葉云,徐鈺興,馮兆池,王秀麗*,李燦*

(1.中國(guó)科學(xué)院大連化學(xué)物理研究所,催化基礎(chǔ)國(guó)家重點(diǎn)實(shí)驗(yàn)室,潔凈能源國(guó)家實(shí)驗(yàn)室,大連　116023；2.中國(guó)科學(xué)院大學(xué),北京　100049)

發(fā)光光譜研究不同粒徑CdS量子點(diǎn)與鈷肟分子間的電荷轉(zhuǎn)移

葉云1,2,徐鈺興1,2,馮兆池1,王秀麗1*,李燦1*

(1.中國(guó)科學(xué)院大連化學(xué)物理研究所,催化基礎(chǔ)國(guó)家重點(diǎn)實(shí)驗(yàn)室,潔凈能源國(guó)家實(shí)驗(yàn)室,大連116023；2.中國(guó)科學(xué)院大學(xué),北京100049)

摘要:本文使用穩(wěn)態(tài)及時(shí)間分辨發(fā)光光譜研究CdS量子點(diǎn)鈷肟(CoⅢ(dmgH)2(3-(OH)py)Cl)分子耦合光催化產(chǎn)氫體系中CdS量子點(diǎn)粒徑大小在CdS-鈷肟分子間電荷轉(zhuǎn)移中的作用。粒徑為3.9,4.4 和5.0 nm的CdS量子點(diǎn)都表現(xiàn)出帶邊發(fā)光及長(zhǎng)波處的橙色缺陷發(fā)光。隨CdS量子點(diǎn)粒徑的減小,帶邊發(fā)光減弱,缺陷發(fā)光增強(qiáng),且小粒徑量子點(diǎn)表現(xiàn)出了更長(zhǎng)的發(fā)光壽命。鈷肟分子的引入迅速猝滅CdS量子點(diǎn)的帶邊發(fā)光和缺陷發(fā)光,且缺陷發(fā)光的猝滅常數(shù)比帶邊發(fā)光更大。CdS量子點(diǎn)的粒徑越小,發(fā)光猝滅的效率越高。這些結(jié)果證明,CdS量子點(diǎn)的自由載流子和束縛載流子都可以轉(zhuǎn)移到鈷肟分子上,且束縛載流子的轉(zhuǎn)移效率更高。CdS量子點(diǎn)粒徑越小,CdS向鈷肟分子的光生電荷轉(zhuǎn)移效率越高。

關(guān)鍵詞:CdS量子點(diǎn)鈷肟；粒徑大??；發(fā)光光譜；電荷轉(zhuǎn)移

1引言

基于半導(dǎo)體的光催化制氫是解決能源危機(jī)和緩解環(huán)境問(wèn)題的重要途徑之一。由于光催化反應(yīng)涉及到半導(dǎo)體吸光,光生載流子的復(fù)合及遷移,表面化學(xué)反應(yīng)等一系列復(fù)雜的過(guò)程,絕大多數(shù)光催化材料的量子利用效率很低。半導(dǎo)體量子點(diǎn)由于其獨(dú)特的光學(xué)性質(zhì)和高效的發(fā)光量子產(chǎn)率而被越來(lái)越多的應(yīng)用到光催化領(lǐng)域中來(lái)。量子點(diǎn)材料在光催化反應(yīng)中,通常作為吸光組分與金屬、半導(dǎo)體、分子催化劑等耦合來(lái)分解水產(chǎn)氫[1-2]。

2實(shí)驗(yàn)

2.1主要化學(xué)試劑

氧化鎘(CdO),硫粉(S),十八烯(ODE),巰基丙酸(MPA),四甲基氫氧化銨(TMAH)購(gòu)于Sigma公司；油酸(OA)購(gòu)于天津市博迪化學(xué)試劑有限公司；甲醇,丙酮,正己烷,氯仿,氯化鈷(CoCl2·6H2O),N,N-二甲基甲酰胺(DMF)購(gòu)于天津市科密歐化學(xué)試劑有限公司。所有試劑皆為分析純,使用之前沒(méi)有經(jīng)過(guò)進(jìn)一步的分離純化。實(shí)驗(yàn)用水經(jīng)過(guò)ELGA-ULTRA GE MK2純化后使用(>18.0 MΩ·cm)。

2.2CdS量子點(diǎn)及鈷肟分子的制備

CdS量子點(diǎn)的制備參考Huang等報(bào)道的方法[7-8]。具體過(guò)程如下:取CdO,OA和溶劑ODE共計(jì)21.5 g懸濁液放入三口燒瓶中,密封后加熱并保持在120℃反復(fù)抽真空、通Ar氣,直至將懸濁液中氣泡除凈。Ar氣氣氛下繼續(xù)加熱到280℃并保持,快速注入溶解在ODE中的S粉5 mL,此時(shí)反應(yīng)液的溫度迅速降低到260℃,保持一段時(shí)間使CdS量子點(diǎn)充分生長(zhǎng)。反應(yīng)通過(guò)調(diào)變OA的濃度來(lái)控制CdS量子點(diǎn)的粒徑大小,為了得到不同粒徑的CdS量子點(diǎn)材料,實(shí)驗(yàn)中用到OA的濃度分別為0.25,0.50和0.65 mol/kg。反應(yīng)完成后的混合液用體積比1∶1的正己烷/甲醇萃取溶劑來(lái)反復(fù)幾次洗滌,然后用丙酮將其中的CdS量子點(diǎn)沉淀下來(lái),8000 rpm離心分離,得到的沉淀物分散在氯仿中備用。由于光催化分解水產(chǎn)氫反應(yīng)在水中進(jìn)行,需要進(jìn)一步將CdS量子點(diǎn)轉(zhuǎn)移到水相中。相轉(zhuǎn)移過(guò)程如下:將0.4 mL MPA加入到20 mL甲醇中,通過(guò)不斷滴加TMAH來(lái)調(diào)節(jié)其pH值到11.0左右,加入分散于氯仿中的CdS量子點(diǎn)5 mL,在避光、室溫下攪拌8 h,交換完全后用丙酮來(lái)使其絮凝,8000 rpm離心分離,得到的沉淀物分散在水溶液中,即得到水相CdS量子點(diǎn)樣品。

鈷肟分子CoⅢ(dmgH)2(3-(OH)py)Cl的制備:參照文獻(xiàn)[9],將等摩爾量的CoCl2·6H2O和丁二酮肟快速溶于丙酮,攪拌5 min后過(guò)濾,保留濾液并放置一天后,得到綠色的CoⅢ(dmgH2)(dmgH)Cl2。將等摩爾量的CoⅢ(dmgH2)(dmgH)Cl2和3-羥基吡啶加入95%的乙醇溶液中,攪拌后溶液呈棕紅色,濃縮析出沉淀并過(guò)濾,然后用水和丙酮洗滌,真空40℃干燥后即得到CoⅢ(dmgH)2(3-(OH)py)Cl粉末,分散在DMF溶液中待用。

2.3樣品表征

穩(wěn)態(tài)及時(shí)間分辨光致發(fā)光光譜的采集使用愛(ài)丁堡儀器公司的FLS920熒光光譜儀,光源分別是450 W的氙燈和皮秒脈沖激光器(激發(fā)光波長(zhǎng)406.8 nm,脈沖寬度為64.2 ps)。

3結(jié)果與討論

圖1(a)給出了實(shí)驗(yàn)合成的三種CdS量子點(diǎn)的紫外可見(jiàn)吸收光譜,其第一激子吸收峰的位置分別是414,428和445 nm,使用參考文獻(xiàn)[10]中的經(jīng)驗(yàn)公式,估算得到相應(yīng)的CdS量子點(diǎn)的粒徑分別為3.9,4.4及5.0 nm；隨著量子點(diǎn)粒徑的減小,其吸收帶邊的位置向短波方向移動(dòng),說(shuō)明CdS量子點(diǎn)的帶隙寬度增加,這是量子尺寸效應(yīng)的結(jié)果。圖1(b)中給出了鈷肟分子CoⅢ(dmgH)2(3-(OH)py)Cl(以下簡(jiǎn)寫(xiě)為Co-OH)的結(jié)構(gòu)式及其紫外可見(jiàn)吸收光譜。

Fig.1(a)UV-vis absorption spectra of CdS QDs with three different particle size dispersed in water(Dia.is short for diameter);(b)UV-vis absorption spectrum of cobaloxime.The inset figure is the structural representation of cobaloxime:CoⅢ(dmgH)2(3-(OH)py)Cl,it is simplified as Co-OH

圖2是三種不同粒徑CdS量子點(diǎn)的穩(wěn)態(tài)及時(shí)間分辨光致發(fā)光光譜。圖2(a)中三種量子點(diǎn)都顯示出典型的帶邊發(fā)光和缺陷發(fā)光,例如,粒徑為5.0 nm的CdS量子點(diǎn)具有位于463 nm處窄而對(duì)稱的帶邊發(fā)光峰及645 nm處寬而對(duì)稱的缺陷發(fā)光峰。發(fā)光光譜隨量子點(diǎn)粒徑減小出現(xiàn)的整體藍(lán)移現(xiàn)象同其吸收光譜一樣,也是由量子尺寸效應(yīng)所致。由于一束光激發(fā)半導(dǎo)體量子點(diǎn)產(chǎn)生的載流子數(shù)目有限,隨之產(chǎn)生的帶邊發(fā)光與缺陷發(fā)光存在相互競(jìng)爭(zhēng),表1給出了不同粒徑量子點(diǎn)的帶邊發(fā)光峰與缺陷發(fā)光峰的相對(duì)強(qiáng)度比值。由表可知,CdS量子點(diǎn)尺寸越小,其缺陷態(tài)發(fā)光越強(qiáng)。這是因?yàn)?隨量子點(diǎn)粒徑的減小,其比表面積增大,此時(shí)對(duì)于水相中表面缺陷位點(diǎn)較多的CdS量子點(diǎn)來(lái)說(shuō),由光激發(fā)量子點(diǎn)產(chǎn)生的束縛態(tài)載流子復(fù)合發(fā)光的幾率會(huì)大大高于自由載流子的復(fù)合發(fā)光,即缺陷發(fā)光具有相對(duì)高的發(fā)光量子產(chǎn)率。

Fig.2(a)The steady-state photoluminescence(PL)spectra of CdS QDs with three different particle size dispersed in water,the excitation wavelength was 355 nm; Time-resolved PL decay curves for the band-edge emission(b)and trap-related emission(c)of all three kinds of CdS QDs with an excitation wavelength of 406.8 nm.The concentration of all the three kinds of CdS QDs was 0.5μM

Tab.1　PL characteristics of CdS QDs with different particle sizes

(Iis the PL intensity of CdS QDs; <τ> is the average lifetime of CdS QDs)

圖2(b)和(c)分別為三種不同粒徑CdS量子點(diǎn)帶邊發(fā)光及缺陷發(fā)光相應(yīng)的動(dòng)力學(xué)衰減曲線,分別代表量子點(diǎn)自由載流子與束縛態(tài)載流子的壽命。表1給出了指數(shù)擬合得到的不同粒徑量子點(diǎn)帶邊發(fā)光及缺陷發(fā)光的平均壽命<τ>。整體而言,CdS量子點(diǎn)缺陷發(fā)光的壽命要遠(yuǎn)遠(yuǎn)長(zhǎng)于帶邊發(fā)光的壽命,而小粒徑量子點(diǎn)的帶邊發(fā)光及缺陷發(fā)光壽命相對(duì)更長(zhǎng)。

圖3為三種不同粒徑CdS量子點(diǎn)分別加入等量Co-OH后導(dǎo)致的帶邊發(fā)光及缺陷發(fā)光猝滅。表2列出了加入Co-OH后三種不同粒徑CdS量子點(diǎn)帶邊發(fā)光和缺陷發(fā)光的猝滅程度D(D=1-I/I0),其中I0與I分別為CdS量子點(diǎn)及加入Co-OH后CdS量子點(diǎn)的發(fā)光強(qiáng)度),加入適量的Co-OH后,三種粒徑量子點(diǎn)發(fā)光均有一定程度的猝滅,且粒徑越小,發(fā)光猝滅程度越強(qiáng),本征發(fā)光和缺陷發(fā)光均存在這種趨勢(shì),且缺陷發(fā)光的猝滅程度更強(qiáng)。我們還利用本征發(fā)光和缺陷發(fā)光猝滅程度的比值D1/D2對(duì)其所代表的自由及束縛態(tài)載流子的轉(zhuǎn)移比進(jìn)行了半定量估算。粒徑從小到大,兩種不同載流子的轉(zhuǎn)移比分別為1∶1.4,1∶1.4及1∶9.3,這個(gè)結(jié)果說(shuō)明三種不同粒徑量子點(diǎn)中,均是束縛態(tài)載流子具有更高的轉(zhuǎn)移效率,且由于大粒徑量子點(diǎn)帶邊發(fā)光轉(zhuǎn)移效率很低,其缺陷發(fā)光轉(zhuǎn)移比更高。實(shí)驗(yàn)過(guò)程中三種不同粒徑CdS量子點(diǎn)的濃度相同。需要說(shuō)明的是,發(fā)光光譜測(cè)試時(shí)采用的激發(fā)光波長(zhǎng)為355 nm,從圖1(b)中可以看出,加入的Co-OH也會(huì)吸收這一波長(zhǎng)的光,因此,在對(duì)發(fā)光光譜進(jìn)行分析之前,已經(jīng)扣除因Co-OH吸光所造成的CdS量子點(diǎn)發(fā)光強(qiáng)度降低的影響。

圖4為圖3中不同發(fā)光峰相應(yīng)的動(dòng)力學(xué)衰減曲線,加入Co-OH后其動(dòng)力學(xué)衰減過(guò)程均有不同程度的加快,根據(jù)發(fā)光猝滅機(jī)理,Co-OH的存在所引起的電荷轉(zhuǎn)移會(huì)使得CdS量子點(diǎn)的發(fā)光猝滅。

Fig.3Steady-state PL spectra of all three kinds of CdS QDs with the addition of Co-OH,the excitation wavelength was 355 nm.The effects caused by the absorption of Co-OH at 355 nm were already compensated

Fig.4Time-resolved PL decay curves for the band-edge emission(a)(b)(c)and trap-related emission(d)(e)(f)of all three kinds of CdS QDs with Co-OH.The excitation wavelength was 406.8 nm

Tab.2　PL characteristics of CdS QDs with the addition of Co-OH

(Dis the quenching degree of PL intensity for CdS QDs in the presence of Co-OH;<τ> is the average lifetime of CdS QDs with the addition of Co-OH while <τ>0for CdS QDs)

帶邊發(fā)光與缺陷發(fā)光動(dòng)力學(xué)的衰減程度由表2中的<τ>0/<τ>簡(jiǎn)單表示,其數(shù)值越大,表示動(dòng)力學(xué)衰減越快,與發(fā)光強(qiáng)度猝滅程度的規(guī)律相一致。加入Co-OH后,小粒徑量子點(diǎn)的帶邊發(fā)光與缺陷發(fā)光動(dòng)力學(xué)衰減最快,因此從CdS量子點(diǎn)到Co-OH的光生電荷轉(zhuǎn)移效果也最好。與帶邊發(fā)光相比,同一粒徑量子點(diǎn)缺陷態(tài)發(fā)光的動(dòng)力學(xué)衰減更快,進(jìn)一步說(shuō)明束縛態(tài)載流子比自由載流子具有更好的電荷轉(zhuǎn)移效果。

總的來(lái)說(shuō),利用CdS量子點(diǎn)作為吸光組分與鈷肟分子構(gòu)建的光催化產(chǎn)氫耦合體系中,粒徑較小的CdS量子點(diǎn)具有更好的光生電荷分離與轉(zhuǎn)移效果。分析原因,一方面,與體相材料相比,由于量子尺寸效應(yīng),量子點(diǎn)具有更寬的帶隙寬度。CdS體相材料的導(dǎo)帶電位為-0.9 V vs NHE(pH=7)[11],相應(yīng)的CdS量子點(diǎn)的導(dǎo)帶電位應(yīng)位于更負(fù)的位置。而我們通過(guò)循環(huán)伏安法測(cè)試得到的Co-OH的第一、第二還原電位分別為-0.24 V及-0.6 V vs NHE,說(shuō)明激發(fā)態(tài)電子從CdS量子點(diǎn)轉(zhuǎn)移到Co-OH是熱力學(xué)允許的,且量子點(diǎn)粒徑越小,其導(dǎo)帶電位越負(fù),光生電荷從量子點(diǎn)向Co-OH轉(zhuǎn)移的驅(qū)動(dòng)力越大。另一方面,由表1中給出的三種CdS量子點(diǎn)帶邊發(fā)光峰與缺陷發(fā)光峰的相對(duì)強(qiáng)度比值可知,小粒徑量子點(diǎn)表面缺陷位最多,即束縛態(tài)載流子數(shù)目最多,而位于量子點(diǎn)表面的束縛態(tài)載流子不但具有長(zhǎng)的壽命,與大多數(shù)體相自由載流子相比,從CdS量子點(diǎn)到Co-OH活性位點(diǎn)的空間遷移距離也有大大縮短,這二者均會(huì)促進(jìn)CdS量子點(diǎn)-鈷肟分子耦合光催化體系中光生電荷的轉(zhuǎn)移。

4結(jié)論

利用穩(wěn)態(tài)及時(shí)間分辨光致發(fā)光光譜,我們對(duì)CdS量子點(diǎn)粒徑大小在CdS-鈷肟分子間電荷轉(zhuǎn)移中的作用進(jìn)行了研究。發(fā)現(xiàn)小粒徑量子點(diǎn)具有更長(zhǎng)的光生載流子壽命,且量子點(diǎn)束縛態(tài)載流子壽命更長(zhǎng)。與鈷肟分子耦合后,小粒徑量子點(diǎn)更利于其光生電荷的轉(zhuǎn)移,且粒徑越小,電荷轉(zhuǎn)移效果越明顯,同時(shí),束縛態(tài)載流子更易轉(zhuǎn)移到鈷肟分子上。以上結(jié)果證明,對(duì)半導(dǎo)體量子點(diǎn)尺寸的合適調(diào)控可以促進(jìn)光催化體系光生電荷的分離和轉(zhuǎn)移,從而進(jìn)一步提高光催化活性。

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Effects of Different Particle Size on the Charge Transfer between CdS QDs and Cobaloxime Studied by Photoluminescence Spectroscopy

YE Yun1,2,XU Yu-xing1,2,FENG Zhao-chi1,WANG Xiu-li1*,LI Can1*

(1.StateKeyLaboratoryofCatalysis,DalianNationalLaboratoryforCleanEnergy,DalianInstituteofChemicalPhysics,ChineseAcademyofSciences,Dalian116023,China; 2.UniversityofChineseAcademyofSciences,Beijing100049,China)

Abstract:In this work,steady-state and time-resolved photoluminescence(PL)spectroscopy were applied to study the effects of CdS particle size on the charge transfer between CdS quantum dots(QDs)and cobaloxime(CoⅢ(dmgH)2(3-(OH)py)Cl)in CdS QDs-cobaloxime hybrid photocatalytic hydrogen production system.All three kinds of CdS QDs with different particle size(3.9,4.4 and 5.0 nm)show a band-edge emission and a broad trap-related emission.When the particle size of CdS QDs is smaller,the band-edge emissionintensity is reduced while the trap-related emission enhanced,and CdS QDs with smaller particle size exhibit longer emission lifetime.Upon mixing with cobaloxime,the band-edge emission and trap-related emission of CdS QDs are quenched quickly,and the quenching constant of the trap-related emission is bigger than that of the band-edge emission.The emission intensity of CdS QDs is quenched more efficiently with the particle size decreasing.These results demonstrate that,both the free and trapped charge carriers of CdS QDs could transfer to cobaloxime,and the trapped ones transfer to cobaloxime more efficiently; CdS QDs with smaller particle size exhibit higher charge transfer efficiency.

Key words:CdS QDs-cobaloxime;particle size;photoluminescence;charge transfer

收稿日期:2015-07-14; 修改稿日期:2015-10-16

基金項(xiàng)目:973項(xiàng)目基金(2014CB239400)；國(guó)家自然科學(xué)基金(21203185)

作者簡(jiǎn)介:葉云(1988-),女,博士,主要從事半導(dǎo)體材料的時(shí)間分辨光譜研究。E-mail:yye@dicp.ac.cn 通訊作者:王秀麗(1982-),女,副研究員,主要從事光(電)催化機(jī)理的時(shí)間分辨光譜表征研究。E-mail:xiuliwang@dicp.ac.cn;李燦(1960-),男,院士,主要從事催化材料、催化反應(yīng)和催化光譜表征方面的研究。E-mail:canli@dicp.ac.cn

文章編號(hào):1004-5929(2016)02-0190-05

中圖分類號(hào):O643.12

文獻(xiàn)標(biāo)志碼:A

doi:10.13883/j.issn1004-5929.201602017