Preparation of Upconversion Nanocrystals NaYF 4 : Yb / Tm with Hydro ( solvo ) thermal Methods

A upconversion nanocrystal of NaYF4:Yb/Tm was synthesized successfully by two different methods of solvothermal and hydrothermal, respectively. The properties of the products were characterized and compared. The results showed that the nanocrystal prepared by hydrothermal method exhibited uniform hexagonal phase and large size, while the nanocrystal prepared by solvothermal method displayed high upconversion luminescence (UCL) and small size. The UCL intensity of the nanocrystal from solvothermal method was higher than that of hydrothermal method. This is the first time to systematically compare the performances of the upconversion nanocrystal prepared by solvothermal and hydrothermal methods, which provides some new insight into the preparation of upconversion nanomaterials with intense UCL and controllable morphology.

At present, many approaches for preparing lanthanide-doped UCNs have been developed, such as hydrothermal, solvothermal, thermal decomposition, and ionic liquids methods [24].Among these approaches, thermal decomposition of trifluoroacetate precursors has the advantages of controllable size and morphology.However, it exhibits drawbacks of rigorous reaction conditions, hazardous precursors and coordinating solvents [25][26][27].Ionic liquids method has the merit of mild reaction condition and user-friendly regents, but exhibits drawbacks of poor shape-control and broad size distribution [14].
http://www.nanobe.orgUsually, hydrothermal and solvothermal are considered as popular and effective methods, which own the merits of facile, mild reaction condition, no toxicity, narrow size distribution and modulatable shape [28][29][30].However, no study has concretely compared the performances of the upconversion nanocrystal prepared by solvothermal and hydrothermal methods.In this study, we concretely compared the performances of the upconversion nanocrystal NaYF 4 :Yb, Tm prepared by the two different methods.This work could provide new insight into the preparation of upconversion nanomaterials with strong upconversion luminescence (UCL) and uniform morphology.

Synthesis of NaYF 4 :Yb/Tm by solvothermal method
To synthesize NaYF 4 :Yb/Tm, 0.78 mmol of YCl 3 , 0.20 mmol of YbCl 3 , and 0.02 mmol of TmCl 3 were mixed with 6 mL oleic acid (OA) and 15 mL octadecene (ODE) in 50 mL three-necked flask.The solution was heated to 160 °C for 30 min and then cooled to room temperature.Subsequently, 10 mL methanol solution containing NaOH (4 mmol) and NH 4 F (2.5 mmol) were slowly added into the flask and kept for 30 min.The solution was heated to 100 °C and kept for 30 min to remove methanol and water, then heated to 300 °C under nitrogen atmosphere and kept for 1 h.After the solution was allowed to cool down to room temperature, nanocrystals were precipitated from the solution with ethanol, and collected after centrifuging and washing with ethanol/water (1:1 v/v) for three times.

Synthesis of NaYF 4 :Yb/Tm by hydrothermal method
In a typical synthesis, 0.78 mmol of YCl 3 , 0.20 mmol of YbCl 3 , and 0.02 mmol of TmCl 3 were mixed with 10 mL de-ionized water in flask.Then 20 mL of aqueous solution containing 2 mmol sodium citrate were added to the flask to form metal citrate complex.After vigorous stirring for 30 min, 30 mL of aqueous solution containing 25 mmol of NaF was introduced into the above solution.Vigorous stirring was maintained for another 60 min.Then, the mixing solution was transferred into a Teflon bottle held in a stainless steel autoclave, sealed and maintained at 180 °C for 24 h.As the autoclave was cooled to room temperature naturally, the precipitates were separated by centrifugation, washed with ethanol and deionized water in sequence, and then dried in air at 60 °C.Finally, the desired products was obtained.

Characterization
Scanning electron microscopy (SEM) images were measured with a JSM-6700F electron microscope.X-ray powder diffraction (XRD) measurements were performed on a Rigaku D/max-2500 X-ray diffractometer using Cu Kα radiation.Transmission electron microscopy (TEM) analyses were performed on a JEOL JEM-2010F electron microscope operating at 200 kV.The upconversion luminescence emission spectra were recorded with an Edinburgh LFS-920 fluorescence spectrometer by using an external 0-2 W adjustable laser (980 nm, Beijing Hi-Tech Optoelectronic Co., China) as the excitation source instead of the Xenon source in the spectrophotometer.

Structure and morphology of the nanocrystals
The phase compositions of the as-prepared samples were detected by the powder XRD.As shown in Fig. 1, the diffraction peaks of the NaYF 4 :Yb/Tm prepared by solvothermal method could be indexed as mixed phase crystals including cubic (JCPDS: 77-2042) and hexagonal (JCPDS: 16-0334) phases [29], while the diffraction peaks of the product prepared by hydrothermal method could be indexed as pure hexagonal phase crystals.
TEM and SEM were used to observe the morphology and size of the nanocrystals.As shown in Fig. 2, NaYF 4 :Yb/Tm prepared by solvothermal method displayed mixed phase structures, including cubic and hexagonal phases, with average diameter of about 25 nm.Whereas NaYF 4 :Yb/Tm prepared by hydrothermal method exhibited pure hexagonal phase structure, with average diameter of about 500 nm.These results agree well with the XRD results, revealing that hydrothermal method is facile to generate uniform pure hexagonalphase nanocrystals with relatively large sizes, while the solvothermal method is facile to generate mixed phase nanocrystals with relatively small sizes.

Upconversion luminescence properties of the nanocrystals
The upconversion luminescenne properties of the as-prepared nanocrystals were investigated.As shown in Fig. 3, upon continuous-wave excitation at 980 nm, nanocrystals prepared by solvothermal and hydrothermal methods exhibited similar UCL spectra with an excellent 800 nm near-infrared (NIR) UCL and an weak emission band at 700 nm.The emission band peaked at 700 nm was caused by 3 F 3 -3 H 6 transitions of Tm 3+ , while the predominant NIR emission peak at 800 nm was caused by 3 H 4 -3 H 6 transition, respectively [22,24].The UCL intensities of the prepared samples were measured at the same concentration of 1.0 mg/ mL in cyclohexane.Interestingly, it was found that the UCL intensity of the product prepared by solvothermal method was much higher than that by hydrothermal method, though the size of the product by solvothermal method was much smaller than that of hydrothermal counterpart.
The difference between the UCL intensities of the nanocrystals prepared by solvothermal method and hydrothermal method are supposed to be mainly due to the differences of capping reagent and the reaction temperature involving in the two synthetic methods.In contrast with citric acid, OA is considered as a more powerful capping reagent which can eliminate the surface defects of the nanocrystals efficiently, leading the nanocrystals to exhibiting a higher upconversion luminescence [31].The carboxyl group of the citric acid molecule is a high energy vibration group with a high energy vibration mode, which can cause a quench of upconversion luminescence of the Ln 3+ through a multi-phonon relaxation process [32,33].The reaction temperature in solvethermal method is higher than that of hydrothermal method.A high reaction temperature favors the generation of high crystallinity and the elimination of surface defects of the nanocrystals, resulting in a high upconversion luminescence [34][35][36].

Conclusions
In summary, upconversion nanocrystal NaYF 4 :Yb/ Tm was synthesized successfully by hydrothermal and solvothermal methods respectively.The properties of  the products prepared by the two different synthesis methods were systematically compared.The results show that the UCL intensity of the nanocrystal prepared by solvothermal method was higher than that by hydrothermal method.Under the experimental conditions, the hydrothermal method made the nanocrystal exhibit a uniform pure hexagonal phase and large sizes, while the solvothermal method made the nanocrystals display a high UCL intensity and small sizes.