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Cite this: NewJ.Chem., 2014,

38, 2629

Hydrothermally derived NaLuF4:Yb3+, Ln3+

(Ln3+ = Er3+, Tm3+ and Ho3+) microstructures withcontrollable synthesis, morphology evolution andmulticolor luminescence properties†

Yu Gao,*a Qian Zhao,b Zhenhe Xub and Yaguang Sun*b

A series of NaLuF4:Yb3+, Ln3+ (Ln3+ = Er3+, Tm3+, and Ho3+) microstructures with diverse morphologies

and sizes were successfully prepared via a designed hydrothermal route with the assistance of trisodium

citrate (Na3Cit). The phases, morphologies, sizes and luminescent properties of the as-prepared products

were fully characterized by means of X-ray diffraction (XRD), field scanning electron microscopy (FESEM),

transmission electron microscopy (TEM), energy dispersive X-ray spectroscopy (EDX) and up-conversion

(UC) photoluminescence spectroscopy, respectively. It can be found that the as-prepared microcrystals

can be rationally modified in phase, size and morphology by tuning the pH value, Na3Cit content, and

reaction time. Based on the experimental results, the possible formation mechanism of the crystal growth

process was proposed. Moreover, a systematic study of the up-conversion (UC) luminescent properties

of NaLuF4 has also been performed through doping different rare earth ions (Yb3+/Er3+, Yb3+/Tm3+ and

Yb3+/Ho3+). It is found that under 980 nm NIR excitation, the emission intensity and the corresponding

luminescent colors of Yb3+/Er3+, Yb3+/Tm3+, and Yb3+/Ho3+ co-doped NaLuF4 can be precisely

modulated by changing the Yb3+ doping concentration. These merits of multicolor emissions in the

visible region endow this kind of material with potential applications in the fields of light display systems,

lasers, and optoelectronic devices.

1. Introduction

The preparation of inorganic nano/microcrystals with control-lable size and morphology has been extensively pursued, notonly because of the fundamental scientific interest, but alsobecause of their technological applications.1 During the pastfew decades, much effort has been exerted to develop differentmethods for the design and fabrication of a large range ofnano/microstructures, such as the molten salt method,2,3

template-directed synthesis,4 the sol–gel process,5 the thermo-lysis approach,6 and the hydrothermal method.7 Among thesemethods, the hydrothermal method has been widely applied tofabricate different structured materials with high crystallinityand wide dimensions. As a typical solution-based approach,this method has been demonstrated to be an environmentallyacceptable process with relatively high yield of desired products.

Furthermore, it has been widely recognized that the crystalgrowth process is not only determined by its intrinsic structurebut also significantly influenced by a series of external para-meters, such as the pH value, Na3Cit content, and reaction time.In particular, the addition of a special chelating agent, which hasa significant effect on the kinetics of the crystal growth, has beendemonstrated to be an effective strategy to achieve control overthe morphology of final products.8,9 Because of their highthermal stability and ability to form complexes with other metalions, the citrate ions (Cit3�) play a dual role as a ligand and ashape modifier by adjusting the growth rate of different facetsunder hydrothermal conditions, resulting in the formation ofvarious geometries of the final products.10–12

Recently, rare earth ion (Ln3+) doped luminescent materialshave been extensively studied due to their unique opticalcharacteristics and stabilities.13–20 Particularly, due to theirhigh refractive indices and low phonon energies, NaREF 4 (RE =rare earth) phosphors have received more and more attentionbecause of their enormous potential applications in diversefields, such as solid-state lasers,21,22 3D flat-panel displays,23,24

biological detection,25,26 low-intensity IR imaging,27 sensing,28

and so forth. Among them, NaLuF4 is expected to be an excellenthost matrix for up-conversion luminescence, due to its good

a College of Materials Science and Engineering, Shenyang University of

Chemical Technology, Shenyang, 110142, P.R. Chinab College of Applied Chemistry, Shenyang University of Chemical Technology,

Shenyang, 100142, P.R. China. E-mail: gaoy777@126.com

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c4nj00058g

Received (in Montpellier, France)12th January 2014,Accepted 17th March 2014

DOI: 10.1039/c4nj00058g

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chemical durability, thermal stability, and low phonon energy.The rare earth ion Ln3+-doped NaLuF4 materials have beenproven to be important up-conversion phosphors. Moreover,compared with Y3+, the Lu3+ may be more favorable for trivalentlanthanide (Ln3+) dopant emission due to the intensity-borrowing mechanism mixing the 4f and 5d orbitals of theLn3+ ions via the lattice valence band level.29,30 It was also found thatthe Lu3+ ion doping can remarkably tune the luminescent properties(e.g. brightness and long afterglow performance) through influen-cing the emitters’ electronic population lifetime.31,32 Therefore,it is expected that the NaLuF4 can exhibit some interesting andunexpected optical properties when substituted by variouslanthanide ions.

Herein, we proposed a facile process for the synthesis ofuniform b-NaLuF4:Yb3+, Ln3+ (Ln3+ = Er3+, Tm3+ and Ho3+)microstructures, using Na3Cit as the chelating agent. Thephase, morphology, and sizes of the samples have been wellcharacterized by various analysis techniques. In addition, theup-conversion properties of the as-prepared b-NaLuF4:Yb3+,Ln3+ (Ln3+ = Er3+, Tm3+ and Ho3+) samples are investigated,which display UC emissions and can serve as efficient hosts forthe UC luminescent materials. This approach not only proposesan economical, environmentally friendly route for the develop-ment of multicolor light, but also provides an effective wayto gain any desired colors in a broad range, which will alsoprovide evidence for tuning the UC emission colors in manyother fluorides.

2. Results and discussion2.1. Phase identification, morphology and composition

In this work, Yb3+, Ln3+ (Ln3+ = Er3+, Tm3+ and Ho3+) ionsco-doped NaLuF4 samples with different phases and morpho-logies, and up-conversion photoluminescence were preparedthrough a facile and mild hydrothermal route. The compositionand phase purity of the studied samples were first investigatedby XRD. Fig. 1 shows the representative XRD patterns of theYb3+ and Ln3+ (Ln3+ = Er3+, Tm3+ and Ho3+) co-doped samplesobtained at 180 1C for 24 h with 0.85 g of Na3Cit, respectively.Vertical bars show the standard hexagonal b-NaLuF4 (JCPDSNo. 27-0726) for comparison. As shown, the diffraction peaks ofall these samples can be assigned to a pure hexagonal NaLuF4

phase, which coincides well with the literature values (JCPDSNo. 27-0726). It should be noticed that no peaks from otherphases or the doping are observed, indicating that the highpurity and Yb3+, Ln3+ (Ln3+ = Er3+, Tm3+ and Ho3+) ions havebeen effectively incorporated into the b-NaLuF4 host by sub-stituting Lu3+ sites. The well-resolved diffraction peaks indicatethe high crystallinity of the products. High crystallinity canbe obtained at a relatively low hydrothermal treatment tem-perature (180 1C). This is important for phosphors becausehigher crystallinity generally means less defects and strongerluminescence.

Fig. 2A–D show the SEM, TEM and HRTEM images of theb-NaLuF4 sample, which was prepared using 0.85 g of Na3Cit at

180 1C for 24 h. From the SEM image of NaLuF4 in Fig. 2A,it can be seen that the as-prepared NaLuF4 sample consists of alarge scale of uniform and well-dispersed hexagonal microrods.The magnified SEM image in Fig. 2B shows a close-up view ofone isolated microrod, which clearly displays the prismaticstructure of the microrods with a length of about 8 mm andan average diameter of 3 mm. To provide further insight into theNaLuF4 microstructures, TEM investigation was also performed.The TEM image in Fig. 2C exhibits the obvious hexagonalmicrostructure with dentate edges on both ends of the microrodsand no chromatic aberration can be detected on the shadow partof the image, indicating the solid structure of the microstructure.As disclosed by the corresponding HRTEM image (Fig. 2D), theinterplanar distance between the adjacent lattice fringes isdetermined to be 0.299 nm, which corresponds to the (110)plane of the b-NaLuF4 phase. The elemental compositions of

Fig. 1 XRD patterns of b-NaLuF4:Yb3+, Ln3+ (Ln3+ = Er3+, Tm3+, Ho3+)samples prepared at 180 1C for 24 h using 0.85 g of Na3Cit at a pH valueof 10, and the standard data for hexagonal NaLuF4 (JCPDS 27-0726) as areference.

Fig. 2 (A) Low magnification and (B) high magnification SEM images,(C) TEM image, and (D) HRTEM image of the b-NaLuF4 samples.

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b-NaLuF4:Yb3+, Er3+ (Fig. S1A, ESI†) and b-NaLuF4:Yb3+, Tm3+

(Fig. S1B, ESI†) samples are characterized by EDX, which revealsthe presence of the corresponding elements in the samples.

2.2. Influential factors and the possible formationmechanism

In previous reports, the effects of different reaction parametersin a hydrothermal system on the size and morphology of otherproducts have been discussed.33–35 In the present work, theinfluence of some reaction conditions including the pH valueof the initial solution, Na3Cit content and reaction time on themorphologies and dimensions of the products has been inves-tigated in detail. Moreover, the possible formation mechanismfor NaLuF4 samples is presented.

2.2.1. Effect of the pH value of the initial solution. Fig. 3Ashows the XRD patterns of the products prepared at 180 1Cfor 24 h using 0.85 g of Na3Cit at pH values of 2, 5, and 10,respectively. For the sample prepared at a pH value of 2, thediffraction peaks display the presence of cubic (a-) phaseNaLuF4 (JCPDS No. 27-0725), while pure hexagonal (b-) phaseNaLuF4 indexed to the standard data (JCPDS No. 27-0726) canbe obtained at the pH values of 5 and 10. It is worth noting thatthe XRD patterns also indicate that there are some differencesin the relative intensities based on (100), (101), (201) and (211)for the samples, indicating the possibility of different prefer-ential growth orientations under different pH conditions.The SEM images of the NaLuF4 crystals prepared at differentpH values are also given in Fig. 3B–D. For the cubic NaLuF4

sample prepared at a pH value of 2 (Fig. 3B), the self-assembled,hierarchical spheroid structure with different sizes is dis-covered. Nevertheless, if the pH value of the initial solution isincreased to 5, not only the phase of the NaLuF4 productchanged from cubic to hexagonal, but the morphology of theproduct transforms remarkably. As shown in Fig. 3C, manymonodisperse hexagonal microtubes with thick walls can beobserved. The average length and diameter of these micro-tubes are calculated to be about 8 mm and 2 mm, respectively.

When the pH value of the initial solution is adjusted to 10,a similar structure to that prepared at pH 5 can be observed,but the product present hexagonal prismatic microrods withrelatively perfect uniformity and monodispersity (Fig. 3D).On the basis of the above analysis, it can be concluded thatthe conversion from the cubic (a-) phase NaLuF4 to the hexa-gonal (b-) phase NaLuF4 directly leads to a dramatic change inthe morphology of the products from microspheres to hexa-gonal prismatic microrods. On the other hand, this experimenthas proved that the inherent crystal structure of the seeds plays asignificant role in the formation of nano- and microstructures.The a-NaLuF4 seeds have isotropic unit cell structures, whichgenerally induce the isotropic growth of particles, and thereforespherical particles are obtained. In contrast, b-NaLuF4 seedshave anisotropic unit cell structures, which can induce aniso-tropic growth along crystallographically reactive directions,contributing to the formation of hexagonal structures. Further-more, facets of spherical a-NaLuF4 particles tend to form on theparticles’ surface to increase the portion of the low-index planesto minimize the total surface energy due to the surfacescontaining high-index crystallographic planes with a high sur-face energy. Consequently, the b-NaLuF4 hexagonal shape withstable {10%10} and {0001} facets appears. Thus, it is logical thatthe larger pH value of reaction solution is more suitable forobtaining the desirable crystalline pure products of b-NaLuF4.The crystallographic structures of these phases have been deter-mined by Thoma and co-workers.36 In the case of a-NaREF 4, Na+

and RE3+ cations are randomly distributed in the cationic sub-lattice, whereas for b-NaREF 4 microtubes, there are three types ofcation sites: a one-fold site occupied by RE3+ (1a), a one-fold siteoccupied randomly by 1/2Na+ and 1/2RE3+ (1f), and a two-fold siteoccupied randomly by Na+ (2h). Thus, for NaLuF4, the transfor-mation from the a-phase to the b-phase is of a disorder-to-ordercharacter with respect to cations.37

2.2.2. Effect of sodium citrate. As one of the most efficientchelating reagents, Na3Cit has been reported to regulate themorphology and size of the samples in the hydrothermalsystem.38 In the present system, Na3Cit also plays an importantrole in the growth process of hexagonal b-NaLuF4 crystals withdifferent morphologies and dimensions. Fig. 4 shows the XRDpatterns and SEM images of the products prepared usingdifferent contents of Na3Cit (0, 0.3, 0.7, 0.85 and 0.9 g) with apH value of 10 at the same hydrothermal temperature (180 1C)for 24 h. The samples display distinctively different XRDpatterns in Fig. 4A associated with the distinct morphologicalvariation with different contents of Na3Cit in Fig. 4B–D. Whenthe reaction was carried out without the use of Na3Cit, a uniqueXRD pattern due to the orthorhombic LuF3 phase (JCPDSNo. 32-0612) and the corresponding irregularly cobblestone-like shaped nanoparticles with an average size of about 100 nmwere obtained as shown in Fig. 4B. However, once a smallquantity of Na3Cit (Na3Cit = 0.3 g) was introduced into thereaction system, the orthorhombic LuF3 phase was replaced bythe cubic (a-) NaLuF4 phase (JCPDS No. 27-0725) and themorphology of the product evolves into a microsphere shapewith different sizes (Fig. 4C). By further addition of 0.7 g,

Fig. 3 XRD patterns (A) and SEM images of the samples obtained at 180 1Cfor 24 h using 0.85 g of Na3Cit and different pH values (B: 2; C: 5; D: 10).

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0.85 g and 0.9 g of sodium citrate, the cubic (a-) NaLuF4 phasetransforms completely into the hexagonal (b-) NaLuF4 phase(JCPDS No. 27-0726). As shown in Fig. 4D, the morphology ofthe product evolves into uniform and well-dispersed hexagonalprismatic microtubes with a length of 20 mm and a diameter of3 mm with 0.7 g of Na3Cit. As its content is increased to 0.85 g,the microrods with hexagonal prismatic morphology display ashorter length of about 8 mm and a similar diameter comparedwith that prepared using 0.7 g of Na3Cit (Fig. 4E). When thecontent of Na3Cit is increased to 0.9 g, the product is composedof hexagonal prismatic microrods, microplates with diversesizes and some irregular aggregation (Fig. 4F). The effect ofCit3� ions on the crystal phase can be discussed as follows.When there is no Na3Cit (or Na3Cit = 0 g) existing in the initialsolution, NaBF4 in aqueous solution is slowly hydrolyzed toproduce BO3

3� and F� anions, as shown in eqn (1), which hasbeen confirmed by other groups.39,40 Thereafter, the Lu3+ in thesolution directly reacts with F� and then it promotes theformation of LuF3 (eqn (2)).

BF4� + H2O 2 3HF + F� + BO3

3� (1)

Lu3+ + 3F� - LuF3 (2)

When a small amount of Na3Cit was added into the initialsolution, Na3Cit can combine with some of the Lu3+ ions toform the Lu3+–Cit3� complex, resulting in the slow release ofLu3+ in the solution. In this case where the nucleation rate isrelatively slow, Cit3� adsorbs onto the crystal surfaces of theformed a-NaLuF4 nuclei tightly in all directions and thusprevents their anisotropic growth. Then, large amounts of the

newly formed NaLuF4 nuclei grow by a diffusive mechanisminto the primary particle units, which will aggregate together toform much larger particles in a process dominated by irrever-sible capture of the single particles. With the Na3Cit contentincreasing and under the hydrothermal conditions of highertemperature and pressure, Lu3+ will be released gradually andwill react with Na+ and F� to generate NaLuF4 nuclei. The phasetransformation takes place from the cubic phase to the hexa-gonal phase upon the dissolution of solid crystals of a-NaLuF4,the mass transfer in liquid solution, and the renucleationand growth of solid crystals of b-phase via a dissolution–renucleation process accompanied by the morphological varia-tion from spherical nanoparticles to final microtubes. It can beconcluded that Cit3� plays a crucial role in forming b-NaLuF4

microtubes.The possible reaction process for the formation of the

NaLuF4 crystal nuclei can be summarized as follows:

NaBF4 + H2O 2 Na+ + F� + 3HF + H3BO3 (3)

Lu3+ + Cit3� - Lu3+–Cit3� (complex) (4)

Na+ + 4F� + Lu3+–Cit3� - NaLuF4 + Cit3� (5)

2.2.3. Effect of reaction time. Because the final micro-crystal structure of b-NaLuF4 is formed by small components,the self-assembly process was investigated using a series oftime-dependent experiments. Fig. S2 (ESI†) shows the XRDpatterns of the samples obtained with different reaction times.It can be observed that the XRD patterns of the samples pre-pared with a reaction time of 45 min and 1.5 h can be fullyassigned to the a-NaLuF4 standard card (JCPDS No. 27-0725).When the reaction time is prolonged to 12 h or 24 h, thediffraction peaks of the diffractions of the as-prepared samplecan be indexed to the pure b-NaLuF4 phase (JCPDS No. 27-0726),indicating a complete phase transition. The XRD results are wellsupported by the SEM images, as shown in Fig. 5. Fig. 5 showsthe SEM images of the NaLuF4 samples prepared using 0.85 g ofNa3Cit at different reaction times when the pH value is 10. Forthe sample prepared at a reaction time of 45 min (Fig. 5A), amass of nanoparticles with wide particle size distribution andirregular morphology are found. When the reaction time isincreased to 1.5 h, the as-prepared sample is composed ofuniform and monodisperse oblate particles with smooth sur-faces approximately 1.5 mm in diameter, as shown in Fig. 5B.If the reaction time is prolonged to 12 or 24 h, a uniform andmonodisperse architecture with hexagonal prismatic morpho-logy was obtained. However, a careful observation of themagnified images (Fig. 5C and D) indicates that the aggregatesincrease gradually in size as the reaction proceeds, and thecorresponding length is 3.5 mm and 8.0 mm, respectively,and the average thickness increases from 2.0 to 3.5 mm. It isnoticeable that the product in Fig. 5D has high uniformity andgood monodispersity.

2.2.4. Formation mechanism of the b-NaLuF4 microtubes.On the basis of the above analysis, we can confirm that thegrowth of the b-NaLuF4 microtubes is directly related to the

Fig. 4 XRD patterns (A) and SEM images of the samples obtained at 180 1Cfor 24 h with different amounts of Na3Cit at a pH of 10 (B: 0 g; C: 0.3 g;D: 0.7 g; E: 0.85 g; F: 0.9 g).

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function of the organic molecules (Na3Cit). Laudise et al.41,42

claimed that the growth of crystals is related to the relativegrowth rate of different crystal facets, and the difference in thegrowth rates of various crystal facets results in a different shapeof the crystallite. In a solution-phase synthesis, organic addi-tives acting as capping agents can change the order of the freeenergies of different facets through their interaction with themetal surface. This alteration may significantly affect therelative growth rates of different facets. Enlightened by ourexperimental results and the previous reports,43,44 a plausibleprocess for the self-assembly of the b-NaLuF4 microtubes isshown in Scheme 1. At first, Na3Cit reacts with Lu3+ to form

Lu3+–Cit3� complexes. According to LaMer’s model, the for-mation of such complexes could control the concentration offree Lu3+ ions in solution and thus help to control the nuclea-tion and growth of the crystals in view of the dynamic process.45

Then under hydrothermal conditions, the Lu3+–Cit3� complexis attacked by F�, and Lu3+ would be released gradually. Thisprocess can slow down the nucleation and subsequent crystalgrowth of precursor particles. Then, Cit3� anions possibly bindselectively to the active (001) facet of growing precursor nano-particles. Thus, the crystal growth along the [001] direction isconfined, and it grows preferentially along the [100] and [010]directions. This kinetic control leads to the formation of theoriginal nanoparticles with preferential growth directions.To sum up, Na3Cit species may have a double effect on thegrowth of the b-NaLuF4 nanoparticles. First, as a strong ligand,it can form a stable complex with Lu3+ ions, which slows downthe nucleation and subsequent crystal growth of the b-NaLuF4

nanoparticles. Second, Na3Cit acts as a structure-directingreagent binding to the surface of crystals, which directly affectsthe growth of different crystal facets by adjusting the growthrate of different facets, resulting in the formation of the micro-tubes. In fact, the mechanism for the formation of microtubesstructures is very complicated, because several factors, includ-ing crystal-face attraction, electrostatic and dipolar fields asso-ciated with the aggregate, van der Waals forces, hydrophobicinteractions, and hydrogen bonds, may have various effects onthe self-assembly.46–48

2.3. Luminescent properties

Lanthanide fluoride compounds have been extensively employedas an efficient host lattice for doping other lanthanide ions,bringing about various degrees of luminescence because different

Fig. 5 SEM images of the b-NaLuF4 samples obtained at different reac-tion times using 0.85 g of Na3Cit at a pH of 10 (A: 45 min; B: 1.5 h; C: 12 h;D 24 h).

Scheme 1 Schematic illustration of the formation process of samples under different conditions.

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doping modes may lead to quite different emission behaviors.49,50

In our present work, we focus on investigating the UC emissionsof b-NaLuF4:Yb3+, Ln3+ (Ln3+ = Er3+, Tm3+, and Ho3+) of themicrotubes prepared at 180 1C for 24 h using 0.85 g of Na3Cit atpH 10. It should be noted that the small number of dopinglanthanide ions have not changed the crystal structures andmorphology of the host materials. All the doping ratios of Ln3+

are molar in our experiments. In order to investigate the UCemission properties of b-NaLuF4:Yb3+, Ln3+ (Ln3+ = Er3+, Tm3+,and Ho3+), samples with different concentrations of Yb3+ co-doping with 2 mol% Er3+, Tm3+ and Ho3+ in b-NaLuF4 havebeen prepared. Co-doping with sensitizer ions can not onlyincrease the photoluminescence efficiency but also induceup-conversion luminescence between the donor and acceptorions.51,52 Generally, Yb3+ is chosen as the sensitizer ion inup-conversion luminescence systems, because it possesses alarge absorption cross section at 980 nm, and can then transferenergy to the other activator ions such as Er3+, Tm3+, Ho3+, etc.to give visible light emissions. Fig. 6 and 7 show the UC emissionspectra of b-NaLuF4:x mol% Yb3+, 2 mol% Ln3+ (Ln3+ = Er3+, Tm3+,and Ho3+) with six different Yb3+ concentrations and their corre-sponding CIE chromaticities diagram.

In the UC emission spectrum of b-NaLuF4:x mol% Yb3+,2 mol% Er3+ (x = 2, 10, 20, 30 35, 40) microtubes under 980 nmNIR excitation (Fig. 6A), two bands in the green emissionregion peaking at 520 (529) and 540 nm account for the

Er3+ 2H11/2 - 4I15/2 and 4S3/2 - 4I15/2 transitions, respectively,with some red emission bands at 654 and 661 nm originatingfrom the 4F9/2 - 4I15/2 transition of Er3+. These peaks corre-sponding to the respective proportion of red and green emis-sions result in the multicolor output. By tuning the doped Yb3+

concentrations (2–40 mol%), we can precisely manipulate therelative emission intensities, thus resulting in a tunable coloroutput from green to yellow. Additionally, with the increase ofYb3+ concentration, the relative intensity of the 4F9/2 - 4I15/2

transition is enhanced and the ratios of the red to greenemission are consequently increased. We reasonably infer thatthe interatomic distance of Yb3+–Er3+ decreases with theenhanced amount of Yb3+ sensitizer ions in the b-NaLuF4 host,which efficiently facilitates the back-energy transfer process4S3/2 (Er3+) + 2F7/2 (Yb3+) - 4I13/2 (Er3+) + 2F5/2 (Yb3+). The energytransfer should subsequently suppress the population inexcited levels of 2H9/2, 2H11/2, and 4S3/2, resulting in the decreaseof green (2H11/2, 4S3/2 -

4I15/2) light emissions. Simultaneously,the energy transfer leads to the saturation of 4I13/2 (Er3+), andthen the excited Yb3+ ions transfer energy to Er3+ ions throughthe energy-transfer process 2F5/2 (Yb3+) + 4F13/2 (Er3+) - 2F7/2

(Yb3+) + 4F9/2 (Er3+), which can directly populate the 4F9/2 level,resulting in the enhancement of red emission (4F9/2 -

4I15/2).53–55

In addition, the populated 4I13/2 level might be excited to the4F9/2 red emitting level in Er3+ ions by the cross-relaxationprocess 4I13/2 + 4I11/2 - 4F9/2 + 4I15/2. Another possible routeis the efficiency of the cross relaxation in Er3+ ions, that is,4F7/2 + 4I11/2 -

4F9/2 + 4F9/2, which can also directly populate the4F9/2 red-emitting level and indirectly depopulate the 2H11/2 and4S3/2 green-emitting levels. From the above results and analysis,the emission color and emission intensity can be tuned bydoping with different Yb3+ concentrations in b-NaLuF4:Yb3+,Tm3+ systems, which can be confirmed by the correspondingCIE (Commission Internationale de I’Eclairage 1931 chromati-city) coordinates of the UC emission spectra of b-NaLuF4:x mol%Yb3+, 2 mol% Er3+ (x = 2, 10, 20, 30, 35, and 40) samples withdifferent Yb3+ doping concentrations in Fig. 7B (points a–f).

As for the UC emission spectra of b-NaLuF4:x mol% Yb3+,2 mol% Tm3+ (x = 2, 10, 20, 30, 35, and 40) in Fig. 6B, it can be

Fig. 6 Room-temperature UC emission spectra of (A) b-NaLuF4:x mol%Yb3+, 2 mol% Er3+ (x = 2, 10, 20, 30, 35 and 40); (B) b-NaLuF4:x mol% Yb3+,2 mol% Tm3+ (x = 2, 10, 20, 30, 35 and 40) samples.

Fig. 7 Room-temperature UC emission spectra of b-NaLuF4:x mol%Yb3+, 2 mol% Ho3+ (x = 2, 10, 20, 30, 35 and 40) samples.

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seen that all six b-NaLuF4:Yb3+, Tm3+ samples can emit thecharacteristic Tm3+ transition bands centered at 450, 477,646, and 803 nm, which can be attributed to the 1D2 - 3F4,1G4 -

3H6, 1G4 -3F4 and 3H4 -

3H6 transitions of Tm3+. FromFig. 6B, we can find that the line positions do not change as theYb3+ concentration changed, indicating that the nature of theTm3+ activation did not change with changing concentration.However, the emission intensity of the blue colors due to the1G4 - 3H6 transitions and red colors arising from 1G4 - 3F4

transition decreases with the increase of Yb3+ concentration. TheCIE (Commission Internationale de I’Eclairage 1931 chromaticity)coordinates for the UC emission spectra of b-NaLuF4:x mol%Yb3+, 2 mol% Tm3+ (2, 10, 20, 30, 35, and 40) products with

different Yb3+ doping concentrations are given in Fig. 8(point g–l), respectively. Their CIE coordinates are located inthe light blue and dark blue regions, as shown by points g–lin Fig. 8. Therefore, the multicolor emission can be adjusted bythe precise control of the Yb3+-doping concentration in theb-NaLuF4:Yb3+, Tm3+ system.

Fig. 7 shows the UC emission spectra of b-NaLuF4:x mol%Yb3+, 2 mol% Ho3+ (x = 2, 10, 20, 30, 35 and 40). As demon-strated, a broad emission band centered at around 541 nm anda relatively weak emission band at 647 nm can be observed,which can be associated with the 5F4, 5S2 - 5I8, and 5F5 - 5I8

transitions of Ho3+ ions, respectively. It can be seen that whenthe doping concentration of Ho3+ is fixed at 2 mol%, theemission intensity increases with the increased Yb3+ concen-tration from 2 mol% to 20 mol%, and decreases with a furtherincrease from 20 mol% to 40 mol% of the Yb3+ concentration,indicating that the b-NaLuF4:20 mol% Yb3+, 2 mol% Ho3+

microtubes have the highest emission intensity. A possibleinterpretation is proposed as follows. When the Yb3+ concen-tration increases from 2 mol% to 20 mol%, more Yb3+ ionsbecome available to furnish and transfer the energy to the Ho3+

ions, resulting in the higher emission intensity. When theYb3+ concentration is further increased, the emission intensitydecreases gradually, which may be ascribed to the cross-relaxation process of superfluous Yb3+ ions. With the additionalincrease of the Yb3+ concentration, the distance betweenneighboring Yb3+ ions becomes shorter, which can enhancethe interaction of the neighboring Yb3+ ions and intensify thecross-relaxation process of Ho3+ ions, thus resulting in theconcentration-dependent quenching. The CIE coordinates ofb-NaLuF4:20 mol% Yb3+, 2 mol% Ho3+ are determined to be(x = 0.2265, y = 0.7269), as shown by points m in Fig. 8. Fig. 9shows the schematic energy level diagram of Yb3+ to Er3+,Tm3+ and Ho3+ ions in the b-NaLuF4 system with a detaileddescription of the up-conversion luminescence mechanism.

Fig. 8 The CIE chromaticity of b-NaLuF4:Yb3+, Ln3+ (Ln = Er3+, Tm3+, Ho3+)showing the tunable color region of Yb3+/Er3+ (point a–f), Yb3+/Tm3+

(point g–l) and Yb3+/Ho3+ (point m) doped crystals.

Fig. 9 Energy level diagrams of the Yb3+, Er3+, Ho3+ and Tm3+ ions and the proposed UC emission mechanism.

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The excitation signal (980 nm) is initially absorbed by Yb3+ ions toincrease the 2F7/2 to the 2F5/2 excited state. For b-NaLuF4:Yb3+/Er3+

(Fig. 9, left), the 4I11/2 energy level of the Er3+ ions is excited by aninitial energy transfer from Yb3+ ions in the 2F5 state. Meanwhile,some of the excited Er3+ ions relax rapidly to the low-lying levelsof the 4I13/2 states. Once these states are populated, a sub-sequent 980 nm photon transferred from the excited-state Yb3+

ions can populate a higher 4F7/2 energetic state of the Er3+ ions.The Er3+ ions can then decay non-radiatively to the low-lying2H11/2 and 4S3/2 states of the Er3+ ions, which result in thedominant green 2H11/2 - 4I15/2 and 4S3/2 - 4I15/2 emissions orfurther relax and populate a red 4F9/2 -

4I15/2 emission. Similarly,in the case of b-NaLuF4:Yb3+/Tm3+ and b-NaLuF4:Yb3+/Ho3+,the energy levels of the Tm3+ and Ho3+ ions are also excited byan initial energy transfer from the excited state Yb3+ ions. Thena few subsequent energy transfer processes from Yb3+ ionspopulate the upper Tm3+ and Ho3+ levels, resulting in thevarious emissions of Tm3+ and Ho3+.

3. Conclusions

In summary, we demonstrated a simple and effective hydro-thermal method to synthesize well-defined b-NaLuF4 micro-structures. The investigation of the synthesis parameters showthat the reaction conditions have a significant role in determin-ing the phase and morphology of the final products. We believethat the synthetic strategy demonstrated here can also be extendedto the controllable synthesis of other inorganic compounds byadjusting the amount of organic additive and other reactionparameters. Under 980 nm excitation, Yb3+/Er3+, Yb3+/Tm3+ andYb3+/Ho3+ co-doped b-NaLuF4 samples exhibit strong green toyellow, light blue to blue and green UC luminescence, respec-tively. These merits of multicolor emissions in the visibleregion endow this kind of material with potential applicationsin the fields of light display systems, lasers, and optoelectronicdevices.

4. Experimental section4.1. Materials

The rare-earth oxides Ln2O3 (99.99%) (Ln = Lu, Yb, Er, Tmand Ho) were purchased from the Science and TechnologyParent Company of Changchun Institute of Applied Chemistry,other chemicals were purchased from Sinopharm ChemicalReagent Co., Ltd and used as received without further purifica-tion. All chemicals were analytical-grade reagents and used aspurchased without further purification.

4.2. Synthesis of b-NaLuF4 samples

The as-obtained b-NaLuF4 microcrystals were prepared by thehydrothermal process. Here we take the fabrication of b-NaLuF4

as an example to illustrate the process. In a typical procedurefor the synthesis of b-NaLuF4, 1 mmol of LuCl3 was added to10 mL of aqueous solution containing 3 mmol Na3Cit toform the RE-Cit3� complex. After vigorous stirring for 30 min,

3.125 mmol of NaBF4 was added into the above solution. ThepH of the mixture was adjusted to 10 by adding 1 mol L�1

NaOH. After additional agitation for 15 min, the as-obtainedmixing solution was transferred to a Teflon bottle held in astainless steel autoclave, which was sealed and maintained at180 1C for 24 h. As the autoclave was naturally cooled to roomtemperature, the precipitates at the bottom were separated bycentrifugation, washed with deionized water and ethanol insequence, and then dried in air at 80 1C for 12 h. Other sampleswere prepared by a similar procedure, except for differentpH values, Na3Cit content, and reaction times. Additionally,b-NaLuF4:x mol% Yb3+, 2 mol% Er3+, b-NaLuF4:x mol% Yb3+,2 mol% Tm3+ and b-NaLuF4:x mol% Yb3+, 2 mol% Ho3+ (x = 2,10, 20, 30, 35 and 40) were also prepared through a similarprocess except for adding a stoichiometric amount of corre-sponding RECl3 instead of LuCl3 aqueous solution at the initialstage. All the doping ratios of rare-earth ions are molar ratios inour experiments.

4.3. Characterization

The X-ray diffraction (XRD) patterns of the samples wererecorded on a D8 Focus diffractometer (Bruker) with Cu Karadiation (l = 0.15405 nm). The morphologies and compositionof the as-prepared samples were inspected using a field emis-sion scanning electron microscope (FESEM, S4800, Hitachi)equipped with an energy-dispersive X-ray spectrometer (EDX,JEOL JXA-840). Low- and high-resolution transmission electronmicroscopy (TEM) was performed by using an FEI Tecnai G2

S-Twin instrument equipped with a field emission gun operat-ing at 200 kV. Images were acquired digitally using a Gatanmultiple CCD camera. The UC emission spectra were obtainedusing a 980 nm laser from an OPO (optical parametric oscillator,Continuum Surelite, U.S.A.) as the excitation source and detectedusing a photomultiplier tube (HAMAMATSU R955) from 400to 900 nm. All the measurements were performed at roomtemperature.

References

1 (a) A. P. Alivisatos, Science, 1996, 271, 933–937;(b) K. J. Klabunde, Nanoscale Materials in Chemistry, Wiley-Interscience, New York, 2001; (c) Z. Liu, L. N. Sun, F. Y. Li,Q. Liu, L. Y. Shi, D. S. Zhang, S. A. Yuan, T. Liu andY. N. Qiu, J. Mater. Chem., 2011, 21, 17615–17618; (d) L. N.Sun, H. S. Peng, M. I. J. Stich, D. Achatz and O. S. Wolfbeis,Chem. Commun., 2009, 5000–5002; (e) J. L. Liu, Y. Liu, Q. Liu,C. Y. Li, L. N. Sun and F. Y. Li, J. Am. Chem. Soc., 2011, 133,15276–15279; ( f ) Z. W. Wei, L. N. Sun, J. L. Liu, J. Zhang,H. R. Yang, Y. Yang and L. Y. Shi, Biomaterials, 2014, 35,387–392; (g) X. C. Ye, J. E. Collins, Y. J. Kang, J. Chen,D. T. N. Chen, A. G. Yodh and C. B. Murray, Proc. Natl. Acad.Sci. U. S. A., 107, 22430–22435; (h) X. C. Ye, J. Chen,M. Engel, J. A. Millan, W. B. Li, L. Qi, G. Z. Xing, J. E.Collins, C. R. Kagan, J. Li, S. C. Glotzer and C. B. Murray,Nat. Chem., 2013, 5, 466–473; (i) Y. S. Liu, D. T. Tu,

Paper NJC

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014.

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hris

tian

Alb

rech

ts U

nive

rsita

t zu

Kie

l on

26/1

0/20

14 1

3:11

:15.

View Article Online

This journal is©The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2014 New J. Chem., 2014, 38, 2629--2638 | 2637

H. M. Zhu and X. Y. Chen, Chem. Soc. Rev., 2013, 42,6924–6958.

2 N. Niu, P. P. Yang, F. He, X. Zhang, S. L. Gai, C. X. Li andJ. Lin, J. Mater. Chem., 2012, 22, 10889–10899.

3 X. Zhang, P. P. Yang, C. X. Li, D. Wang, J. Xu, S. L. Gai andJ. Lin, Chem. Commun., 2011, 47, 12143–12145.

4 (a) Z. H. Xu, Y. Gao, T. Liu, L. M. Wang, S. S. Bian and J. Lin,J. Mater. Chem., 2012, 22, 21695–21703; (b) Z. H. Xu,Q. Zhao, Y. G. Sun, B. Y. Ren, L. X. You, S. J. Wang andF. Ding, RSC Adv., 2013, 3, 8407–8416; (c) Y. Gao, Q. Zhao,Q. H. Fang and Z. H. Xu, Dalton Trans., 2013, 42,11082–11091.

5 Z. H. Xu, B. Feng, S. S. Bian, T. Liu, L. M. Wang, Y. Gao,D. Sun, X. Gao and Y. G. Sun, J. Solid State Chem., 2012, 196,301–308.

6 F. Y. Li, W. Gu, H. Wang, Y. Z. Qi, Y. L. Deng, N. Xiao, Y. Liu,Q. Y. Xu and L. Ye, RSC Adv., 2013, 3, 5386–5392.

7 (a) Z. H. Xu, B. Feng, Y. Gao, Q. Zhao, D. Sun, X. Gao, K. l. Li,F. Ding and Y. G. Sun, CrystEngComm, 2012, 14, 5530–5538;(b) Z. H. Xu, S. S. Bian, T. Liu, L. M. Wang, Y. Gao, H. Z. Lianand J. Lin, RSC Adv., 2012, 2, 11067–11077; (c) Z. H. Xu,Q. Zhao, F. Ding and Y. G. Sun, Mater. Res. Bull., 2013, 48,2797–2803.

8 D. L. Jiang, D. Li, J. M. Xie, J. J. Zhu, M. Chen,X. M. Lu and S. C. Dang, J. Colloid Interface Sci., 2010, 350,30–38.

9 Y. G. Sun and Y. N. Xia, Adv. Mater., 2002, 14, 833–837.10 Y. J. Sun, H. J. Liu, X. Wang, X. G. Kong and H. Zhang,

Chem. Mater., 2006, 18, 2726–2732.11 Q. Hao, L. Q. Xu, G. D. Li and Y. T. Qian, Langmuir, 2009, 25,

6363–6367.12 T.-U. Kim, J.-A. Kim, S. M. Pawar, J.-H. Moon and J. H. Kim,

Cryst. Growth Des., 2010, 10, 4256–4261.13 G. Blasse and B. C. Grabmaier, Springer-Verlag, Berlin,

Germany, 1994.14 F. Wang, Y. Han, C. S. Lim, Y. H. Lu, J. Wang, J. Xu,

H. Y. Chen, C. Zhang, M. H. Hong and X. G. Liu, Nature,2010, 463, 1061–1065.

15 K. S. Sohn, J. M. Lee and N. S. Shin, Adv. Mater., 2003, 15,2081–2084.

16 P. P. Yang, S. L. Gai and J. Lin, Chem. Soc. Rev., 2012, 41,3679–3698.

17 S. L. Gai, P. P. Yang, C. X. Li, W. X. Wang, Y. L. Dai,N. Niu and J. Lin, Adv. Funct. Mater., 2010, 20,1166–1172.

18 S. H. Huang, J. Xu, Z. G. Zhang, X. Zhang, L. Z. Wang,S. L. Gai, F. He, N. Niu, M. L. Zhang and P. P. Yang, J. Mater.Chem., 2012, 22, 16136–16144.

19 N. Niu, P. P. Yang, F. He, X. Zhang, S. L. Gai, C. X. Li andJ. Lin, J. Mater. Chem., 2012, 22, 10889–10899.

20 N. Niu, F. He, S. L. Gai, C. X. Li, X. Zhang, S. H. Huang andP. P. Yang, J. Mater. Chem., 2012, 22, 21613–21623.

21 S. Ghosh, T. F. Rosenbaum, G. Aeppli and S. N. Coppersmith,Nature, 2003, 425, 48–51.

22 T.-C. Liu, B.-M. Cheng, S.-F. Hu and R.-S. Liu, Chem. Mater.,2011, 23, 3698–3705.

23 E. Downing, L. Hesselink, J. Ralston and R. A. Macfarlane,Science, 1996, 273, 1185–1189.

24 C. C. Lin and R.-S. Liu, J. Phys. Chem. Lett., 2011, 2,1268–1277.

25 G. S. Yi, H. C. Lu, S. Y. Zhao, Y. Ge, W. J. Yang, D. P. Chenand L. H. Guo, Nano Lett., 2004, 4, 2191–2196.

26 L. Y. Wang and Y. D. Li, Chem. Commun., 2006, 2557–2559.27 Z. Y. Hou, C. X. Li, P. A. Ma, G. G. Li, Z. Y. Cheng, C. Peng,

D. M. Yang, P. P. Yang and J. Lin, Adv. Funct. Mater., 2011,21, 2356.

28 F. Vetrone, R. Naccache, A. Zamarron and A. J. Fuente,ACS Nano, 2010, 4, 3254–3258.

29 O. Guillot-Noel, B. Bellamy, B. Viana and D. Vivien,Phys. Rev. B: Condens. Matter Mater. Phys., 1999, 60,1668–1677.

30 J. A. Capobianco, F. Vetrone and J. C. Boyer, Opt. Mater.,2002, 19, 259–268.

31 D. Q. Chen, Y. L. Yu, Y. S. Wang, P. Huang and F. Weng,J. Phys. Chem. C, 2009, 113, 6406–6410.

32 A. Xia, M. Chen, Y. Gao, D. M. Wu, W. Feng and F. Y. Li,Biomaterials, 2012, 33, 5394–5405.

33 F. He, P. P. Yang, D. Wang, N. Niu, S. L. Gai, X. B. Li andM. L. Zhang, Dalton Trans., 2011, 40, 11023.

34 H. Deng, C. M. Liu, S. H. Yang, S. Xiao, Z.-K. Zhou andQ.-Q. Wang, Cryst. Growth Des., 2008, 8, 4432–4439.

35 C. X. Li, C. M. Zhang, Z. Y. Hou, L. L. Wang, Z. W.Quan, H. Z. Lian and J. Lin, J. Phys. Chem. C, 2009, 113,2332–2339.

36 R. E. Thoma, H. Insley and G. M. Hbert, Inorg. Chem., 1966,5, 1222–1229.

37 H. X. Mai, Y. W. Zhang, R. Si, Z. G. Yan, L. D. Sun,L. P. You and C. H. Yan, J. Am. Chem. Soc., 2006, 128,6426–6436.

38 C. M. Zhang, C. X. Li, C. Peng, R. T. Chai, S. S. Huang,D. M. Yang, Z. Y. Cheng and J. Lin, Chem. – Eur. J., 2010, 16,5672–5680.

39 (a) L. Zhu, L. Qin, X. D. Liu, J. Y. Li, Y. F. Zhang, J. Meng andX. Q. Cao, J. Phys. Chem. C, 2007, 111, 5898–5903;(b) L. Zhu, J. Meng and X. Q. Cao, Eur. J. Inorg. Chem.,2007, 3863–3867.

40 Z. J. Miao, Z. M. Liu, K. L. Ding, B. X. Han, S. D. Miao andG. M. An, Nanotechnology, 2007, 18, 125605.

41 R. A. Laudise and A. A. Ballman, J. Phys. Chem., 1960, 64,688–691.

42 R. A. Laudise, E. D. Kolb and A. J. Caporaso, J. Am. Ceram.Soc., 1964, 47, 9–12.

43 C. X. Li, Z. W. Quan, J. Yang, P. P. Yang and J. Lin, Inorg.Chem., 2007, 46, 6329–6337.

44 C. X. Li, Z. W. Quan, P. P. Yang, J. Yang, H. Z. Lian andJ. Lin, J. Mater. Chem., 2008, 18, 1353–1361.

45 C. X. Li, J. Yang, P. P. Yang, X. M. Zhang, H. Z. Lian andJ. Lin, Cryst. Growth Des., 2008, 8, 923–929.

46 Y. Politi, T. Arad, E. Klein, S. Weiner and L. Addadi, Science,2004, 306, 1161–1164.

47 H. Colfen and M. Antonietti, Angew. Chem., Int. Ed., 2005,44, 5576–5591.

NJC Paper

Publ

ishe

d on

18

Mar

ch 2

014.

Dow

nloa

ded

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Alb

rech

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nive

rsita

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Kie

l on

26/1

0/20

14 1

3:11

:15.

View Article Online

2638 | New J. Chem., 2014, 38, 2629--2638 This journal is©The Royal Society of Chemistry and the Centre National de la Recherche Scientifique 2014

48 H. Colfen and S. Mann, Angew. Chem., Int. Ed., 2003, 42,2350–2365.

49 F. He, P. P. Yang, D. Wang, N. Niu, S. L. Gai and X. B. Li,Inorg. Chem., 2011, 50, 4116–4124.

50 Y. S. Liu, D. T. Tu, H. M. Zhu, R. F. Li, W. Q. Luo andX. Y. Chen, Adv. Mater., 2010, 22, 3266–3271.

51 C. M. Zhang, P. A. Ma, C. X. Li, G. G. Li, S. S. Huang,D. M. Yang, M. M. Shang, X. J. Kang and J. Lin, J. Mater.Chem., 2011, 21, 717–723.

52 C. Y. Cao, W. P. Qin, J. S. Zhang, Y. Wang, G. F. Wang,G. D. Wei, P. F. Zhu, L. L. Wang and L. Z. Jin, Opt. Commun.,2008, 281, 1716–1719.

53 Z. L. Wang, J. H. Hao and H. L. W. Chan, J. Mater. Chem.,2010, 20, 3178–3185.

54 X. Y. Chen, E. Ma and G. K. Liu, J. Phys. Chem. C, 2007, 111,10404–10411.

55 J. Yang, C. M. Zhang, C. Peng, C. X. Li, L. L. Wang, R. T. Chaiand J. Lin, Chem. – Eur. J., 2009, 15, 4649–4655.

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