Promising lanthanide-doped double molybdates KYb(MoO4)2 phosphors for highly efficient upconversion luminescence and temperature sensing.

Here we report the highly efficient upconversion luminescence (UCL) and optical temperature sensing based on the novel host of KYb(MoO4)2 doped with trivalent lanthanide (Ln3+) ions at 980 nm excitation. The high Yb3+ concentration and unique ordered layer structure in KYb(MoO4)2 host are beneficial for the enhancement of UCL efficiency by improving the absorption and the negative migration of excitation energy. Ho3+, Er3+, and Tm3+ ions were selected to singly dope the KYb(MoO4)2 host, achieving three primary colors of red, green, blue UCL, respectively. At the optimal doping concentration, the blue upconversion quantum yield (UCQY) of the KYb(MoO4)2: 1.0%Tm3+ phosphor reaches 0.13%, which is rare for the Tm3+-doped oxides. By leveraging the efficient blue light, we achieved high-brightness white UCL by co-doping Ho3+ in KYb(MoO4)2: Tm3+. Furthermore, the temperature sensing performance of the KYb(MoO4)2: Tm3+, Ho3+ phosphors operating within the first biological window (BW-I) was evaluated based on a thermo-responsive fluorescence intensity ratio (FIR) of far-red to near-infrared (NIR) emission from completely separated 3F2,3/3H4 → 3H6 transitions of Tm3+. At the excitation of 980 nm, the maximum absolute and relative sensitivities were determined as 0.25 × 10-3 K-1 at 673 K and 2.84% K-1 at 303 K, respectively. These results indicate that the double alkali-rare-earth molybdate KYb(MoO4)2 can be used as a promising host to achieve highly efficient UCL and temperature sensing, suggesting potential applications in the fields of anti-counterfeiting, displays, and non-contact temperature sensors.

Color-tunable luminescence based on the efficient energy transfer of a Tm-Dy system for optical thermometry and white LED lighting.

The development of phosphors with color-tunable luminescence including white emission is at the forefront of lighting and display technologies. Herein, Dy3+,Tm3+ single-doped or co-doped K3Y(PO4)2 phosphors are synthesized through the solid-state reaction method. By properly adjusting the ratio of Dy3+,Tm3+ co-doping concentrations, color-tunable luminescence from blue to yellow, including white luminescence, is realized under 359 nm excitation. The mechanism of energy transfer between Tm3+ and Dy3+ is further investigated via measuring the luminescence decay curve. Based on efficient energy transfer from Tm3+ to Dy3+, the emission of Dy3+ exhibits an abnormal thermal enhancement phenomenon as the temperature increases. The optical thermometry behaviors of various emission combinations for the Dy3+,Tm3+ co-doped system are analyzed. The maximum sensitivity can be obtained as a constant of 4.8 × 10-3 K-1, which is conducive to improve the measurement accuracy of optical temperature sensing at high temperatures. Furthermore, we also demonstrate the applicability of K3Y(PO4)2:Tm3+,Dy3+ phosphors in white LEDs, providing proof-of-concept for the lighting and display fields.

Winning color-tunable upconversion luminescence and high-sensitive optical thermometry in K3Gd(PO4)2:Yb3+,Er3+,Tm3.

A series of Yb3+, Er3+, Tm3+ co-doped K3Gd(PO4)2 are prepared via the solid-state reaction method. Upon 980 nm excitation, the synthesized samples present color-tunable upconversion luminescence ranging from yellow to blue with the increment of Tm3+ doping contents. The upconversion mechanisms of Yb3+, Er3+, Tm3+ co-doped system are systematically investigated in detail. Varying contents of Tm3+ can appropriately alter the upconversion emissions of blue, green and red via possible energy transfer processes. Furthermore, the thermometric performances of phosphors associated with upconversion luminescence are analyzed in the temperature region of 300-675 K. By employing non-thermally coupled energy levels (2H11/2/4F9/2 of Er3+), the maximum relative and absolute sensitivity reaches 0.78 % K-1 and 0.168 K-1. Combining the sensitivity characteristic and repeatability of thermometer, the luminescence intensity ratio technology based on non-thermally coupled energy levels may be a more effective choice for optical thermometry. These excellent results will pave an avenue to K3Gd(PO4)2:Yb3+,Er3+,Tm3+ phosphors for the fields of color-tunable luminescence and non-contact temperature sensing.

Vanadium Hexacyanoferrate as a High-Capacity and High-Voltage Cathode for Aqueous Rechargeable Zinc Ion Batteries.

Prussian blue analogs (PBAs) are widely used as electrode materials for secondary batteries because of their cheapness, ease of synthesis, and unique structural properties. Nevertheless, the unsatisfactory capacity and cyclic stability of PBAs are seriously preventing their practical applications. Here, vanadium hexacyanoferrate (VHCF) is successfully prepared and used as a cathode for aqueous zinc-ion batteries (AZIBs). When using 3 M Zn(CF3SO3)2 as the electrolyte, a high capacity of ~230 mA h g-1 and a high voltage of ~1.2 V can be achieved. The XRD result and XPS analysis indicate that the outstanding Zn2+ storage capability is due to the presence of dual electrochemical redox centers in VHCF (Fe2+ ⇋ Fe3+ and V5+ ⇋ V4+ ⇋ V3+). However, the battery shows a short cycle life (7.1% remaining capacity after 1000 cycles) due to the dissolution of VHCF. To elongate the cycle life of the battery, a high-concentration hybrid electrolyte is used to reduce the activity of water molecules. The improved battery exhibits an impressive capacity of 235.8 mA h g-1 and good capacity retention (92.9%) after 1000 cycles.

Towards highly efficient NIR II response up-conversion phosphor enabled by long lifetimes of Er3.

The second near-infrared (NIR II) response photon up-conversion (UC) materials show great application prospects in the fields of biology and optical communication. However, it is still an enormous challenge to obtain efficient NIR II response materials. Herein, we develop a series of Er3+ doped ternary sulfides phosphors with highly efficient UC emissions under 1532 nm irradiation. ß-NaYS2:Er3+ achieves a visible UC efficiency as high as 2.6%, along with high brightness, spectral stability of lights illumination and temperature. Such efficient UC is dominated by excited state absorption, accompanied by the advantage of long lifetimes (4I9/2, 9.24 ms; 4I13/2, 30.27 ms) of excited state levels of Er3+, instead of the well-recognized energy transfer UC between sensitizer and activator. NaYS2:Er3+ phosphors are further developed for high-performance underwater communication and narrowband NIR photodetectors. Our findings suggest a novel approach for developing NIR II response UC materials, and simulate new applications, eg., simultaneous NIR and visible optical communication.

Synthesis of core-shell nanoparticles based on interfacial energy transfer for red emission and highly sensitive temperature sensing.

Red up-conversion luminescence with both emission and excitation peaks within the "tissue transparency window" makes for ideal fluorescent labels for deep tissue penetration and low-background biological imaging. In this work, an efficient strategy is proposed to realize bright red up-conversion luminescence based on interfacial energy transfer (IET), and the luminescence centers (Er3+ and Tm3+) and sensitizers (Yb3+) are doped in separate layers to avoid deleterious cross-relaxation. The IET process between Yb3+ and Er3+ can enable bright red photon up-conversion under 980 nm excitation via fine control and manipulation of the lanthanide ion concentration and shell thickness. By appropriately employing the fluorescence intensity ratio technique, an optical thermometer based on non-thermally coupled energy levels demonstrates a superior relative sensitivity of 2.02% K-1 across the whole temperature region. NaYF4:Er3+,Tm3+@NaYF4:Yb3+ with strong red UC luminescence and highly sensitive performance exhibits potential for application in the field of non-contact temperature sensing.

High-sensitive temperature sensing based on thermal-enhanced emission and non-thermally coupled energy levels of white upconversion luminescence system.

Na3Y(VO4)2:Nd3+,Yb3+,Ho3+,Tm3+ phosphors present significantly improved sensitivity of optical temperature sensing based on thermal-enhanced upconversion luminescence and non-thermally coupled energy levels. Under 808 nm excitation, white upconversion luminescence is successfully achieved in Nd3+-sensitized system. In addition, the emissions intensities originated from 4G5/2→4I9/2 transition of Nd3+ and 3F2,3→3H6 transition of Tm3+ gradually increase with elevating temperature owning to the thermal population effects, as opposed to the blue (1G4→3H6 transition of Tm3+), green (5S2,5F4→5I8 transition of Ho3+) and red (5F5→5I8 transition of Ho3+) emissions intensities show continuous decreasing trend. The temperature sensing behaviors are investigated by employing multiple non-thermally coupled energy levels. Based on non-thermally coupled energy levels of 4G5/2 (Nd3+)/1G4 (Tm3+), the absolute and relative sensitivities are obtained to be 0.433 K-1 and 2.81 % K-1. These results demonstrate that the Na3Y(VO4)2:Nd3+,Yb3+,Ho3+,Tm3+ phosphors with superior optical thermometry performance and white luminescence within a relatively wide temperature range can achieve optical applications in many fields.

Engineering Er3+-sensitized nanocrystals to enhance NIR II-responsive upconversion luminescence.

An Er3+-sensitized system with a high response to 1550 nm radiation in the second near-infrared window (NIR II) has been considered for a new class of potential candidates for applications in bio-imaging and advanced anti-counterfeiting, yet the achievement of highly efficient upconversion emission still remains a challenge. Here, we constructed a novel Er3+-sensitized core-shell-shell upconversion nanostructure with a Yb3+-enriched core as the emitting layer. This designed nanostructure allows the Yb3+ emitting layer to more efficiently trap and lock excitation energy by combining the interfacial energy transfer (IET) from the shell (Er3+) to the core (Yb3+), high activator Yb3+ content, and minimized energy back-transfer. As a result, the NIR II emission at 1000 nm is remarkably enhanced with a high quantum yield (QY) of 11.5%. Based on this trap and lock-in effect of the excitation energy in the Yb3+-enriched core, highly efficient 1550 nm-responsive visible and NIR upconversion emissions are also achieved by co-doping with other activator ions (e.g., Ho3+ and Tm3+). Our research provides a new functional design for improving NIR II-responsive upconversion luminescence, which is significant for developing practical applications.

Near-infrared-emitting upconverting BiVO4 nanoprobes for in vivo fluorescent imaging.

Near-infrared (NIR) emitting BiVO4:Yb3+,Tm3+ nanoparticles are synthesized by a new solvothermal strategy using solvents of oleic acid and methanol. The obtained BiVO4:Yb3+,Tm3+ samples show an average particle size of ≈164 nm and exhibit an asymmetry monoclinic crystal structure of BiVO4. At NIR excitation of 980 nm, the BiVO4:Yb3+,Tm3+ sample exhibits a nearly single NIR emission at ≈796 nm with extremely weak blue emissions from Tm3+ ions. These high-energy visible emissions are absorbed by the semiconducting host of BiVO4 that possesses a bandgap of ≈2.2 eV. Therefore, the NIR excitation to a single intense NIR emission fluorescent BiVO4 materials could be a potential ideal probe for deep-tissue high-resolution bioimaging. To validate the ability of BiVO4 materials for bio-applications, we conduct the cytotoxicity experiments. The results show that the cytotoxicity of HeLa cells is negligible at a concentration of 0.2 mg/ml of BiVO4:Yb3+,Tm3+ , and the cell viability approaches 90% at a high dosage of 0.5 mg/ml. The Daphnia magna and Zebrafish treated with nanoparticles (0.5 mg/ml) display bright NIR emission without any background, indicating the excellent in vivo fluorescent imaging capacity of BiVO4:Yb3+,Tm3+ nanoparticles. Our findings offer an environment-friendly strategy to synthesize BiVO4 UCL nanophosphors and provide a promising new class of fluorescent probes for biological applications.

A thiocarbonate-caged fluorescent probe for specific visualization of peroxynitrite in living cells and zebrafish.

Peroxynitrite (ONOO-), a highly reactive oxygen species (ROS), is implicated with many physiological and pathological processes including cancer, neurodegenerative diseases and inflammation. In this regard, developing effective tools for highly selective tracking of ONOO- is urgently needed. Herein, we constructed a concise and specific fluorescent probe NA-ONOO for sensing ONOO- by conjugating an ONOO--specific recognition group ((4-methoxyphenylthio)carbonyl, a thiocarbonate derivative) with a naphthalene fluorophore. The probe, NA-ONOO, was in a dark state because the high electrophilicity of (4-methoxyphenylthio)carbonyl disturbs the intramolecular charge transfer (ICT) in the fluorophore. Upon treatment with ONOO-, the fluorescent emission was sharply boosted (quantum yield Φ: 3% to 56.6%) owing to an ONOO- triggered release of (4-methoxyphenylthio)carbonyl from NA-ONOO. Optical analyses showed that NA-ONOO presented high selectivity and sensitivity toward ONOO-. With good cell permeability and biocompatibility, the NA-ONOO probe was successfully applied to imaging and tracing exogenous and endogenous ONOO- in living cells and zebrafish. The probe NA-ONOO presents a new recognition group and a promising method for further investigating ONOO- in living systems.

Enhancing red luminescence by doping Yb3+ into Er3+ self-sensitized Gd2O2S upconverting nanoparticles under excitation at 1530 nm.

Red upconversion luminescence (UCL) nanoparticles are of significant importance for applications in the fields of deep tissue imaging, photothermal therapy and security ink. In this work, a highly efficient red emission was achieved by introducing Yb3+ ions as mediators in Er3+ self-sensitized Gd2O2S nanoparticles under excitation at 1530 nm. The results show that the Gd2O2S:Yb3+,Er3+ nanoparticles synthesized by a homogeneous precipitation method exhibit a uniform spherical shape and narrow size distribution with a mean particle diameter of ≈65 nm. Moreover, the integral emission intensity ratio of red to green of the Gd2O2S:Yb3+,Er3+ sample is significantly enhanced 3-fold compared with the Gd2O2S:Er3+ sample without Yb3+ doping. The enhancement mechanisms are discussed in detail on the basis of steady-state luminescence spectra and decay dynamics measurements under various excitations at 380, 808, 980 and 1530 nm, respectively. It has been demonstrated that the enhanced red luminescence is induced by cross-relaxation energy transfer from Er3+ to Yb3via4S3/2 (Er3+) + 2F7/2 (Yb3+) → 4I13/2 (Er3+) + 2F5/2 (Yb3+) and 4I11/2 (Er3+) + 2F7/2 (Yb3+) → 4I15/2 (Er3+) + 2F5/2 (Yb3+), and further followed by back energy transfer from Yb3+ to Er3+ through 4I13/2 (Er3+) + 2F5/2 (Yb3+) → 4F9/2 (Er3+) + 2F7/2 (Yb3+). The former cross-relaxation procedure effectively populates the red emission level of 4F9/2 by depopulating the green emission level of 3S3/2. Our findings provide a feasible way to enhance the red UCL and new insights into red UCL mechanisms in the Er3+ self-sensitized system under ≈1500 nm excitation, by combining with the nontoxic oxysulfide host, indicating their potential application as safe fluorescent nanoprobes in the bio-field.

High-Capacity and Long-Lifespan Aqueous LiV3O8/Zn Battery Using Zn/Li Hybrid Electrolyte.

Aqueous zinc-ion batteries (AZIBs) are promising candidates for large-scale energy storage because of their low cost and high safety. However, their practical applications are impeded by low energy density and short service life. Here, an aqueous Zn2+/Li+ hybrid-ion battery is fabricated using the LiV3O8 nanorods as the cathode, metallic Zn as the anode, and 3 M Zn(OTf)2 + 0.5 M LiOTf aqueous solution as the electrolyte. Compared with the batteries using pure 3 M Zn(OTf)2 electrolyte, the cycle performance of the hybrid-ion battery is significantly improved. After 4000 cycles at 5 A g1, the remaining capacity is 163.9 mA h g-1 with impressive capacity retention of 87.0%. Ex-situ XRD, ex-situ XPS, and SEM tests demonstrate that the hybrid electrolyte can inhibit the formation of the irreversible Zn3(OH)2V2O7·2H2O by-product and restrict Zn dendrite growth during cycling, thereby improving the cycle performance of the batteries.

Promising lanthanide-doped BiVO4 phosphors for highly efficient upconversion luminescence and temperature sensing.

The semiconductor oxide BiVO4 has been intensively studied as a highly efficient photocatalyst. Here we attempt to adopt trivalent lanthanide (Ln3+)-doped BiVO4 as a novel upconversion luminescence (UCL) material for achieving high-efficiency UCL and temperature sensing under near-infrared (NIR) irradiation. Er3+/Tm3+, Yb3+/Er3+, and Yb3+/Tm3+ ions were selected to co-dope the BiVO4 phosphors, achieving three primary colors of red, green, and blue (RGB) with high color-purity. At an optimal doping concentration, the upconversion quantum yield of the BiVO4:8%Yb3+,18%Er3+ phosphor reaches as high as 2.9%. Furthermore, we, for the first time, demonstrate the non-contact temperature sensing properties of a BiVO4:Er3+,Tm3+ phosphor via employing fluorescence intensity ratio technology. The results show that the maximum absolute thermal sensitivity is ≈70 × 10-4 K-1 at 473 K under 980 nm excitation, with high and stable sensitivity of more than 60 × 10-4 K-1 over a wide temperature range of 333-493 K. In addition, at a much safer wavelength of 1550 nm, this sample achieves maximum absolute sensitivity of 56 × 10-4 K-1 at 453 K. Moreover, the BiVO4:Er3+,Tm3+ phosphor presents high relative sensitivity of about 1.1% K-1 under both 980 and 1550 nm excitation at 293 K. These results indicate that the BiVO4 semiconductor oxide can be used as a novel host to achieve high UCL efficiency and promising thermal sensing performance, suggesting potential applications in the new fields of anti-counterfeiting, displays, and non-contact temperature sensors.

Hierarchical Aluminum Vanadate Microspheres with Structural Water: High-Performance Cathode Materials for Aqueous Rechargeable Zinc Batteries.

Controlling morphology, adopting metal cations and introducing crystal water are three effective strategies to improve the electrochemical performance of various battery electrodes. However, the effects of simultaneously applying these three strategies to aqueous rechargeable zinc batteries (ARZBs) are rarely demonstrated. Herein, hierarchical H11 Al2 V6 O23.2 (HAVO) microspheres were successfully prepared using a simple hydrothermal method, and used as cathode material for ARZBs. The as-prepared HAVO microspheres exhibited superior electrochemical performance than the dehydrated AlV3 O9 (AVO) microspheres, i. e. they have a larger specific capacity of 390.4 mA h g-1 at 100 mA g-1 , a better rate capability of 191.4 mA h g-1 at 5000 mA g-1 and a higher cycling stability of up to 1000 cycles with a capacity retention of 80.9 %. The excellent electrochemical performance of HAVO is due to the synergistic effects of the shortened ion diffusion distance in primary HAVO nanosheets, the improved electronic conductivity, and structural stability by adopting Al3+ into the lattice, the enhanced charge transfer properties and ion diffusion coefficient of the electrode due to the existence of crystal water. Therefore, this work may offer a new route for the design and synthesis of more advanced electrode materials for ARZBs.

Three primary color emissions from single multilayered nanocrystals.

The achievement of three-primary-color luminescence in a single material will lead to revolutionary developments of many advanced applications such as dynamic display with ultra-high resolution, and complex anti-counterfeiting. Here we report the realization of steady-state three-primary-color emission in single multilayered NaYF4 upconversion (UC) nanoparticles. In this core-shell structure, a novel design of a tri-sensitizer, i.e., Nd3+, Yb3+ and Er3+ ions, is utilized, which effectively absorbs the excitation photons of 808, 980 and 1550 nm, and then exhibits blue, red and green emissions, respectively. By simply combining the three primary color emissions, tunable full-color luminescence was achieved in this single material. These nanoparticles have demonstrated promising potential applications in dynamic display and multiple anti-counterfeiting.

Luminescence property tuning of Yb3+-Er3+ doped oxysulfide using multiple-band co-excitation.

Lanthanide ions have abundant excited-state channels which result in a radiation relaxation process generally accompanied by a non-radiation relaxation process. However, non-radiation relaxation processes will consume the activation energy and reduce the luminescence efficiency of the phosphor. Two lasers with an excitation energy which matched the ground state absorption and excited state absorption of ions were used to excite the phosphors to avoid the non-radiation relaxation process. This approach can achieve the purpose of populating specific states of the lanthanide ions, and furthermore effectively tunes the luminescence intensity and color output of the sample. Results show that the red emission intensity of the sample is significantly improved and this is caused by populating the 4F9/2 level under simultaneous 1510 nm and 980 nm excitation. Then when the 1510 nm and 808 nm co-operate to excite the sample, the green emission obtained increased sharply because the 2H11/2/4S3/2 states were efficiently populated. As a proof-of-concept experiment, this new approach has potential in the applications of solar cells.

Correction: Luminescence property tuning of Yb3+-Er3+ doped oxysulfide using multiple-band co-excitation.

[This corrects the article DOI: 10.1039/C8RA02503G.].

Investigation on the thermal effects of NaYF4:Er under 1550 nm irradiation.

The thermal effects of NaYF4:Er microcrystals under 1550 nm laser diode irradiation were investigated using an infrared thermal imaging method. The results revealed that the temperature of the LD irradiation beam area was non-uniform. The NaYF4:Er sample exhibits a rapid temperature increase within 5-10 s, and the heating process reaches its steady state after 60 s of irradiation when exposed to 1550 nm LD. The response time, temperature increase, and emission intensity of the samples depend on the excitation power density and concentration of doped Er3+ ions. Low excitation power density is required to avoid the influence of thermal effects on the up-conversion emission spectra measurement. Rapid scanning in up-conversion emission spectra measurement is not effective in preventing the heating effect under high excitation power pumping. In addition, a relative deviation of temperature, existing in fluorescence intensity ratio technique, decreased with the increasing irradiation time.

Study on Fabrication and UV Photoelectric Property of TiO2 Nanotube Array Electrodes.

Highly ordered TiO2 nanotube array electrodes were successfully fabricated by a two-step anodization method on Ti sheet substrates in an electrolyte composed of ammonium fluoride, deionized water, and glycol. The tube wall was smooth, and the average internal and external diameters, wall thickness, and tube length achieved were 80 nm, 90 nm, 10 nm, and 9 µm, respectively. X-ray diffraction and field emission scanning electron microscopy results revealed that the TiO2 nanotube arrays presented an amorphous structure. When calcined at 300 °C, the arrays crystallized into the anatase phase, and the crystallization degree of the oxide layer increased as the temperature rose. Calcinating at 400 °C did not obviously disrupt the porous structure of the highly ordered arrays. However, higher temperature enlarged the diameter of the nanotube array and roughened the tube wall. When the temperature reached 600 °C, the nanotube mouth broke because of the excessive stress, causing the oxide layer's thinness and nanotube mouth clogging. The photoelectric test showed that the electrode presented obvious photoresponse under 300-400 nm UV excitation (maximized at 360 nm). The degree of crystallization and the micro-structure of the oxide layer can significantly affect the photoelectric properties of the electrode. After calcination at 400 °C, the TiO2 nanotube arrays, with highly ordered tubular structure directly connected to the Ti substrate, can ensure the rapid transportation of photo-induced electrons to the Ti substrate, while the high crystallinity of the arrays can help reduce the defect density of the nanotube and extend the lifetime of the photo-induced carriers. The electrode showed the best photoelectric property, and the photocurrent intensity was maximized (29.6 µA). However, the calcination process with over-temperature resulted in substantial loss of the TiO2 oxide layer, mouth clogging, and a severe decline in the photoelectric properties.