English

• 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 TiO₂ has 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⁺ and x mol% Li⁺ (or y mol% Mn²⁺ or z mol% 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 ℃ 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 ℃. Here, TiO₂: Yb³⁺/Tm³⁺/x mol% Li⁺ (x mol% = 0.3 and 1.0), TiO₂: Yb³⁺/Tm³⁺/y mol% Mn²⁺ (y mol% = 0.2 and 1.0) and TiO₂: Yb³⁺/Tm³⁺/z mol% Cu²⁺ (z mol% = 0.2, 0.4 and 0.6) were respectively named as Li-x, Mn-y and 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 CuΚα radiation source (40 kV, 30 mA, λ = 1.5406 Å, 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 2θ values ranging from 22° to 28° of Li-x, Mn-y and Cu-z are 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 2θ values 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 2θ reflection 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-1.0 shift 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 2θ angles 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 Å) by the larger Tm³⁺ (CN = 6, r = 0.88 Å), Yb³⁺ (CN = 6, r = 0.868 Å) and Li⁺ (CN = 6, r = 0.76 Å) results in the expansion in the crystal lattice, leading to the shift of the diffraction peaks towards a lower 2θ value^[17]. In contrast, the shift of diffraction peaks to the larger angles in Cu-z caused by the lattice contraction is attributed to the fact that Cu²⁺ ions with the smaller ionic radii (CN = 4, r = 0.57 Å) 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.

  Figure 1

  

  Figure 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

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  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-x and Mn-y also 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-z nanocrystals 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).

  Figure 2

  

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

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  3.2 Optical characteristics

  Fig. 3 (a-c) displays the UC emissions spectra of Li-x, Mn-y and Cu-z nanocrystals 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 the ³H₄ → ³H₆ and ¹G₄ → ³F₄/³F₃ → ³H₆ transitions 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-x and Mn-y nanocrystals. 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.

  Figure 3

  

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

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  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. The ¹G₄ state of Tm³⁺ ions is populated from Yb³⁺ ions via energy transition (ET) processes of ET1: ³H₆ (Tm³⁺) + ²F_5/2 (Yb³⁺) → ³H₅ (Tm³⁺) + ²F_7/2 (Yb³⁺), ET2: ³F₄ (Tm³⁺) + ²F_5/2 (Yb³⁺) → ³F₃ (Tm³⁺) + ²F_7/2 (Yb³⁺) and ET3: ³H₄ (Tm³⁺) + ²F_5/2 (Yb³⁺) → ¹G₄ (Tm³⁺) + ²F_7/2 (Yb³⁺)^{[20, 21]}. Radiatively relaxing processes from the ¹G₄ state to the ³H₆ and ³F₄ states of Tm³⁺ ions, respectively, yield the blue emission at 478 nm and red one at 647 nm. The Tm³⁺ ions at the ³F₃ state decay radiatively to the ³H₆ ground 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⁺) + |²F_7/2, ⁶A_1g > (Mn²⁺-Yb³⁺ dimer) → ³H₆ (Tm³⁺) + |²F_7/2, ⁴T_1g > (Mn²⁺-Yb³⁺ dimer)) and back energy transfer (BET) process (³H₆ (Tm³⁺) + |²F_7/2, ⁴T_1g > (Mn²⁺-Yb³⁺ dimer) → ³F₂ (Tm³⁺) + |²F_7/2, ⁶A_1g > (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 |²F_7/2, ⁴T_1g > of Mn²⁺-Yb³⁺ dimer, which arose from the ground state absorption (GSA: |²F_7/2, ⁶A_1g > → |²F_5/2, ⁶A_1g > ) and excited state absorption (ESA: |²F_5/2, ⁶A_1g > → |²F_7/2, ⁴T_1g > ), lead to an efficient BET process to also populate the ³F₃ state of Tm³⁺ ions. Subsequently, the Tm³⁺ ions at the ³F₃ state partly decay radiatively to the ground ³H₆ state, yielding the strongly increased red UC emission at 695 nm in Mn-1.0, and partly are promoted to the UC emitting ¹G₄ state via ET5 process. The ET5 process would compensate the reduced populations of ¹G₄ state 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 3d¹⁰4s⁰ state of the Cu²⁺ ions to the ¹G₄ state of Tm³⁺ ions also contributes to populate the ¹G₄ state.

  Figure 4

  

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

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  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 2θ angles 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 of ³H₆ (Tm³⁺) + |²F_7/2, ⁴T_1g > (Mn²⁺-Yb³⁺ dimer) → ³F₂ (Tm³⁺) + |²F_7/2, ⁶A_1g > (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 3d¹⁰4s⁰ state of the Cu²⁺ ions to the ¹G₄ state of Tm³⁺ ions are responsible for the increased intensities of blue and red UC emissions under 980 nm excitation.

  
  
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• 

  Figure 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

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

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

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

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