Hydrothermal Synthesis and Upconversion Properties of Nanocrystals

Delivered by Publishing Technology to: McMaster University IP: 130.113.111.210 On: Tue, 14 Jul 2015 07:28:54Copyright: American Scientific Publishers Table I.Lists mass of precursors Ca(NO 3 2,Yb(NO 3 3,Er(NO 3 3,F127,P123and NaBF 4,and reaction temperature,reaction time and pH of the solution.

Mass of Mass of Mass of Mass of Mass of V olume of Reaction

Samples Ca(NO 3 2(g)Er(NO 3 3(g)Yb(NO 3 3(g)NaBF 4(0.4M)(mL)surfactant (g)autoclave (mL)temperature ( C)pH S10.14800.00930.089810P1231.0000251807.0S20.14800.00930.089810F1271.0000251807.0S30.14800.00930.089810Sodium citrate 1.0000251807.0while F ?ions lie at the centers of the octants.22Due to the well-known good optical properties of the CaF 2host,lanthanide doped CaF 2crystals are widely used as laser media.In the last decade,investigations of the properties of co-doped (with lanthanide ions)crystals have shown energy transfers between the lanthanide ions that improve the laser ef?ciency.Recently,CaF 2doped with certain triply ionized lanthanide,namely Er 3+,Tm 3+,Pr 3+,etc.,has been widely studied for laser applications and opto-electronic devices.In lanthanide doped CaF 2nanocrystals,the substitution of a cation requires a negative ion for com-pensation,therefore,some interesting luminescence prop-erties can be expected.23

Herein,we introduce a series of lanthanide co-doped upconversion luminescence nanomaterials of Er 3+,Yb 3+co-doped CaF 2with controllable morphology via a facile hydrothermal https://www.wendangku.net/doc/c615554712.html,ing surfactants could simultane-ously control the crystallographic phase,size and optical emission properties.The reaction parameters were sum-marized in Table I.The results indicated that we have successfully syn-

thesized the monodispersive CaF 2:Er 3+/Yb 3+nanocrystals with different morphology using different surfactants.At the same time,we also found that all samples have sim-ilar luminescent upconversion properties under 980nm excitation.It is then expected that these monodispersive

CaF 2:Yb 3+/Er 3+nanocrystals can be employed as multi-

channel luminescent probes in biological applications.242.EXPERIMENTAL DETAILS 2.1.Preparation All starting materials were commercially available prod-ucts (produced by Aladdin-reagent):hydrated calcium nitrates (99.90%),hydrated erbium nitrates (99.99%)ytter-bium nitrates (99.99%),surfactant pluronic F127(F127)(99.99%),pluronic P123(P123)(Poly(ethylene glycol)-block-poly(propylene glycol)-block-poly(ethylene glycol))(99.99%),Sodium citrate (99.99%),and sodium ?uorobo-rate (99.90%).Water was distilled.There was no further puri?cation of any of the chemicals used in this study.In a typically procedure to prepare CaF 2:Er 3+/Yb 3+sample,0.1842g Ca(NO 3 2was ?rst dissolved in 8.00ml deion-ized water,followed by the addition of 0.0898g ytterbium nitrates and 0.0093g erbium nitrates to the above solution.After 10minutes of ultrasonication,1.0000g surfactant was dissolved in the above solution,followed by addi-tion of 10.00ml NaBF 4aqueous solution (0.4M).After 30min strong stirring,the pH of the solution was adjusted to 7.0.Subsequently,the ?nal solution was transferred into a te?on bottle held in a stainless steel autoclave,sealed,and keep in an oven at 180 C for 2h,8h,12h,18h,24h and 36h.After the autoclave was cooled down to ambient temperature naturally,the precipitates were sepa-rated by centrifugation,washed with deionized water and ethanol for several times,and vacuum dried at 60 C for 10hours.Additionally,surfactant and pH of the solution were selected according to the process as shown in Table I.2.2.Characterizations

The X-ray powder diffraction (XRD)patterns were carried out on a XD-3diffractometer at a scanning rate of 2 /min in 2 range from 10to 80 with graphite monochrom-atized Cu K radiation ( =1.540598?),tube voltage 40kV ,tube current 40mA.The size and morphology of the samples were inspected using a ?eld-emission scanning electron microscope FE-SEM (HITACHI S-4800).Photo-luminescence (PL)spectra were performed on an Edin-burgh Instruments FLS 920spectrophotometer equipped with a 980nm laser as the excitation source.All measure-ments were performed at room temperature.

3.RESULTS AND DISCUSSION

3.1.Structure and Morphology

The composition and phase purity of a series of sam-

ples prepared under various surfactant conditions were ?rst

examined by XRD,as shown in Figures 1–3.The CaF

2nanocrystals doped with 0.2%Er 3+,20%Yb 3+ions are isostructural with CaF

2crystal.All the diffraction peaks

of {111}{220}{311}planes correspond to the JCPDS ?le PDF #04-0864of pure cubic phase of CaF 2,which can be well indexed to the nanocrystals belong to the pure cubic phase.25The strong and sharp diffraction peaks of the sam-ples indicate that as-prepared nanocrystals are well crys-tallized.At the same time,the Er 3+ions doped in crystal lattice substituted the Ca 2+ions,leading to the increased intensity of the {200}diffraction peak because of the dif-ference in the atomic number between erbium and calcium element,especially at a high doping level.This result is consistent with that reported in the literature,26con?rm-ing the argument that the appearance of this {200}peak J.Nanosci.Nanotechnol.14,3380–3386,20143381

Delivered by Publishing Technology to: McMaster University IP: 130.113.111.210 On: Tue, 14 Jul 2015 07:28:54Copyright: American Scientific Publishers 10203040506070802 Theta (degree)PDF#04-0864(220)

36 h 24 h 18 h 12 h 8 h I n t e n s i t y (a .u .)(111)(200)(311) 2 h Figure 1.XRD patterns of CaF 2:Er 3+/Yb 3+nanocrystals prepared at 180 C using P123as the surfactant at different reaction time.The stan-dard diffraction peaks cards PDF #04-0864of pure cubic phase CaF 2is the black histogram at the bottom.in the XRD diagram of the lanthanide ions doped CaF 2is an indicator of the lanthanide ions’incorporation into the CaF 2lattice.As shown in Figure 1,it can be seen that the XRD peaks of samples has not changed obviously after 2h reaction time.Similarly,XRD pattern of CaF 2:Er 3+/Yb 3+nanocrystals prepared at 180 C using F127as surfactant at differ-ent reaction time was shown in Figure 2,which is in good agreement with the JCPDS ?le PDF #04-0864.Slight difference is that the diffraction peaks gradually become sharper and sharper with the reaction time increasing,indi-cating that the crystallization of samples was

getting bet-ter and better.This result also reveals that the lanthanide ions (Er 3+,Yb 3+ were incorporated into the CaF 2lattice to reform cubic shape nanocrystals with a mean size of

～3um.

1020304050607080

2 Theta (degree)

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(111)(200)(220)

(311)

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PDF#04-0864

Figure 2.XRD patterns of CaF 2:Er 3+/Yb 3+nanocrystals prepared at

180 C using F127as the surfactant at different reaction time.The stan-dard diffraction peaks cards PDF #04-0864of pure cubic phase CaF 2is the black histogram at the bottom.1020304050607080

2 Theta (degree)

2 h

PDF#04-0864

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8 h

I n

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(111)

(220)

(311)

(200)Figure 3.XRD patterns of CaF 2:Er 3+/Yb 3+nanocrystals prepared at 180 C using sodium citrate as the surfactant at different reaction time.The standard diffraction peaks cards PDF #04-0864of pure cubic phase CaF 2is the black histogram at the bottom.

It is noteworthy that the relative intensities of XRD pat-terns based on {200}peak for CaF 2:Er 3+/Yb 3+nanocrystals prepared at 180

C using sodium citrate as surfactant at different reaction time in Figure 3show a large differ-ence from those in Figures 1and 2.The XR

D pattern (Fig.3)shows that a slight difference of {200}plane peak appears,which is still well identi?ed as cubic PDF #04-0864except for that {200}plane peak intensity become not as strong as shown in Figures 1and 2.The reason can be assigned to the effect of surfactant.The samples are composed of spherical particles with a mean diame-ter of 700nm,as shown in Figure 6.This demonstrates that the surfactant plays an important role in the phase and morphology of CaF 2:Er 3+/Yb 3+nanocrystals.Also it Figure 4.SEM images of CaF 2:Er 3+/Yb 3+nanocrystals prepared at 180 C using P123as surfactant at different reaction time.(A)8h (B)12h (C)18h (D)36h.

3382J.Nanosci.Nanotechnol.14,3380–3386,2014

Delivered by Publishing Technology to: McMaster University IP: 130.113.111.210 On: Tue, 14 Jul 2015 07:28:54Copyright: American Scientific Publishers

Figure 5.SEM images of CaF 2:Er 3+/Yb 3+nanocrystals prepared at 180 C using F127as surfactant at different reaction time.(A)2h (B)8h (C)12h (D)18h.can be seen from Figure 3that the XRD peaks of samples remained unchanged obviously after 2h reaction time.The morphologies of the as-prepared products are illus-trated in Figures 4–6.The scanning electron microscopy images of the as-synthesized CaF 2:Er 3+/Yb 3+nanocrystals

using P123as surfactant at different reaction times were shown in Figure 4.Figure 4(a)is the

image of the sam-ple with reaction time of 8h,and it is showing cubic morphology with an average size of ～400nm.The inset showed the higher magni?cation SEM images,demonstrat-ing the presence of

irregular polyhedron with an average Figure 6.SEM images of CaF 2:Er 3+/Yb 3

+nanocrystals prepared at 180 C using sodium citrate as surfactant.(A)2h (B)8h (C)12h (D)18h,it can be seen from that the structure and size of samples almost no more changed obviously after 2h.size of ～3 m.Figure 4(b)is the image of the sam-

ple with reaction time of 12h,and it is showing relative

uniform and regular cubic morphology with an average

size of ～200nm,Figure 4(c)is the image of the sample

with reaction time of 18h,and it is showing more uni-

form and regular cubes with an average size of ～200nm,

Figure 4(d)is the image of the sample with reaction time

of 18h It clearly indicates that samples are composed of

monodisperse and regular cubic structures with uniform

size of ～200nm.

Figure 5displays the SEM images of the as-synthesized

CaF 2:Er 3+/Yb 3+nanocrystals using F127as surfactant at

different reaction times.Figure 5(a)is the image of the

sample with reaction time of 2h,and it is showing cubic

and sphere nanocrystals with a diameter of ～500nm.

A more careful examination of the magni?ed SEM image

shows clearly the mixture of cubic and irregular micro-

sphere with a mean size of ～500nm.At reaction time of 8h,the irregular microsphere disappears completely and

only the cubic structure exists as shown in Figure 5(b).The

corresponding morphology is fairly uniform cubic with a

mean size of 5 m,and the inset shows the nanocrystals under higher magni?cation.As shown in Figure 5(c),

CaF 2:Er 3+/Yb 3+nanocrystals become highly smooth and

regular cubic morphology at reaction time of 12h with a uniform size of 3 m.A careful examination indicates

that

their surfaces are extremely smooth without obvious

defects.From the above analysis,it can be concluded that,

with the increase of reaction time,the mean size of cubes

decreases and the morphology becomes highly uniform systematically.However,it can be seen that the cubic mor-phology and the uniform size of the samples almost did

not change obviously at reaction time of 18h as shown in

Figure 5(d).In addition,the experimental results show that

after 12h–36h the morphology and size always remain

stable with the increasing of reaction time.

Figure 6show the scanning electron microscopy images

of the as-synthesized CaF 2:Er 3+/Yb 3+nanocrystals using

sodium citrate as surfactant at different reaction times.

It clearly indicates that samples are composed of reg-

ular and monodispersive spheres with uniform size of

～700nm as shown in Figures 6(a)and (b).A more

careful examination of the magni?ed SEM images in

Figures 6(c)and (d)shows clearly that the micro-

spheres have smooth surfaces,remarkable uniformity and

monodispersity.It illustrated that if the reaction time is

adjusted (2h–36h),the as-prepared product almost no

longer changed,which is quite different from the samples

using P123/F127as the surfactant.

In summary,a controlled experiment was carried out in

the absence of surfactant and the other reaction param-

eters remained unchanged and using the same surfactant

at different reaction time.(this is very confusing,please

re-edit it).We can obtain various samples with different morphology and size.

J.Nanosci.Nanotechnol.14,3380–3386,20143383

Delivered by Publishing Technology to: McMaster University IP: 130.113.111.210 On: Tue, 14 Jul 2015 07:28:54Copyright: American Scientific Publishers Figure 7.Schematic energy level diagrams showing typical UC pro-cesses for Er 3+.The dashed-dotted,dotted,and full arrows represent excitation,multiphonon relaxation,and emission processes,respectively.

The excitation originates from either direct photo excitation or energy transfer.Since energy transfer can occur with the assistance of phonons,the energy differences between each key excited level and its key lower-lying level can be a little inconsistent.The 2S +1L J notations used to label

levels refer to spin (S ),orbital (L )and angular (J )momentum quantum

numbers respectively according to the Russel–Saunders notation.273.2.Luminescent Properties 3.2.1.Luminescent Mechanism As well know,the lanthanides are associated with the ?lling of the 4f -shell,so they are well-suited for upconversion applications because of the multiple metastable levels.Er 3+is a frequently used activator because of the typically feature of ladder-like arranged energy levels as shown in Figure 7.For Er 3+,the energy difference (～10350cm ?1 between the 4I 11/2and 4I 15/2levels is similar to that (～10370cm ?1 between the 4F 7/2and 4I 11/2levels.Thus,the energy levels of 4I 15/2,Figure 8.The proposed energy transfer mechanisms of the upconver-

sion processes in Er 3+and Yb 3+doped crystals under 980-nm diode laser excitation.The dashed-dotted,dashed,dotted,and full arrows represent photon excitation,energy transfer,multiphonon relaxation,and emission processes,respectively.Only visible and NIR emissions are shown here.5005506006507007502 h

8 h

12 h

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Figure 9.Upconversion luminescence spectra of cubic CaF 2:

Yb 3+(20%)/Er 3+(2%)nanocrystals prepared at 180 C using P123

as surfactant at different reaction time excited under 980nm after normalized.

4I 11/2,and 4F 7/2can be used to generate upconversion

emission using ～970nm excitation.Instead of being

directly excited to the 4F 7/2state,Er 3+ion in the 4I 11/2

state can relax to the 4I 13/2state,followed by excitation to the 4F 9/2state with phonon-assisted energy transfer.To enhance UC luminescence ef?ciency,a sensitizer with a suf?cient absorption cross-section in the NIR region is usually co-doped along with the activator to take advan-tage of the ef?cient ETU process between the sensitizer and activator.Trivalent Yb possesses an extremely sim-ple energy level scheme with only one excited 4f level of

2

F 5/2(Fig.8).The absorption band of Yb 3+that is located

around 980nm due to the 2

F 7/2→2F 5/2transition has a larger absorption crosssection than that of other lanthanide

ions.Additionally,the 2F 7/2→2F 5/2transition of Yb 3+

is well resonant with many f →f transitions of typical

500550600650700750

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Figure 10.Upconversion luminescence spectra of cubic CaF 2:

Yb 3+(20%)/Er 3+(2%)nanocrystals prepared at 180 C using F127as sur-

factant at different reaction time excited under 980nm after normalized.3384J.Nanosci.Nanotechnol.14,3380–3386,2014

Delivered by Publishing Technology to: McMaster University IP: 130.113.111.210 On: Tue, 14 Jul 2015 07:28:54Copyright: American Scientific Publishers 5005506006507007502 h 8 h 12 h 18 h 24 h I n t

e

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Wavelength (nm)36 h Figure 11.Upconversion luminescence spectra of cubic CaF 2:

Yb 3+(20%)/Er 3+(2%)nanocrystals prepared at 180 C using sodium cit-rate as surfactant at different reaction time excited under 980nm after normalized.

upconverting lanthanide ions (Er 3+,Tm 3+,and Ho 3+),thus facilitating ef?cient energy transfer from Yb 3+particularly suitable for use as a UC sensitizer.The sensitizer content is normally kept high (20mol%)in doubly or triply doped nanocrystals,while the activator content is relatively low (2mol%),minimizing cross relaxation energy loss.3.2.2.Luminescent Spectra The upconversion luminescence spectra of the as-prepared CaF 2:Er 3+/Yb 3+nanocrystals prepared at 180 C using P123,F127and sodium citrate as surfactant at different reaction time under 980nm excitation after normalized was shown in Figures 9–11.These samples have similar spectra pro?le corresponding to the same wavelength posi-tions except for different emission intensities.The obvious green emission peaks observed at 524and 546nm are assigned to the (2H 11/2,4S 3/2 –4I 15/2transitions of the Er 3+ions,while the observed red emission peak centered at 660nm is attributed to the Er 3+4F 9/2–4I 15/2transition.4.CONCLUSION A series of monodispersed,highly uniform CaF 2:Er 3+/Yb 3+nanocrystals have been prepared via a facile hydrothermal method.These as-prepared nano-crystals not only can have controlled morphology and sizes at 180 C using P123,F127and sodium citrate as various surfactants at different reaction time,but also have strong emission intensities of upconversion luminescence.The phase,morphology,mean size and upconversion luminescence properties of various samples were com-pared with different reaction parameters.The different surfactant in?uences the growth process and results in various morphology of CaF 2:Er 3+/Yb 3+nanocrystals.At the same time,the possible mechanisms of upconversion luminescence are analyzed by diagrams of the proposed energy transfer mechanisms showing the upconversion

processes in Er 3+and Yb 3+doped crystals under 980-nm

diode laser excitation and the schematic energy level

diagrams showing typical upconversion processes for

Er 3+.The results also reveal that the as-synthesized

CaF 2:Er 3+/Yb 3+nanomaterials may be in the cubic struc-

ture with space group of Fm3m,while Ln 3+cations occupy

crystal lattice positions with lower point symmetry,which

may lead to high upconversion ef?ciency under the exci-

tation of a 980nm diode laser.In addition,the results

also indicated that CaF

2is a promising host material for producing desirable upconversion luminescence.

Acknowledgment:The authors are grateful to the ?nan-

cial aid from the National Natural Science Foundation of

China (Grant No.20961006),Inner Mongolia Natural Sci-

ence Foundation (Grant No.20080404MS0201),and Inner Mongolia Technology Innovation and Guidance Funds.References and Notes

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Received:1October 2012.Accepted:24December 2012.

3386J.Nanosci.Nanotechnol.14,3380–3386,2014