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Timestamp: 2019-04-21 10:25:24+00:00

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Qingyong Tian received his BS degree at Hubei University in 2013. He is currently pursuing a PhD degree under the supervision of Prof. Changzhong Jiang and Prof. Wei Wu in the School of Physics and Technology at Wuhan University. His research focuses on the design and synthesis of multifunctional nanostructured materials for applications in photocatalysts and advanced energy devices.
Weijing Yao received her BS in 2015 from Inner Mongolia Agricultural University. She is currently pursuing a PhD degree under the supervision of Prof. Wei Wu in the School of Printing and Packaging at Wuhan University. Her research involves the design and synthesis of upconversion materials for anti-counterfeiting applications.
Wei Wu received his PhD in 2011 from the Department of Physics, Wuhan University, China. He then joined the group of Prof. Daiwen Pang at Wuhan University (2011) and Prof. V. A. L. Roy at City University of Hong Kong (2014) as a postdoctoral fellow. Now he is a full professor and Director of Laboratory of Printable Functional Nanomaterials and Printed Electronics, School of Printing and Packaging, Wuhan University. He has published over 80 papers, which have received over 3000 citations. He is also the topical editor of Frontiers in Materials. His research interests include the synthesis and application of functional nanomaterials, printed electronics and intelligent packaging.
Chang Zhong Jiang received his BS in 1983 from Huazhong University of Science and Technology and MS in 1990 from Wuhan University. He obtained his PhD in 1999 from Université Claude Bernard Lyon 1, France. He has been a full professor in the Department of Physics, Wuhan University since 2001, and he is also the Director of Center for Ion Beam Application, Wuhan University. He has published, as an author and co-author, more than 100 publications in various reputed international journals, such as Physical Review Letters, Nano Letters, ACS Nano, and Advanced Materials. His research interests include the synthesis and application of low-dimension nanomaterials, magnetic materials and ion beam modification of materials.
One increasingly appealing and practical approach is coupling photocatalytic semiconductors with nonlinear optical materials that could utilize the abundant NIR light in photocatalysis, to improve the utilization of solar energy, and finally enhance the photoreactions.24,25 Of especial note are the anti-Stokes luminescence mechanism-based upconversion materials, with the unique property to sequentially absorb two or multiple low-energy photons and radiate out specific types of photons with shorter wavelength, which are considered as potentially efficient candidates for exploitation of NIR excitation. Lanthanide ion (Ln3+)-doped upconversion materials with abundant f-orbital configurations have narrow fluorescence emission bands via intra-4f (4fn–4fn) or 4f–5d (4fn–4fn−15d) orbital transitions.26,27 The involved symmetries of inner electronic quantum states would yield metastable energy levels with long lifetimes that favor the incidence of successive populations in the energized states of Ln3+ ions. Under NIR laser irradiation, the pump photons in the ground state of sensitizer ions will be sequentially excited to the excited states of activator ions. Then, the population will relax non-radiatively or radiatively to the ground state of activator ions to achieve the upconversion emissions. The perfect shielding effect of outer 5s and 5p shells of Ln3+ prevents the influence of surrounding environment on electronic transitions, resulting in perfect physical and chemical stability against photobleaching and photochemical degradation. Doping with different Ln3+ ions in an inorganic substrate will induce designable upconversion emissions, from the UV, passing through the entire visible and extending to the NIR region under irradiation.28–30Fig. 1 presents the occupation of Ln3+ ion-doped upconversion materials with large absorption cross section for converting the NIR light in the solar spectrum into visible light. The main adsorption centers of the Ln3+ ions, i.e., Nd3+, Yb3+, Ho3+ and Er3+, are located at 808 (Nd3+: 4I9/2–4F5/2), 980 (Yb3+: 2F7/2–2F5/2), 1160 (Ho3+: 5I8–5I6) and 1530 (Er3+: 4I15/2–4I13/2) nm, respectively.31 These ions will act as sensitizers that absorb the centered NIR photons and emit UV or visible photons through multiple photon absorption or energy transfer processes. For instance, tunable emissions varying from blue to red colors can be realized by fine-tuning the concentration of sensitizer Yb3+ ions and changing the doped activator Tm3+ and Er3+ ions.32 The spectral power from the normalized air mass (AM) 1.5 spectrum yield is nearly 25 W m−2 ranging from 1480 to 1580 nm when Er3+ acts as the activator.33 It will be extremely attractive to combine upconversion materials as frequency conversion elements with UV- or visible-active semiconductor photocatalysts to further harvest solar energy in a broader spectral range, and thus promoting the utilization of solar energy efficiently.
Fig. 1 The schematic conversion processing of NIR light into visible light via Ln3+ ions, referring to standard AM 1.5 solar radiation spectrum, and the tunable emissions of upconversion materials.
The upconversion emissions are based on the transition of 4f electronic configurations between rare earth (RE) ions (Sc, Y and La are technically d-shield elements).34 Each kind of RE ion has its definite energy level positions, and the upconversion processes between different RE ions are different from each other, while all of them represent similar property of chemical elements attributed to the dominant trivalent oxidation state. The characteristic energy level positions of different Ln3+ ions are illustrated in Fig. 2, which was well defined by experimental measurement by Dieke (Dieke diagram) and Carnall et al.31,35 The energy level positions give detailed illustration of absorption and excitation spectra of Ln3+ ion-doped upconversion materials with distinguishable spectroscopic fingerprints. The upconversion process has been recognized to be grouped into three stages: excited state absorption (ESA), energy transfer (ET), and phonon avalanche (PA).
Fig. 2 Characteristic energy level positions of a sequence of Ln3+ ions.
where n is the number of pump photons.
Fig. 3 Principal diagram for the upconversion processes of Ln3+-doped crystals. (a) ESA, (b) SET, (c) CR, (d) CU, (e) PA and (f) EMU. The red, green, and purple lines stand for photon excitation, energy transfer, and emission processes, respectively.
The ETU process generally occurs between two types of ions (donor and acceptor) in close proximity via dipole–dipole interaction. High concentration of dopants is required in ETU process-dominated upconversion materials, as the dopant content determines the distance between the proximal ions. ETU can be classified into three categories based on diverse energy transfer types: successive energy transfer (SET), cross relaxation (CR) and cooperative upconversion (CU).
2.2.1 Successive energy transfer (SET). The SET process is illustrated in Fig. 3b. The donor ions in the excited state should match the energy requirement with proximal acceptor ions in the ground state. Consequently, the donor ions will transfer energy to the acceptor ions and excite them into the excited state (E1), albeit themselves will return to the GS through non-radiative relaxation. The excited acceptor ions may be promoted to higher excited state (E2) via successive energy transfer.
2.2.2 Cross relaxation (CR). The CR process between one or two types of neighboring ions is displayed in Fig. 3c. Both Ion1 and Ion2 (Ion1) are populating the intermediate energy level (E1 and E2). Then, Ion2 (Ion1) would be motivated to higher intermediate energy level (E3) in accordance with the non-radiatively energy transfer processes of proximal Ion1, accompanied by the relaxation of Ion1 to the ground state.
In addition, with the rapid development of core–shell nanostructured upconversion materials, a new energy migration-mediated upconversion (EMU) mechanism was proposed recently (Fig. 3f).40,41 This involves successive energy transfer from the sensitizers (S) to accumulators (A′) and migrators (M), and trapped by activators (A), finally inducing the upconversion process.
where P is the order of phonons (P = ΔE/hw), ΔE is the gap between two energy levels, w is the phonon frequency, h is the Boltzmann constant, and α and C are constants related to matrix materials. P will increase with decreasing phonon energy, resulting in the decrease of non-radiative relaxation possibility, thus enhancing the upconversion luminescence efficiency. Furthermore, the increased lifetime of metastable energy levels will prohibit the quenching of radiative transitions in the upconversion process. The phonon energy depends on the crystal lattice of host matrixes, and charge and diameter of anions implanted in the crystal. When the phonon energy matches the frequency of excitation or emission, the absorption of the crystal lattice can reduce the emission efficiency, and thus the host matrix is desirable to occupy with lower phonon energy. The most investigated host matrixes include low phonon energy (fluorides ∼355 cm−1, chlorides ∼260 cm−1, oxides ∼600 cm−1, iodides ∼144 cm−1, bromides ∼172 cm−1, etc.) and high phonon energy (silicates ∼1000 cm−1, borates ∼1100 cm−1, phosphates, etc.) materials,35,45–47 as listed in Table 1. Considering the effect of the photon energy of host matrixes on the upconversion luminescence, the efficiency increases in the order of oxides < fluorides < chlorides < bromides < iodides. Comparatively, heavy halides are hygroscopic, and the structural stability of the matrixes decreased in inversed proportional of oxides > fluorides > chlorides. Therefore, a series of investigations were carried out, expecting to find new-fashioned host matrixes with advantages of high upconversion efficiency (like fluorides and chlorides) and high stability (like oxides), to achieve practical applications. Typically, the hexagonal NaYF4 crystal with low phonon energy and excellent chemical stability is accepted to be the most efficient host material hitherto.
In the following section, the effectiveness of commonly investigated upconversion materials with different host matrixes that are applied in the field of photocatalysis, including a sequence of NaYF4-based, fluoride-based, oxide-based and Ln3+ ion-doped semiconductor-based photocatalysts, is discussed in detail.
The proposed photocatalytic mechanism of semiconductor-based upconversion photocatalysts is illustrated in Fig. 4. The upconversion materials serve as the frequency conversion element and harvest two or multiple NIR light photons and then convert them into UV or visible light photons via an efficient fluorescent resonance energy transfer (FRET) process. Subsequently, the converted photons can be re-absorbed by the surrounding semiconductors and inject the electrons from VB to CB of semiconductors, thereby increasing the yield of photoinduced carriers. The direct energy migration between the combination of UC materials and semiconductors under NIR light irradiation could extend the utilization of the solar spectrum and promote the photocatalytic efficiency.
Fig. 4 The proposed photocatalytic enhancement mechanism for semiconductor-based upconversion photocatalysts.
RE ion-doped sodium fluoride matrix contains two different types of crystal phases, including cubic and hexagonal phases (Fig. 5a and b). In the cubic phase, the host F− ions form a high-symmetry cubic crystal structure, with Na+ and RE3+ ions randomly occupying the cation sites. Comparatively, the Na+ and RE3+ ions are selectively occupying the two types of relatively low-symmetry cation sites that are contained in regular crystal sublattices of F− ions in hexagonal NaYF4.42 Generally, host crystal lattices with low site symmetry of doping lanthanoids are preferable for upconversion luminescence over their higher symmetry counterparts, because low-symmetry lattices ordinarily have a crystal field with lots of uneven components surrounding the doping ions. The uneven components strengthen the electronic coupling among 4f electronic orbits and their higher electronic configuration and subsequently promote 4f–4f transition possibilities of the doping ions.48 The density functional theory (DFT)-calculated charge densities for cubic and hexagonal NaREF4 crystals are presented in Fig. 5c and d. In the hexagonal phase, the Ln3+ dopants increase the asymmetry distribution and improve the charge transfer, resulting in an enhancement of polarizability and formation energy. Therefore, hexagonal phase NaREF4 nanocrystals exhibited an order of magnitude augmentation in upconversion efficiency when compared with cubic phase.49 Especially, the hexagonal phase NaYF4, having the advantages of wide energy bandgap (∼8 eV)50 and a very low phonon energy (∼360 cm−1 ≈ 45 meV),51,52 has been considered as one of the most efficient upconversion host matrixes for decades. And most photocatalysts have been combined with hexagonal phase NaYF4 to achieve efficient NIR-active photocatalytic systems.
a MB represents methylene blue; RhB represents Rhodamine B; MO represents methyl orange; R6G represents Rhodamine 6G.
Other Ln3+ ion-doped sodium fluoride-based phosphors such as NaGdF4 and NaLuF4 also have been investigated as promising frequency conversion candidates for NIR photocatalysts.77–80 For example, β-NaLuF4 and tetragonal-phase LiLuF4 are reported as novel host matrixes that may show higher yields of upconversion efficiency than β-NaYF4,81–83 ascribed to the distinctive electronic states at the top valence band of Lu3+ ions and the smaller lattice-cell volume counterparts.
Besides, to construct core–shell structural upconversion nanocrystals through epitaxial growth of small lattice-mismatched layers is excepted as a fascinating approach for improving the upconversion features, by reducing the non-radiative decay losses. The outer layers would confine the doping ions in the interior core crystals and thus inhibit the surface energy loss, resulting in improved upconversion yields. For example, excellent upconversion efficiency improvements of about 7- and 79-fold were realized after epitaxial coating of NaYF4 shell on the NaYF4:Yb,Er(Tm) core, respectively.84 Attributed to valid surface passivation, Huang et al. achieved record high upconversion quantum yield of 7.6% for LiLuF4:Yb,Tm@LiLuF4 core/shell upconversion nanoparticles.81 Until now, there are only two studies applied to the core/shell structured NaYF4:Yb/Tm@NaYF4 and NaYF4:Tm/Yb@Yb/Nd luminescent cores as energy transducers that combine with TiO2 semiconductor for environmental purification.85,86 Therefore, there are still bright prospects for development of core/shell upconversion nanoparticles for photocatalytic application.
Because of the closely arranged energy gaps of Ln3+ ions, host matrixes with low photonic energy are needed to overcome the non-radiative loss between the excited levels for realizing sensitive fluorescent transitions. Yttrium fluoride (YF3) takes advantage of low vibration energy (∼355 cm−1) that could reduce the quenching of electron transitions within the 4f shell, and has been selected as an appropriate candidate for matrix materials.25 A model of the orthorhombic YF3 structure is displayed in Fig. 8a, where the Ln3+ ions will mainly occupy the interstitial sites of YF3 crystals at low doping concentration and substitute the sites of Y3+ ions at high doping concentration.87,88 Various trivalent lanthanide ion-doped YF3 crystals with micro- and nanostructural morphologies (Fig. 8b–h) have been synthesized with hydrothermal process, reverse micelle method, thermal decomposition method, micro-emulsion method and microwave method by alternatively parameterizing the reaction conditions.89–93 Additionally, the sizes, morphologies and phases could be manipulated for colorful emitting luminescence properties of Ln3+ ion-doped YF3 nanocrystals which could act as frequency conversion elements for wide application in NIR-driven photocatalysis.
Fig. 8 (a) Schematic representation of orthorhombic structure. (Reproduced with permission.87 Copyright 2017, American Chemical Society.) Typical YF3 morphologies of (b) nanospheres (reproduced with permission;88 copyright 2014, Royal Society of Chemistry), (c) rhombic shapes (reproduced with permission;89 copyright 2014, Royal Society of Chemistry), (d) nanorods (reproduced with permission;91 copyright 2017, American Chemical Society), (e) quadrilateral-shaped particles (reproduced with permission;90 copyright 2005, American Chemical Society), (f) hexagonal particles (reproduced with permission;90 copyright 2005, American Chemical Society), (g) dumbbell-shaped particles (reproduced with permission;92 copyright 2017, Royal Society of Chemistry) and (h) peanut-like nanocrystals (reproduced with permission;95 copyright 2013, Elsevier BV). The photocatalytic performances of as-fabricated NIR-driven photocatalyst ETY-FCZ for degradation of (i) SA and (j) MO, and (k) the proposed photocatalytic mechanism (reproduced with permission;104 copyright 2015, Elsevier BV).
Also, Ln3+ ion-doped fluoride nanocrystals are coupled with conventional semiconductors to fabricate NIR-active photocatalysts.94 For instance, peanut-like YF3:Yb3+,Tm3+ nanocrystals with high specific surface were coated with uniform TiO2 shell, expanding the light utilization threshold to the NIR region.95 Bi3+ ions were further doped in YF3 by substituting Y3+ ions to tailor the energy transfer process between Yb3+ with Er3+/Tm3+. The Bi3+ ions possessing larger ionic radius will increase the cell volume of matrixes and adjust the crystal field symmetry, therefore prolonging the lifetimes of the metastable energy states by changing their wave functions, and finally enhancing the upconversion emissions.96,97 Additionally, the vacancies and point defects in Ln3+ ion-doped calcium fluoride (CaF2) matrix will induce charge compensation effects, thus increasing the energy transfer rates and the upconversion probability.98,99 The non-toxic, stable, abundant and nonhygroscopic features of CaF2 have received increasing attention in fabrication of NIR-driven photocatalysts.100–105 For example, by integrating Er3+/Tm3+/Yb3+ tri-doped CaF2 nanocrystal with magnetic ZnFe2O4 and ZnO, a multi-component and multi-functional NIR photocatalyst, ETY-FCZ, was fabricated.104 The ETY-FCZ presents continuous degradation of salicylic acid (SA) (Fig. 8i) and MO (Fig. 8j) with 71.9% and 43.73% removal rates when exposed to visible-NIR light irradiation for 180 min, respectively. Fig. 8k presents the proposed photocatalytic mechanism of the ETY-FCZ system, where the upconversion emitted UV-visible photons boosted the generation rates of electron–hole pairs of the ZnO–ZnFe2O4 heterostructures. Examples of fluoride-based upconversion photocatalysts based on the recent literature are presented in Table 3.
where the nearest interaction distances of RE ions confined in the host lattice of Y2O3, Gd2O3, Lu2O3 and La2O3 are presented in Table 5. Comparatively, it can be predicted that a more efficient energy transfer could be achieved with the same lanthanide dopants in the Lu2O3 host than in the Gd2O3 and La2O3 hosts, and their upconversion efficiency may obey the sequence of Lu2O3 > Gd2O3 > La2O3. As mentioned above, the low vibration energy of Y2O3 matrix improves the probability of radiative transitions among electronic energy levels of lanthanide dopants, inducing the highest interacting energy transfer rate in sesquioxides.
Schematic illustrations of RE sesquioxide unit cells modeled according to the structural parameters in Table 5 are shown in Fig. 9.108–110 The nanocrystalline systems of RE2O3 (RE = Y, Gd, Lu) prefer the formation of cubic structure with space group of Ia3 (No. 206) (Fig. 9a and b), while La2O3 is predisposed to unconventional hexagonal phase with space group of P3m1 (No. 164) (Fig. 9c and d). Theoretically, the polymorphism of RE sesquioxides with lighter atoms (La–Sm) generally means that they adopt the hexagonal structure and the heavier atoms (Ho–Lu) prefer the cubic, whereas the intermediate atoms (Sm–Dy) can be stabilized in both cubic and monoclinic structures (Fig. 9e).108 The lighter Ln3+ ions with larger ionic radii prefer the formation of hexagonal crystals in virtue of higher electron cloud distortion induced by increased dipole polarizability.42 The results will offer us reasonable guidance for accommodating appropriate Ln3+ dopants in morphological construction engineering.
Fig. 9 Schematic illustration of the unit cell projected on a–b plane and the RE coordination polyhedra: (a and b) RE2O3 (RE = Y, Gd, Lu), (c and d) La2O3 (reproduced with permission;108 copyright 2018, Elsevier BV). (e) Ionic radius (or polarizability) induced phase transition from cubic to hexagonal of Ln3+ ion-doped upconversion crystals.
Fig. 10 Photocatalytic mechanism of Ln3+-doped semiconductor photocatalysts.
In summary, the synthesis of upconversion materials with effective quantum yields and fabrication of upconversion-based semiconductor photocatalysts are considerable challenges. Despite the significant developments in this direction, there are several tough challenges associated with upconversion materials that limit their applications in basic sciences, including lower upconversion yields and confined excitation wavelengths. However, the reported highest upconversion efficiency is 3.0 ± 0.3% for Yb3+,Er3+ co-doped bulk NaYF4 materials.143 Nevertheless, the micro- or nanomaterials possess lower upconversion efficiency than bulk materials because of the synergistic effect of surface state defects induced by the small particle sizes and adhering water reducing the optical efficiency. Therefore, much effort is required to design and develop high-efficiency upconversion materials. To achieve this purpose, the first method is to select host materials with low phonon energy, which prefer to decrease multi-phonon relaxation between closely spaced energy levels, thus generating more effective upconversion. The host materials provide a surrounding crystalline field for Ln3+ ions but do not determine the emissions. The type of lanthanide dopant and doping concentration are vital for tailoring the upconverted luminescence. In addition, the SPR effect of noble metals is proved theoretically and experimentally to boost the upconversion luminescence by enhancing the absorption of the sensitizer through electric-field coupling, improving the radiative decay rate of the activator, and increasing energy transfer from the sensitizer to the activator. The intrinsic features of SPR are greatly beneficial to the photocatalytic performance of semiconductor photocatalysts by boosting the yield of photogenerated electron/hole pairs and promoting their effective separation. In addition, the SPR of noble metal nanoparticles could effectively promote the efficiency of upconversion materials. Interestingly, researchers have recently discovered localized surface plasmon resonances (LSPRs) in heavily doped semiconductor nanoparticles, quantum dots and amorphous two-dimensional nanomaterials.144–146 The occurrence of LSPR originates from collective oscillations of excess free carriers, the concentration of which (1019–1021 cm−3) can be regulated by adjusting stoichiometric ratios, vacancy or doping concentrations, phase structures, etc.147 Therefore, the LSPR-induced electric field in semiconductors provides a powerful strategy to enhance the upconversion efficiency. More importantly, the unique interactions based on the process of plasmonic energy transfer will further improve the NIR light-active photocatalytic performance.148,149 The marriage of upconversion with semiconductor LSPR could be a novel frontier in fundamental investigations and practical solar energy conversion applications.
Also, epitaxial growth of a shell with small lattice mismatch around a core can reduce non-radiative decay losses of the surface fluorescence, which would offer a useful way to improve the fluorescence intensity of upconversion crystals. In a core–shell structure, the dopant RE ions are incorporated into the interior core layer of particles. The shell layer can effectively suppress energy loss on the crystal surface, resulting in higher luminescence efficiency. These attractive features of core–shell structures make them suitable candidates for photocatalytic application.
The preceding discussion is concentrated entirely on Yb3+ ion (absorption cross section ≈10−20 cm2)-sensitized upconversion materials with the photo-absorption band centered at 980 nm. By paying attention to the drawbacks of confined excitation wavelength, innovative strategies that incorporate replaceable sensitizers have been proposed for tuning the excitation wavelengths of upconversion crystals. Nd3+ is ideal as an alternative sensitizer since it has a one order of magnitude larger absorption cross section (≈10−19 cm2) at ≈800 nm than Yb3+ at ≈980 nm.27 Benefiting from the unique features of deep tissue penetration depth and low photothermal effect of ≈800 nm light (first NIR window), the Nd3+-sensitized upconversion nanoparticles have attracted considerable interdisciplinary attention in biosensing, bioimaging, drug delivery, therapy, and three-dimensional displays.150 Unfortunately, there is no literature focusing on the Nd3+-sensitized upconversion materials as energy conversion elements that are composited with semiconductor photocatalysts, for further utilizing solar light centered at 800 nm. The emergence of Nd3+-sensitized upconversion nanoparticles would open other possible avenues of future works on upconversion semiconductor-based photocatalytic systems. We anticipate new breakthroughs in the construction of novel and highly efficient NIR-light-activated photocatalysts.
All authors contributed during the preparation of the manuscript. All authors have given approval to the final version of the manuscript.
This work was supported by the NSFC (51471121), Hubei Provincial Natural Science Foundation (2014CFB261), Basic Research Plan Program of Shenzhen City (JCYJ20160517104459444), Natural Science Foundation of Jiangsu Province (BK20160383) and Wuhan University.
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