ZnO/CdS/MoS2異質(zhì)結(jié)納米棒陣列的制備及性能

傅艷玨, 王春瑞

(東華大學(xué) 理學(xué)院, 上海 201620)

ZnO/CdS/MoS2異質(zhì)結(jié)納米棒陣列的制備及性能

傅艷玨, 王春瑞

(東華大學(xué) 理學(xué)院, 上海 201620)

采用連續(xù)離子層吸附法, 以ZnO納米棒陣列為模板, 將窄帶CdS和更窄帶層狀MoS2依次包覆在ZnO納米棒表面, 得到一種ZnO/CdS/MoS2異質(zhì)結(jié)納米棒陣列. 利用X射線衍射、掃描電子顯微鏡、高分辨透射電子顯微鏡和拉曼光譜對產(chǎn)物進(jìn)行了表征. 結(jié)果表明, CdS鞘和MoS2鞘依次均勻地包覆在垂直生長的ZnO納米棒上, 且結(jié)合過程中未明顯影響內(nèi)芯ZnO的棒狀結(jié)構(gòu). 在異質(zhì)結(jié)結(jié)構(gòu)中, ZnO/CdS界面、CdS/MoS2界面接觸良好且晶格失配率低. 此外, 一維ZnO納米棒陣列的存在也抑制了MoS2在c軸方向上的堆積, 使包覆在納米棒上的MoS2以少層的形式存在. 原位電輸運特性表明, 與ZnO納米棒和ZnO/CdS異質(zhì)結(jié)納米棒相比, ZnO/CdS/MoS2異質(zhì)結(jié)納米棒則呈現(xiàn)出了更優(yōu)的電學(xué)特性.

連續(xù)離子層吸附法; 少層二硫化鉬; ZnO/CdS/MoS2異質(zhì)結(jié); 原位電性能

近年來, 芯/鞘異質(zhì)結(jié)納米棒的研究受到國內(nèi)外廣泛關(guān)注[1-5]. 這是因為在芯/鞘納米結(jié)構(gòu)中, 激發(fā)的電子和空穴在空間上分離并分別進(jìn)入芯納米棒和包裹鞘層, 有利于改善器件的輸運特性和光電轉(zhuǎn)換效率, 所以其在新型光電子、電子器件等方面具有重要應(yīng)用價值.

過渡金屬氧化物/金屬硫化物因具有優(yōu)良的性能, 其被認(rèn)為是一種理想的構(gòu)筑芯/鞘異質(zhì)結(jié)結(jié)構(gòu)的材料[6-7]. 基于這樣的理念, 構(gòu)筑結(jié)構(gòu)上匹配、性能上協(xié)同的兩種或兩種以上的不同金屬氧化物/金屬硫化物芯/鞘異質(zhì)結(jié)的研究已成為該領(lǐng)域的研究熱點和前沿. 文獻(xiàn)[8]用泡沫鎳為襯底, 以種子層輔助的水熱法, 制成了三維Co3O4/MnO2核殼納米針陣列, 它的綜合電化學(xué)性能明顯由于純Co3O4.文獻(xiàn)[9]則應(yīng)用簡單的水熱法制備了少層的MoS2納米片, 隨后將其包覆在TiO2納米帶之上, 形成了TiO2/MoS2的異質(zhì)結(jié), 它具有優(yōu)異的儲電性和穩(wěn)定的充/放電循環(huán)性能. 文獻(xiàn)[10]利用兩步水熱法成功合成了SnO2/MoS2異質(zhì)結(jié), MoS2納米片能有效緩沖鋰離子進(jìn)入SnO2納米顆粒之后引起的結(jié)構(gòu)變化, 另一個方面, SnO2納米顆粒插層于MoS2的層與層之間能夠有效地防止MoS2納米片的堆積, 其結(jié)構(gòu)非常穩(wěn)定且具有優(yōu)良的電學(xué)性能.

MoS2是一種層狀結(jié)構(gòu)材料, 層間由比較弱的范德華力連接, 層內(nèi)則由共價鍵相連[11-12]. 當(dāng)MoS2從體相向單層轉(zhuǎn)變時, 由間接帶隙半導(dǎo)體變?yōu)橹苯訋栋雽?dǎo)體并伴隨著光致發(fā)光的增強(qiáng)[13-14]. 這些獨特的光學(xué)和電學(xué)特性使得少層MoS2有著廣泛的應(yīng)用, 如鋰電池陽極[15]和超級電容器電極[16]等. MoS2納米片的可逆容量雖然較石墨有很大的提高[17-18], 但其循環(huán)穩(wěn)定性較差且倍率性較低, 而ZnO則可以很好地彌補(bǔ)MoS2這一不足之處.

本文利用水熱法和連續(xù)離子層吸附法制備ZnO/CdS/MoS2異質(zhì)結(jié)納米棒陣列, 并利用拉曼光譜、掃描電子顯微鏡及高分辨透射電子顯微鏡等手段研究了ZnO/CdS/MoS2異質(zhì)結(jié)的結(jié)構(gòu)特性和形貌變化, 同時研究了ZnO/CdS/MoS2異質(zhì)結(jié)納米棒原位電輸運特性, 為進(jìn)一步研究構(gòu)筑基于芯/鞘異質(zhì)結(jié)納米棒器件奠定理論與試驗的基礎(chǔ).

1 試 驗

1.1 ZnO納米棒陣列的制備

將0.000 5 mol二水乙酸鋅溶于100 mL無水乙醇中, 獲得二水乙酸鋅溶液. 利用旋轉(zhuǎn)法將二水乙酸鋅溶液涂覆到玻片襯底表面, 然后在420 ℃的恒溫鼓風(fēng)干燥箱中退火20 min. 重復(fù)旋涂和退火6次, 得到ZnO種子層. 稱取0.01 mol六水合硝酸鋅和0.01 mol六次甲基四胺, 分別溶于200 mL去離子水中, 并將兩者混合. 將含晶種層的玻片置于聚四氟乙烯內(nèi)襯不銹鋼高壓釜(50 mL)中, 加入混合溶液. 將高壓反應(yīng)釜放入80 ℃的恒溫鼓風(fēng)干燥箱中, 反應(yīng)12 h. 反應(yīng)結(jié)束后清洗基片, 在420 ℃干燥箱中退火20 min.

1.2 ZnO/CdS異質(zhì)結(jié)納米棒陣列的制備

稱取1 mol硝酸鎘加入到200 mL無水乙醇中, 1 mol硫化鈉溶于200 mL甲醇中. 兩溶液分別超聲震蕩30 min后分別倒入兩只培養(yǎng)皿中. 將ZnO納米棒的基片放入盛有硝酸鎘溶液的培養(yǎng)皿中浸泡2.5 min, 之后取出并用無水乙醇沖洗, 再放入盛有硫化鈉溶液的培養(yǎng)皿中浸泡2.5 min, 取出用甲醇沖洗. 重復(fù)操作4次. 浸泡結(jié)束后, 放入420 ℃干燥箱中退火20 min.

1.3 ZnO/CdS/MoS2異質(zhì)結(jié)納米棒陣列的制備

將0.025 mol鉬粉加入燒杯, 在冰水浴下逐滴滴入30 mL的H2O2(質(zhì)量分?jǐn)?shù)為30%), 所得黃色溶液磁力攪拌4 h以上. 將硫脲溶于120 mL去離子水中, 在磁力攪拌下將上述氧化鉬前驅(qū)體溶液逐滴加入并繼續(xù)攪拌使溶液均勻. 得到的混合溶液使用旋轉(zhuǎn)涂膜機(jī)旋涂在ZnO/CdS異質(zhì)結(jié)納米棒的基片上, 將玻片放入電熱恒溫鼓風(fēng)干燥箱中, 在氮氣環(huán)境下300 ℃退火5 min.

1.4 樣品表征

試驗產(chǎn)物用X射線衍射(XRD)、掃描電子顯微鏡(SEM)、激光拉曼光譜、透射電子顯微鏡(TEM)和原位電性能等手段進(jìn)行表征. X射線衍射儀使用日本電子株式會社D/Max-2550型, 射線源為Cu靶Kα(λ=0.154 056 nm). 掃描電子顯微鏡采用HITACHI公司S-4700型, 操作電壓為10 kV. 激光拉曼光譜儀則用英國Renishaw公司的inVia-Reflex型以發(fā)射波長為532 nm的He-Ne激光器作為光源. 透射電子顯微鏡采用FEI公司的Tecnai G2S-TWIN型, 配備能譜色散儀. 原位電性能使用Nanofactory Instruments AB 制造的掃描隧道顯微鏡-透射電鏡支架測量得到.

2 結(jié)果與分析

圖1 ZnO、ZnO/CdS以及ZnO/CdS/MoS2異質(zhì)結(jié)納米棒的XRD衍射譜圖

Fig.1 XRD patterns of as-prepared ZnO, ZnO/CdS and ZnO/CdS/MoS2heterostructure nanorod

圖1是生長在玻片表面的ZnO、ZnO/CdS和ZnO/CdS/MoS2樣品的XRD衍射譜圖. 由圖1可知, ZnO樣品所有的衍射峰均可指標(biāo)為六方相的ZnO(JCPDS No.65-3411), 且無明顯雜峰. ZnO納米棒沿(002)方向擇優(yōu)生長明顯, 具有明顯的c軸取向, 同時發(fā)現(xiàn)(101)和(100)的衍射峰信號較強(qiáng), 表明部分ZnO納米棒可能出現(xiàn)了歪斜和倒塌. ZnO/CdS樣品的XRD衍射譜圖中能觀察到ZnO納米棒衍射峰, 同時出現(xiàn)的CdS衍射峰可指標(biāo)為六方相的CdS(JCPDS No.1-0781), 其中ZnO納米棒的(002)衍射峰依然較強(qiáng)且尖銳, 表明CdS的包覆并沒有破壞垂直生長的ZnO納米棒陣列結(jié)構(gòu). ZnO/CdS/MoS2樣品的XRD衍射譜圖中MoS2的衍射峰出現(xiàn)了不同程度的偏移, 可能是由于MoS2在包覆ZnO/CdS異質(zhì)結(jié)納米棒表面時發(fā)生了彎曲形變引起的[19], 此外仍可以觀察到CdS的衍射峰.

ZnO、ZnO/CdS和ZnO/CdS/MoS2樣品的SEM照片如圖2所示. 從圖2(a)中可看出, ZnO呈六棱柱棒狀結(jié)構(gòu)且較為均勻有序地生長在玻璃襯底上. 從圖2(b)中可以清晰地看到, 包覆CdS之后的ZnO納米棒表面呈粗糙狀, 說明CdS均勻地包覆在ZnO納米棒的表面. 從圖2(c)仍可以清楚地看到ZnO納米棒的六棱柱棒狀結(jié)構(gòu), 但觀察單根的ZnO/CdS/MoS2異質(zhì)結(jié)納米棒發(fā)現(xiàn), 相比于ZnO/CdS異質(zhì)結(jié)納米棒, ZnO/CdS/MoS2納米棒的表面因包覆MoS2變得更為粗糙.

圖3～5分別是ZnO、 ZnO/CdS和ZnO/CdS/MoS2樣品的TEM照片、高分辨率的透射電子顯微鏡(HRTEM)照片和對應(yīng)的選區(qū)電子衍射(SAED)圖譜. 從圖3(a)可以清楚地看到合成的ZnO呈棒狀結(jié)構(gòu), 長度約為1 μm. 觀察圖3(b)發(fā)現(xiàn)樣品的晶面間距與六方相結(jié)構(gòu)ZnO的(001)面一致, 約為0.526 nm, 說明ZnO納米棒沿著{001}方向生長.

圖2 ZnO、ZnO/CdS和ZnO/CdS/MoS2異質(zhì)結(jié)納米棒陣列的SEM照片F(xiàn)ig.2 SEM images of ZnO, ZnO/CdS and ZnO/CdS/MoS2 heterostructure nanorod arrays

圖3 ZnO納米棒的TEM照片、HRTEM照片和SAED圖譜Fig.3 TEM image, HRTEM image and corresponding SAED pattern of as-prepared ZnO nanorod

圖4 ZnO/CdS異質(zhì)結(jié)納米棒的TEM照片、HRTEM照片和SAED圖譜Fig.4 TEM image, HRTEM image and corresponding SAED pattern of as-prepared ZnO/CdS heterostructure nanorod

圖5 ZnO/CdS/MoS2異質(zhì)結(jié)納米棒的TEM照片、HRTEM照片和SAED圖譜Fig.5 TEM image, HRTEM image and corresponding SAED pattern of as-prepared ZnO/CdS/MoS2 heterostructure nanorod

由圖3(c)可以看出ZnO納米棒是一種單晶, 其結(jié)晶性較好且沒有缺陷. 從圖4(a)可以看到包裹CdS后的異質(zhì)結(jié)仍保持較好的棒狀結(jié)構(gòu). 從圖4(b)可看到CdS鞘層的晶面間距約為0.329 nm, 與六方相CdS的(002)面一致. 從圖4(c)可以看出ZnO和CdS皆為單晶且結(jié)晶性較好. 從圖5(a)中可以看到, 經(jīng)MoS2包覆后的ZnO/CdS異質(zhì)結(jié)納米棒表面有明顯的層狀結(jié)構(gòu). 從圖5(b)可以看到MoS2納米片包裹在CdS的外部且呈少層結(jié)構(gòu), CdS的晶面間距約為0.328 nm, MoS2的晶面間距約為0.645 nm. 從圖5(c)可以看出MoS2是一種多晶.

圖6 ZnO、ZnO/CdS以及ZnO/CdS/MoS2異質(zhì)結(jié)納米棒的拉曼光譜Fig.6 Raman spectra of as-prepared ZnO, ZnO/CdS and ZnO/CdS/MoS2 heterostructure nanorod

3 樣品的電學(xué)性能

原位電性能測試示意圖如圖7所示. 由圖7可知, 金探針連接在一個固定的電子傳感器上, 黏有一定數(shù)量納米棒的鉑探針連接在另一個可移動的支架上, 在透射電子顯微鏡下, 通過納米級的操作, 在兩根探針之間搭建納米棒橋梁, 異質(zhì)結(jié)納米棒的電性能由軟件測試得到.

圖中標(biāo)尺為ZnO: 100 nm; ZnO/CdS: 100 nm; ZnO/CdS/MoS2: 200 nm圖7 原位電性能測試示意圖Fig.7 Schematic plot of the in-situ measurements

ZnO、 ZnO/CdS和ZnO/CdS/MoS2異質(zhì)結(jié)納米棒的伏安特性曲線如圖8所示. 通過觀察可以發(fā)現(xiàn), 與ZnO納米棒相比, ZnO/CdS異質(zhì)結(jié)納米棒在-3 000～3 000 mV的電壓范圍內(nèi)展現(xiàn)出的電流強(qiáng)度更強(qiáng), 是ZnO納米棒的9倍, 并且表現(xiàn)出非歐姆接觸的性質(zhì)[33], 這可能是由于探針與納米棒之間形成了肖特基接觸[34-36]. 而與ZnO納米棒和ZnO/CdS異質(zhì)結(jié)納米棒相比, ZnO/CdS/MoS2異質(zhì)結(jié)納米棒則展現(xiàn)出了更佳的電學(xué)特性. 在-3 000～3 000 mV的電壓范圍內(nèi), ZnO/CdS/MoS2異質(zhì)結(jié)納米棒的電流強(qiáng)度是ZnO/CdS異質(zhì)結(jié)納米棒的3倍, 顯示了更優(yōu)的電學(xué)特性.

圖8 ZnO、ZnO/CdS和ZnO/CdS/MoS2異質(zhì)結(jié)納米棒的I-V特性曲線Fig.8 I-V curves of as-prepared ZnO, ZnO/CdS and ZnO/CdS/MoS2 heterostructure nanorod

4 結(jié) 語

本文采用簡單的水熱法制備出了較為均勻的ZnO納米棒陣列, 以其為模板, 利用連續(xù)離子層吸附法和旋轉(zhuǎn)涂膜法使CdS和MoS2均勻地吸附在ZnO納米棒的表面, 合成了ZnO/CdS/MoS2異質(zhì)結(jié)納米棒陣列. 此外, ZnO納米棒為模板有效地抑止了MoS2納米片的堆積, 以少層的結(jié)構(gòu)存在. 通過原位電性能測試發(fā)現(xiàn), ZnO/CdS/MoS2異質(zhì)結(jié)納米棒的電學(xué)特性優(yōu)于ZnO納米棒和ZnO/CdS異質(zhì)結(jié)納米棒. 目前的工作主要側(cè)重于ZnO/CdS/MoS2異質(zhì)結(jié)納米棒的合成與表征, 對于其性能的研究并不深入. 因此今后的工作方向?qū)⒅饕獓@ZnO/CdS/MoS2異質(zhì)結(jié)納米棒在鋰電池等方面的應(yīng)用展開. 此外, 如何選擇合適的材料組成較為穩(wěn)定且性能良好的異質(zhì)結(jié), 以及其性能反饋和調(diào)試也將成為今后的研究重點.

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(責(zé)任編輯: 徐惠華)

Fabrication and Properties of ZnO/CdS/MoS2Heterostructure Nanorod Arrays

FUYanjue,WANGChunrui

(College of Science, Donghua University, Shanghai 201620, China)

ZnO nanorod arrays were fabricated as a template, which were coated with CdS with narrow bandgap and few-layer MoS2with a narrower bandgap by the successive ionic layer adsorption and reaction (SILAR) method, respectively. The ZnO/CdS/MoS2heterostructure nanorods arrays were obtained. The samples were characterized by X-ray powder diffraction, scanning electron microscope, high resolution transmission electron microscope and Raman spectrum. The results show that CdS and MoS2are attached to the vertically grown ZnO nanorods uniformly, and ZnO remains to be nanorods arrays. Meanwhile, ZnO/CdS and CdS/MoS2keep good contact interfaces and have small lattice mismatches. In addition, the existence of ZnO nanorod arrays prevents the restacking of MoS2nanosheets alongcaxis, which makes MoS2few-layer flakes. The I-V curves of the in-situ measurements show that ZnO/CdS/MoS2heterostructure nanorods present better electrical properties than ZnO nanorods and ZnO/CdS heterostructure nanorods.

successive ionic layer adsorption and reaction method; few-layer MoS2; ZnO/CdS/MoS2heterostructure; in-situ measurements

1671-0444 (2017)02-0286-07

2016-04-11

國家自然科學(xué)基金資助項目(11174049，61376017)

傅艷玨(1990—)，女，上海人，碩士研究生，研究方向為納米光電材料與器件.E-mail: fuyanjue@163.com 王春瑞(聯(lián)系人)，男，教授，E-mail: crwang@dhu.edu.cn

O 472.4

A