| In recent years, rare earth upconversion luminescent nanomaterials have beenwidely used in the fields of solar cell, bioimaging and photocatalysis. Among theserare earth upconversion luminescent nanomaterials, rare earth fluorides have attractedmany attentions due to their low phono energy and high chemical stability. It is wellknown that the fluorescent properties of the unconversion luminescence materialslargely depend on the matrix, doping element and the preparation method. It is a vitalproject to synthesize upconversion materials with various sizes and morphologiesusing different matrices, doped elements and preparation methods, as well as study oftheir upconversion luminescent properties and applications.As a potential photocatalyst, titanium dioxide (TiO2) has been widely used fortreating organic pollutions. However, TiO2can only absorb the ultraviolet light insolar light due to its board band-gap. The ultraviolet light constitutes only3%5%ofthe solar light. Upconversion luminescent materials can turn the visible and infraredlights into the ultraviolet light which can be absorbed by the TiO2. Thus, combiningthe upconversion luminescent materials with TiO2can expand the light response rangeand elevate the reaction activity of the photo-generated electrons and holes,resultingin a significant improvement of photocatalytic activity of TiO2.In this study, two kinds of methods were utilized to synthesize rare earthfluorides upconversion luminescent materials with various matrices and dopingelements. Their morphologies, grain sizes and crystallines were characterized, andtheir upconversion luminescent properties were investigated. Finally, a novelcomposite photocatalyst of NaLuF4: Gd, Yb, Tm/TiO2was prepared throughcombining the as-prepared rare earth upconversion luminescent nanocrystals and TiO2.The photocatalytic properties of the composite were studied by photodegradation ofdyes and dye wastewater. The main contents of this study are listed below:(1) Five kinds of upconversion nanocrystals of NaYF4:Yb, Tm，NaYF4:Gd, Yb,Tm，NaGdF4:Yb, Tm，NaLuF4:Yb, Tm and NaLuF4:Gd, Yb, Tm were synthesizedsuccessfully by a solvothermal method, using NaYF4, NaGdF4and NaLuF4as matrices, respectively. Their morphologies, grain sizes, crystalline phases andcompositions were characterized and their upconversion luminescent (UCL)properties were investigated. It is found that the crystalline phase and upconversionluminescent property of the nanocrystals are largely impacted by the matrix anddoping element. NaYF4:Yb, Tm is mixed-phase including cubic and hexagonal phases,NaYF4:Gd, Yb, Tm is pure hexagonal phase, while NaLuF4-based nanocrystals arecubic phases and NaGdF4-based nanocrystals are pure hexagonal phases. It indicatesthat Gd3+can induce a phase transformation from cubic to hexagonal phase and hostsof NaGdF4:Yb, Tm，NaLuF4are apt to form pure hexagonal. The UCL intensities ofthe nanocrystals are in the sequence (from strong to weak) of NaLuF4:Gd, Yb, Tm，NaLuF4:Yb, Tm，NaGdF4:Yb, Tm，NaYF4:Gd, Yb, Tm and NaYF4:Yb, Tm. Theseresults demonatrate that doping Gd3+is favorable for the UCL and the NaLuF4is themost excellent host for UCL among the three kinds of matrices.(2)The above five kinds of upconversion luminescence materials were synthesizedsuccessfully by hydrothermal method. The results show that the nanocrystals preparedby hydrothermal method are all hexagonal phase crystallines, with uniformmorphologies and big sizes (300-1400nm), while their upconversion luminescentintensities are all weaker than that of counter-parts prepared by solvothermal method.The upconversion luminescent intensities of the nanocrystals are in the sequence(from strong to weak) of NaLuF4:Gd, Yb, Tm，NaLuF4:Yb, Tm，NaGdF4:Yb, Tm，NaYF4:Gd, Yb, Tm and NaYF4:Yb, Tm, it is similar to the solvothermal method.（3）A novel nanocomposite photocatalyst of NaLuF4: Gd, Yb, Tm/TiO2has beendeveloped. The nanocomposite is composed of hexagonal phase nanocrystal NaLuF4:Gd, Yb, Tm (45nm) and anatase TiO2(10nm), where the small sized TiO2grains areformed and dispersed upon around the surface of the nanocrystals. Its photocatalyticactivity is evaluated through photocatalytic degradation dyes of rhodamine B (RhB),methyl blue (MB), methyl orange (MO), dye mixture and dye wastewater under thesimulated solar light irradiation, with measurement of degradation ratios and chemicaloxygen demand (COD) removal ratios. The results show that the as-preparednanocomposite displays high photocatalytic activity which is significant higher than that of commercial P25. Under simulated sunlight irradiation250min using thecomposite as catalyst, the degradation rate of composite photocatalys for the dyes anddye mixture were98.7%,95.5%,82.9%and92.4%respectively; while the CODremoval ratio of dyes, dye mixture and wastewater were77.5%,75.4%,63.5%,56.3%and44.7%respectively. The results imply that the prepared composite is a potentialphotocatalyst that can be applied in photocatalytic degradation of dye wastewater andother organic pollutions under solar light irradiation. |
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