【作者】 李静；

【导师】 李继光；

【作者基本信息】 东北大学 ， 材料学， 2020， 博士

【摘要】 本工作以硝酸盐型稀土层状氢氧化物Ln2(OH)5NOnH2O(以下略记为LLnH,Ln=Pr～Lu镧系元素及Y)为研究对象,就其常压下的可控合成、氟化反应以及氟化产物的性状和典型稀土激活离子在所得氟化物中的上、下转换发光性能进行了较系统研究。开发了两步结晶新技术,成功解决了小半径镧系元素(Ln=Tm～Lu)的纯相LLnH难以合成的难题;分别以NaF、KF和NH4F为氟源,在室温至90℃的较低温度下研究了LLnH的氟化动力学和氟化机理,明确了镧系收缩(Ln3+的半径渐变)和氟源的影响规律,成功制备出了Ln(OH)3-xFx、LnF 3、AF-LnF3系复式氟化物(A分别为NH4+、Na+、K+)和Na(x+y-3)Ln(OH)xFy·nH2O类新型化合物;研究了典型氟化产物在空气中的热分解行为和煅烧过程中的物相演化;以直接氟化产物及其煅烧产物为基质,分析了重要发光离子En3+、Tb3+和Yb3+/Er3+的上转换或下转换发光性能和机理。主要创新性成果总结如下:(1)研究了LLnH纳米片的低温(约4℃)一步沉淀合成,发现产物的相组成和晶体结构特征与Ln离子半径密切相关,其中半径最大的La3+的产物为层间NO3-直接参与配位的La(OH)2NO3·nH2O化合物,离子半径居中的Ln元素(Ln=Pr～Er及Y)的产物为层间NO3-属非配位型自由离子的LLnH纳米片晶(厚约4nm),其晶体结构的a-b面和层间距分别倾向于随Ln3+的半径减小而收缩和扩张,而离子半径最小的Tm、Yb、Lu三元素的产物为准晶态纳米颗粒。将后三种低温沉淀产物于50～65℃下晶化处理(二步结晶),获得了结晶性良好的微米级LLnH晶体,发现LTmH和LYbH中的层间NO3-为自由离子、可与十二烷基磺酸根离子(DS-)进行阴离子交换且交换产物可在甲酰胺中自发剥离为直径达微米量级、厚约1.7 nm的单层纳米片,而LLuH因层间硝酸根参与配位而不具有离子交换和剥离性。(2)以L(Y,RE)H(RE=Eu或Yb/Er)为自牺牲模板、NH4F为氟源,探究了室温条件下n(NH4F):n(Ln3+)(R)的值对氟化过程和氟化产物的影响。发现随着R值增大,L(Y,RE)H先经层间阴离子交换生成L(Y,RE)H-F-((Y,RE)2(OH)5F·nH2O)再通过溶解再沉淀等反应过程而逐渐转变为六方晶(Y,RE)(OH)1.57F1.43、立方晶NH4(Y,RE)3F10和立方晶NH4(Y,RE)2F7。氟化反应过程中未出现(Y,RE)F3物相。分析了Eu3+在NH4Y3F10和NH4Y2F7及其煅烧产物YF3晶格中的光谱特性并研究了 Yb3+/Er3+共掺杂YF3在978 nm激光激发下的上转换发光性能和发光机理。(3)以L(Y,Eu)H为自牺牲模板、KF为氟源,经较低温度(约90℃)下的氟化反应获得了六方晶(Y,Eu)(OH)3-xFx(x=1.15～1.51)和立方晶K5Ln9F32。发现产物的氟的摩尔分数与反应条件密切相关且倾向于随n(KF):n(Ln3+)的值的增加而增大;发现空气中450℃煅烧(Y,Eu)(OH)3-xFx所得产物为正交晶(Y,Eu)O(3-y)/2Fy而高于700℃时(Y,Eu)O(3-y)/2Fy逐渐被氧化为六方晶(Y,Eu)OF乃至立方晶(Y,Eu)2O3。研究了上述不同物相中Eu3+的发光性能,明确了F的摩尔分数和煅烧温度的影响及其作用机理。(4)以LLnH为自牺牲模板、NaF为氟源,经约90℃下的氟化反应制备出了结晶性良好的Na(x+y-3)Ln(OH)xFy·nH2O新型氟化物(Ln=La～Lu及Y),发现其生成需经历溶解再沉淀等过程且晶粒倾向于沿[001]生长。XRD谱图的初步精修结果表明该类化合物属六方晶系(P63(173)空间群),其晶格常数a和c均随Ln3+的半径递减而渐小。分析了 Na0.56Eu(OH)1.06F2.50·0.63H2O和Nao.50Tb(OH)1.11F2.39·1.0H2O的下转换发光性能,发现Eu3+和Tb3+分别以617 nm(5D0→7F2跃迁)红光和545 nm(5D4→7F5跃迁)绿光发射为主。978 nm激光激发下,Yb3+/Er3+共掺杂的Na0.60Y(OH)0.94F2.66·0.14H2O呈现Er3+的红光和绿光上转换发射,其发光过程均属双光子机制。(5)以LLnH为自牺牲模板、NaF为氟源(n(NaF):n(Ln3+)=167),经室温下3 h氟化反应而合成出了一系列具有六方晶系结构的β-NaLnF4化合物(Ln=Pr～Lu镧系元素及Y)。巧妙利用其与LLnH晶体结构上的相似性克服了 β相生成所需的较高势垒,从而避免了不利于上转换发光的立方晶体结构α相的生成。以LYH为例研究了氟化反应动力学,发现NaF与Ln3+的摩尔比对β-NaYF4成相起到至关重要的作用。以β-NaYF4为典型基质,研究了 Yb3+/Er3+掺杂对的上转换发射光谱和发光机理,发现Er3+的红光与绿光发射均属双光子过程。以β-NaGdF4为基质,研究了 Tb3+、Eu3+和Tb3+/Eu3+激活剂的发光性能,分析了 Gd3+至Tb3+、Gd3+至Eu3+以及Tb3+至Eu3+的能量传递现象和传递机理,并通过改变Tb3+/Eu3+共掺杂离子对中的Eu3+的摩尔分数而实现了 274 nm激发下(Gd3+的8S7/2→6IJ受激跃迁)的荧光调色。更多还原

【Abstract】 This Doctoral Dissertation focuses on the nitrate-type layered rare-earth hydroxide of Ln2(OH)5NO3·nH2O(abridged as LLnH,Ln=Pr-Lu lanthanides and Y),including controllable synthesis and fluorination,characteristics of the fluorination products,and down-/up-conversion luminescence of typical rare-earth activators in the derived fluoride compounds.Through a novel two-step crystallization technique,the LLnH of smaller Ln3+(Ln=Tm-Lu),which is difficult to synthesize,was obtained in a phase-pure form.With NaF,KF and NH4F as fluorine sources,respectively,the kinetics and mechanisms of LLnH fluorination and the influence of fluorine source and lanthanide contraction were clarified,and the various products of Ln(OH)3-xFx,LnF3,complex fluorides in the AF-LnF3 binary system(A=NH4+,Na+and K+,respectively)and Na(x+y-3)Ln(OH)xFy·nH2O new compound(Ln=Pr-Lu and Y)were successfully produced under relative low temperatures of up to 90℃.The thermal behavior and phase transition during air calcination of typical fluorination products were also investigated.With the direct products of fluorination and their calcination derivatives as host lattices,the properties and mechanisms of down-/up-conversion photoluminescence were systematically investigated for the important activators of Eu3+,Tb3+ and Yb3+/Er3+.The main achievements of this work are summarized as follows.(1)LLnH synthesis via low-temperature(～4℃)precipitation revealed that phase constituent and structural feature of the product were closely related to the ionic radius of Ln3+.It was found that La(OH)2NO3·nH2O was formed for the largest La3+ ion,where the interlayer NO3-is coordinated to La3+,LLnH nanosheets(about 4 nm thick)were crystallized for relatively smaller Ln3+of Ln=Pr-Er and Y,where the interlayer NO3-is free anion,and quasi-crystalline masses were generated for the three smallest Ln3+ of Ln=Tm-Lu.Aging the Ln=Tm-Lu products at 50-65℃ produced highly crystalline and micron-sized LLnH crystals.It was further demonstrated that LTmH and LYbH accommodate free NO3-anions in the interlayer gallery,which are readily exchangeable with dodecyl sulfonate(DS-)anions.Delamination of the DS-derivatives in formamide produced micron-sized unilamellar nanosheets of～1.7 nm thick.A similar anion-exchange and exfoliation operation were found to hardly proceed for LLuH owing to direct coordination of the interlayer NO3-with Lu3+.(2)With LLnH as sacrificial template and NH4F as fluorine source,the effect of NH4F/Ln molar ratio(R)on the course of fluorination and characteristics of the product were investigated.It was found that,with increasing R,LLnH would experience anion exchange of the interlayer NO3-t o form LLnH-F(Ln2(OH)5F·nH2O)and then dissolution and reprecipitation to yield hexagonal structured Ln(OH)1.57F1.43,cubic NH4Ln3F10 and finally cubic NH4Ln2F7.The LnF3 phase was not identified during the course of fluorination.The photoluminescence of Eu3+ in NH4LnF10 and NH4Ln2F7 and their calcinationderived YF3 was investigated,and the property and mechanism.of up-conversion luminescence of Yb3+/Er3+pair in YF3 were also studied.(3)Fluorination of LLnH with KF at the relatively low temperature of 90℃produced hexagonal structured Ln(OH)3-xFx(x=1.15-1.51)and cubic K5Ln9F32.It was shown that the actual F content of the product is closely dependent on reaction conditions and tends to increase with increasing KF/Ln molar ratio.Calcining Ln(OH)3-xFx at 450℃ in air produced LnO(3-y)/2Fy,and oxidation of LnO(3-y)/2Fy to form hexagonal LnOF and even cubic Ln2O3 was observed at temperatures above 700℃.The photoluminescence of Eu3+in the above different phases was thoroughly analyzed to clarify the influence of F content and calcination temperature.(4)Fluorination of LLnH with NaF at 90℃ produced a series of well-crystallized Na(x+y-3)Ln(OH)xFy·nH2O new compounds,which were shown to form via dissolution-reprecipitation and tend to grow up along the[001]crystallographic direction.Preliminary analysis of the XRD pattern via structure refinement suggested that the group of compounds were crystallized in the hexagonal system(space group:P63(173)),whose lattice parameters a and c decrease towards a smaller Ln3+.The Na0.56Eu(OH)1.06F2.50·0.63H2O and Na0.50Tb(OH)1.11F2.39·1.0H2O compounds were analyzed to show luminescence dominated by the 617 nm red(5D0→7F2 transition)and 545 nm green(5D4→7F5)emissions,respectively.Yb3+/Er3+ codoped Na0.60Y(OH)0.94F2.66·0.14H2O,on the other hand,exhibits green and red up-conversion luminescence of Er3+ under 980 nm laser excitation,which were analyzed to involve a two-photon process.(5)Fluorination of LLnH with NaF(NaF/Ln=167 molar ratio)at room temperature for 3 h produced a series of hexagonal structured β-NaLnF4 compounds(Ln=Pr-Lu and Y).With the structure similarities between LLnH andβ-NaLnF4,the high-energy barrier for the β-phase to crystallize was overcome and formation of the a-phase,which is less-attractive as a host for un-conversion luminescence,was avoided.With LYH as a typical example,it was shown that NaF/Ln molar ratio played an essential role in β-phase crystallization.The spectrum and mechanism of Yb3+/Er3+ up-conversion luminescence were investigated with the most important β-NaYF4 host lattice,and the red and green emissions of Er3+ were analyzed to involve a two-photon process.With β-NaGdF4 as host,the down-conversion photoluminescence of Tb3+,Eu3+ and Tb3+/Eu3+ pair were thoroughly investigated,and the energy transfers from Gd3+ to Tb3+,from Gd3+ to Eu3+ and from Tb3+ to Eu3+ were discussed.Furthermore,tailoring the emission color under 274 nm excitation(the 8S7/2→6IJ transition of Gd3+)was achieved for β-NaGdF4:Tb/Eu phosphor through adjusting the Eu content.更多还原