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CdSe nanocrystal-TiO2 nanowire

Enhanced Photochemical Response of TiO2/CdSe Heterostructured

Nanowires

Jung-Chul Lee,?Tae Geun Kim,?Heon-Jin Choi,§and Yun-Mo Sung*,?

Department of Materials Science&Engineering,Korea Uni V ersity,Seoul136-713,South Korea,

Department of Electronics Engineering,Korea Uni V ersity,Seoul136-713,South Korea,and School of

Ad V anced Materials Science&Engineering,Yonsei Uni V ersity,Seoul120-749,South Korea

Recei V ed June26,2007;Re V ised Manuscript Recei V ed September20,2007

ABSTRACT:High-density single-crystalline TiO2nanowires(~50nm diam)were successfully grown on Ti substrates by chemical vapor deposition at a low temperature of700°C and within a remarkably short time period of5min.They were combined with CdSe nanocrystals(~5nm diam)to form TiO2/CdSe heterotructured nanowires by overcoating the nanowires with the CdSe-containing solution and subsequent annealing at600°C.The TiO2/CdSe nanowires showed uniformly distributed CdSe nanocrystals, and high crystallinity of rutile and wurtzite from the TiO2and the CdSe,respectively.Owing to the heterostructure of the TiO2/ CdSe,they demonstrate almost full visible-range light absorption and thus enhanced photocatalytic activity by charge separation via electron and hole transfer between the CdSe and the TiO2.

Introduction

TiO2is one of the most important oxide semiconductors showing distinct photochemical activities due to its unique energy band gap characteristics.1,2Thus,for many years,it has been intensively exploited for the puri?cation of contaminated water and air3–6and for the generation of renewable energy.7–11 Recently,TiO2nanostructures,including nanoparticles,nano-tubes,and organic–inorganic nanohybrids,have attracted a great deal of interest due to the large surface-to-volume ratio that is bene?cial to most of the TiO2-based devices.As the surface absorbing the light and reacting with surrounding substances increases,the photochemical activity can be considerably enhanced.12,13However,the poor natural light absorption capability due to its intrinsically large energy band gap(~3.2 eV)has been signi?cantly limiting the broad applications of the TiO2nanostructure for photovoltaics and photocatalysts.To overcome this drawback,organic or inorganic dye-sensitized TiO2heterostructures have been developed,and the improve-ment of their natural light absorption capability has been reported.14,15Also,the dye-sensitized TiO2can show active photochemical reactions by charge separation.16The free electrons generated in the dye by the visible light excitation can be injected to the TiO2,and the holes from the TiO2can transfer to the dye,which can prevent the electron–hole recombination effectively and provide high photovoltaic and photocatalytic ef?ciency.Semiconductors,such as CdSe,CdS, and PbS,have energy band gaps corresponding to the energy of visible-range light and can serve as the inorganic dye for the TiO2.17–21In contrast to the aforementioned nanostructures,one-dimensional TiO2nanowires have been relatively less exploited probably due to the dif?culty in synthesis,although they also possess a very bright future in various applications.Furthermore, to date,there have been only scattered reports on the well-controlled vapor-phase synthesis of TiO2nanowires,22–24con-trary to the wet-chemical methods such as sol–gel process using anodic aluminum oxide(AAO)templates25–28and anodizing of titanium.29–32The vapor-phase growth is highly desirable not only for the synthesis of high-purity and high-crystallinity nanowires but also for the achievement of high-density nanowire arrays on the limited area of a substrate,which is crucial for their various electrical,optical,and chemical device applications. However,despite the presence of reports on the vapor-phase growth of TiO2nanowires,33,34there still exist drawbacks to be overcome for their wide-range applications,such as high-processing temperature(~1000°C),low growth density,and slow growth rate of the nanowires,all of which signi?cantly limit the broad applications of the TiO2nanowires to many devices.These critical problems could come most probably from the insuf?cient supply of Ti vapor source and the slow nucleation and growth kinetics of TiO2crystals,respectively. In this paper,we report a successful approach to obtain high-density arrays of single-crystalline TiO2nanowires at a low temperature for a short time period.Single-crystalline TiO2 nanowires were prepared on titanium substrates using the vapor–liquid–solid(VLS)mechanism by chemical vapor trans-port of TiCl4.

It has been well-known that due to the intrinsic energy band gap structure,CdSe-TiO2is a good combination to obtain high photoconversion and high photodecomposition ef?ciency.In the TiO2/CdSe heterostructure,prior to the electron–hole recombi-nation,photogenerated electrons can transfer from CdSe to TiO2, while the holes transfer from the TiO2to the CdSe.In this paper, we report the successful synthesis of TiO2/CdSe nanowires through combining the CdSe nanocrystals with the surface of TiO2nanowires grown on Ti substrates.The light absorption and photocatalytic characteristics of the heterostructure were investigated and compared with those of the bare TiO2 nanowires.

Experimental Procedures

For the TiO2nanowire growth,TiCl4was used as a Ti source gas, and sapphire,quartz,and titanium were used as substrates.Ti buffer layer(~250nm thick)and Au catalyst layer(~25nm thick)were subsequently deposited onto the substrates by radio frequency(RF)-magnetron sputtering.H2(10sccm)/TiCl4(Aldrich,99.9%)and O2(0.5 sccm)were used as reactive gases,and H2(20sccm)/Ar(100sccm) was used as a carrier gas.The substrate temperature was~700°C, and the time period for the growth of TiO2nanowires was5min.

Cadmium oxide(99.99%)and selenium shot(99.999%)(Aldrich Chemical,Milwaukee,WI)were used as precursors for the synthesis

*Corresponding author.Tel:+82-2-3298-3284.Fax:+82-2-928-3584.E-mail address:ymsung@korea.ac.kr.

?Department of Materials Science&Engineering,Korea University.

?Department of Electronics Engineering,Korea University.

§Yonsei University.

CRYSTAL GROWTH &DESIGN

2007 VOL.7,NO.12 2588–2593

10.1021/cg070588m CCC:$37.00 2007American Chemical Society

Published on Web11/15/2007

of

CdSe nanocrystals.Paraf?n oil and oleic acid(Aldrich Chemical,

Milwaukee,WI)were used as a solvent and a surfactant,respectively. Most details of the synthesis were similar to the CdSe nanocrystal preparation previously reported in the literature.11In this study,CdO was added into a mixture of paraf?n oil and oleic acid(45:5)to5and 1mM,respectively,in a three-neck?ask.The solution was heated to 160°C under Ar?ow and then distilled in vacuum to remove the remaining acetone.The Se metal was dissolved in paraf?n oil at2mM

Figure1.Field emission scanning electron microscopy(FESEM)images of TiO2nanostructures grown on(a)sapphire,(b)quartz,and(c)titanium and(d)an enlarged image of a portion of panel c.

Figure2.High-resolution transmission electron microscopy(HRTEM)image(a),selected area electron diffraction(SAED)patterns(b),and X-ray diffraction(XRD)patterns(c)of TiO2nanowires grown on titanium substrates at700°C.Here,A,R,and T denote anatase,rutile,and Ti,respectively. Enhanced Photochemical Response of TiO2/CdSe Nanowires Crystal Growth&Design,Vol.7,No.12,20072589

at 220°C,and 20mL of Cd solution was rapidly injected into the Se -paraf?n oil solution,which allowed fast nucleation and slow growth of the CdSe nanocrystals.

The TiO 2nanowires were overcoated by the solution bearing the CdSe nanocrystals in which the surface was passivated by oleic acid molecules at room temperature.Then,they were heat treated at 600°C for 30min in Ar gas for strong bonding of the nanocrystals to the nanowires through the sintering process and removal of remaining organic material.

The morphological features and crystallinity of the TiO 2/CdSe nanowires were investigated using ?eld-emission scanning electron microscopy (FESEM;Hitachi S4300,Tokyo,Japan),high-resolution transmission electron microscopy (HRTEM;JEOL JEM 4010,Tokyo,Japan),and X-ray diffraction (XRD;Rigaku Ultima-2000,Tokyo,Japan).The chemical composition was examined using energy-dispersive X-ray spectroscopy (EDS;Oxford,Inca,Oxon,UK)analyses.

The light absorption characteristics of the TiO 2and TiO 2/CdSe nanowires were investigated using the UV–visible spectrometer (JASCO UV–visible spectrophotometer:V530,Tokyo,Japan)with a full visible and UV range light source.Photocatalytic ef?ciency of the TiO 2and TiO 2/CdSe nanowires was evaluated using the degradation of a methylene blue solution.The solution containing the nanowires was irradiated by an ultraviolet and visible (UV–visible)light source (mercury -xenon lamp,Newport)having light irradiation range from ~200to 900nm for photocatalytic reactions.The decrease in the intensity of UV–visible light absorbance peaks at 664nm,corresponding to the decomposition of methylene blue,was monitored according to UV-light exposure time period.

Results and Discussion

Distinct TiO 2nanostructures were formed on the three different substrates by chemical vapor transport of TiCl 4gas at 700°C for 5to 30min.Figure 1shows FESEM images of the TiO 2nanostructures prepared on the (a)sapphire,(b)quartz,and (c,d)titanium substrates.Nanorods with a diameter of ~30–50nm and a length of ~1μm were formed together with nanodisks at the surface of the sapphire after 30min,and TiO 2nanoparticles with a size of ~300–400nm were formed at the surface of the quartz after 30min.On the other hand,straight nanowires with a diameter of ~30–70nm and a length of ~5–10μm were grown on the surface of the titanium after 5min.XRD and HRTEM analysis results presented in Figure 2show that the straight nanowire grown on the titanium has high crystallinity of rutile phase.Also,the existence of a trace of anatase phase was identi?ed in the XRD patterns probably due to the relatively low nanowire growth temperature of 700°C.The nanowires show a strong tendency to grow in the [110]direction,and they were defect-free and single-crystalline.

The possible mechanism for the enhanced TiO 2nanowire growth on the Ti substrates was considered as two aspects,as depicted in Figure 3.One is the increased Ti vapor pressure due to the additional vapor supplied from the Ti substrates.The

Figure 3.A suggested mechanism for the enhanced growth of the TiO 2nanowires on Ti substrates:(a)preheating of Au ?lm to break up into catalysts,(b)Ti vapor penetration into Au catalysts and TiO 2seed layer foramtion,(c)TiO 2nucleation at the interface between catalysts and TiO 2seed layer,and (d)TiO 2nanowire growth.

Figure 4.Field emission scanning electron microscopy (FESEM)images of TiO 2/CdSe nanowires.

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other,believed to be more effective than the ?rst one,is the TiO 2seed layer formation at the interfacial regions between Au catalysts and Ti substrates (b).The TiO 2seed layer can be crystalline rutile since the processing temperature was high enough (700°C),and this layer can enhance the formation

kinetics of rutile nuclei (c).The possible epitaxy in the rutile nucleation can lower the activation energy barrier for it and thus signi?cantly enhance its kinetics.The considerably lowered synthesis temperature and increased growth rate of the nanwores is the evidence of the lowered activation energy barrier for the nucleation of rutile TiO 2.Further,supplying Ti vapor to the Au catalysts causes the growth of the nuclei into the straight rutile nanowires (d).Due to the additional Ti vapor from the substrate,the growth rate of the nanowires can also be substantially increased.Overall nanowire growth kinetics is much enhanced using the titanium substrates.

Next,the TiO 2nanowires were overcoated with the prepared CdSe nanocrystals and heat treated.Figure 4shows a FESEM image of the heterostructured TiO 2/CdSe nanowires showing avery rough surface (see the insert)due to the attachment of the CdSe nanocrystals onto the TiO 2nanowires grown on the Ti substrates.Quantitative chemical analysis of the heterosturc-tured TiO 2/CdSe nanowires was performed using EDS,and the results indicated that the average content of Ti,O,Cd,and Se was 32.81,64.42,1.58,and 1.18atom %,respectively.

Detailed morphological features and crystallinity of the TiO 2/CdSe nanostructues were investigated using HRTEM and SAED analyses as shown in Figure 5.The HRTEM image of the TiO 2/CdSe nanostructure shows that the high-density CdSe nanoc-rystals are uniformly bonded to the surface of the TiO 2nanowire.

Figure 5.High-resolution transmission electron microscopy (HRTEM)image (a),its enlarged image (b),selected area electron diffraction (SAED)patterns (c),and X-ray diffraction (XRD)patterns (d)of TiO 2/CdSe nanowires.The white circles indicate the locations of the CdSe nanocrystals.Here,A,C,R,and T denote anatase,CdSe,rutile,and Ti,respectively.

Figure 6.UV–visible light absorption spectra of TiO 2and TiO 2/CdSe naowires.

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The oleic acid must have played a major role in uniformly dispersing the CdSe nanocrystals.This is critically important not only to maintain their particle size to obtain the quantum con?nement but also to secure the surface area of the nanoc-rystals for maximum light absorption.The HRTEM images show the strong bonding between the TiO 2nanowires and the CdSe nanocrystals via the sintering process during annealing,which can provide the excited electrons in the CdSe with a path to be injected into the TiO 2.Selected area electron diffraction (SAED)on the TiO 2/CdSe nanostructure shows the apparent ring diffraction patterns from the crystallinity of the wurtzite CdSe nanocrystals,as well as the spot diffraction patterns from the rutile TiO 2nanowires.Also,the crystallinity of the TiO 2/CdSe nanowires was investigated in Figure 5using XRD,and the rutile and wurtzite phases were identi?ed.Other secondary phases such as CdO that may form during the annealing step for the CdSe nanocrystal attachment were not identi?ed in the XRD patterns.Since the annealing was performed in Ar gas atmosphere at 600°C and the time period was 30min,oxidation or interdiffusion between the TiO 2and CdSe did not occur seriously.

Figure 6shows the comparison in the UV–visible light absorption spectroscopy results of the bare TiO 2and TiO 2/CdSe heterostructured nanowires.The TiO 2/CdSe nanowires show an apparent increase in the light absorbance of the visible light ranging from ~400to ~700nm.This much enhanced visible-range light absorption in the TiO 2/CdSe nanowires is attributed to the excitation of electrons in the CdSe.Since the size distribution of the CdSe nanocrystals attached to the surface of

the TiO 2nanowires was ~5–10nm,their energy band gap ranges from 2.48to 1.74eV according to the so-called,quantum con?nement effect,35–38by which electrons and holes are three-dimensionally con?ned in a nanocrystal quantum dot and its energy band gap increases with decrease in the size.Therefore,the corresponding light absorption wavelength ranges from 500to 714nm,and almost full visible-range light can be absorbed by the CdSe nanocrystals.The exited electrons in the nano-crystals can be effectively injected into the TiO 2due to the strong interfacial bonding between them as shown in Figure 5a.The bare TiO 2and the TiO 2/CdSe nanowires show light-absorption peaks at 335and 353nm,which correspond to energy band gaps of 3.70and 3.52eV,respectively.Both of the nanostructures show quantum con?nement effect,since the pure rutile possesses an energy band gap of 3.2eV.The slight red shift in the UV–visible light absorption of the TiO 2/CdSe nanowires can come from the leakage of the electron wave function of TiO 2into CdSe.39,40The TiO 2/CdSe nanowires show a second light absorption peak around 662nm,which corre-sponds to an energy band gap of 1.86eV and the average particle size of 8.2nm of the CdSe nanocrystals.

Photocatalytic ef?ciency of the TiO 2/CdSe nanowires was evaluated using the degradation of a methylene blue (MB)solution,and it was compared with that of the bare TiO 2.The MB solution containing the nanostructures was irradiated by ultraviolet (UV)light for photocatalytic reactions.The decrease in the intensity of visible light absorbance peaks at 664nm,corresponding to the decomposition of the MB,was monitored according to the UV-light exposure time period.Figure 7a shows the normalized variation in the absorbance of the MB solutions containing the bare TiO 2and TiO 2/CdSe nanowires.The TiO 2/CdSe nanowires almost completely decomposed the MB solu-tion within 80min,while the bare TiO 2nanowires decomposed it within 110min.It is certain that the visible-light absorption could contribute to the photodecomposition reaction in the heterostructured TiO 2/CdSe nanowires.

The possible mechanism for the enhanced photocatalytic activity even under visible-range light irradiation was considered using the energy band diagram for the heterostructured TiO 2/CdSe nanowires as also indicated in Figure 7b.The diagram explains how the loaded CdSe nanocrystals can act as a catalyst for both the reductive and oxidative reactions.The essential point is that the energy barrier height at the TiO 2/CdSe interface is changeable by the light illumination.When the light is il-luminated,photoexcited electrons in the conduction band of CdSe mostly enter the Fermi level of TiO 2,and they act as catalyst for a reductive reaction.Absorption of a unit of the visible-range light,associated with the formation of a conduction band free electron and a valence band hole,occurs in the CdSe nanocrystals during the visible-light irradiation,and the migra-tion of the photogenerated electron to the surface and the transfer to the TiO 2nanowire occurs subsequently.There is avery low concentration of holes with which to recombine,and therefore the electron has high opportunity to participate in the reduction reaction to form oxygen radicals,which are very strong oxidants and can decompose organic substances effectively.Also,the photogenerated holes in the CdSe nanocrystals theoretically migrate to the surface and participate in the oxidation reaction to form hydroxyl radicals,which are more effective than oxygen radicals to decompose organic substances.The TiO 2nanowire itself can show photodecomposition activity only under UV-light irradiation due to the high energy band gap.The holes created by UV light irradiation of the TiO 2can transfer to the

Figure https://www.sodocs.net/doc/af11692419.html,parison of the photocatalytic ef?ciency of bare TiO 2and TiO 2/CdSe nanowires (a)and schematic diagram showing the energy band structure and electron–hole separation in the nanowires (b).

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CdSe,and the opportunities for electron–hole recombination can be substantially reduced.

Conclusions

In this study,straight and high-density single-crystalline TiO2 nanowires were successfully grown on titanium substrates at a remarkably lowered temperature over a reduced time period. In addition to the TiCl4gas,the Ti vapor source from the titanium substrates can accelerate the TiO2nanowire growth. More possibly,the rutile layer formed by the oxidation of the titanium substrates could induce epitaxy for the nucleation of rutile nanowires.The lowered activation energy for nucleation by epitaxy can signi?cantly enhance the nucleation and growth kinetics of the TiO2nanowires.Heterostructured TiO2/CdSe nanostructures were achieved through overcoating the surface of the single-crystalline TiO2nanowires with CdSe nanocrystal-containing solution and subsequent annealing at600°C.The TiO2/CdSe nanowires show the absorption of UV–visible light ranging from353to614nm.The enhanced visible light absorption in the TiO2/CdSe is attributed to the charge separation between the CdSe and TiO2.Due to visible-light absorbing capability,the TiO2/CdSe nanowires show enhanced photocata-lytic ef?ciency compared with the bare TiO2nanowires.This TiO2/CdSe heterostructure also can be a strong candidate for dye-sensitized solar cells due to the high quantum yield from the charge separation under visible-light irradiation. Acknowledgment.This work was supported by the Korea Research Foundation(KRF)Grant by the Korean Government (MOEHRD;KRF-2006-311-D00568).Also,this work was supported through the Post-BK21program by the Korean Government(MOEHRD).

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