葛夢(mèng)然 李丹丹 姚 濤 潘國(guó)強(qiáng)

（中國(guó)科學(xué)技術(shù)大學(xué) 國(guó)家同步輻射實(shí)驗(yàn)室 合肥 230029）

Cu 納米團(tuán)簇發(fā)光性能和結(jié)構(gòu)的研究

葛夢(mèng)然 李丹丹 姚 濤 潘國(guó)強(qiáng)

（中國(guó)科學(xué)技術(shù)大學(xué) 國(guó)家同步輻射實(shí)驗(yàn)室 合肥 230029）

利用光致發(fā)光譜(Photoluminescence, PL)、X射線吸收精細(xì)結(jié)構(gòu)(X-ray Absorption Fine Structure, XAFS)和MALDI-TOF質(zhì)譜等技術(shù)研究了化學(xué)還原法合成的Cu金屬納米團(tuán)簇的發(fā)光性能及其結(jié)構(gòu)。PL譜表明隨著反應(yīng)溶液中十二硫醇(C12SH)和2-巰基-5-正丙烷基嘧啶(C7H10N2S, 2-mercapto-5-n-propylpyrimidine, MPP)配體比例的增加，Cu納米團(tuán)簇主發(fā)光峰的波長(zhǎng)從618 nm逐漸藍(lán)移到571 nm。質(zhì)譜的結(jié)果說(shuō)明以MPP作為單一配體時(shí)的主要產(chǎn)物為Cu5[MPP]3；而當(dāng)為MPP與C12SH兩種混合配體時(shí)，Cu納米團(tuán)簇中Cu原子數(shù)變小，組成變?yōu)镃u4[MPP][C12SH]；并且隨著C12SH比例增加，Cu4[MPP][C12SH]產(chǎn)物的組成保持不變。XAFS結(jié)果則進(jìn)一步表明隨著C12SH比例的增加，Cu納米團(tuán)簇的Cu-S鍵長(zhǎng)從0.228 nm縮短到0.224 nm，原子構(gòu)型從雙三角錐結(jié)構(gòu)轉(zhuǎn)變?yōu)樗拿骟w。綜合以上結(jié)果，認(rèn)為Cu納米團(tuán)簇的原子數(shù)的減少導(dǎo)致團(tuán)簇的光致發(fā)光從618nm藍(lán)移至597 nm；而Cu-S鍵長(zhǎng)的縮短引起Cu(I)-S雜化的HOMO-LUMO帶隙增大，從而導(dǎo)致團(tuán)簇的發(fā)光波長(zhǎng)進(jìn)一步從597 nm藍(lán)移至571 nm。

Cu納米團(tuán)簇，發(fā)光可調(diào)，X射線吸收精細(xì)結(jié)構(gòu)(XAFS)，同步輻射

金屬納米團(tuán)簇（尺寸通常小于2 nm）因其獨(dú)特的物理和化學(xué)性質(zhì)，以及在發(fā)光和催化等領(lǐng)域的廣泛應(yīng)用前景，近年來(lái)逐漸成為納米材料研究領(lǐng)域的熱點(diǎn)[1-3]。由于特殊的量子尺寸效應(yīng)，金屬納米團(tuán)簇的電子能級(jí)將呈現(xiàn)出與大的納米晶所不同的離散型特征，主要體現(xiàn)在光致發(fā)光和光吸收的變化[4-7]。長(zhǎng)期以來(lái)，該領(lǐng)域的研究主要集中在貴金屬（如Au、Ag等）納米團(tuán)簇的合成和結(jié)構(gòu)表征上，例如Au25(SR)18、Au102(SR)44（SR=巰基配體）等納米團(tuán)簇已被合成，并確定其結(jié)構(gòu)[8-9]。然而，由于固有的不穩(wěn)定性，關(guān)于Cu納米團(tuán)簇的合成和性能調(diào)控研究卻很少[8-11]，Cu納米團(tuán)簇具有獨(dú)特的電子[12]、光學(xué)[13]和催化性能[14]，同時(shí)作為過(guò)渡族金屬，儲(chǔ)量豐富，實(shí)現(xiàn)對(duì)Cu納米團(tuán)簇的可控合成并對(duì)其性能進(jìn)行深入調(diào)控將具有重要的應(yīng)用價(jià)值。

近年來(lái)，一些穩(wěn)定的不同原子個(gè)數(shù)的Cu納米團(tuán)簇合成已經(jīng)相繼被報(bào)道[14-16]，例如Salorinne等[17]最近成功制備出苯并三唑包覆的CumBTAn(m≤6)納米團(tuán)簇，并計(jì)算出Cu6BTA4是正四面體的Cu4金屬內(nèi)核被兩個(gè)BTA-Cu(I)-BTA保護(hù)的結(jié)構(gòu)。陳衛(wèi)課題組[18]利用2-巰基-5-正丙烷基嘧啶作為表面活性劑，通過(guò)單相法合成出具有425 nm和593 nm雙發(fā)射波長(zhǎng)的Cun(n≤8)納米團(tuán)簇，并在氧還原反應(yīng)中表現(xiàn)出良好的電催化性能。但是從發(fā)光性能來(lái)看，目前報(bào)道的Cu納米團(tuán)簇發(fā)光波長(zhǎng)無(wú)法調(diào)節(jié)，對(duì)發(fā)光機(jī)理研究也一直是一個(gè)難于解決的問(wèn)題。最近，對(duì)納米團(tuán)簇發(fā)光機(jī)理認(rèn)識(shí)有了一定的進(jìn)展。例如，Wu等[19]基于[Au25(SR)18]q納米團(tuán)簇（q是團(tuán)簇的電荷值）的發(fā)光性研究，發(fā)現(xiàn)金屬納米團(tuán)簇的發(fā)光主要來(lái)自于團(tuán)簇表面配體-金屬作用和金屬內(nèi)核。

本文通過(guò)調(diào)節(jié)初始配體2-巰基-5-正丙烷基嘧啶(MPP)和十二硫醇(C12SH)的比例，合成出從618nm到571 nm可控發(fā)光的Cun(n≤6)納米團(tuán)簇。利用紫外可見吸收光譜(Ultraviolet-visible spectroscopy, UV-vis)、基質(zhì)輔助激光解吸-飛行時(shí)間質(zhì)譜儀(Matrix Assisted Laser Desorption Ionization Time of Flight Spectrometer, MALDI-TOF)質(zhì)譜和X射線吸收精細(xì)結(jié)構(gòu)(X-ray Absorption Fine Structure, XAFS)技術(shù)對(duì)不同初始比例生成的Cu納米團(tuán)簇的結(jié)構(gòu)進(jìn)行研究，利用最小二乘法對(duì)XAFS函數(shù)FT(k3χ(k))進(jìn)行模擬計(jì)算，明確初始配體中C12SH量對(duì)Cu納米團(tuán)簇的結(jié)構(gòu)影響，并了解其發(fā)光機(jī)理。

1 實(shí)驗(yàn)

硝酸銅、硼氫化鈉購(gòu)于上海國(guó)藥化學(xué)；四正辛基溴化銨購(gòu)于阿拉?。籑PP購(gòu)于Alfa Aesar?？煽匕l(fā)光的Cu納米團(tuán)簇的合成方法基于對(duì)已有的方法進(jìn)行改進(jìn)得到[18]。硝酸銅(0.025 g，0.103 mmol)和四正辛基溴化銨(0.136 g，0.249 mmol)共溶在乙醇中(25 mL)，再將其在80 °C環(huán)境下攪拌30 min。然后把該溶液在冰水中冷卻后加入MPP或MPP和C12SH的初始混合配體。溶液在氬氣流下劇烈攪拌反應(yīng)6 h后加入硼氫化鈉(0.047 g，1.241 mmol)和乙醇(5 mL)繼續(xù)反應(yīng)7 h。經(jīng)離心分離10 min、轉(zhuǎn)速16 000 r·min-1和乙醇洗滌多次，最后將沉淀的產(chǎn)物再分散在乙醇中以供進(jìn)一步使用。

Cu納米團(tuán)簇的MALDI-TOF質(zhì)譜在Bruker Autoflex III質(zhì)譜儀上測(cè)得，基質(zhì)采用反式-2-[3-(4-叔丁基苯基)-2-甲基-2-丙烯]丙二腈(DCTB)，與樣品摩爾比例是1:1000，測(cè)量范圍：500-1100 Da（避免基質(zhì)在465 Da的峰）。Cu納米團(tuán)簇的Cu邊X射線吸收譜在北京同步輻射裝置(Beijing Synchrotron Radiation Facility, BSRF)1W1B線站測(cè)得，儲(chǔ)存環(huán)電子能量為2.5 GeV，最大電流強(qiáng)度為250 mA。XAFS實(shí)驗(yàn)數(shù)據(jù)通過(guò)IFEFFIT的Athena分析后，利用IFEFFIT的Artermis進(jìn)行擬合。

2 結(jié)果與討論

圖1(a)展示不同初始配體比例的Cu納米團(tuán)簇在365 nm激發(fā)光下的光致發(fā)光譜(Photoluminescence, PL)及其白光下照片（最大發(fā)射波長(zhǎng)和吸收峰位置歸納見表1）。由圖1(a)可知，樣品的最大發(fā)光波長(zhǎng)位置隨著初始配體中C12SH比例的增大從618 nm逐漸藍(lán)移到571nm。由于反應(yīng)中的其他參數(shù)都保持一致，表明Cu納米團(tuán)簇發(fā)光波長(zhǎng)的改變是由于初始配體比例的變化導(dǎo)致的。此外，自然光下的Cu納米團(tuán)簇的溶液顏色也隨著初始配體中C12SH比例的增大從深黃色變到淡黃色。根據(jù)陳衛(wèi)等[18,20]對(duì)Cu納米團(tuán)簇的電子結(jié)構(gòu)分析可知，Cu納米團(tuán)簇通常會(huì)表現(xiàn)出400 nm和600 nm附近的兩種發(fā)光，其中400 nm附近的發(fā)光來(lái)源于sp帶到d帶的躍遷，而600 nm附近的發(fā)光來(lái)源于sp帶內(nèi)HOMO-LUMO 躍遷。因此，這里的PL譜藍(lán)移可能歸因于HOMO-LUMO帶隙的變化。

圖1 不同初始配體比例樣品光致發(fā)光譜（激發(fā)波長(zhǎng)365 nm）和白光下照片(a)以及UV-vis吸收譜(b)Fig.1 Photoluminescence spectra (excited at 365 nm) and optical photograph (a) and UV-vis absorption spectra of the samples (b).

表1 Cu納米團(tuán)簇合成參數(shù)及相應(yīng)的吸收峰位置、最大發(fā)光峰位置Table 1 Synthesis parameters, absorption peaks and emission peaks of copper clusters.

圖1(b)顯示了不同初始配體比例的Cu納米團(tuán)簇的UV-vis吸收譜，未發(fā)現(xiàn)存在560 nm附近的Cu納米晶體的特征表面等離子共振(Surface Plasmon Resonance, SPR)吸收峰和由Cu2+的d-d躍遷產(chǎn)生的750 nm附近處的特征吸收峰，證實(shí)了團(tuán)簇尺寸較?。ㄐ∮? nm），并且樣品中的Cu原子至少被還原到+1價(jià)[17,21-22]。隨著初始配體中C12SH所占比例的增加，所有樣品在350 nm和280 nm附近的吸收峰發(fā)生藍(lán)移，這兩個(gè)吸收峰已被確定來(lái)自于Cu納米團(tuán)簇的d帶到sp帶的帶間吸收躍遷[18,20]，這說(shuō)明初始配體比例的不同會(huì)對(duì)Cu納米團(tuán)簇的電子結(jié)構(gòu)產(chǎn)生影響。因此，PL譜中的藍(lán)移主要來(lái)源于產(chǎn)物的電子結(jié)構(gòu)變化。由于金屬納米團(tuán)簇的電子結(jié)構(gòu)和團(tuán)簇的化學(xué)組成和尺寸以及結(jié)構(gòu)有關(guān)[18-19]，為了了解PL譜中藍(lán)移現(xiàn)象的機(jī)理，分別采用質(zhì)譜和具有元素分辨的XAFS技術(shù)研究不同初始配體比例的Cu納米團(tuán)簇的化學(xué)組成和尺寸以及結(jié)構(gòu)信息。

圖2給出了樣品的MALDI-TOF質(zhì)譜在550-1150 Da的結(jié)果。從圖2可知，初始配體MPP和C12SH 比例為1:0制備的Cu納米團(tuán)簇展現(xiàn)出995.2 Da、778.5 Da和561.7 Da三個(gè)峰，分別對(duì)應(yīng)于Cu6M4、Cu5M3和Cu4M2納米團(tuán)簇（M代表MPP配體）。根據(jù)Salorinne等[17]對(duì)銅納米團(tuán)簇的質(zhì)譜分析，由于三個(gè)質(zhì)譜峰各相差一個(gè)CuM(216.8 Da)碎片單元，因MALDI-TOF質(zhì)譜工作中存在激光對(duì)樣品電離造成的解離，可認(rèn)為無(wú)C12SH制備的Cu納米團(tuán)簇是Cu5M3占主導(dǎo)的納米團(tuán)簇。當(dāng)C12SH加入之后，三個(gè)樣品都含有較強(qiáng)的608.8 Da和較弱的561.7 Da峰，對(duì)應(yīng)于Cu4ML（L代表C12SH配體）和Cu4M2納米團(tuán)簇，并且以Cu4ML占主導(dǎo)。MALDI-TOF質(zhì)譜表明初始配體中引入C12SH會(huì)減少Cu納米團(tuán)簇的Cu原子數(shù)，但進(jìn)一步增加C12SH在初始配體中比例則不會(huì)對(duì)Cu納米團(tuán)簇的Cu原子個(gè)數(shù)產(chǎn)生影響。根據(jù)文獻(xiàn)報(bào)道[23-25]，金屬納米團(tuán)簇的發(fā)光能量會(huì)隨著團(tuán)簇中金屬原子數(shù)目的增加而降低。由于無(wú)C12SH制備的Cu納米團(tuán)簇，且Cu原子數(shù)數(shù)目最多，所以發(fā)光能量最低，波長(zhǎng)最大。當(dāng)配體中引入C12SH時(shí)，由于Cu納米團(tuán)簇中Cu原子數(shù)減少到4，因此納米團(tuán)簇的發(fā)光波長(zhǎng)相應(yīng)的從618 nm藍(lán)移到597 nm。然而對(duì)于其他三個(gè)初始配體中引入C12SH的樣品，它們擁有相同的化學(xué)組成，卻表現(xiàn)出不同發(fā)光。

圖3(a)和(b)顯示了Cu納米團(tuán)簇Cu原子的k空間EXAFS函數(shù)kχ(k)曲線和R空間的FT(k3χ(k))函數(shù)曲線及相應(yīng)的擬合曲線。圖3(a)表明，不同初始配體比例的Cu納米團(tuán)簇的kχ(k)曲線與參照物體相Cu表現(xiàn)出很大差別，但與參照物CuS類似。這表明Cu納米團(tuán)簇的尺寸很小，并且納米團(tuán)簇的Cu原子與S原子有較強(qiáng)成鍵。從圖3(b)可以進(jìn)一步看出，在所有樣品的RSF曲線中，都存在一個(gè)0.18 nm附近的主振幅峰。與參比樣品CuS的RSF曲線對(duì)比可知，0.18 nm處的主振幅峰對(duì)應(yīng)著Cu-S配位，并且主振幅隨著C12SH所占比例的增大向低R方向有微弱移動(dòng)。對(duì)于初始配體無(wú)C12SH得到的Cu納米團(tuán)簇，Cu-S配位峰強(qiáng)度較低并在0.22 nm附近表現(xiàn)出明顯肩膀峰，對(duì)比體相Cu的RSF可知，該肩膀峰對(duì)應(yīng)于Cu-Cu配位；當(dāng)C12SH引入時(shí)，Cu-Cu配位峰減弱并消失，并且Cu-S向低R方向微弱移動(dòng)，表明C12SH的引入導(dǎo)致納米團(tuán)簇的原子結(jié)構(gòu)發(fā)生較大變化，相對(duì)于無(wú)C12SH引入的Cu納米團(tuán)簇，Cu-Cu配位數(shù)的減少。

圖2 不同初始配體比例樣品的MALDI-TOF質(zhì)譜Fig.2 MALDI-TOF mass spectra of the samples showing the main clusters and fragments.

為獲得Cu納米團(tuán)簇結(jié)構(gòu)的局域結(jié)構(gòu)信息，通過(guò)IFEFFIT中的ARTEMIS對(duì)FT(k3χ(k))函數(shù)曲線進(jìn)行最小二乘法的擬合（擬合區(qū)域0.14-0.30 nm）。擬合結(jié)果如圖3(b)（空心圈）所示，結(jié)構(gòu)參數(shù)歸納于表2。由表2可知，當(dāng)初始配體中引入C12SH時(shí)，Cu納米團(tuán)簇中的Cu-Cu的鍵長(zhǎng)從0.254 nm 縮短到0.24 nm，Cu-Cu配位數(shù)從3.6降到3.0，Cu-S的配位數(shù)從1.2降到1.0。隨著C12SH在初始配體中的比例升高，除了Cu-S鍵長(zhǎng)從0.228 nm 逐漸縮短到0.224 nm，Cu納米團(tuán)簇的整體結(jié)構(gòu)參數(shù)保持不變。綜合MALDI-TOF質(zhì)譜和XAFS擬合結(jié)果，認(rèn)為配體僅為MPP的Cu5M3納米團(tuán)簇，它是一種類雙三角錐結(jié)構(gòu)(圖4(a))；當(dāng)初始配體中引入C12SH，Cu納米團(tuán)簇變?yōu)樗拿骟w結(jié)構(gòu)(圖4(b))，雖然C12SH和MPP中S原子與Cu原子結(jié)合的Cu-S鍵長(zhǎng)可能不同，但是圖3(b)中樣品的RSF曲線僅有一個(gè)配位峰，因此我們認(rèn)為這兩種Cu-S鍵長(zhǎng)十分接近；此外，隨著C12SH所占比例的繼續(xù)增大，Cu納米團(tuán)簇除Cu-S鍵長(zhǎng)外，四面體的結(jié)構(gòu)保持不變。

圖3 不同初始配比Cu納米團(tuán)簇EXAFS譜(a) k空間kχ(k)譜，(b) 徑向結(jié)構(gòu)函數(shù)k3χ(k)譜（實(shí)線）及其擬合曲線（空心圈）Fig.3 Structure determination EXAFS k-space spectra kχ(k) (a), RSF curves (solid lines) and simulated fit (open circles) (b).

表2 Cu納米團(tuán)簇對(duì)Cu邊的EXAFS進(jìn)行擬合所得的結(jié)構(gòu)參數(shù)Table 2 Structural parameters of Cu clusters fitting for Cu K-edge EXAFS spectra.

圖4 Cu5M3 (a)和Cu4ML (b)納米團(tuán)簇的晶體結(jié)構(gòu)Fig.4 Computational Cu5M3 cluster model (a) and Cu4ML cluster model (b).

根據(jù)以往對(duì)Au納米團(tuán)簇電子結(jié)構(gòu)分析[26-28]，我們認(rèn)為Cu納米團(tuán)簇的Cu原子的3d10軌道組成d帶；表面Cu原子的4sp軌道和S原子的3p軌道形成的Cu(I)-S混合態(tài)組成sp帶內(nèi)的HOMO能級(jí)和LUMO能級(jí)的低能量部分；內(nèi)部Cu原子的sp軌道組成了LUMO能級(jí)的高能量部分。因此，我們認(rèn)為樣品中的600 nm發(fā)光是由LUMO能級(jí)中高能量部分的激發(fā)態(tài)電子向sp帶內(nèi)的HOMO能級(jí)躍遷產(chǎn)生，即發(fā)光來(lái)源于內(nèi)部Cu原子向表面Cu(I)-S電荷轉(zhuǎn)移。反應(yīng)中，C12SH在初始配體中的比例增大導(dǎo)致生成的Cu納米團(tuán)簇的Cu-S鍵長(zhǎng)發(fā)生縮短，從而導(dǎo)致了sp帶內(nèi)的HOMO能級(jí)的降低，使HOMO-LUMO帶隙增大，進(jìn)而導(dǎo)致Cu納米團(tuán)簇的發(fā)光波長(zhǎng)的藍(lán)移。

3 結(jié)語(yǔ)

通過(guò)調(diào)節(jié)初始配體MPP和C12SH的比例，合成出618-571 nm可控發(fā)光的Cun(n≤6)納米團(tuán)簇。根據(jù)UV-vis吸收譜、MALDI-TOF質(zhì)譜以及XAFS譜的結(jié)果分析，認(rèn)為僅有MPP作為配體時(shí)的產(chǎn)物主要為雙三角錐結(jié)構(gòu)的Cu5MPP3；當(dāng)初始配體中引入C12SH后則變?yōu)樗拿骟w結(jié)構(gòu)的Cu4[MPP][C12SH]，納米團(tuán)簇中的Cu原子數(shù)的減少導(dǎo)致Cu納米團(tuán)簇的發(fā)光從618 nm藍(lán)移至597 nm。隨著C12SH在初始配體中所占比例的繼續(xù)增加，Cu納米團(tuán)簇除Cu-S鍵長(zhǎng)從0.228 nm縮短到0.224 nm外，四面體結(jié)構(gòu)保持不變。但是Cu-S鍵長(zhǎng)的縮短導(dǎo)致了由Cu(I)-S所構(gòu)成的HOMO能級(jí)的降低，HOMO-LUMO帶隙增大導(dǎo)致Cu納米團(tuán)簇的發(fā)光波長(zhǎng)從597 nm逐漸藍(lán)移至571 nm。

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CLCTL99

Study on photoluminescence and structure of copper nanoclusters

GE Mengran LI Dandan YAO Tao PAN Guoqiang
(National Synchrotron Radiation Laboratory, University of Science and Technology of China, Hefei 230029, China)

Background:During the past few decades, metal nanoclusters have

considerable research interests for their distinct optical, catalytic and electronic properties. However, till today, most studies are focused on the synthesis, structure and properties of Au and Ag clusters, while the studies on Cu nanoclusters are rather rare because of its inherent instability.Purpose:We aim to synthesize the Cu nanocluster that has the tunable photoluminescence.Methods:Through varying the initial legend molar ratio of 2-mercapto-5-n-propylpyrimidine and dodecanethiol, a series of Cu nanoclusters with different emission wavelength were prepared. The photoluminescence property and structure of these nanoclusters were studied by the photoluminescence spectrometry (PL), X-ray absorption fine structure (XAFS) and Matrix Assisted Laser Desorption Ionization Time of Flight Spectrometer (MALDI-TOF) mass spectrometry.Results:(1) PL spectra showed that with the increase of the proportion of dodecanethiol, the maximum emission wavelength of Cu nanoclusters shifted from 618 nm to 571 nm. (2) Mass spectra results indicated that the main products were changed from Cu5MPP3(MPP: 2-mercapto-5-n-propylpyrimidine) in the presence of MPP alone to Cu4[MPP][C12SH] when the dodecanethiol was introduced, and remains unchanged with increasing the dodecanethiol. (3) XAFS results showed that with the increase of dodecanethiol, the Cu-S bond length was shortened from 0.228 nm to 0.224 nm.Conclusion:The reduction of the amount of Cu atomic number would lead to the shift of emission wavelength from 618 nm to 597 nm, and the shortening of Cu-S bond length pushed down the HOMO level which was formed by the Cu(I)-S. With the increase of the HOMO-LUMO band gap, the emission wavelength of the products shifted from 597 nm to 571 nm.

Copper nanoclusters, Tunable photoluminescence, X-ray absorption fine structure (XAFS), Synchrotron radiation

TL99

10.11889/j.0253-3219.2014.hjs.37.090104

國(guó)家自然科學(xué)基金課題(No.11079032、No.U1232132)資助

葛夢(mèng)然，男，1990年出生，現(xiàn)就讀于中國(guó)科學(xué)技術(shù)大學(xué)國(guó)家同步輻射實(shí)驗(yàn)室

潘國(guó)強(qiáng)，E-mail: gqpan@ustc.edu.cn

2014-04-15，

2014-05-08