The perfect energy level overlap of 2 H 11/2 , 4 S 3/2 , and 4 F 9/2 in Er 3+ ions with those of 5 F 3 , 5 F 4 / 5 S 2 , and 5 F 5 in adjacently codoped Ho 3+ ions allows efficient interenergy transfer. Therefore, in addition to routine activators, Er 3+ or Ho 3+ can further act as sensitizers to transfer the upconverted energy to nearby Ho 3+ or Er 3+ , resulting in enhanced upconversion luminescence due to the emission overlap. Proper codoping of Er 3+ /Ho 3+ or Ho 3+ /Er 3+ obviously elevates the maximum doping concentration (thus producing additional upconverted photons) to a level higher than that causing luminescence quenching and significantly enhances upconversion emissions compared with those of singly Er 3+ or Ho 3+ -doped host materials. Indeed, the so-far strongest red upconversion emission under 1532 nm excitation was obtained in LiYF 4 :Er/Ho@LiYF 4 nanoparticles and Ho 3+ -sensitized Er 3+ upconversion emissions excited by 1150 nm laser was simultaneously discovered. With great enhancement compared with that of singly Ho 3+ doped counterparts, this work demonstrates the generality and rationality of our design strategy.

The attractive features of lanthanide-doped upconversion luminescence (UCL), such as high photostability, nonphotobleaching or photoblinking, and large anti-Stokes shift, have shown great potentials in life science, information technology, and energy materials. Therefore, UCL modulation is highly demanded toward expected emission wavelength, lifetime, and relative intensity in order to satisfy stringent requirements raised from a wide variety of areas. Unfortunately, the majority of efforts have been devoted to either simple codoping of multiple activators or variation of hosts, while very little attention has been paid to the critical role that sensitizers have been playing. In fact, different sensitizers possess different excitation wavelengths and different energy transfer pathways (to different activators), which will lead to different UCL features. Thus, rational design of sensitizers shall provide extra opportunities for UCL tuning, particularly from the excitation side. In this review, we specifically focus on advances in sensitizers, including the current status, working mechanisms, design principles, as well as future challenges and endeavor directions.

Electrochemical and photoelectrochemical (PEC) oxygen evolution reactions (OER) are receiving considerable attention owing to their important roles in the overall water splitting reaction. In this contribution, ternary NiFeCo-layered double hydroxide (LDH) nanoplates were in situ hybridized with Ti 3 C 2 T x (the MXene phase) via a simple solvothermal process during which Ti 3 C 2 T x was partially oxidized to form anatase TiO 2 nanoparticles. The obtained Ti 3 C 2 T x /TiO 2 /NiFeCo-LDH composite (denoted as TTL) showed a superb OER performance as compared with pristine NiFeCo-LDH and comercial IrO 2 catalyst, achieving a current density of 10 mA cm À2 at a potential of 1.55 V versus a reversible hydrogen electrode (vs. RHE) in 0.1 M KOH. Importantly, the composite was further deposited on a standard BiVO 4 film to construct a TTL/BiVO 4 photoanode which showed a significantly enhanced photocurrent density of 2.25 mA cm À2 at 1.23 V vs. RHE under 100 mW cm À2 illumination. The excellent PEC-OER performance can be attributed to the presence of TiO 2 nanoparticles which broadened the light adsorption to improve the generation of electron/hole pairs, while the ternary LDH nanoplates were efficient hole scavengers and the metallic Ti 3 C 2 T x nanosheets were effective shuttles for transporting electrons/ions. Our in situ synthetic method provides a facile way to prepare multi-component catalysts for effective water oxidation and solar energy conversion.

It is the nature of crystals to exist in different polymorphs. The recent emergence of two-dimensional (2D) materials has evoked the discovery of a number of new crystal phases that are different from their bulk structures at ambient conditions, and revealed novel structure-dependent properties, which deserve in-depth understanding and further exploration. In this contribution, we review the recent development of crystal phase control in 2D materials, including group VI transition metal dichalcogenides (TMDs), group IVA metal chalcogenides and noble metals. For each group of materials, we begin with introducing the various existing crystal phases and their structure-related properties, followed by a detailed discussion on factors that influence these crystal structures and thus the possible strategies for phase control. Finally, after summarizing the whole paper, we present the challenges and opportunities in this research direction.