Enhanced Upconversion Emissions of TiO2:Yb3+/Tm3+ Nanocrystals:Comparison with Different Effects of Li+,Mn2+ and Cu2+ Ions①

JI He-Ming XU Ming-Gung ZHANG Hi-Yn LI Xio-Long QIAN Yn-Nn②

a (Guangdong Proνincial Key Laboratory of Functional Soft Condensed Matter,School of Materials and Energy,Guangdong Uniνersity of Technology,Guangzhou 510006,China)

b (School of Mechanical and Electrical Engineering,Yunnan Agricultural Uniνersity,Kunming 650201,China)

ABSTRACT Codoping with Mn+ ions (Mn+=Li+,Mn2+ and Cu2+) enhanced the blue and red upconversion (UC) emissions in TiO2:Yb3+/Tm3+ nanocrystals under 980 nm excitation.The different effects of Li+,Mn2+ and Cu2+ ions on the phase structures,morphologies and optical characteristics of TiO2:Yb3+/Tm3+ were discussed.The minor shifting in the diffraction peaks at 25.2° was observed for TiO2:Yb3+/Tm3+/Li+,and adding Mn2+ ions remained almost the same position of diffraction peaks,while the introduction of Cu2+ ions resulted in the shift of the diffraction peaks towards the larger angles.TiO2:Yb3+/Tm3+/Li+ and TiO2:Yb3+/Tm3+/Mn2+ nanosheets and the sphere-like TiO2:Yb3+/Tm3+/Cu2+ were observed.The mechanisms for increased UC emissions caused by adding Li+,Mn2+ and Cu2+ ions were attributed to the tailored local environment around Tm3+ ions,efficient energy transition between Mn2+-Yb3+ dimer and Tm3+ ions,and the localized surface plasmon resonance (LSPR) of Cu2+ ions,respectively.

Keywords:TiO2:Yb3+/Tm3+,Li+,Mn2+ and Cu2+,optical characteristics;DOI:10.14102/j.cnki.0254-5861.2011-3185

1 INTRODUCTION

The utilization of solar energy by dye-sensitized solar cells (DSSCs) and photocatalysts in dealing with environmental pollution has been explored for improving the energy consumption and filling the gap left by fossil fuels[1,2].However,owing to the limitation of the wide bandgap (～3.2 eV) of semiconductor TiO2,the DSSCs and photocatalysts absorb only a part of the solar spectrum,suppressing their efficiency[3].In order to enlarge the solar spectrum response,rare earth ions (RE3+) doped anatase TiO2has been widely applied in DSSCs and photocatalysts since TiO2,exhibiting the excellent electron mobility,lower dielectric constant,high chemical and photo stability,a high refractive index at visible wavelength and non-toxicity,successfully combines with the upconverting near infrared (NIR) sunlight into ultraviolet (UV) and visible emissions presented by RE3+ions[4-6].J.Z.Huang and X.J.Xu reported that TiO2:Yb3+/Er3+thin film showed an increased photocatalytic degradation of Rhodamine B[7].F.Trabelsi synthesized the anatase TiO2:Er3+/Yb3+nano-spherical particles to compensate the mismatch of the solar spectrum in NIR range and further enhance the efficiency of optoelectronic devices[8].It has been reported by P.Qu that the introduction of TiO2:Yb3+/Er3+spheres into the photoanodes of QDSCs increased the photoelectric efficiency by 30%[9].

Among many RE3+ions,Yb3+/Tm3+codoping system becomes an ideal candidate to obtain the efficient blue and red UC emissions under NIR laser excitation[10,11].This is because the Yb3+ions,which possess a larger absorption cross section of NIR,could efficiently transfer their energy to the Tm3+ions[12,13].However,the low efficiency of UC emissions still limits the practical applications in photoelectric devices.It is well known that the UC optical characteristics of RE3+ions are affected by the concentrations of RE3+ions,the local environmental around RE3+ions,the crystal surface chemical and the non-radiative energy transition (ET) between RE3+and codoping ions[14].Recently,it is a useful strategy for enhancing the intensities of UC emissions through adding metal ions (Li+,K+and Mg2+) and transition metal ions (Mn2+,Cu2+,Ag+and Au+)[15,16].

In this work,for the first time to our knowledge,the different mechanisms of Li+,Mn2+and Cu2+ions for improving the UC performance of TiO2:Yb3+/Tm3+are discussed.The crystal structure,the morphology and the optical characteristics of TiO2:Yb3+/Tm3+/Mn+nanocrystals are studied.

2 EXPERIMENTAL

2.1 Materials and methods

TiO2:Yb3+/Tm3+/Mn+(Mn+=Li+,Mn2+and Cu2+) nanocrystals were synthesized by the hydrothermal method.Firstly,2 mol% Yb3+,0.3 mol% Tm3+andxmol% Li+(orymol% Mn2+orzmol% Cu2+) were dissolved into the mixing solution containing 0.6 mL HF and 5 mL TTIP under stirring.The above mixture was transferred into a 100 mL autoclave and heated at 200 °C for 24 h.The white precipitates TiO2:Yb3+/Tm3+/Mn+were centrifuged and washed with deionized water and ethanol for three times,and dried at 80 °C.Here,TiO2:Yb3+/Tm3+/xmol% Li+(xmol%=0.3 and 1.0),TiO2:Yb3+/Tm3+/ymol% Mn2+(ymol%=0.2 and 1.0) and TiO2:Yb3+/Tm3+/zmol% Cu2+(zmol%=0.2,0.4 and 0.6) were respectively named as Li-x,Mn-yand Cu-z.Titanium (IV) isopropoxide (TTIP,99.99%),thulium trinitrate pentahydrate (Tm(NO3)3·5H2O,99.99%),ytterbium nitrate pentahydrate (Yb(NO3)3·5H2O,99.99%) and hydrofluoric acid (HF,40%) were purchased from Aladdin.All chemicals were used without further purification.

2.2 Characteristics

The powder X-ray diffraction (XRD) spectra were measured by using a powder diffractometer equipped with CuKαradiation source (40 kV,30 mA,λ=1.5406 ?,Bruker AXS D8-Advance,Germany).The morphologies of all samples were measured by field emission scanning electron microscopy (FESEM,Hitachi SU8010,Japan).The UC emission spectra were measured by a fluorescence spectrometer system (Zolix FV-CFR-A-1707,China) under 980 nm excitation.

3 RESULTS AND DISCUSSION

3.1 Structure and morphology

Fig.1 (a) shows the effect of Cu2+,Mn2+and Li+ions on the crystalline structure of TiO2:Yb3+/Tm3+nanocrystals,which is investigated by using XRD technique.Enlarged patterns of the diffraction peaks at 2θvalues ranging from 22° to 28° of Li-x,Mn-yand Cu-zare displayed in Fig.1 (b-d),respectively.As illustrated in Fig.1(a),the XRD patterns of all samples can be assigned to the anatase phase of TiO2((JCPDS no.21-1272).Obviously,the diffraction peaks at 2θvalues of 25.2°,36.9°,37.8°,38.6°,48.8°,53.9°,55.1°,62.7°,68.8°,70.4° and 75.2° are corresponding to (101),(103),(004),(112),(200),(105),(211),(204),(116),(220) and (215) reflection planes in turn.However,an impurity phase of YbF3(JCPDS No.49-1805) from its 2θreflection at 27.8° and 31.7° is observed in all samples.As shown in Fig.1 (b),compared with the standard JCPDS card of anatase TiO2,the diffraction peaks at 25.2° of Li-0.3 and Li-1.0shift slightly toward smaller angles.As for the introduction of Mn2+ions (shown in Fig.1 (c)),the diffraction peaks still remain at 25.2°.It can be seen from Fig.1 (d) that the (101) peaks of TiO2:Yb3+/Tm3+/Cu2+nanospheres exhibit drastic shift toward larger 2θangles with increasing the concentrations of Cu2+ions.The shifting in the diffraction peaks observed in TiO2:Yb3+/Tm3+/Li+and TiO2:Yb3+/Tm3+/Cu2+is dependence on the ionic radii of doping ions.The substitution of the Ti4+ions (CN=6,r=0.605 ?) by the larger Tm3+(CN=6,r=0.88 ?),Yb3+(CN=6,r=0.868 ?) and Li+(CN=6,r=0.76 ?) results in the expansion in the crystal lattice,leading to the shift of the diffraction peaks towards a lower 2θvalue[17].In contrast,the shift of diffraction peaks to the larger angles in Cu-zcaused by the lattice contraction is attributed to the fact that Cu2+ions with the smaller ionic radii (CN=4,r=0.57 ?) replace Ti4+ions.Consequently,doping Li+and Cu2+ions tailors the local environment of Tm3+ions,which would improve the UC optical properties of TiO2:Yb3+/Tm3+nanocrystals.

Fig.1.XRD patterns of TiO2:Yb3+/Tm3+ codoped with Li+,Mn2+ and Cu2+ ions.Fig.1 (b-d) the enlarged patterns of the diffraction peaks at 2θ values ranged from 20° to 30° of Li-x,Mn-y and Cu-z,respectively

The influences of codoping Li+,Mn2+and Cu2+ions on the morphologies of the synthesized TiO2:Yb3+/Tm3+are shown in Fig.2.Utilizing the HF as the capping agent,the uniform nanosheets are investigated in TiO2:Yb3+/Tm3+.Similarly,Li-xand Mn-yalso exhibit the nanosheets in shape,suggesting that Li+and Mn2+ions have little effects on the morphologies of TiO2:Yb3+/Tm3+.Contrastively,Cu2+ions play a key role in the change of the morphology of TiO2:Yb3+/Tm3+.Cu-znanocrystals are featured spheres-like in smaller size,which is consistent with the crystal lattice contraction of TiO2:Yb3+/Tm3+/Cu2+shown in Fig.1 (c).

Fig.2.SEM images of TiO2:Yb3+/Tm3+ codoped with Cu2+,Mn2+ and Li+ ions

3.2 Optical characteristics

Fig.3 (a-c) displays the UC emissions spectra of Li-x,Mn-yand Cu-znanocrystals under 980 nm excitation,respectively.As for all samples,a strong blue UC emission centered at 478 nm and two red UC emissions at 647 nm/695 nm are attributed to the3H4→3H6and1G4→3F4/3F3→3H6transitions of Tm3+ions,respectively[18,19].As shown in Fig.3 (a-b),the intensities of blue and red UC emissions are increased with the increasing concentrations of Li+and Mn2+ions in Li-xand Mn-ynanocrystals.Furthermore,a drastically enhanced red UC emission at 695 nm is observed in Mn-1.0 nanocrystal under 980 nm excitation.Different from codoping Li+and Mn2+ions,the intensities of blue and red UC emissions are increased with Cu2+ions of 0.2 mol%,whereas decreased at higher concentrations of Cu2+ions of 0.4 and 0.6 mol% (Shown in Fig.3 (c)).The effects of Li+,Mn2+and Cu2+ions on the luminescence properties of Tm3+are understood in the next section.

Fig.3.UC emissions of TiO2:Yb3+/Tm3+ codoped with dopant under 980 nm excitation (a) Li+ ions (b) Mn2+ ions (c) Cu2+ ions

The energy levels of Yb3+,Tm3+,Mn2+and Cu2+ions,as well as the UC mechanism under 980 nm excitation are shown in Fig.4.The1G4state of Tm3+ions is populated from Yb3+ionsνiaenergy transition (ET) processes of ET1:3H6(Tm3+)+2F5/2(Yb3+) →3H5(Tm3+)+2F7/2(Yb3+),ET2:3F4(Tm3+)+2F5/2(Yb3+) →3F3(Tm3+)+2F7/2(Yb3+) and ET3:3H4(Tm3+)+2F5/2(Yb3+) →1G4(Tm3+)+2F7/2(Yb3+)[20,21].Radiatively relaxing processes from the1G4state to the3H6and3F4states of Tm3+ions,respectively,yield the blue emission at 478 nm and red one at 647 nm.The Tm3+ions at the3F3state decay radiatively to the3H6ground state,producing red UC emission at 695 nm.On the basis of the above analytical results,the mechanism for increased blue and red UC emissions in Li-x(shown in Fig.3(a)) results from the fact that the local environment of Tm3+ions is tailored by adding metal Li+ions because the Li+ion,which is the smallest metallic ion with the smallest cationic radius,is benefit for its movement and localization in the host lattice.In the case of TiO2:Yb3+/Tm3+/Mn2+,it is proposed that the bidirectional energy transfer between Yb3+-Mn2+dimer and Tm3+ions contributes to the increase in blue and red UC emissions.The bidirectional energy transfer processes include the ET4 process (1G4(Tm3+)+|2F7/2,6A1g＞(Mn2+-Yb3+dimer) →3H6(Tm3+)+|2F7/2,4T1g＞(Mn2+-Yb3+dimer)) and back energy transfer (BET) process (3H6(Tm3+)+|2F7/2,4T1g＞(Mn2+-Yb3+dimer) →3F2(Tm3+)+|2F7/2,6A1g＞(Mn2+-Yb3+dimer))[22,23].Since the rate of energy transition is inversely proportional to the distance between two neighboring ions,the increasing concentrations of Mn2+ions shorten the distance between Tm3+and Mn2+ions,resulting in the fast ET and BET processes in Mn-1.0 nanocrystal.Consequently,the decreased blue emission at 476 nm and red one at 647 nm are obtained in Mn-1.0.Additionally,the increased populations of |2F7/2,4T1g＞of Mn2+-Yb3+dimer,which arose from the ground state absorption (GSA:|2F7/2,6A1g＞→ |2F5/2,6A1g＞) and excited state absorption (ESA:|2F5/2,6A1g＞→ |2F7/2,4T1g＞),lead to an efficient BET process to also populate the3F3state of Tm3+ions.Subsequently,the Tm3+ions at the3F3state partly decay radiatively to the ground3H6state,yielding the strongly increased red UC emission at 695 nm in Mn-1.0,and partly are promoted to the UC emitting1G4state via ET5 process.The ET5 process would compensate the reduced populations of1G4state due to the ET4 process.Consequently,the overall results are the increased blue (476 nm)/red (647 nm) UC emissions and a drastic enhancement of red emission at 695 nm,as illustrated in Fig.3(b).As for the TiO2:Yb3+/Tm3+/Cu2+nanocrystals (seen in Fig.3(c)),the enhanced intensities of blue and red UC emissions in Cu-0.2 are assigned to the modified local environment around Tm3+ions induced by the localized surface plasmon resonance (LSPR) of Cu2+ions.And the ET6 process from 3d104s0state of the Cu2+ions to the1G4state of Tm3+ions also contributes to populate the1G4state.

Fig.4.Energy levels of Yb3+,Tm3+,Mn2+ and Cu2+ ions,as well as the UC mechanism under 980 nm excitation

4 CONCLUSION

In summary,the different effects of Li+,Mn2+and Cu2+ions on the phase structure,morphology and optical characteristics of anatase TiO2:Yb3+/Tm3+are investigated.It has been shown that TiO2:Yb3+/Tm3+codoped with Li+and Mn2+ions display the nanosheets in shape,and TiO2:Yb3+/Tm3+/Cu2+exhibits the spheres-like in smaller size.The XRD diffraction peaks at 25.2° shift slightly toward lower angles in TiO2:Yb3+/Tm3+/Li+,and almost remain unchanged for TiO2:Yb3+/Tm3+/Mn2+,while shows a drastic shift to larger 2θangles with increasing the Cu2+ion concentrations.Li+,Mn2+and Cu2+ions in anatase TiO2:Yb3+/Tm3+present the dissimilar mechanisms for the enhancement of UC emissions.The increased blue and red UC emissions in TiO2:Yb3+/Tm3+/Li+nanosheets are due to the tailored local environment around Tm3+ions by adding Li+ions.In the case of TiO2:Yb3+/Tm3+/Mn2+nanosheets,the BET process of3H6(Tm3+)+|2F7/2,4T1g＞(Mn2+-Yb3+dimer) →3F2(Tm3+)+|2F7/2,6A1g＞(Mn2+-Yb3+dimer) is responsible for the drastically strong red UC emission at 695 nm.As for sphere-like TiO2:Yb3+/Tm3+/Cu2+,the synergistic effects of the LSPR of Cu2+ions and the energy transfer process from 3d104s0state of the Cu2+ions to the1G4state of Tm3+ions are responsible for the increased intensities of blue and red UC emissions under 980 nm excitation.