Facile synthesis and visible photocatalytic activity of single-crystal TiO2PbTiO3 heterostructured

PAPER

Zhaohui Ren, Gaorong Han et al.

Facile synthesis and visible photocatalytic activity of single-crystal TiO 2/Pb TiO 3 heterostructured nanofiber composites

CrystEngComm

PAPER

Cite this:CrystEngComm ,2015,17,1024

Received 11th September 2014,Accepted 17th October 2014DOI:10.1039/c4ce01864h https://www.sodocs.net/doc/e513924936.html,/crystengcomm

Facile synthesis and visible photocatalytic activity of single-crystal TiO 2/PbTiO 3heterostructured nanofiber composites

Yifeng Yu,a Zhaohui Ren,*a Ming Li,a Siyu Gong,a Simin Yin,a Shan Jiang,a Xiang Li,a Xiao Wei,ab Gang Xu,a Ge Shen a and Gaorong Han *a

Single-crystal TiO 2/PbTiO 3nanofiber composites were prepared by a simple hydrothermal synthesis and annealing treatment at 650°C,where pre-perovskite (PP)PbTiO 3(PTO)and tetrabutyl titanate (TBOT)were used as precursors.In the composites,anatase TiO 2nanorods grew on the surface of tetragonal perovskite (TP)PTO nanofibers and formed sharp TiO 2/PbTiO 3interfaces,leading to single-crystal heterostructures.The as-synthesized heterostructured nanofiber composites exhibited excellent photocatalytic activity for degradation of methylene blue (MB)under visible light irradiation (λ>400nm).Especially,the composites

TiO 2/PbTiO 3TBOT:0.4mL showed the highest photocatalytic activity,and the degradation rate of MB was 0.02392min ?1.Such photocatalytic activity of the heterostructured nanofiber composites was supposedly attributed to the large-scale formation of the sharp interfaces,which could be critical for the photogenerated charge carrier separation and its transfer from the PTO phase to TiO 2.It is suggested that the obtained hetero-structured nanofiber composites TiO 2/PbTiO 3can be exploited as an alternative visible-light-driven photocatalyst.

Introduction

Among many candidates for photocatalysts,titania has been extensively explored because of its excellent photocatalytic performance in organic pollutant degradation and water splitting to produce hydrogen.1–5However,the large band gap (3.0eV for rutile and 3.2eV for anatase)6–8of TiO 2signifi-cantly hinders its applications in the visible light region,which consequently limits the efficient utilization of solar energy.In order to extend applications of TiO 2to the visible light region,different approaches have been proposed and developed to tailor the crystal structure and band structures of TiO 2,including doping 9–11and hydrogenation.1,12In addi-tion,coupling of TiO 2with other semiconductors capable of being sensitized by visible light is often used,13,14where heterojunctions can promote charge separation through favor-able band alignments,possibly leading to the reduction of recombination losses.15Up to now,many efforts have been devoted to fabricating TiO 2-based heterostructured composites,such as CdS/TiO 2,16CdSe/TiO 2,17Cu 2O/TiO 2,18and Bi 2S 3/TiO 2,19which exhibited enhanced visible light https://www.sodocs.net/doc/e513924936.html,pared with these semiconductors (CdS,Cu 2O,etc.),the motivation

for using the perovskite oxides to couple with TiO 2derives from their high chemical stability 20and wide variety,and many oxides can be chosen to couple with TiO 2because of narrower band gaps compared to that of TiO 2.Recently,hierarchically core –shell heterostructured photocatalysts based on TiO 2and perovskite oxides,such as BiFeO 3/TiO 2,21BaTiO 3/TiO 2,22and PbTiO 3/TiO 2,15have been successfully synthesized.The composites offer a new and effective strategy for overcoming the barrier caused by the large band gap of TiO 2and significantly enhancing the visible light photocata-lytic efficiency.

Previous findings 15,23,24suggest that in the heterostructured composites of TiO 2and perovskite oxides,visible light is absorbed by the perovskite component,carriers generated in the perovskite and possibly separated by an internal field at the interface,and dye degradation occurred on the TiO 2shell.Internal fields in composites can arise from ferroelectric polarization phenomena,p –n junctions and so on.25There-fore,considering the photogenerated charge carrier separa-tion and its transfer from the perovskites to TiO 2,large-scale sharp interfaces with good crystallization between perovskite oxides and TiO 2are highly desirable to enhance photocata-lytic activity in the visible light region.

In this work,we report a facile preparation of single-crystal heterostructured nanofiber composites TiO 2/PbTiO 3consisting of the TP-PTO nanofiber and anatase TiO 2nano-rods using a hydrothermal approach and then annealing at 650°C,where PP-PTO and TBOT were used as precursors.The

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a

State Key Laboratory of Silicon Materials,Department of Materials Science and Engineering,Cyrus Tang Center for Sensor Materials and Application,Zhejiang University,Hangzhou,PR China.E-mail:renzh@https://www.sodocs.net/doc/e513924936.html,,hgr@https://www.sodocs.net/doc/e513924936.html,;Fax:+8657187952341;Tel:+8657187951649b

Electron Microscope Center of Zhejiang University,Hangzhou,310027,PR China

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This journal is ?The Royal Society of Chemistry 2015scenario for the heterostructured nanofiber composite preparation is described in Fig.1,and the preparation details are described in the Experimental section.To the best of our knowledge,the one-dimensional (1D)ferroelectric nanocomposites,consist-ing of monocrystalline component phases,have not yet been realized.The heterostructured composites TiO 2/PbTiO 3show greatly enhanced photocatalytic activity for degradation of MB,compared to PTO and P25,under visible light irradiation.

Experimental details

Synthesis

Single-crystal PP-PTO nanofibers 26were loaded with TiO 2nanorods using the hydrothermal method by the hydrolysis of TBOT as a Ti source.In detail,solutions were prepared by mixing different amounts of TBOT (0.3mL,0.4mL,0.5mL,and 0.7mL)with 25mL of ethanol and stirring for 30min.Next,1.0g of PP-PTO nanofibers were introduced into the obtained solutions and stirred for another 180min to get homogeneous suspensions.5.0mL of NH 3·H 2O was then added dropwise into the suspensions as a mineralizer with stirring for 20min,and 5.0mL of deionized water was added to promote the hydrolysis of TBOT while stirring for another 20min.Thereafter,the suspensions were transferred to 50mL Teflon-sealed autoclaves and maintained at 200°C for 12h under autogenerated pressures.After natural cooling to room temperature,the resulting composites were collected,rinsed with ethanol several times,and dried at 60°C for 12h.Finally,these composites were annealed in air to trans-form the single-crystal PP-PTO nanofibers into single-crystal TP-PTO nanofibers and crystallize the TiO 2nanorods on the nanofibers.The composites were heated rapidly to 650°C,annealed for 1h at 650°C,and then naturally cooled to room temperature.As a comparison,single-crystal TP-PTO nano-fibers were prepared using the same procedure but without the addition of TBOT.The diagrammatic sketch in Fig.1rep-resents the formation process of nanofiber composites.Characterization

The crystal structures of the final products were characterized by means of X-ray diffraction using a RIGAKU D-MAX-C with

Cu K α(λ=1.54056?)radiation;a step size of 0.02°and a scanning rate of 4°min ?1were used.SEM images were taken with a Hitachi field emission SEM MODEL S-4800.The struc-tures and morphologies of the heterostructured composite were analyzed by transmission electron microscopy (TEM)and high-resolution TEM (HRTEM)with a Tecnai G2F20using an accelerating voltage of 200kV.The TEM sample was prepared by dispersing the powder ultrasonically in ethanol and then distributing a small drop of the suspension onto

a

Fig.1Proposed scheme for fabrication of the nanofiber composites consisting of the TP-PTO nanofiber and TiO 2

nanorods.

Fig.2XRD patterns of (a)TiO 2/PbTiO 3TBOT :0.3mL (the molar ratio of TiO 2/PbTiO 3is 26.7%),(b)TiO 2/PbTiO 3TBOT :0.4mL (the molar ratio of TiO 2/PbTiO 3is 35.6%),(c)TiO 2/PbTiO 3TBOT :0.5mL (the molar ratio of TiO 2/PbTiO 3is 44.5%),(d)TiO 2/PbTiO 3TBOT :0.7mL (the molar ratio of TiO 2/PbTiO 3is 62.3%).The explanation after TiO 2/PbTiO 3represents the added amount of TBOT in the preparation of the

composite.

Fig.3SEM images of (a)TiO 2/PbTiO 3TBOT :0.3mL (the molar ratio of TiO 2/PbTiO 3is 26.7%),(b)TiO 2/PbTiO 3TBOT :0.4mL (the molar ratio of TiO 2/PbTiO 3is 35.6%),(c)TiO 2/PbTiO 3TBOT :0.5mL (the molar ratio of TiO 2/PbTiO 3is 44.5%),(d)TiO 2/PbTiO 3TBOT :0.7mL (the molar ratio of TiO 2/PbTiO 3is 62.3%).The explanation after TiO 2/PbTiO 3represents the added amount of TBOT in the preparation of the composite.

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copper microgrid.Diffuse reflectance spectroscopy (DRS)was employed to investigate the optical properties of samples using a UV-vis-NIR spectrophotometer (UV-3600,Shimadzu).Photocatalytic experiments

The rate of photodegradation of MB under visible light irradi-ation was used to evaluate the photocatalytic activity of the powders based on the absorption spectroscopic technique.In a typical process,0.1g of the photocatalyst and 100mL of the 1×10?5M MB aqueous solution were mixed in a 100mL quartz vessel.After reaching absorption/desorption equilib-rium between the photocatalyst and the dye in the dark for 40min,the vessel was exposed to visible light produced by a 400W Xenon lamp equipped with a UV cutoff filter (λ>400nm)under ambient conditions with stirring.After every given time interval,5mL of the photoreacted suspension was taken,centrifuged and analyzed using a UV-visible spectrophotometer (TU1901).The absorption peak at 664nm was recorded,and the peak intensity was related to the MB photodegradation.

Results and discussion

3.1.Structure and morphology of the nanofiber composites

Fig.2presents the XRD patterns of the single-crystal TiO 2/PbTiO 3heterostructured nanofiber composites prepared by adding different amounts of TBOT.As shown in Fig.2,all of the reflection peaks in these patterns can be readily indexed to perovskite TP-PTO (JPCDS 06-0452)and

anatase

Fig.4Microstructure analysis of the heterostructured nanofiber composites TiO 2/PbTiO 3TBOT :0.4mL (the molar ratio of TiO 2/PbTiO 3is 35.6%):a typical SEM image (a)and TEM image (b)of the PTO nanofiber with dispersed TiO 2nanorods on their surface,(c)HRTEM image of the heterostructured

interface.

Fig.5(a)Enlarged HRTEM image caught from the area marked with a purple rectangle in Fig.4c.Fast Fourier transform (FFT)pattern corresponding to area 1(b)and area 2(c)in (a).(d)FFT pattern of the heterostructured interface area in (a).

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This journal is ?The Royal Society of Chemistry 2015TiO 2(JCPDS 21-1272).It is revealed that the TP-PTO nanofibers are highly crystallized.No diffraction peaks corresponding to other impurities are detected,indicating that the TiO 2/PbTiO 3heterostructured nanofiber composites with high purity have been fabricated by our experimental scheme.

SEM images of the nanofiber composites with 26.7%,35.6%,44.5%and 62.3%molar ratio of TiO 2/PbTiO 3were prepared (Fig.3a –d).As shown in Fig.3a,the TiO 2nanorods grow on the surface of the PTO nanofibers sparsely when the molar ratio of TiO 2/PbTiO 3is 26.7%.As the molar ratio of TiO 2/PbTiO 3is increased from 26.7%to 35.6%,distribution of the nanorods on the surface of the nanofibers becomes dense and relatively uniform,although TiO 2aggregation could not be avoided (Fig.3b).With the increase of the molar ratio of TiO 2/PbTiO 3from 35.6%to 44.5%and 62.3%,nano-rod aggregation significantly develops and can be observed even on the surface of the nanofibers (Fig.3c –d).The PTO nanofibers have a diameter of 200–500nm and a length of a few to tens of micrometers,while the size of TiO 2nanorods is about 30nm in diameter and 50–100nm in length.

Fig.4a and b present typical SEM and TEM images of the heterostructured nanofiber composites TiO 2/PbTiO 3TBOT:0.4mL.As shown in Fig.4b,the single heterostructured nanofiber is composed of the PTO nanofiber (~300nm in diameter)and TiO 2nanorods (50–70nm in length).The HRTEM image in Fig.4c shows a clear and sharp interface with good crystallization between the PTO nanofiber and the TiO 2nanorods (the interface is marked with the white dot line).To get further insight into the microstructure of the heterostructured interface,an enlarged HRTEM image caught from the area marked with a purple rectangle in Fig.4c is shown in Fig.5a.The 0.353nm lattice fringe interval observed in Fig.5a agrees well with the (101)spacings of anatase TiO 2(JCPDS 21-1272),and the observed lattice spacings in the PTO nanofiber are 0.391nm and 0.284nm,which can be indexed

to the (100)and (011)crystal planes of perovskite TP-PTO (JPCDS 06-0452),respectively.As seen in Fig.5a,the differ-ence between the interplanar distances of the planes (101)in anatase TiO 2and (011)in perovskite PTO is large,and thus a disordered region with a width of 1–2nm within the hetero-structured interface is formed to reduce the growth barrier of the TiO 2nanorods on the surface of the PTO nanofiber.The fast Fourier transform (FFT)patterns (Fig.5b,c and d)corre-sponding to area 1,area 2and the heterostructured interface area in Fig.5a,respectively,show sharp diffraction spots,indicating that the TiO 2nanorods and the PTO nanofibers are well-developed single crystals in nature.3.2.Optical and photocatalytic performance of the nanofiber composites

The optical absorption spectrum of semiconductor catalysts is critical to acquire their band gap and determine their photocatalytic performance.Fig.6shows the absorption spectra of the samples transformed from the diffuse reflectance spectra (DRS)according to the Kubelka –Munk (K –M)method.27It is clear that there is almost no absorption for P25when the wavelength of light is beyond 400nm.In the case of

PTO

Fig.6Absorbance data converted from diffuse reflectance spectra (DRS)using the Kubelka –Munk function.(a)PTO,(b)TiO 2/PbTiO 3TBOT:0.3mL,(c)TiO 2/PbTiO 3TBOT:0.4mL,(d)TiO 2/PbTiO 3TBOT:0.5mL,(e)TiO 2/PbTiO 3TBOT:0.7mL,(f)

P25.Fig.7(a)Photodegradation of MB with the heterostructured nanofiber composites under visible light illumination;(b)absorption changes of MB solution during the photocatalytic process with the TiO 2/PbTiO 3TBOT:0.4mL under visible light illumination.

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nanofibers,the absorption edge is about 2.63eV and the value is lower than the previous results (2.75–3.60eV),28–31possibly due to a different morphology.32,33Such a band gap of PTO nanofibers suggests a possibility of utilizing more visible light for photocatalysis.For sample TiO 2/PbTiO 3TBOT:0.4mL,the absorption edge is near 445nm,corre-sponding to a band gap of 2.78eV and indicating that the composites have an absorption range extended to the visible-light spectrum as compared to that of P25.

The photocatalytic activity of the heterostructured nano-fiber composites for MB degradation is shown in Fig.7,and their reaction rates (including PTO and P25),K MB ,which is represented as the slope of ln ?C 0/C )vs.time,are also shown in Table 1.As an organic heterocyclic dye,MB is relatively stable under visible light if there is no catalyst added,and the degradation rate of MB is 0.00497min ?1,which is denoted as “Blank ”in Table 1.The PTO nanofibers demonstrate relatively high photocatalytic activity (0.00757min ?1)com-pared to the P25(0.00614min ?1),which is mainly due to their smaller band gap.In stark contrast,remarkable degradation is achieved in the nanofiber composites as a photocatalyst.Especially,the composite TiO 2/PbTiO 3TBOT:0.4mL exhibits the highest activity,0.02392min ?1.About 90%of MB decomposed after 100min irradiation.When the content of TiO 2exceeds 35.6mol%(TBOT:0.4mL),the photocatalytic activity of the nanofiber composites decreased owing to the excess amounts of TiO 2in the composites,possibly shielding the visible light absorption of PTO nanofibers from visible light irradiation.The composite TiO 2/PbTiO 3TBOT:0.4mL demonstrated higher photocatalytic activity compared to the heterostructured PbTiO 3/TiO 215core –shell particles,which is very likely due to its shape interface and monocrystalline component phases.It has been proposed that carriers generated by the visible-light-absorbing phase would be transferred to the TiO 2surface,via the interface,to participate in dye degrada-tion.The heterostructured nanofiber composites presented here have large-scale sharp crystalline interfaces,which are highly beneficial to the charge transfer and subsequent pho-tocatalytic activity.

Furthermore,large-scale sharp interfaces with good crys-tallization can promote charge separation and decrease the electron –hole recombination rate with the aid of internal electric fields.25For example,it has been discussed that the internal bipolar field normal to the heterostructured interface,which arises from the spontaneous ferroelectric polarization,could lead to band bending in the ferroelectric and,thus,separate photogenerated carriers,15,22,34–36such as the heterostructured BaTiO 3/TiO 2(ref.22)and PbTiO 3/TiO 2(ref.15)core –shell particles.In contrast,in our prepared

nanofiber composites,the polarization orientation of TP-PTO nanofibers is parallel to the interfaces.37Therefore,such polarization can hardly contribute to the separation of charges across the interface.On the other hand,Kim et al.38believed that the p –n diode formation in the CaFe 2O 4/PbBi 2Nb 1.9W 0.1O 9composite was the reason that led to the high visible-light catalytic performance.Considering that PbTiO 3is a p-type semiconductor 39–41and TiO 2is n-type,42,43the possible for-mation of p –n junctions between the PTO and the TiO 2phase could result in the formation of a space charge region and efficient separation of electron –hole pairs.The exact mecha-nism is not clear,and it will be explored in future work.

Conclusions

The single-crystal heterostructured nanofiber composite TiO 2/PbTiO 3with a novel configuration was designed and synthesized by a hydrothermal approach combined with annealing treatment at 650°C.Such heterostructured com-posites (especially TiO 2/PbTiO 3TBOT:0.4mL)demonstrate a high visible-light catalytic performance,which should be ascribed to its single-crystal component phases and shape interfaces,suggesting that this processing method could offer a possibility to acquire good crystallization and sharp inter-faces between perovskite oxides and titania.

Acknowledgements

This work was financially supported by the National Natural Science Foundation of China (no.51232006,51102212and 51102208)and the Fundamental Research Funds for the Central Universities (no.2014FZA4009).

Notes and references

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Table 1Reaction rates of the heterostructured nanofiber composites,P25and PTO for the photocatalytic degradation of MB (KMB)

Blank

P25PTO TiO 2/PbTiO 3TBOT :0.3mL TiO 2/PbTiO 3TBOT :0.4mL TiO 2/PbTiO 3TBOT :0.5mL TiO 2/PbTiO 3TBOT :0.7mL K MB

(min ?1)

0.00497

0.00614

0.00757

0.01216

0.02392

0.01052

0.00891

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