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Energy migration (energy transfer among identical luminescence centers) is always thought to be related to the concentration quenching in luminescence materials. However, the novel Eu 3+ -doped Ba 6 Gd 2 Ti 4 O 17 phosphor seems to be an exception. In the series of Ba 6 Gd 2(1−x) Ti 4 O 17 :xEu 3+ (x = 0.1, 0.3, 0.5, 0.7, and 0.9) phosphors prepared and investigated, no concentration quenching is found. Detailed investigations of the crystal structure and the luminescence properties of Ba 6 Gd 2(1−x) Ti 4 O 17 :xEu 3+ reveal that the nonoccurrence of concentration quenching is related to the dimensional restriction of energy migration inside the crystal lattices. In Ba 6 Gd 2 Ti 4 O 17 , directly increasing the number of Eu 3+ ions to absorb as much excitation energy as possible allows to achieve a higher brightness. The highly Eu 3+ -doped Ba 6 Gd 2(1−x) Ti 4 O 17 :xEu 3+ (x = 0.9) sample can convert near-UV excitation into red light, whose Commission Internationale de l'Eclairage (CIE) coordinates are (0.64, 0.36) and the color purity can reach up to 94.4%. Moreover, warm white light with the CIE chromaticity coordinates of (0.39, 0.39), the correlated color temperature of 3756 K, and the color rendering index of 82.2 is successfully generated by fabricating this highly Eu 3+ -doped phosphor in a near-UV light-emitting diode chip together with the green YGAB:Tb 3+ and blue BAM:Eu 2+ phosphors. KEYWORDS: Eu 3+ -doped phosphor, Ba 6 Gd 2 Ti 4 O 17 host, nonconcentration quenching, energy migration, light-emitting diodes

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bottleneck of the two half-cell reactions, i.e., hydrogen evolution reaction (HER) [3] and oxygen evolution reaction (OER) [4] .The key issue lies in developing highly efficient and stable electrocatalysts to reduce the overpotential, accelerate reaction kinetics, and thus improve the energy conversion efficiency. [1,2,5] Currently, Pt and RuO 2 (or IrO 2 ) are the benchmark electrocatalysts for the HER and OER, respectively. However, the high cost and scarcity of these precious metal-based catalysts greatly impede their commercialization. [6,7] Moreover, using two different catalysts for the respective half reaction is undesirable for actual electrolyzers requiring the same electrolyte environment, restrained by the incongruity of the optimum pH for each side. [8][9][10] Therefore, breakthroughs in the production of new bifunctional electrocatalysts will greatly facilitate the advancement of electrochemical water splitting.Among various nonprecious catalysts explored, transition metal phosphides (TMPs), such as FeP [11] , CoP [12] , NiP [13] , and MoP [14] , show distinct advantages for overall water splitting by virtue of their elemental abundance and favorable activity. However, monometallic phosphides often show good performance in a half reaction (HER or OER), with poor activity on the other side. [15,16] A number of strategies have therefore been proposed to boost the catalytic performance of TMPs toward overall water splitting. One strategy is to introduce secondary metals to afford heterometallic phosphides, which can optimize the electronic structure of TMPs and thus tailor the kinetic energy barriers, due to the synergistic effect between the two metal components. [9,[17][18][19] Engineering the micro/nanostructures of TMPs has also proven to be capable of effectively reducing the overpotential and facilitating the reaction kinetics. [15,20] To this end, hierarchical porous TMPs have attracted enormous attention owing to their interior voids and large surface area, which can substantially enhance the exposure of active sites, accelerate electrolyte penetration, and shorten ion/electron diffusion pathways during the electrochemical process. [5,20,21] In addition, hybridizing TMPs with conductive components such as carbon, which can facilitate charge transfer and prevent TMPs from aggregation and oxidation during electrocatalysis, has also been widely employed to boost the electrocatalytic performance. [22,23] Designing highly-efficient, cost-effective, and stable electrocatalysts for water splitting is of great significance for implementing renewable energy technologies. Herein, a self-templated strategy is employed to fabricate 2D porous electrocatalysts of heterometallic phosphides featuring a cage-in-cage superlattice architecture. The as-made heterometallic phosphide electrocatalysts, comprising a layer of close-packed CoFeP nanocages intimately embedded in an interconnected carbon-cage framework, are converted from carbon-coated CoFeO nanocrystal superlattices by one-step phosphidation. Benefiting f...