前驱溶液的pH值对制备Ca2Zn4Ti16O38:Pr(上标 3+)，Na(上标 +)发光粉物相、形貌和发光性质的影响

The phosphor Ca2Zn4Ti16O38:Pr(superscript 3+), Na(superscript +) has received increasing interest owing to its high chemical stability, broad red emission and special long afterglow properties. However, the sintering temperature for obtaining pure Ca2Zn4Ti16O38:Pr(superscript 3+), Na(superscript +) is confirmed to be 950℃ and the sintering period is 21 h for sol-gel synthesis of this phosphor. So, optimization of synthesis conditions of Ca2Zn4Ti16O38 must be carried out. It is most important to get uniform and transparent gel in the sol-gel process. The results show that the mixture solution is clarification when pH≤3 (adjusted by HNO3 solution). While increasing the pH value (adjusted by ammonia solution) the mixture solution begin to come forth muddy. After dried in 150℃, the exrogel shows yellow color under pH≤3 and others present black. After further heated at 400℃ for 2 h, a dark dry gel is obtained and ground in an agate mortar to black powder serving as the precursor for the present work. The final phosphor powder is obtained after reheated at 900℃ for 8 h. Crystalline phases, morphology and photoluminescence properties have been characterized by using powder X-ray diffractometry (XRD), scanning electron microscopy (SEM) and fluorescence spectrophotometer, respectively. XRD patterns show that the synthesized Ca2Zn4Ti16O38 powders possess well structure when pH≤3. Extra-resultants such as CaTiO3, Zn2TiO4 and TiO2 appeared when pH≥5. The SEM results showed that the samples obtained at pH≤3 are fleecy and uniform. With the increasing pH value, particles gradually become larger, the reunion and sinter phenomenon become seriousevident. TG-DAT curves reveal the different various trends for the precursors of pH=3 and pH=9. The electrical alkoxy-OR of Ti(OC4H9)4 make metal ions Ti(superscript 4+) highly vulnerable by nucleophilic attack. Ti(OC4H9)4 easily hydrolyzes or polymerizes no matter what in acid or alkaline conditions. When pH≤3, H(superscript +) speeds up the hydrolysis of Ti(OC4H9)4 and to replace the form of coordination of tetrabutyl titanate alkoxy, direct bonding with titanium ion formed by hydrolysis of Ti(OH) (subscript x)(AC)(subscript y)(with x+y=4). However, when pH=9, a large number of OH-ions exists in Ti(OC4H9)4 hydrolysis and produces Ti(OH)4 or TiO(OH)2, and that the other cations Ca(superscript 2+), Zn(superscript 2+) and Pr(superscript 3+) from Ca(OH)2, Zn(OH)2 and Pr(OH)3 will not mixe with each other effectively and will not form other phases. Thereby, keeping the pH≤3 is necessary for cations to uniformly mix to inhibit Ti(OC4H9) directly hydrolyzed to form Ti(OH)4 precipitation. Excitation and emission spectra indicate that the phosphors exhibit the highest excitation bands in the visible region in the ranges of 450~495 nm originated from 4f→4f transition, as well as red emitting of Pr(superscript 3+) at 613 nm(1D2→3H4) and 644 nm(3P0→3F2) when pH≤3, which result in, upon excitation with 475 nm, the red emission at 644 nm decays slower than that of pH≥5. This fact suggests that Ca2Zn4Ti16O38:Pr(superscript 3+), Na(superscript +) obtained via acid sol-gel process (pH≤3) gives the good red long afterglow under visible light irradiation.