One-step synthesis and upconversion luminescence properties of hierarchical In2O3:Yb3+,Er3+ nanorod flowers

In2O3:Yb ,Er nanorod flowers (NRFs) are prepared by a simple hydrothermal method, where sucrose was used as a ligand. The obtained In2O3:Yb ,Er NRFs were carefully characterized by scanning electron microscopy (SEM), power X-ray diffraction (XRD), transmission electron microscopy (TEM) and steady/ transient spectroscopy. The dependence of the upconversion luminescence (UCL) of In2O3:Yb ,Er NRFs on morphology, Yb concentration and excitation power was carefully discussed. It is found that the luminescence intensity ratio of the red emission to green emission (R/G) depended on the morphology and Yb concentration, and the green emissions are dominantly resulting from a threephoton populating process with higher Yb doping concentration. More importantly, the concentration quenching in the In2O3:Yb ,Er NRFs was greatly suppressed due to boundary effects, and is beneficial for lighting and photon energy conversion devices.


Introduction
][6][7][8][9][10] The inorganic host is the carrier of UC and it is very important for improving the efficiency of UCL, which is affected by the morphology of UCNPs.8][19] At present, the controllable preparation of high effective UCL of rare earth (RE) doped In 2 O 3 NCs is still difficult.Several strategies have been proposed to solve these problems.For example, Chen's group synthesized Er 3+ ions doped cubic In 2 O 3 nanoparticles via a simple solvothermal method, and rstly demonstrated its UC and downconversion emissions under 808 and 980 nm excitation, respectively. 20Xu et al. successfully fabricated In 2 O 3 :RE nanotubes by the electrospinning method for the rst time, and the sensitivity of In 2 O 3 nanotubes to H 2 S were signicantly improved due to the RE ions doping. 21Dutta's group also prepared Eu/Dy doped In 2 O 3 nanoparticles by a sonochemical technique.Relatively weak Eu 3+ emission could be detected in In 2 O 3 :Eu nanoparticles, which is attributed to the distorted environment around the Eu ions in the In 2 O 3 lattice. 22So far, although a few works on In 2 O 3 :RE nanoparticles have been performed, the mechanism of UC processes of In 2 O 3 NCs doped with Yb 3+ , Er 3+ ions is not fully understood.Especially, the dependence of UCL on cross relaxation, population processes, excitation power density and RE doping concentration are not clear.Therefore, it is necessary to detailedly investigate the UCL property of In 2 -O 3 :Yb 3+ ,Er 3+ NCs.
In this article, we prepared In 2 O 3 :Yb 3+ ,Er 3+ hierarchical NRFs by a facile hydrothermal method using sucrose as surfactant. 23The formation, morphology and structure of the In 2 O 3 :Yb 3+ ,Er 3+ NRFs were examined by SEM, TEM, and XRD.And the inuence of sucrose on the hydrothermally prepared In 2 O 3 :Yb 3+ ,Er 3+ NRFs is further elucidated.The investigation of UCL property of the In 2 O 3 :Yb 3+ ,Er 3+ NRF demonstrated that the UCL intensity and R/G ratios depended on particle size, excitation power and Yb 3+ concentration.The as-obtained In 2 O 3 :Yb 3+ ,Er 3+ NRFs exhibited a high effective UC emission, which is a promising candidate for display and solar cells.

Synthesis In 2 O 3 nanorods owers
All the reagents (analytical-grade purity) were used without any further purication.In the preparation of (10 mol%)Yb 3+ , (1 mol%)Er 3+ co-doped In 2 O 3 NCs, InCl 3 $4.5H 2 O (266 mg), YbCl 3 $6H 2 O (33 mg), ErCl 3 $6H 2 O (3 mg), urea (15 mg) and moderate sucrose (342 mg) were dissolved in deionized water (36 mL) with adequately stirring for 30 min.In this reaction, sucrose acts as a template and linker to bind the nanoparticles, which leads to the formation of ower-like nanostructures.The presence of both sucrose and urea is a prerequisite for the formation of this porous hierarchical In 2 O 3 .Then, a Teonlined stainless steel autoclave was used for placed for the mixture solution and placed in 160 C oven for 12 h.Aer the autoclave was cooled to room temperature naturally, the result product was obtained through centrifuging the result product using deionized water and ethanol for three times, and then dried at 80 C for 1 day.In 2 O 3 :Yb 3+ ,Er 3+ NCs were obtained through calcining the precursor at 500 C for 2 h.The same route is adopted for the preparation of (10 mol%)Yb 3+ and xEr 3+ (x ¼ 2, 3, 5, 8 mol%) doped In 2 O 3 NCs.The preparation process of In 2 O 3 :Yb 3+ ,Er 3+ NRFs is presented in Scheme 1.

Characterization and measurements
The SEM images were obtained on a JEOL JSM-7500F microscope operating at 15 kV.The TEM, HR-TEM and selected-area electron diffraction (SAED) images were measured on a JEOL JEM-2100 microscope operated at 200 kV.The XRD pattern was recorded on a Rigaku D/max-rA power diffractometer.The UC emission spectra and dynamics of rare earth ions were measured on an Edinburgh uorescence spectrometer (FLS980).2 shows the SEM images of In 2 O 3 :Yb 3+ ,Er 3+ NCs with varying amount of sucrose.In the absence of sucrose, the product is the mixture of irregular nanocubes and ower-like nanostructures (Fig. 2a), as the reference (REF) samples.When the moderate sucrose is added, the NRFs were appeared.Fig. 2b shows the low magni-cation SEM image of In 2 O 3 :Yb 3+ ,Er 3+ NRFs, demonstrated that the NRFs are high uniformity.The enlarged-magnication SEM images are presented in Fig. 2c and d, and we can see that each nanorod is uniform.Fig. 3 records the elaborate structural analysis of samples using TEM.In Fig. 3a, it can be seen that each nanorod of NRFs is homogenous, and the size of these nanorods is $100 nm in diameter and $1 mm in length (Fig. 3b).From the TEM images, we can deduce that the NRFs are self-assembled by many single nanorods.The inter-planar spacing is $0.292 nm through measuring the lattice fringes (Fig. 3c), matching with the distance of the (2 2 2) plane of the cubic In 2 O 3 crystal.The SAED pattern of the In 2 O 3 :Yb 3+ ,Er 3+ NRFs (Fig. 3d) conrms that the obtained products are polycrystalline in structure.In order to further determine the structure and dopant concentrations of In 2 O 3 :Yb 3+ ,Er 3+ NRFs, the energy-dispersive X-ray (EDX) analysis of In 2 O 3 :(10 mol%) Scheme 1 Schematic illustration for the synthesis of In 2 O 3 :Yb 3+ ,Er 3+ NRFs.Yb 3+ ,(2 mol%)Er 3+ NRFs is measured in Fig. S1.† It shows the presence of In, Yb, Er and O element.Furthermore, the spectrum conrms that the mole ratio of Yb and Er in the NRFs is 8.47 and 1.7, respectively.According to this measuring result, we can deduce that the exact doping concentration of Er 3+ ions is 0.85 mol%, 2.55 mol%, 4.24 mol%, 6.79 mol% in In 2 O 3 :(10-mol%)Yb 3+ ,xEr 3+ (x ¼ 1, 3, 5, 8 mol%) NRFs.

Effect of morphology, Yb 3+ concentration and excitation power on UCL
The typical UCL spectra of In 2 O 3 :Yb 3+ ,(1 mol%)Er 3+ REF and NRFs samples were investigated (Fig. 4).In the two samples, the 4 4).This phenomenon is known as concentration quenching and quite common in RE-doping NCs, which is due to an energy migration process among the same type of RE ions. 24In In 2 O 3 :Yb 3+ ,Er 3+ NRFs, the nanorod is only $100 nm and dispersive each other, which can be clearly recognized by HR-TEM images (as shown in Fig. 3).The boundary effects in other traditional NCs would be unavoidably occurred.For example, in the Y 2 O 3 :Eu 3+ ,Y 2 SiO 5 :Eu 3+ , and LaPO 4 :Ce 3+ ,Tb 3+ inverse opal photonic crystals, [25][26][27] the quenching concentration of the RE emission increases more than relative to the corresponding bulk phosphors.In other words, the concentration quenching was suppressed considerably due to the boundary effects.
To further understand the UCL mechanism, the UCL dynamics of 4 F 9/2 -4 I 15/2 transition of In 2 O 3 :Yb 3+ ,(1 mol%)Er 3+ REF and NRFs were measured (Fig. 5), and all the decay curves of Er 3+ ions were tted by the double-exponential function. 28rom the lifetime constants, we can concluded that: (1) in the lower doped concentration of Er 3+ ions, the lifetimes in REF samples are longer than that in In 2 O 3 :Yb 3+ ,Er 3+ NRFs.As we known, the decay time constant is the reverse of the nonradiative and radiative decay rates.The non-radiative decay rate is signicantly affected by the large phonon groups in the surface, crystal phase, lattice defect state and phonon energy.In In 2 O 3 :Yb 3+ ,Er 3+ NRFs, the non-radiative channels nearby Er 3+   ions, such as lattice defect states/surface large phonon bands would be increased due to the increasing of the volume to surface ratio. 29(2) In the In 2 O 3 :Yb 3+ ,Er 3+ NRFs, the lifetime constants decrease gradually with the increasing of Er 3+ concentration compared to those of REF samples (as shown in the insert of Fig. 5), implying that the concentration quenching in the NRFs are suppressed.In the traditional In 2 O 3 :Yb 3+ ,Er 3+ phosphors, the long-range resonant energy transfer (ET) processes among Er 3+ ions are very severe.Inevitably, luminescent quenching could occur because of the ET from Er 3+ ions to defect states, which is dependent on doped concentrations.In the In 2 O 3 :Yb 3+ ,Er 3+ NRFs, these ET process should be restrained largely due to the structure of NRFs, because the size of each nanorod is $100 nm and the nanorods are dispersive.In this case, the ET among Er 3+ ions can only happen within one nanorod, then, the photons will be scattered into the air instead of capturing by the defect states.
Fig. 6 shows the Yb 3+ concentration dependent UCL spectra of the In 2 O 3 :Yb 3+ ,Er 3+ NRFs.From the spectra analysis of Fig. 6, the integral intensity of red emission gradually increased, whereas that of green emissions diminished gradually as the Yb 3+ concentration increases.The emission spectra of the In 2 O 3 :Yb 3+ ,Er 3+ NRFs at different excitation powers were also measured (Fig. 7).We can found that the red emission is dominant in the low excitation power, and the R/G ratio is as high as $35 when the excitation power is 330 mW.The blue and green emissions increased gradually with increasing of excitation power.The R/G ratio versus excitation power is shown in the insert of Fig. 7, it is clear seen that the R/G ratios decreased with increasing of excitation power.
The ln-ln plots of the integral intensity versus excitation power for the red and green emissions of In 2 O 3 :(15 mol%)Yb 3+ ,xEr 3+ (x ¼ 1, 3, 5, 8 mol%) NRFs are investigated (Fig. 8).As is well known, the visible output power intensity (I V ) will be proportional to power (n) of the infrared excitation (I IR ) power if the saturation effect can be neglected: 30 , where n is the number of IR photons absorbed per visible photon emitted.The slopes n are determined to be 2.68, 2.62, 2.65, 2.4 for 2 H 11/2 -4 I 15/2 transition, 2.87, 2.62, 2.73, 2.02 for 4 S 3/2 -4 I 15/2 transition and 1.17, 1.71, 1.79, 1.86 for 4 F 9/2 -4 I 15/2 transition in the In 2 O 3 :Yb 3+ ,Er 3+ NRFs at different Er 3+ ions concentration of 1 mol%, 3 mol%, 5 mol% and 8 mol%, respectively.We can see that the slopes n are >2 for the green emissions and >1 for the red emission of Er 3+ .Thus the 2 H 11/2 / 4 S 3/2 levels are populating through three photons processes, and the 4 F 9/2 level is populating through two-photons ET processes for all the In 2 O 3 :Yb 3+ ,Er 3+ NRFs.As far as we know, the three-photon populating phenomenon for the 2 H 11/2 / 4 S 3/2 levels is few reported in Er 3+ /Yb 3+ codoped materials.The population mechanisms of 4 S 3/2 and 4 F 9/2 levels in In 2 O 3 :Yb 3+ ,Er 3+ NRFs are quite different to other UC nanocrystals from the above analysis.Fig. 9 presents UC population and emission diagram of the Er 3+ /Yb 3+ doping system under 980 nm excitation.The 4 I 11/2 level is populated by the electrons on the ground state 4 I 15/2 via ET of neighboring Yb 3+ ions.Subsequently, the 4 I 13/2 level is populating through the nonradiative relaxation process of 4 I 11/2 -4 I 13/2 via the assistance of large phonon groups, which is widely existed in NCs prepared via the hydrothermal method. 31In the second-step, the UC population of 4 I 11/2 -4 F 7/2 and 4 I 13/2 -4 F 9/2 occurred via ET and excitedstate absorption, generating the red emission ( 4 F 9/2 -4 I 15/2 ).The cross relaxation of 4 F 7/2 + 4 I 11/2 À 4 F 9/2 + 4 F 9/2 is signicant for populating the 4 F 9/2 level, when the excitation power or the Yb 3+ concentration is sufficiently high.Then the Yb 3+ concentration is lower in In 2 O 3 :Yb 3+ ,Er 3+ NRFs, the electrons in 4    of 4 I 13/2 -4 F 9/2 to be more effective.This is a main reason for increasing the R/G ratio in In 2 O 3 :Yb 3+ ,Er 3+ NRFs.
The relation of R/G ratio and Yb 3+ concentration can be explained below, the population of 4 F 9/2 level is mainly originated from ET upconversion processes.The ET transfer from Yb 3+ ions to Er 3+ ions could be increased with the increase of Yb 3+ concentration, thus the red emission ( 4 F 9/2 -4 I 15/2 ) was accelerated because of an increase of the ET process.The quenching of green emission with the increasing Yb 3+ concentration is due to the reversed ET from Er 3+ to Yb 3+ ions ( 4 S 3/2 (Er 3+ ) + 2 F 7/2 (Yb 3+ ) À 4 I 13/2 (Er 3+ ) + 2 F 5/2 (Er 3+ )), which leads to the depopulation of the 4 S 3/2 level.As we all known, the phonon energy of In 2 O 3 is larger than that of Yb 2 O 3 , which may lead to the increase of nonradiative relaxation.However, all the emissions (including red emission and green emission) will be quenching due to the nonradiative relaxation.The quenching of the green emissions is mainly attributed to the reversed ET from Yb 3+ ions to Er 3+ ions, which can be conrmed by the uorescence decay of 4 S 3/2 level.Finally, the power-dependent UCL in In 2 O 3 :Yb 3+ ,Er 3+ NRFs can be well explained.The three-photon population processes 4 F 9/2 -4 G 11/2 become more effective with the increasing of excitation power, leading the depopulation of the 4 F 9/2 level and the population enhancement of 4 G 11/2 .The populations for 4 F 3/2 and 4 S 3/2 / 2 H 11/2 levels also increase due to multi-phonon relaxation, so the R/G ratio decreases as the excitation power increases.

Conclusion
In 2 O 3 :Yb 3+ ,Er 3+ NRFs has been successfully prepared by a simple hydrothermal method combined with a succedent calcining process.It is also found that in In 2 O 3 :Yb 3+ ,Er 3+ NRFs, the concentration quenching is dramatically suppressed.The R/ G ratio changed depending on the NP morphology, Yb 3+ concentration and the excitation power.The green emissions are mainly originated from three-photon excitation in In 2 O 3 :-Yb 3+ ,Er 3+ NRFs with higher Yb 3+ concentration, which is different from other substrate materials.The In 2 O 3 :Yb 3+ ,Er 3+ NRFs are potential candidate for the application of lighting and solar cell.

3. 1
Fig. 1 shows the XRD patterns of the calcining In 2 O 3 :Yb 3+ ,Er 3+ products and the corresponding reference samples (REF).It can be seen that the products are the pure cubic structure according to JCPDS card no.06-416.With space group Ia 3 (no.206) and lattice parameters of a ¼ 10.118 Å.No diffraction peaks from any other impurities were observed, indicating the high purity
F 3/2 -4 I 15/2 (blue), 2 H 11/2 , 4 S 3/2 -4 I 15/2 (green) and 4 F 9/2 -4 I 15/2 (red) transitions can be clearly identied.The intensity ratio of the red ( 4 F 9/2 -4 I 15/2 ) emission to the green ( 2 H 11/2 , 4 S 3/2 -4 I 15/2 ) emissions (R/G) in In 2 O 3 :Yb 3+ ,Er 3+ NRFs is higher than that in REF sample.The UCL property could be inuenced by the following three factors: the nanocrystal size, crystallinity, and morphology.From the XRD and SEM analysis, we can deduce that the crystallinity and the size of In 2 O 3 :Yb 3+ ,Er 3+ NRFs and REF samples are both nearly same, indicating that the effect of the size and crystallinity might not be the dominant reason to the change of R/G ratio.The remarkable change in the R/G ratio between In 2 O 3 :Yb 3+ ,Er 3+ NRFs and REF samples could be attributed to the change of morphology.The In 2 O 3 :Yb 3+ ,Er 3+ NRFs constituted with dispersive nanorods has a large specic surface area, which introduced more surface ligands and surface defects.These groups and defects can bridge some non-radiative relaxation channels, such as 4 I 13/2 -4 I 11/2 and 4 S 3/2 -4 F 9/2 , favoring the generation of the 4 F 9/2 -4 I 15/2 red emission.In order to verify the conclusion, the In 2 O 3 :(10 mol%)Yb 3+ ,(1 mol%)Er 3+ power was prepared by the sol-gel method, the UCL spectra of In 2 O 3 :Yb 3+ ,Er 3+ power (the size > 10 mm) was measured under 980 nm excitation, shown in Fig. S2.† We can nd that the R/G in the power is also dramatically lower than that in In 2 O 3 :Yb 3+ ,Er 3+ NRFs.The integral intensity of UCL for In 2 O 3 :Yb 3+ ,Er 3+ NRFs is lower than REF sample.Then the UCL intensity of In 2 O 3 :Yb 3+ ,Er 3+ NRFs exceeds REF samples with further increase the doping concentration of Er 3+ (shown in the insert of Fig.