Development of MOF-based photocatalysts is intriguing research due to their structural flexibility and tremendous catalytic sites, whereas most MOFs only can take use of UV/visible light and lack of response to NIR light. Herein, we present a facile approach to integrate upconversion nanoparticles (UCNPs) with MOF to build a NIR-responsive composite photocatalyst. The MOF shell with controllable thickness can be grown on the UCNPs, thus exhibiting tunable photocatalytic activities under NIR irradiation. Furthermore, we extend visible absorption of the MOF shell by adding −NH2 groups so that the composite photocatalysts have a better utilization of UC emissions and sunlight to improve their activities. The developed composite photocatalysts have been characterized by XRD, TEM, PL, etc., and their photocatalytic performances were systematically explored. The formation and working mechanism of the composite photocatalysts were also elucidated.Keywords: core−shell structure; MIL-53(Fe); MOF; photocatalysts; upconversion;

Combination of upconversion nanocrystals (UCNs) with CeO2 is a decent choice to construct NIR-activated photocatalysts for utilizing the NIR light in the solar spectrum. Herein we present a facile approach to deposit a CeO2 layer with controllable thickness on the plate-shaped NaYF4:Yb,Tm UCNs. The developed core–shell nanocomposites display obvious photocatalytic activity under the NIR light and exhibit enhanced activity under the full solar spectrum. For enhancing the separation of photogenerated electrons and holes on the CeO2 surface, we sequentially coat a ZnO shell on the nanocomposites so as to form a heterojunction structure for achieving a better activity. The developed hybrid photocatalysts have been characterized with TEM, SEM, PL, etc., and the working mechanism of such UCN-semiconductor heterojunction photocatalysts has been proposed.

Fe-Based MOFs can serve as low cost, stable, eco-friendly and visible light-driven photocatalysts, exhibiting promising applications in a broad range of fields. However, the present modification or semiconductor-coupling routes are still unable to extend their light response to the NIR region. Herein we present a microwave-assisted method to prepare MIL-53(Fe) octahedra with uniform concave facets. This unique structural geometry enables MIL-53(Fe) to trap upconversion nanocrystals (UCNPs) on their concave surface with a stable morphology. The developed MIL-53(Fe)/UCNP composites exhibit remarkable photocatalytic activity under NIR light and enhanced performance under Vis-NIR light. Through modifying the organic linkers with amino groups, both light absorption and utilization of upconverted emission of the composites can be enhanced, showing an improved activity. The formation and working mechanisms of the MOF/UCNP composite photocatalysts are also explored.

•Core-shell NaYF4:Yb,Tm@SiO2@TiO2 NPs were synthesized via a sequential coating method.•Hybrid NaYF4:Yb,Tm@SiO2@TiO2 NPs show NIR-light enhanced photocatalytic activity.•NIR-driven antibacterial performance has been realized with NaYF4:Yb,Tm@SiO2@TiO2 NPs.•Working mechanism of the hybrid photocatalysts as antibacterial agents was proposed.ZnO is one of the most promising materials for both photocatalytic and antibacterial applications, but its wide bandgap requires the excitation of UV light which limits their applications under visible and NIR bands. Herein, we demonstrate a facile approach to synthesize core-shell-shell hybrid nanoparticles consisting of hexagonal NaYF4:Yb,Tm, amorphous SiO2 and wurtzite ZnO. The upconversion nanocrystals are used as the core seeds and sequentially coated with an insulting shell and a semiconductor layer. Such hybrid nanoparticles can efficiently utilize the NIR light through the upconverting process, and display notable photocatalytic performance and antibacterial activity under NIR irradiation. The developed NaYF4:Yb,Tm@SiO2@ZnO nanoparticles are characterized with TEM, XRD, EDS, XPS and PL spectra, and their working mechanism is also elucidated.Download high-res image (96KB)Download full-size image

•The g-C3N4-based NaYF4:Yb,Tm@TiO2 nanocomposite was fabricated by a facile approach.•The as-prepared composites exhibit enhanced activities under Vis and/or NIR lights.•Upconversion and semiconductor heterojunction lie behind the improved photocatalysis.•Photocatalytic working mechanism of this ternary nanocomposite was proposed.Upconversion (UC) NaYF4:Yb,Tm nanocrystals (NCs) are capable of converting low-energy near-infrared (NIR) photons to high-energy ultraviolet (UV) and visible (Vis) photons. Integration of NaYF4:Yb,Tm with graphitic carbon nitride (g-C3N4) can extend the spectral response of g-C3N4 to the NIR range. However, photocatalytic activity of NaYF4:Yb,Tm/g-C3N4 is still severely limited by the high recombination rate of photo-generated (PG) electrons and holes (e–/h+) in the g-C3N4. Herein, we report a facile approach to fabricate a ternary nanocomposite consisting of NaYF4:Yb,Tm, TiO2 and g-C3N4. When NaYF4:Yb,Tm NCs were coated with a TiO2 shell and sequentially assembled with g-C3N4 nanosheets (NSs), a semiconductor heterojunction can be fabricated on the UC particles. The as-prepared nanocomposites possess an enhanced photocatalytic activity under Vis and/or NIR lights due to the formation of heterojunction and UC effect. The ternary nanocomposites have been characterized in detail and their photocatalytic mechanism is proposed. Such kind of ternary nanocomposites may provide a new scenario for the design and synthesis of composite photocatalysts for efficiently utilizing the Vis/NIR lights in environmental remedy.

High-quality hexagonal NaYF4:Yb,Tm upconversion nanocrystals (UCNs) prepared in organic solutions display uniform sizes and strong UC emissions, but they possess a hydrophobic surface which hinders combining them with various semiconductor nanocrystals (NCs) to form a hybrid NIR-activated photocatalyst. Herein we present a facile approach to modify hydrophobic UCNs with a uniform carbon layer and enable them with hydrophilicity and surface functionalization. The carbon shell provides a good substrate for enriching with metal ions and in situ generation of CdS nanoclusters on the particle surface which can utilize both the upconverted UV and visible emissions. The developed NaYF4:Yb,Tm@C@CdS nanoparticles are characterized with TEM, SEM, XRD, PL and UV-Vis spectra and their formation mechanism is elucidated. The products display good photocatalytic activity under visible light and obviously enhanced performance under Vis-NIR light, due to the efficient utilization of UC emissions and the strong adsorption capacity of the carbon shell. The working mechanism of the hybrid photocatalysts is also proposed.

The construction of a p-n heterojunction is an efficient strategy to resolve the limited light absorption and serious charge-carrier recombination in semiconductors and enhance the photocatalytic activity. However, the promotion effect is greatly limited by poor interfacial charge transfer efficiency as well as reduced redox ability of charge carriers. In this work, we demonstrate that the embedding of metal Pd into the interface between n-type C3N4 and p-type Cu2O can further enhance the interfacial charge transfer and increase the redox ability of charge carriers through the design of the C3N4-Pd-Cu2O stack nanostructure. The embedded Pd nanocubes in the stack structure not only trap the charge carriers from the semiconductors in promoting the electron–hole separation but also act as a Z-scheme “bridge” in keeping the strong reduction/oxidation ability of the electrons/holes for surface reactions. Furthermore, Pd nanocubes also increase the bonding strength between the two semiconductors. Enabled by this unique design, the hydrogen evolution achieved is dramatically higher than that of its counterpart C3N4-Cu2O structure without Pd embedding. The apparent quantum efficiency (AQE) is 0.9% at 420 nm for the designed C3N4-Pd-Cu2O. This work highlights the rational interfacial design of heterojunctions for enhanced photocatalytic performance.Keywords: heterojunction; hydrogen evolution; interface design; metal; photocatalysis; stack nanostructure; Z-scheme

The combination of a metal with a semiconductor is a promising route to improve the solar-to-chemical conversion efficiency of photocatalysts. In this article, ultrathin Pd nanosheets are integrated with semiconductor TiO2 nanosheets for photocatalytic hydrogen evolution, which acts as a cocatalyst and plasmonic agent in ultraviolet and visible-near-infrared spectral regions, respectively. Owing to the unique two-dimensional (2D) nanostructure, the Pd nanosheet cocatalyst realizes the large TiO2–Pd interfacial area for electron transfer as well as a large Pd exposed area for reduction reactions, while the plasmonic Pd nanosheets offer strong vis-NIR light absorption for “hot” electron production as well as a large interfacial area for “hot” electron injection. As a result, the Pd nanosheets achieve improved photocatalytic activity in comparison with three-dimensional Pd nanotetrahedrons under both light irradiations. This work underlines the importance in choosing a suitable shape of metal in the surface and interface design of semiconductor–metal hybrid photocatalysts as well as the advantages of 2D metal nanostructures in realizing high photocatalytic performance.

•Ultrathin TiO2 hollow spheres were synthesized with an etching and reconstruction method.•Shell thickness of the hollow spheres was tuned through adjusting the etching temperature.•Shell thickness-dependent activity of photocatalytic hydrogen production was investigated.•The reasons for the higher photocatalytic activity with thinner shell were studied.Decreasing the shell thickness of TiO2 hollow spheres is a promising route to achieve an overall enhanced photocatalytic performance. However, in traditional synthetic approaches, the thin shell can not guarantee the structural stability of the TiO2 hollow spheres in the following calcination process to achieve a high degree of crystallinity. In this paper, ultrathin porous TiO2 hollow spheres with good structural stability, high degree of crystallinity as well as in small grain size were prepared through a novel etching and reconstruction route using amorphous SiO2/TiO2 hollow spheres as precursor. With hot water as etchant, the removal of SiO2 leads to the reconstruction of TiO2 hollow spheres and receiving a thinner and thinner shell. Such thin shell enables the exposure of more catalytic active sites of TiO2 nanocrystals and the improved photogenerated charge transfer and separation. Meanwhile, the residual SiO2 on the TiO2 shell protects the TiO2 nanocrystals from growing larger during high-temperature calcinations. The as-obtained ultrathin porous TiO2 hollow spheres exhibited enhanced photocatalytic activity in hydrogen production from water in comparison with those with thicker shell.

Synthesis of N-doped TiO2 hollow spheres with a small grain size, large surface area and high degree of crystallinity is favorable to achieve an overall high photocatalytic activity. But traditional synthetic approaches generally involve many tedious steps and are time-consuming. Here we present a facile method to synthesize N-doped TiO2 hollow spheres using organic silane-doped silica nanoparticles as templates. The organic–inorganic silica can be simultaneously etched during the deposition of TiO2 shell to create a thin silica layer on the TiO2 surface. Such a thin silica layer can efficiently protect the TiO2 nanocrystals from growing larger during high-temperature calcination. At the same time, the organic silane can serve as a nitrogen source to be doped in the TiO2 shell. The developed silica-protected and N-doped TiO2 hollow spheres can be directly employed as photocatalysts without SiO2 etching and show obvious light utilization in both UV and visible bands.

Development of upconversion nanocrystals (UCNs) under 808 nm excitation rather than 980 nm is very important to biological applications for avoiding tissue over-heating. Nd3+ and Yb3+ dual-sensitized UCNs are proven to be promising candidates but how to select host materials to construct core–shell UCNs with strong UC emissions still remains unexplored. Herein, we prepare a series of homogeneous and heterogeneous core–shell UCNs using NaYF4 and NaGdF4 as the core–host and/or as the shell–host, respectively, through the seed-mediated synthetic approach. Our results show that selecting the core–host in the core–shell UCNs plays the key role in determining their final UC intensities. Furthermore, homogeneous core–shell UCNs can give stronger UC fluorescence than the heterogeneous ones due to the low crystal defects at the core–shell interface. Moreover, the concentration effect of activator ions and sensitizer ions in these core–shell UCNs on their UC emissions is analyzed, and optimal doping under different NIR excitation (808 nm, 980 nm or 808 nm/980 nm) is achieved.

•An electrostatic assembly method is demonstrated for the preparation of hybrid materials consisting of both hydrophobic and hydrophilic nanostructures.•A hybrid photocatalyst of g-C3N4 nanosheets and NaYF4:Yb, Tm upconversion nanocrystals is developed.•Hybrid g-C3N4/NaYF4:Yb, Tm photocatatlysts can be activated under both visible and NIR lights.•Working mechanism of g-C3N4/NaYF4:Yb, Tm photocatalysts is proposed.Graphitic (g-) C3N4 is a promising visible light-driven photocatatalyst but it has a narrow absorption in the solar spectrum. To broaden its light utilization, here we present a strategy to integrate upconversion nanocrystals into g-C3N4 nanosheets by electrostatic assembly. This hybrid photocatalyst can be activated under both visible and near-infrared (NIR) lights, showing improved photocatalytic activity in the full solar spectrum. Phase, morphology and activity of this composite photocatalyst are characterized and its working mechanism is also proposed.

We demonstrate a facile method to directly coat a TiO2 layer on hydrophobic NaYF4:Yb,Tm upconversion nanocrystals. Through modifying the hydrophobic nanocrystals with a surfactant layer, a conventional sol–gel route can be applied to them for TiO2 deposition. The prepared β-NaYF4:Yb,Tm@TiO2 particles show obvious photocatalytic activity under near-infrared light as well as ultraviolet light. Based on the success in preparing upconversion cores with different sizes and subsequent TiO2 coating, we have systematically investigated the effect of core sizes on their photocatalytic performance. The results suggest that upconversion fluorescence of the core and surface of the shell both have a great effect on their photocatalytic activities. A moderate core size is thus preferred, due to the competition between these two effects.

Colloidal nanoparticle clusters (CNCs) of LaF3 nanoplates are developed by an emulsion method. Owing to their highly anisotropic shape, these nanoplates can be assembled into spherical particles with various assembly patterns. Investigation on the formation mechanism suggests that two kinds of surfactant, one from the surface of nanocrystals and one from the emulsion, play important roles in directing the assembly patterns of these nanoplates. After doping with lanthanide ions, LaF3:Yb,Er/Tm CNCs after assembly can show enhanced upconversion fluorescence compared to their counterparts in a free-standing form. Through loading fluorescent dyes in mesoporous pores of these LaF3 CNCs, one can finely tune their upconversion emissions through the FRET process.

Uniform-assembled lanthanide borate nanocrystals have been synthesized via a facile self-assembly process under hydrothermal conditions. All of the prepared lanthanide borate assemblies exhibit a spindle-like profile despite the fact that they belong to different crystal systems and have different formulas for composition. Each assembly is composed of small nanocrystals that are tightly attached along with their lateral surfaces. X-ray diffraction, scanning electron microscopy, and transmission electron microscopy have been used to characterize the structure and morphology of these samples. The mechanism responsible for the growth and assembly of these lanthanide borate assemblies is also demonstrated. After Eu3+ and/or Tb3+ ions are doped inside these assemblies, strong and multicolor emissions can be realized. Notably, tunable emission and a warm-white color can be achieved in the Eu3+/Tb3+ codoped samples.

Porous magnetic-luminescent Fe3O4@Y2O3:Ln (Ln = Eu, Yb/Er) nanoparticles (NPs) have been fabricated using carbonized ferrocene as templates. Through hydrothermal carbonization of ferrocene, carbon-coated Fe3O4 NPs were obtained, providing both a magnetic core and a good depositing surface to establish a uniform layer of Ln(OH)CO3:Ln. After annealing the pre-synthesized Fe3O4@C@Y(OH)CO3:Ln NPs in air, core–shell Fe3O4@Y2O3:Ln NPs with a porous morphology were synthesized. The as-synthesized NPs are uniform in size with a core diameter of 140 nm and a shell thickness of 20 nm. N2 adsorption–desorption isotherms reveal that these porous NPs possess a large surface area of 32 m2 g−1. After being doped with different lanthanide ions, these NPs can exhibit various down- and up-conversion emissions. Magnetic measurements also indicate that these NPs display superparamagnetic behavior and high magnetization at room temperature.

•We developed a facile route to synthesize upconversion YF3:Yb,Tm nanocrystals.•The nanocrystals exhibit an assembly structure with a peanut-like appearance.•Core/shell particles of YF3:Tb,Tm@TiO2 were obtained via a sol–gel process.•The YF3:Tb,Tm@TiO2 particles show good photocatalytic activity under the UV/NIR band.•The core particles of YF3:Yb,Tm can efficiently upconvert NIR light into UV one.We have developed a facile method to synthesize upconversion YF3:Yb,Tm nanocrystals in ethylene glycol. These nanocrystals exhibit an assembly structure with a peanut-like appearance. A layer of TiO2 shell can be facilely coated on the YF3:Yb,Tm nanocrystals via a sol–gel process and crystallized through annealing. The developed YF3:Tb,Tm@TiO2 particles show good photocatalytic activity under the UV band and work better in the UV/NIR band of a Xe lamp, as the core particles can efficiently upconvert NIR light into UV one.

Five-fold twinned structures are a class of important members in the family of metallic nanocrystals with face-centered cubic (fcc) structures, which can anisotropically grow into nanowires when their {100} facets are protected. In this communication, we first discover their unique growth mode that generates a new structure of palladium nanocrystals potentially enclosed by high-index facets, when the growth kinetics is manoeuvred.

Synthesis of NaYF4 nanocrystals with well-defined shapes and/or architectures has been of great interest in recent years, owing to their unique optical properties and wide applications in biological fields. NaYF4 nanocrystals with conventional morphologies have been developed through a variety of chemical or physical methods; however, it still remains challenging to build complex architectures by assembly well-defined NaYF4 nanocrystals in a simple system. Here, we present a facile solution-based method for synthesizing assembled NaYF4 nanocrystals at large scale. These assemblies exhibit a spherical appearance within which NaYF4 nanocrystals are tightly attached and arranged along the same crystalline orientation. This interesting hierarchical structure not only offers a high surface area and easy modification surface, but also provides a new efficient host for doping various lanthanide ions to give strong down- or up-conversion emissions. It is anticipated that these unique assembled NaYF4 nanostructures will serve as biolabels in various biomedical applications.

Sodium lanthanide fluorides (NaLnF4) are important luminescent materials due to their low-photon energies, high refractive indexes and excellent optical stabilities. Nanostructures of NaLnF4 crystals have been developed using a variety of methods. However, assembled NaLnF4 nanocrystals with ordered structures are still rarely obtained to date. Herein, we demonstrate a facile method to prepare NaLnF4 (Ln=Ce, Eu, Tm, Y) assemblies comprising of small NaLnF4 nanocrystals with an ordered structure. Interestingly, small NaLnF4 nanocrystals within a single assembly are tightly attached to one another and orientated along the same direction. Spectral investigation revealed strong down- and up-conversion emissions from these assemblies. A possible mechanism on the formation of these novel assemblies is also proposed here.Highlights► We developed a facile route to synthesize assembled NaLnF4 nanostructures. ► NaLnF4 nanocrystals in a single assembly are orientated along the same direction. ► The prepared NaLnF4 assemblies show strong down- and up-conversion emissions.

Lanthanide-doped upconversion nanocrystals which can convert near-infrared lights to visible lights have attracted growing interest because of their great potentials in biomedical engineering. However, it remains a grand challenge to maneuver the intensity ratio between different emission lines and enable tunable upconversion functions. Herein, we present a facile method to integrate NaYF4:Yb,Er upconversion nanocrystals with gold nanocrystals for constructing NaYF4:Yb,Er@SiO2@Au hybrid nanostructures, in which a silica layer on NaYF4:Yb,Er nanocrystals serves as an interface for gold decoration. The green-to-red emissions of the upconversion nanocrystals can be conveniently tuned by altering the amount of surrounding gold nanocrystals. We further demonstrate the capability of utilizing both green and red emission lines for spontaneous signaling in an emitter−quencher-based bioassay by implementing this novel hybrid nanostructure. It is anticipated that the controllability in upconversion fluorescence in this hybrid nanostructure may provide a platform for widely exploring applications in biological imaging, detection, and sensing.

Synthesis of N-doped TiO2 hollow spheres with a small grain size, large surface area and high degree of crystallinity is favorable to achieve an overall high photocatalytic activity. But traditional synthetic approaches generally involve many tedious steps and are time-consuming. Here we present a facile method to synthesize N-doped TiO2 hollow spheres using organic silane-doped silica nanoparticles as templates. The organic–inorganic silica can be simultaneously etched during the deposition of TiO2 shell to create a thin silica layer on the TiO2 surface. Such a thin silica layer can efficiently protect the TiO2 nanocrystals from growing larger during high-temperature calcination. At the same time, the organic silane can serve as a nitrogen source to be doped in the TiO2 shell. The developed silica-protected and N-doped TiO2 hollow spheres can be directly employed as photocatalysts without SiO2 etching and show obvious light utilization in both UV and visible bands.

We demonstrate a facile method to directly coat a TiO2 layer on hydrophobic NaYF4:Yb,Tm upconversion nanocrystals. Through modifying the hydrophobic nanocrystals with a surfactant layer, a conventional sol–gel route can be applied to them for TiO2 deposition. The prepared β-NaYF4:Yb,Tm@TiO2 particles show obvious photocatalytic activity under near-infrared light as well as ultraviolet light. Based on the success in preparing upconversion cores with different sizes and subsequent TiO2 coating, we have systematically investigated the effect of core sizes on their photocatalytic performance. The results suggest that upconversion fluorescence of the core and surface of the shell both have a great effect on their photocatalytic activities. A moderate core size is thus preferred, due to the competition between these two effects.

Colloidal nanoparticle clusters (CNCs) of LaF3 nanoplates are developed by an emulsion method. Owing to their highly anisotropic shape, these nanoplates can be assembled into spherical particles with various assembly patterns. Investigation on the formation mechanism suggests that two kinds of surfactant, one from the surface of nanocrystals and one from the emulsion, play important roles in directing the assembly patterns of these nanoplates. After doping with lanthanide ions, LaF3:Yb,Er/Tm CNCs after assembly can show enhanced upconversion fluorescence compared to their counterparts in a free-standing form. Through loading fluorescent dyes in mesoporous pores of these LaF3 CNCs, one can finely tune their upconversion emissions through the FRET process.

Development of TiO2-based photocatalysts capable of utilizing NIR light is very important for practical applications using the solar spectrum. The combination of upconversion nanocrystals (UCNs) with TiO2 is a promising route for activation of photocatalysts under NIR irradiation. Here we propose integration of dually sensitized UCNs with N-doped TiO2 with a core–shell–shell structure. In such composite photocatalysts, the core–shell UCNs exhibit enhanced UC emissions in response to two NIR bands and all the UC emissions can be utilized by the N-TiO2 shell. The doping manner of the UCN cores and the N-doping level of the TiO2 shell both have a significant effect on the photocatalytic activity. Our results suggest that rational design of the configuration of the component materials is a feasible way to improve the photocatalytic activity of such UCN/TiO2 nanocomposites.