微波輔助合成色控型CMC-Tb/Eu納米復(fù)合物的熒光性質(zhì)*

葉君 梁偉鵬 熊犍

(1.華南理工大學(xué) 制漿造紙工程國(guó)家重點(diǎn)實(shí)驗(yàn)室, 廣東 廣州 510640;2.華南理工大學(xué) 食品科學(xué)與工程學(xué)院, 廣東 廣州 510640)

稀土金屬配位有機(jī)化合物熒光材料因其亮度高、發(fā)光效率高等優(yōu)良的熒光性能而受到廣泛關(guān)注并得到推廣應(yīng)用,其中Tb3+或Eu3+型單稀土金屬配位化合物因高純度綠和紅光皆屬三原色范圍而得到研究者們的重視[1].然而單稀土金屬配位化合物因熒光材料光色單一、光色變化不可控,無(wú)法滿足現(xiàn)實(shí)需求,于是研究者便致力合成Tb-Eu雙稀土金屬有機(jī)配位化合物材料,通過(guò)調(diào)節(jié)兩者濃度比可得到可見(jiàn)光區(qū)中絕大多數(shù)光色,達(dá)到熒光材料光色可控的目的,以滿足實(shí)際具體需要[2].研究表明,在雙稀土金屬有機(jī)配位化合物材料中,Tb3+與Eu3+之間存在發(fā)生能量轉(zhuǎn)移的可能性[3].

配體是影響稀土金屬有機(jī)配位化合物材料結(jié)構(gòu)和性能的重要因素[4].有機(jī)配體不僅能夠克服稀土離子光吸收系數(shù)小的問(wèn)題,有效地敏化Tb3+和Eu3+,而且,不同于Si—O類的無(wú)機(jī)配體,它們往往具有性質(zhì)獨(dú)特、結(jié)構(gòu)多樣化、配位能力較強(qiáng)和配位方式靈活多變等特點(diǎn)[5- 6],復(fù)合物熒光產(chǎn)率高，熒光壽命也長(zhǎng),如已得到廣泛應(yīng)用的β-二酮、吡啶以及鄰二氮雜菲衍生物等有機(jī)配體[7].羧酸類化合物也受到研究者們的青睞，這得益于此類化合物中的羧基氧原子與Tb3+和Eu3+有強(qiáng)配位作用[8],能夠有效地提高復(fù)合物中Tb3+或Eu3+的發(fā)光效率[9].隨著綠色化學(xué)的興起,尋找無(wú)毒、無(wú)污染、穩(wěn)定、廉價(jià)且能與Tb3+或Eu3+進(jìn)行高效能量傳遞的配體日益顯得重要[10]。

羧甲基纖維素(CMC)是一種含羧基的纖維素衍生物,因具有可再生性、生物降解性和生物相容性,得到了廣泛的應(yīng)用[11].近年來(lái),Ag、Pt、Pd、 Cu、 Eu等金屬與CMC合成出金屬配位有機(jī)化合物的研究成為熱點(diǎn)[12- 14],但對(duì)CMC與Tb3+和Eu3+雙稀土金屬有機(jī)配位化合物的結(jié)構(gòu)、熒光性能和稀土間能量轉(zhuǎn)移過(guò)程的研究則尚鮮見(jiàn)報(bào)道.

微波合成是制備納米復(fù)合材料的綠色合成方法[15- 16].本研究采用微波輔助加熱將CMC與Tb3+/Eu3+反應(yīng),合成CMC/Tb-Eu雙稀土金屬納米復(fù)合物,并考察其結(jié)構(gòu)、熒光性能以及能量轉(zhuǎn)移.

1 實(shí)驗(yàn)

1.1 原料

CMC(純度99.9%,DS=0.92),鷹特化工(石家莊)有限公司產(chǎn)品;NaOH(AR),廣州市東紅化工廠產(chǎn)品;乙醇(AR)、HCl(AR)，廣州化學(xué)試劑廠產(chǎn)品;KBr(光譜純),天津市科密歐化學(xué)試劑有限公司產(chǎn)品;TbCl3·6H2O(99.9%),阿拉丁試劑有限公司產(chǎn)品;Eu2O3(光譜純),上海躍龍化工廠產(chǎn)品;透析膜(MWCO:2000),上海源葉生物科技有限公司產(chǎn)品.

1.2 實(shí)驗(yàn)方法

參照文獻(xiàn)[14]中的方法配制EuCl3溶液.

稱取2.000 g CMC溶于100 mL去離子水,按照表1加入TbCl3(0.031 8 mol/L)和EuCl3(0.034 6 mol/L)溶液,用1 mol/L的NaOH調(diào)pH值為7,在微波合成催化/萃取儀中,70 ℃下反應(yīng)10 min.將反應(yīng)液倒入透析袋中,浸泡于去離子水中直至用AgNO3溶液無(wú)法在去離子水中檢測(cè)出白色沉淀,將產(chǎn)物置于60 ℃烘箱中烘干至恒重,樣品粉碎備用.

表1 制備樣品時(shí)滴加的TbCl3和EuCl3溶液體積

Table 1 Volume of TbCl3and EuCl3solutions added in preparing samples

樣品nTb3+:nEu3+VTbCl3/mLVEuCl3/mLS110∶015.000.00S29∶113.501.38S37∶310.504.13S45∶57.506.89S53∶74.509.65S61∶91.5012.41S70∶100.0013.79S10∶1015.0013.79

1.3 測(cè)試與表征

采用德國(guó)Carl Zeiss公司生產(chǎn)的 EVO 18型掃描電子顯微鏡進(jìn)行復(fù)合物表面形貌分析,用Nano Measure1.2軟件計(jì)算平均粒徑;采用日本電子株式會(huì)社生產(chǎn)的JEM- 2100F型發(fā)射透射電子顯微鏡在200 kV下觀察復(fù)合物中納米粒子的形貌與粒徑;采用日本 SHIMADZU公司生產(chǎn)的UV- 2450型紫外可見(jiàn)分光光度計(jì)進(jìn)行紫外吸收檢測(cè),樣品研磨成粉裝入積分球中,測(cè)量波長(zhǎng)范圍200～600 nm;采用英國(guó) Kratos 公司的 Axis Ultra DLD 多功能X-射線電子能譜儀(XPS)測(cè)定納米復(fù)合物表面的元素化學(xué)狀態(tài)(分析室工作真空度為6.67×10-7Pa,采用單色化 A1 Kα 光源.能量為1 486.6 eV,10 mA×15 kV.束斑大小為700 nm×300 nm,掃描模式為CAE.全譜掃描160 eV,窄譜掃描40 eV);采用法國(guó)HORIBA Jobin Yvon公司生產(chǎn)的FluoroMax- 4型全功能穩(wěn)態(tài)及瞬態(tài)熒光光譜儀測(cè)定復(fù)合物熒光光譜,激發(fā)狹縫為2.5 nm,發(fā)射光狹縫為3 nm，樣品研磨成粉放入石英片中測(cè)定;采用英國(guó)Edinburgh Instruments 公司生產(chǎn)的的FLS920型穩(wěn)態(tài)/瞬態(tài)熒光光譜儀測(cè)定復(fù)合物熒光壽命,激發(fā)波長(zhǎng)350 nm.所有測(cè)試均在室溫下進(jìn)行.

2 結(jié)果與討論

2.1 復(fù)合物納米粒子的形貌結(jié)構(gòu)特征

CMC和復(fù)合物S4的SEM分析結(jié)果如圖1所示.圖1(b)右上角為其EDS譜圖.

由圖1可見(jiàn),CMC的表面形貌較為平滑規(guī)整和緊湊；而產(chǎn)物表面形貌則有很大的改變,表面形貌粗糙,出現(xiàn)許多裂痕,復(fù)合物表面出現(xiàn)均勻分散的小顆粒,因?yàn)槲⒉軌蚴菇饘匐x子高速運(yùn)動(dòng),有效阻礙了大規(guī)模團(tuán)聚使這些粒子分布均勻[18].EDS檢測(cè)出這些小顆粒含有Tb或/和Eu元素,證明這些小粒子正是目標(biāo)產(chǎn)物.SEM圖還表明復(fù)合物的形貌相似,只是粒徑大小和分布不同,利用Nano measurer 1.2軟件計(jì)算出這些粒子平均直徑見(jiàn)表2.

文中采用微波輔助加熱只需10 min即可得到具有納米尺寸的復(fù)合粒子,這比文獻(xiàn)[14,17]中30 min水浴加熱要節(jié)省能源和時(shí)間.這是因?yàn)槲⒉▓?chǎng)誘導(dǎo)能產(chǎn)生更有效地分子間碰撞[16].且受熱體系溫度均勻,這也保證了粒子的粒徑分布較窄[14,17].

圖1 CMC和S4的SEM圖Fig.1 SEM images of CMC and S4

Table 2 Average sizes of particles in each complex on CMC surfaces

樣品粒徑/nmS193S298S384S482樣品粒徑/nmS575S678S785

S4的TEM圖以及局部TEM放大圖如圖2所示.圖2(b)右上角為S4的電子衍射圖.

圖2(a)所圈的粒子在EDX結(jié)果中也顯示含有Tb3+和Eu3+,粒子為實(shí)心結(jié)構(gòu),且在CMC上均一分布,S4的粒子直徑約為1.82 nm(見(jiàn)圖2(a))，該值小于在SEM上所觀察到的粒子的粒徑，這是由于較高的表面能導(dǎo)致粒子在表面的聚集較內(nèi)部嚴(yán)重[14].在選定區(qū)域的晶格條紋中,S4相鄰的條紋距離為0.21 nm,其電子衍射圖呈現(xiàn)出多晶衍射圖樣(見(jiàn)圖2(b)右上角插圖),表明所合成的產(chǎn)物具有微小晶區(qū)的特征.

圖2 S4的TEM圖以及局部TEM放大圖Fig.2 TEM images of S4 and its partial magnified images

2.2 CMC與Tb3+或Eu3+的鍵合方式

CMC的C 1 s、O 1 s的XPS圖譜如圖3所示， S4的C 1s、O 1s以及Tb 3 d與Eu 3 d的XPS圖譜如圖4所示.

圖3 CMC的XPS圖譜Fig.3 XPS patterns of CMC

圖4 S4、Tb 3d、Eu 3d的XPS圖譜Fig.4 XPS patterns of S4,Tb 3d and Eu 3d

對(duì)S4的C 1s峰進(jìn)行分峰擬合得5個(gè)峰:284.36、285.27、286.10、286.84和288.19 eV.對(duì)比CMC的C 1s 圖譜可以看到,歸屬于吡喃糖環(huán)C—OH以及醚鍵中的C—O—C鍵(位于285.27和286.10 eV)變化較大,這是由于Tb3+和Eu3+與C—OH鍵以及醚鍵中的C—O—C鍵中的O原子作用，且Tb3+和Eu3+的離子半徑不同,從而產(chǎn)生兩個(gè)峰.而歸屬于C—C或C—H鍵O—C—O鍵及O—C—O鍵的峰變化不大,表明Tb3+和Eu3+與這些基團(tuán)沒(méi)有作用.

相比CMC的O 1s分峰擬合結(jié)果,S4的O 1s的分峰擬合結(jié)果有較大的變化(見(jiàn)圖4(b))，表明Tb3+和Eu3+與這些基團(tuán)O原子均有作用.

圖4(c)中處于1 276.34 eV的Tb 3d3/2峰歸屬于三價(jià)Tb鹽,而處于1 243.60 eV的Tb 3d5/2峰歸屬于三價(jià)Tb氧化物.光譜中還有兩個(gè)分別位于1 251.98和1 280.39 eV的衛(wèi)星峰,表明S4中不含有Tb2+和Tb4+的氧化態(tài)形式[21].圖4(d)中Eu3+的3d5/2峰在1 134.89 eV,其衛(wèi)星峰在1 125.78 eV處,兩者均歸屬三價(jià)Eu氧化物[22].Eu3+的3d3/2峰在1 165.21 eV,衛(wèi)星峰處于1 157.49 eV,兩者均歸屬于三價(jià)Eu鹽[23].

以上結(jié)果表明Tb3+和Eu3+與CMC間同時(shí)存在離子鍵和共價(jià)鍵兩種鍵合方式,且無(wú)氧化還原反應(yīng).

2.3 復(fù)合物納米粒子的發(fā)光性質(zhì)以及能量轉(zhuǎn)移

圖5是CMC和納米復(fù)合物的紫外吸收?qǐng)D譜.

圖5 CMC和CMC/Tb-Eu納米復(fù)合物的紫外吸收?qǐng)D譜

Fig.5 UV-Vis spectroscopy curves of CMC and CMC/Tb-Eu nanocomplexes

與CMC的紫外吸收?qǐng)D譜相比后發(fā)現(xiàn),納米復(fù)合物保留了CMC在202 nm處的吸收,且在250 nm處有新的吸收峰出現(xiàn),這是因?yàn)镃MC中—COO-,—OH與Tb(Ⅲ)或Eu(Ⅲ)形成的配位鍵所致.由此也證實(shí)了O—Tb(Ⅲ)和O—Eu(Ⅲ)的形成[24],稀土離子因f-f躍遷禁閉導(dǎo)致其在紫外-可見(jiàn)光的吸收系數(shù)小[25],納米復(fù)合物吸收主要為配體CMC的吸收,且新的吸收峰出現(xiàn)也增強(qiáng)了CMC的吸光能力.

圖6 S4在不同波長(zhǎng)光下的激發(fā)光譜

Fig.6 Excitation spectra of the S4 under different emissions

圖7 S1,S4和S7的發(fā)射光譜Fig.7 Emission spectra of S1,S4 and S7

圖9是S1-S7的色度圖.可以觀察到納米復(fù)合物的光色從綠色變化至黃橙再至紅橙終至紅色.這表明可以通過(guò)控制Tb3+和Eu3+的比例來(lái)實(shí)現(xiàn)CMC-Tb/Eu納米復(fù)合物熒光光色可控的目的.

圖8 S2-S6的發(fā)射圖譜(插圖為S6的分峰圖)

Fig.8 Emission spectra of S2-S6(the inset is the deconvolutation of the peak of the spectra of S6)

圖9 S1-S7的色度圖Fig.9 CIE Chromaticity diagram of S1-S7

樣品Ii/I0S20.97S30.80S41.00樣品Ii/I0S50.72S60

在激發(fā)光波長(zhǎng)為350 nm的條件下S1和S位于545 nm處發(fā)射峰的熒光壽命可以利用式(1)計(jì)算[30]:

(1)

3 結(jié)論

通過(guò)對(duì)所合成的一系列CMC-Tb/Eu復(fù)合物的結(jié)構(gòu)和性能的研究,得出以下主要結(jié)論:

(1)所合成的 CMC-Tb/Eu復(fù)合物粒子的平均直徑為75～98 nm,并為實(shí)心結(jié)構(gòu)；

(2)Tb3+和Eu3+與CMC上的—OH、COO-和醚鍵發(fā)生了配位及離子鍵合；

(3)通過(guò)Tb3+—O和Eu3+—O配位鍵CMC可將所吸收的能量傳遞給Tb3+和Eu3+,實(shí)現(xiàn)“天線效應(yīng)”,且傳遞給Tb3+的效率要遠(yuǎn)大于給Eu3+的.

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