2pin pure TiO2,indicating that Ti exists in form of Ti4+[31].In the O1sspectrum of Fig.6c,one peak at around 530.10 eV corresponds to the oxygen in the TiO2lattice,and the other peak at around 531.58 eV corresponds to the hydroxyl(—OH)on the surface of TiO2[32?33].In Fig.6d,V(881.52),V″(888.13),and V?(898.41)correspond to Ce3d5/2spin?orbital bands;U(900.11),U″(906.83),and U? (915.81)correspond to Ce3d3/2spin?orbital bands.The peaks labeled as V,V″,V?,U,U″,and U?are attributed to the existence of Ce4+.The peaks at V'(885.13)and U'(903.12)are attributed to the presence of Ce3+in the composite[34].Ce3+is mainly due to the strong interaction between TiO2and CeO2,which makes Ce4+reduced to Ce3+[35].

Because the intensity of light emission depends on the recombination ability of excited electrons and holes,we can analyze the ability of semiconductor materials to capture and migrate photogenerated holes and electrons.The low intensity of the PL spectrum indicates that the recombination rate of electron?hole pairs is low and the separation efficiency of electron?hole pairs represents reverse.Fig.7 shows the PL spec?tra excited at 350 nm.The PL intensity of CeO2/TiO2?12 was lower than that of CeO2/TiO2?6,indicating that CeO2/TiO2?12 presented high separation efficiency.

Fig.7 PL spectra of CeO2/TiO2?6 and CeO2/TiO2?12

2.2 Photocatalytic activity

To investigate the photocatalytic activity of the sample,the photocatalytic degradation of MO(xenon lamp simulated sunlight)was carried out.The degrada?tion rate of MO was calculated as follows：D=(1-A/A0)×100%,whereDis the degradation rate of MO solution;A0is the absorbance of MO solution before irradiation;Ais the absorbance of MO solution at the wavelength of 464 nm.The experimental results of photocatalysis under light were shown in Fig.8.

Fig.8 Photocatalytic degradation rate of MO for the samples

Fig.8a shows the curve of the photocatalytic degra?dation rate of MO under simulated sunlight for the sam?ples prepared with various molar ratios of CeO2and TiO2.Fig.8b shows the photocatalytic degradation rate curves of MO under simulated sunlight irradiation for the samples prepared under different solvothermal times when the molar ratio of CeO2to TiO2was 0.1(The material prepared without polyethylene glycol,cetyltrimethyl ammonium bromide,and carboxamide was recorded as CeO2/TiO2?B).It can be seen that the degradation rate of MO with catalyst increased with the extension of illumination time.The degradation rate of CeO2/TiO2was better than that of TiO2after 50 min illu?mination.The photocatalytic performance of flower?like CeO2/TiO2was higher than that of CeO2/TiO2?B.0.1CeO2/TiO2had the best photocatalytic performance under 50 min illumination and the photocatalytic activ?ity of CeO2/TiO2?6 was the best,and the degradation rate reached 95% after 50 min illumination(Fig.8b).The degradation rate of MO solution added with pure TiO2or CeO2was only 78% or 70% respectively after 50 min illumination,which indicated that the compos?ite of CeO2and TiO2enhances the photocatalytic activity of TiO2.

Fig.9 is the UV?Vis diffuse reflectance spectra of the samples.It can be seen that the absorption band edges of TiO2,CeO2,CeO2/TiO2?4,CeO2/TiO2?6,and CeO2/TiO2?12 were 393,432,463,481,and 469 nm respectively.According to the formulaEg=1 240/λg(λgis absorption edge),the bandgaps(Eg)of TiO2,CeO2,CeO2/TiO2?4,CeO2/TiO2?6,and CeO2/TiO2?12 were about 3.16,2.87,2.68,2.58,and 2.64 eV respectively,which indicates that CeO2/TiO2broadens the absorp?tion range compared with TiO2and CeO2.

Fig.9 UV?Vis diffuse reflectance spectra of the samples

Fig.10 shows the effects of reuse times of CeO2/TiO2?6 catalyst on photocatalytic activity.It can be seen that the degradation rates of MO by CeO2/TiO2?6 were 95%,94%，and 92% respectively when the cata?lyst was reused for the first time,the second time，and the third time.The catalytic activity was not significantly reduced,indicating that the photocatalyst has certain stability and can be recycled many times.

Fig.10 Effect of reuse degradation times of CeO2/TiO2?6 on the degradation rate of MO

In the process of photocatalysis,water molecules or hydroxyl radicals can be oxidized by holes to gener?ate hydroxyl radicals,and superoxide anion radicals may be generated when dissolved oxygen in water receives photogenerated electrons.Electron spin reso?nance(ESR)is generally used to detect hydroxyl radi?cal(·OH)radical and superoxide radical(·O2-).Fig.11 presents the ESR spectra of DMPO?·OH and DMPO?·O2-obtained with 5,5?dimethyl?1?pyrrolineN?oxide(DMPO)as the radical scavenger.Under xenon lamp irradiation,the ESR spectra of·OH showed four char?acteristic peaks,and that of·O2-showed six character?istic peaks.However,there was no signal in the dark.It indicates that·OH and ·O2-exist in the reaction sys?tem with CeO2/TiO2.

Fig.11 ESR spectra of(a)DMPO?·OH and(b)DMPO?·O2-for CeO2/TiO2?6 in the dark and under xenon lamp irradiation

The interface charge transfer and photogenerated charge recombination of the catalyst were investigated by electrochemical characterization.Fig.12a shows the photocurrent response of the catalyst under xenon lamp irradiation.It suggests that CeO2,TiO2,and CeO2/TiO2?6 all had obvious photocurrent responses.When the light source was turned off,the current signal returned to the original level,and the response current of CeO2/TiO2?6 was higher than that of pure CeO2or pure TiO2under the light.Generally,the stronger the separation ability of photo?generated carriers,the stronger the photocur?rent of the material.That shows the separation ability of CeO2/TiO2?6 photo?generated carriers was better than pure CeO2and TiO2,which is mainly due to the formation of heterojunction between CeO2and TiO2.EIS can further confirm the effective separation of pho?togenerated electrons and holes.The arc radius in EIS(Fig.12b)is related to the charge transfer resistance of the material.In general,the smaller the arc radius,the faster the separation or transfer speed of photogenerat?ed carriers,and the photocurrent intensity is also increased.It can be seen that CeO2/TiO2?6 had the smallest arc radius,which indicates that CeO2/TiO2?6 has the smallest electron transfer resistance and the best charge separation efficiency,which is consistent with the photocurrent response.

Fig.12 (a)Transient photocurrent responses of CeO2,TiO2,and CeO2/TiO2?6;(b)EIS spectra of CeO2,TiO2,and CeO2/TiO2?6

Fig.13 shows the photocatalysis mechanism of CeO2/TiO2.Under simulated sunlight,CeO2/TiO2can absorb not only ultraviolet light but also visible light.Both CeO2and TiO2can be excited by ultraviolet light,then the electrons jump to the conduction band to form the conduction band electron(e-)while leaving holes(H+)in the valence band.Because the conduction band(CB)of CeO2is higher than that of TiO2,the electrons in CB of CeO2transfer to CB of TiO2through the inter?face.On the other hand,the valence gap(VB)of CeO2is lower than that of TiO2,and the holes of VB of TiO2are transferred to VB of CeO2,which is prone to the separation of photogenerated electron?hole pairs[30].Under visible light irradiation,electrons from VB of CeO2are transferred to CB of TiO2,and photogenerated electrons in CB of CeO2can be transferred to CB of TiO2,thus inhibiting the recombination of photogenerat?ed electrons and hole[36].The results were consistent with the photocurrent response and EIS.Subsequently,the e-was reacted with the O2to form·O2-.The H2O could be oxidized by h+to produce·OH.The pollutant was oxidized by·O2-and·OH to produce CO2and H2O.Simultaneously,the h+in VB of CeO2was directly involved in the oxidation of pollutants.

Fig.13 Photocatalysis mechanism of CeO2/TiO2

3 Conclusions

The three?dimensional flower?like CeO2/TiO2heterojunction was prepared by the solvothermal meth?od.Compared with TiO2,flower?like CeO2/TiO2hetero?junction showed better photocatalytic performance un?der simulated sunlight.Among them,the degradation rate of MO reached 95% when CeO2/TiO2?6 was illumi?nated for 50 min,and the photocatalytic performance reached the best.The flower?like CeO2/TiO2heterojunc?tion had excellent catalytic performance,which is mainly due to the following factors.First of all,the three?dimensional hierarchical structure,with a large specific surface area and a different size of pore struc?ture,greatly improves the utilization of light.Secondly,the heterojunction effect can enhance the efficiency of charge separation and interface charge transfer greatly.