The positioning of the Tm²⁺ 4f¹²5d¹ and 4f¹³ energy levels as relative to the conduction band of the orthorhombic BaCl₂ host lattice has been determined. Therefore, the energies of the Tm²⁺ 4f¹²5d¹-and excited 4f¹³-level were retrieved, as relative to the Tm²⁺ 4f¹³ ground state. In addition, the energy for exciton creation in the orthorhombic BaCl₂ host lattice was established, from which the bandgap energy was determined. This value was found to correspond quite well to known literature values. Furthermore, the Tm³⁺-Cl^- charge transfer transition was determined, from which the energy difference between the Tm²⁺ 4f¹³ ground state and the top of the BaCl₂ valence band was deduced. A host referred binding energy scheme deduced for BaCl₂:Tm²⁺ then showed that the lowest energy Tm²⁺ 4f¹²5d¹-levels are positioned 0.3–0.5 eV below the BaCl₂ conduction band. Room temperature photo-excitation into this level will then most likely result in thermal ionization effects that have an impact on the Tm²⁺ 4f¹²5d¹→4f¹³ and 4f¹³→4f¹³ luminescence and corresponding quantum yield.

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In the field of solid-state luminescence, Cu²⁺ has long been widely acknowledged for its capacity to emit infrared light. However, the occurrence of visible emission from Cu²⁺ ions had been infrequently observed and reported. In this study, we made an intriguing discovery by examining the behavior of Cu²⁺ within an irregular coordination environment of Ba in BaGa₂O₄. When excited by UV light, Cu²⁺ unexpectedly gave a vibrant yellow–red emission, covering a wavelength range spanning from 500 to 750 nm. More noteworthy, by simply manipulating the excitation wavelength or adjusting the temperature, the peak wavelength of the emission could be effectively tuned from approximately 600 to 660 nm, which could be attributed to the luminescence nature of the charge transfer (CT) between O²⁻ and Cu²⁺. Moreover, the phosphor material displayed a remarkable persistent luminescence (PerL) lasting up to 12 h after UV light excitation. Through thermoluminescence (TL) measurements and first-principle calculations, we found that the intrinsic defects, such as vacancies of oxygen and gallium (V_O and V_Ga^″), played important roles for the PerL phenomena. These findings highlighted the exceptional tunability and PerL properties of BaGa₂O₄:Cu²⁺. Our study provided a new potential guideline for the design of Cu²⁺-activated phosphors in visible region, and opened up new avenues for the research in related functional luminescence materials.

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A novel Eu²⁺-Eu³⁺ co-activated ratiometric thermo-sensitive phosphor was developed and synthesized by solid-state reaction. The valence state of Eu, photoluminescence and thermo-sensitive performance of the phosphor prepared either in ambient air or carbothermally were investigated and discussed. The phosphor shows high sensitivity (S_a = 0.0173 K^–1, S_r = 0.461%/K) and superior signal discriminability (Δν = ∼10380 cm⁻¹). The thermo-sensitive performance is subject to the dual effects of different thermo-responses by Eu²⁺ versus Eu³⁺ combined with energy transfer from Eu²⁺ to Eu³⁺, so that the sensitivity of the phosphor in the temperature range presents a non-monotonic trend. The development of the BaAl₂B₂O₇:Eu²⁺,Eu³⁺ phosphor is not only expected to be relevant for application in the field of temperature sensing, but also of reference significance for improving the sensitivity by means of energy transfer between co-activator ions over a wider temperature range of Eu²⁺-Eu³⁺ co-activated ratiometric thermo-sensitive phosphors.

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To support the role of proximal and remote sensing in geological rare earth element (REE) resource exploration, we studied the reflectance spectroscopy of synthetic single- and mixed-REE phosphate phases. Synthesis yielded monazite for the elements La to Gd, and xenotime for Dy to Lu and Y. Visible-to-shortwave infrared (350–2500 nm) reflectance spectra of synthetic single-REE monazites and xenotimes can be used to identify the ions responsible for the absorption features in natural monazites and xenotimes. Nd³⁺, Pr³⁺ and Sm³⁺ produce the main absorption features in monazites. In natural xenotime, Dy³⁺, Er³⁺, Ho³⁺ and Tb³⁺ ions cause the prevalent absorptions. The majority of the REE-related absorption features are due to photons exciting electrons within the 4f subshell of the trivalent lanthanide ions to elevated energy levels resulting from spin-orbit coupling. There are small (< 20 nm) shifts in the wavelengths of these absorptions depending on the nature of the ligands. The energy levels are further split by crystal field effects, manifested in the reflectance spectra as closely spaced (∼ 5–20 nm) multiplets within the larger absorption features. Superimposed on the electronic absorptions are vibrational absorptions in the H₂O molecule or within [OH]⁻, [CO₃]²⁻ and [PO₄]³⁻ functional groups, but so far only the carbonate-related spectral features seem usable as a diagnostic tool in REE-bearing minerals. Altogether, our study creates a strengthened knowledge base for detection of REE using reflectance spectroscopy and provides a starting point for the identification of REE and their host minerals in mineral resources by means of hyperspectral methods.

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While Eu²⁺ → Eu³⁺ energy transfer is well known, in this study the energy transfer from Eu³⁺ to Eu²⁺ is reported for the first time. The predominant condition for Eu³⁺ → Eu²⁺ energy transfer is a Eu²⁺ 4f⁵5d band at lower energy than the position of the Eu³⁺ 4f⁶[⁵D₀] level, which is fulfilled in Eu-doped CaO. X-ray powder diffraction, Eu Mössbauer spectroscopy and optical absorption measurements are employed to determine the Eu³⁺ and Eu²⁺ concentrations in the prepared CaO:1at.%Eu samples. Synthesis in an H₂/N₂ atmosphere and addition of graphite powder as a reducing agent to the starting mixture are found to result in respective Eu³⁺ and Eu²⁺ concentrations of 0.6–0.7% and 0.3–0.4%. For this sample, the Eu³⁺ → Eu²⁺ energy transfer efficiency is estimated to be high (> 90%). This is explained by the high oscillator strength of the 4f⁷ → 4f⁶5d excitation transition of the Eu²⁺ ion to which energy is transferred. As the Eu²⁺ 4f⁵5d band lies below the Eu³⁺ 4f⁶[⁵D₀] level, Eu³⁺ does not act as a killer center for the near-infrared (NIR) Eu²⁺ emission at about 720 nm. Therefore, a full reduction of Eu³⁺ is not required to attain a high quantum efficiency. Implications of the demonstrated Eu³⁺ → Eu²⁺ energy transfer for application of long wavelength Eu²⁺ phosphors are discussed.

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The luminescence properties of Tm2+-doped BaCl2 with an orthorhombic structure have been studied as a function of temperature and compared to other Tm2+-doped chlorides. In addition to the 2F5/2 → 2F7/2 (4f13 → 4f13) line emission, two 4f125d1 → 4f13 band emissions are observed at 20 K that can be ascribed to the spin-allowed (3H6,5d1)S=1/2 → 2F7/2 and spin-forbidden (3H6,5d1)S=3/2 → 2F7/2 transitions. So far, the Tm2+ spin-allowed (3H6,5d1)S=1/2 → 2F7/2 transition has only been identified in Tm2+-doped iodides and some bromides but never before in a Tm2+-doped chloride. Its presence in orthorhombic BaCl2:Tm2+ is explained by the absence of a (3H6,5d1)S=1/2 → (3H6,5d1)S=3/2 energy transfer process. As the temperature increases, both 4f125d1 → 4f13 emissions undergo rapid quenching and are no longer observed at 120 K, resulting in an intensity increase of the 4f13 → 4f13 emission. However, above 100 K, the intensity of the 4f13 → 4f13 emission also decreases, most likely due to quenching via (3H6,5d1)S=3/2 → 2F7/2 interband crossing, as enabled by the exceptionally large 4f125d1 Stokes shift.@en

The cyan-emitting BaSi₂O₂N₂:Eu²⁺ phosphor is a promising narrow-band and high-efficiency luminescent material used in wide-color-gamut white light-emitting diodes (wLEDs). However, its serious degradation under thermal attacks hinders its practical applications and needs to be improved. Herein, we proposed to deposit a nano-sized Al₂O₃ film around each BaSi₂O₂N₂:Eu²⁺ particle through atomic layer deposition (ALD) in a fluidized bed reactor to improve its thermal stability. Thermal gravimetric analysis results showed that the Al₂O₃ layer with a thickness of only 11 nm had an obvious anti-oxidization effect, by which the oxidation temperature in air of the Al₂O₃ coated phosphor was largely increased from ∼550 to ∼750 °C. Moreover, the Al₂O₃ coated phosphor remained 93% of its luminescence intensity in comparison to 73% of the uncoated one when degraded under water-steam at 200 °C for 24 h. The oxidization of both the BaSi₂O₂N₂ host matrix and the doped Eu²⁺ ions was reduced by the Al₂O₃ layer. Meanwhile, the wLEDs fabricated with the Al₂O₃ coated phosphor showed a luminous flux of 3 times higher than that of the uncoated one when aged under 100 mA for 300 h. The greatly improved thermal degradation property of BaSi₂O₂N₂:Eu²⁺ phosphor and the reliability of the wLEDs indicate that the ALD approach could be a feasible route to produce uniform and nano layers on phosphors and enhance their stability.

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The red afterglow of current rare-earth-activated long persistent luminescence (LPL) phosphors is largely still less than 6 h, in contrast to the 20 or 30 h long blue- or green-emitting ones, becoming the main obstacle to realize their multiscenario applications in practice. Herein, we report a rare-earth-free red-emitting LPL phosphor SrGa₂O₄:Cu²⁺that can exhibit an afterglow at about 622 nm lasting over 30 h, which can largely match with the luminance of current blue- or green-emitting LPL phosphors. We find that the Cu²⁺ion could be charged by ultraviolet light from 280 to 420 nm, and the emission has a very broad band with a full width at half-maximum of about 150 nm. Combining the thermoluminescence measurement and the first-principles calculation, we find that the O vacancies and the -1 charged Ga vacancies could store the holes and contribute to the LPL of SrGa₂O₄:Cu²⁺. Our results may dramatically promote and expand its potential applications and stimulate the research of the multicolor LPL phosphors in future.

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The photocatalytic mechanism of TiO₂ (P25) nanoparticles coated with SiO₂ (SiO₂:TiO₂) by atomic layer deposition was investigated. The deposition of SiO₂ on TiO₂ not only gives a photocatalytic improvement for the degradation of both Rhodamine B (3.6–fold) and Acid Blue 9 (3–fold). SiO₂ deposition also changes the mechanism from direct oxidation of the pollutant at the surface of TiO₂ to a predominantly OH^⁎ radical based degradation of the pollutants originating from SiO₂:TiO₂. Low SiO₂ loadings on TiO₂, where the coating is incomplete, improve the OH^⁎ radicals generation due to the higher number of acidic Si–OH groups combined with the facilitated charge separation at the TiO₂–SiO₂ interface. As a consequence of incomplete coverage, the TiO₂ surface remains accessible, which allows both the oxidation and reduction reactions at the SiO₂:TiO₂ surface. On the other hand, high loading of SiO₂ (>3 wt.% Si) results in photocatalytic suppression due to the coverage of TiO₂ surface by SiO₂. The degradation of differently charged dyes on the SiO₂:TiO₂ surface demonstrates the independence of the adsorption properties on the photocatalytic improvement. Simultaneous degradation of two dyes demonstrated the advantage of SiO₂:TiO₂ being less selective and, therefore, better suited for general water purification.

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Phosphors with high quantum efficiency and thermal stability are greatly desired for lighting industries. Based on the design strategy of solid solution, a series of deep-blue-emitting phosphors (Sr_0.99-xBa_x)₂P₂O₇:0.02Eu²⁺ (SB_xPE x = 0–0.5) are developed. Upon excitation at 350 nm, the optimized SB_0.3PE phosphor shows a relatively narrow full width at half maximum (FWHM = 32.7 nm) peaking at 420 nm, which matches well with the plant absorption in blue region. Moreover, this phosphor exhibits obvious enhancement of internal quantum efficiency (IQE) (from 74% to 100%) and thermal stability (from 88% to 108% of peak intensity and from 99% to 124% of integrated area intensity at 150°C) compared with the pristine one. The white LED devices using SB_0.3PE as deep-blue-emitting component show good electronic properties, indicating that SB_0.3PE is promising to be used in plant growth lighting, white LEDs, and other photoelectric applications. Inorganic Materials; Materials Application; Optical Materials

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The role of Pt on photocatalytic substrates such as TiO₂ (P25) for the decomposition of organic pollutants is still controversial in the scientific community. The well-observed behavior of an optimum catalytic activity as a function of the Pt loading is usually explained by the shift from charge separation to charge recombination behavior of Pt clusters. However, experiments supporting this explanation are still lacking to give a concise understanding of the effect of Pt on the photocatalytic activity. Here, we present an experimental study that tries to discriminate the different effects influencing the photocatalytic activity. Using atomic layer deposition in a fluidized bed reactor, we prepared TiO₂ (P25) samples with Pt loadings ranging from 0.04 wt % to around 3 wt %. In order to reveal the mechanism behind the photocatalytic behavior of Pt on P25, we investigated the different aspects (i.e., surface area, reactant adsorption, light absorption, charge transfer, and reaction pathway) of heterogeneous photocatalysis individually. In contrast to the often proposed prolonged lifetime of charge carriers in Pt-loaded TiO₂, we found that after collecting the excited electrons, Pt acts more as a recombination center independent of the amount of Pt deposited. Only when dissolved O₂ is present in the solution, charge recombination is suppressed by the subsequential consumption of electrons at the surface of the Pt clusters with the dissolved O₂ benefited by the improved O₂ adsorption on the Pt surface.

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We employed atomic layer deposition (ALD) to deposit ultrathin SiO₂ layers on P25 TiO₂ nanoparticles to fabricate TiO₂/SiO₂ core/shell nanostructures. The ALD process was carried out in a fluidized bed reactor working at atmospheric pressure using SiCl₄ and H₂O as precursors, enabling the deposition of SiO₂ at 100 °C with the ability to control the thickness at the sub-nanometer level. By controlling the thickness of the SiO₂ in a very narrow range, i.e., below 2 nm, the photocatalytic activity of TiO₂ can be tuned. In particular, an enhancement was obtained for the SiO₂ layers with a thickness below 1.4 nm, in which the layer with a thickness of about 0.7 nm exhibited the highest photocatalytic activity; for SiO₂ layers thicker than 1.4 nm, the photocatalytic activity was strongly suppressed. The photocatalytic activity enhancement and the degradation mechanism of RhB by the TiO₂/SiO₂ photocatalysts were investigated by combining X-ray photoelectron spectroscopy, UV–Vis absorption spectroscopy, photoluminescence spectroscopy and the aid of charge carrier and radical scavengers. Our findings have revealed an improvement of photogenerated charge separation due to the SiO₂ coating and the dominating role of hydroxyl radicals in the degradation of RhB.

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Photocatalysts for water purification typically lack efficiency for practical applications. Here we present a multi-component (Pt:SiO₂:TiO₂ (P25)) material that was designed using knowledge of reaction mechanisms of mono-modified catalysts (SiO₂:TiO₂, and Pt:TiO₂ ) combined with the potential of atomic layer deposition (ALD). The deposition of ultrathin SiO₂ layers on TiO₂ nanoparticles, applying ALD in a fluidized bed reactor, demonstrated in earlier studies their beneficial effects for the photocatalytic degradation of organic pollutants due to more acidic surface Si–OH groups which benefit the generation of hydroxyl radicals. Furthermore, our investigation on the role of Pt on TiO₂ (P25), as an improved photocatalyst, demonstrated that suppression of charge recombination by oxygen adsorbed on the Pt particles, reacting with the separated electrons to superoxide radicals, acts as an important factor for the catalytic improvement. Combining both materials into the resulting Pt:SiO₂:TiO₂ (P25) nanopowder exceeded the dye degradation performance of both the individual SiO₂:TiO₂ (P25) (1.5 fold) and Pt:TiO₂ (P25) (4-fold) catalysts by 6-fold as compared to TiO₂ (P25). This approach thus shows that by understanding the individual materials’ behavior and using ALD as an appropriate deposition technique enabling control on the nano-scale, new materials can be designed and developed, further improving the photocatalytic activity. Our research demonstrates that ALD is an attractive technology to synthesize multicomponent catalysts in a precise and scalable way.

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Terbium and europium co-doped Sr ₈ ZnY(PO ₄ ) ₇ phosphors are successfully prepared through a high temperature solid-state reaction (SSR). The crystal structure of the as-prepared samples was identified to be Sr ₈ ZnY(PO ₄ ) ₇ (SZYP) pure phase by an X-ray powder diffraction technique. Under near-ultraviolet light excitation (378 nm), the SZYP:Tb ³⁺ and SZYP:Eu ³⁺ phosphors show green and red emission peaking at 545 and 618 nm, respectively. Moreover, an effective energy transfer process from Tb ³⁺ to Eu ³⁺ could be verified by the concentration dependence of emission intensity and lifetime. The energy transfer mechanism between Tb ³⁺ and Eu ³⁺ is determined to be governed by dipole-dipole interactions. The internal quantum efficiency (IQE) is evaluated to be as high as about 91%. The temperature-dependent spectra indicate that the SZYP:Tb ³⁺ ,Eu ³⁺ phosphor shows a high thermal stability. Furthermore, the as-fabricated white LED devices exhibit an excellent correlated color temperature (CCT) of 3223 K, a color rendering index (R _a ) of 85.8 and a luminance efficiency of 37.4 lm W ^-1 . All results imply that the SZYP:Tb ³⁺ ,Eu ³⁺ phosphors have a great potential for application in white LEDs.

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