發(fā)泡法制備二維材料泡沫體的進(jìn)展

高天, 肖慶林, 許晨陽(yáng), 王學(xué)斌

發(fā)泡法制備二維材料泡沫體的進(jìn)展

高天, 肖慶林, 許晨陽(yáng), 王學(xué)斌

(南京大學(xué) 現(xiàn)代工程與應(yīng)用科學(xué)學(xué)院, 固體微結(jié)構(gòu)物理國(guó)家重點(diǎn)實(shí)驗(yàn)室, 人工微結(jié)構(gòu)科學(xué)與技術(shù)協(xié)同創(chuàng)新中心, 江蘇省功能材料設(shè)計(jì)原理與應(yīng)用技術(shù)重點(diǎn)實(shí)驗(yàn)室, 南京 210093)

以石墨烯為代表的二維材料具有優(yōu)異的本征性質(zhì), 例如高表面積和電導(dǎo)率, 但其宏觀塊體材料的性質(zhì)仍不理想。這是由于石墨烯片層堆疊損失了有效的表面; 片層之間聯(lián)結(jié)較弱導(dǎo)致接觸電阻和熱阻增大。原則上二維材料的三維化設(shè)計(jì)能避免上述問(wèn)題, 將納米尺度的優(yōu)異性質(zhì)傳遞到宏觀尺度, 獲得高表面積、高導(dǎo)電、貫通孔道和優(yōu)良機(jī)械性能的塊體材料。二維材料多孔塊體可用于電極、吸附劑和彈性體等。發(fā)泡法工藝簡(jiǎn)單、成本低, 是近年來(lái)制備二維材料泡沫體的主要方法。本文系統(tǒng)總結(jié)了發(fā)泡法的基本原理, 綜述了石墨烯、氮化硼等二維材料泡沫體的研究進(jìn)展, 展望了二維材料泡沫體在能源、環(huán)境等方面的應(yīng)用前景。

二維材料; 發(fā)泡法; 石墨烯; 氮化硼; 泡沫材料; 綜述

石墨烯是一種由碳原子通過(guò)sp2雜化構(gòu)成的二維新材料[1], 具有超薄結(jié)構(gòu)和獨(dú)特物性, 如高載流子遷移率、表面積、熱導(dǎo)率[2-5], 在電子、儲(chǔ)能和催化等領(lǐng)域具有重要的應(yīng)用前景[6-9]。在石墨烯的宏觀塊體中, 兩個(gè)相鄰片層的面與面之間容易發(fā)生p-p堆疊, 損失表面積; 在面內(nèi)方向, 兩個(gè)相似片層之間通常依靠范德華力聯(lián)結(jié), 接觸電阻、熱阻較大; 此外, 片片之間的堆積孔道曲折無(wú)序, 不利于外來(lái)物質(zhì)的擴(kuò)散。這三點(diǎn)限制了基于石墨烯的電化學(xué)電極等應(yīng)用性能的提升[10]。

三維石墨烯材料是一種石墨烯塊體, 與石墨烯粉體、石墨烯薄膜并列, 如圖1所示。在概念上, 三維石墨烯是將石墨烯單元連接形成sp2雜化的三維網(wǎng)絡(luò)。在理論上三維石墨烯可以繼承二維納米材料的本征高比表面積、電導(dǎo)率、熱導(dǎo)率等優(yōu)點(diǎn)。三維網(wǎng)絡(luò)結(jié)構(gòu)提供的雙聯(lián)通通道為固相網(wǎng)絡(luò)通道和內(nèi)部孔道聯(lián)通形成的空腔通道, 前者用于輸運(yùn)電子、聲子和力; 后者用于傳質(zhì)。三維石墨烯在界面相關(guān)應(yīng)用場(chǎng)景中具有重要意義, 包括電極、吸附等[11]。

學(xué)者們報(bào)導(dǎo)了數(shù)種制備三維石墨烯的方法: 基于氧化還原石墨烯(RGO)的凝膠化方法(包括體相凝膠化[12-15]、界面凝膠化法[16]、冰模板法[17]、模板/交聯(lián)劑輔助法[18-19])、基于泡沫鎳的化學(xué)氣相沉積法(CVD)[11,20-21]、基于鋅分層效應(yīng)的熱裂解法[22]、生物質(zhì)熱裂解法[23]、發(fā)泡法[24]和3D打印[25]等。凝膠化方法主要依靠非共價(jià)鍵組裝, 內(nèi)部聯(lián)結(jié)較弱; 泡沫鎳等多孔模板較難循環(huán)利用。發(fā)泡法工藝簡(jiǎn)單、成本低、易擴(kuò)大生產(chǎn), 其產(chǎn)品的固相網(wǎng)絡(luò)內(nèi)部聯(lián)結(jié)強(qiáng)、表面積大, 是一種極具產(chǎn)業(yè)價(jià)值的制備方法。除了三維石墨烯, 硫化物、雙氫氧化物等二維材料多孔塊體也是二維材料三維化設(shè)計(jì)的熱門(mén)領(lǐng)域[26-28]。

回溯歷史, 發(fā)泡法起源于傳統(tǒng)泡沫塑料加工, 但在近年開(kāi)始用于制備二維材料泡沫體。2011~2013年, 王學(xué)斌等受吹泡啟發(fā), 將發(fā)泡過(guò)程首次引入到二維材料領(lǐng)域, 發(fā)展了化學(xué)發(fā)泡法制備石墨烯泡沫體[24]、氮化硼(BN)納米片泡沫體[29]。隨后, LEI[30]和WANG[31]等利用硝酸鹽輔助法制備了石墨烯泡沫體; WANG[32]、ZHAO[33]和DONG[34]等制備了氮摻雜石墨烯泡沫; DONG[35]、ZHU[36]、WU[37]、CAI[38]和TAN[39-41]等制備了擔(dān)載各種功能物質(zhì)的石墨烯泡沫; LU等[42]使用銨鹽發(fā)泡劑制備了氮化碳泡沫; ZHAO等[43]制備了BN泡沫。

圖1 多種維度的sp2雜化碳納米材料, 石墨烯粉體、薄膜和塊體三種形態(tài), 以及設(shè)計(jì)三維石墨烯的理念

本文總結(jié)了采用發(fā)泡法制備二維材料泡沫體的發(fā)泡原理、不同類(lèi)型泡沫體及其應(yīng)用, 以期闡明未來(lái)的發(fā)展前景。

1 發(fā)泡原理

1.1 發(fā)泡流程

發(fā)泡過(guò)程包括三步(圖2): ①前驅(qū)體與發(fā)泡劑混合; ②在內(nèi)/外部作用下產(chǎn)生氣體源, 使氣泡成核、長(zhǎng)大; ③流體泡沫經(jīng)歷穩(wěn)定化過(guò)程最終變成固體泡沫。根據(jù)氣源不同, 發(fā)泡法分為化學(xué)發(fā)泡和物理發(fā)泡, 前者通過(guò)化學(xué)反應(yīng)產(chǎn)生氣體, 后者則利用沸騰或減壓膨脹產(chǎn)生氣體。流體泡沫目前基本沒(méi)有工程用途, 它需經(jīng)歷固化、硬化和結(jié)晶等穩(wěn)定化過(guò)程。

泡沫的基本單元為氣泡, 在平衡態(tài)時(shí)呈多面體形狀。氣泡由泡壁、筋和交點(diǎn)構(gòu)成, 分別定義為2、3、4個(gè)氣泡的結(jié)合部, 亦即幾何學(xué)中的面、棱和點(diǎn), 如圖2所示。按照Plateau定律, 3個(gè)泡壁相交于1條筋(Plateau通道), 且傾向?qū)ΨQ(chēng)分布; 4條筋相交于1個(gè)交點(diǎn), 也傾向?qū)ΨQ(chēng)分布。

1.2 發(fā)泡幾何學(xué)與靜力學(xué)

泡沫幾何學(xué)主要研究“干泡沫”的多面體堆砌問(wèn)題。歐拉定律認(rèn)為氣泡多面體的大多數(shù)面應(yīng)為五邊形[44]; Aboav-Weaire定律認(rèn)為一個(gè)多面體擁有的面數(shù)越多, 則其相鄰多面體的面數(shù)就越少[45]。Lewis定律認(rèn)為多面體的體積隨面數(shù)的增加而增加[46]。因此, 一個(gè)大氣泡通常被小氣泡所包圍。此外, 表面張力等靜力學(xué)因素也影響泡沫的結(jié)構(gòu)。表面能與表面積成正比, 泡沫表面能最小的為密堆多面體, 因?yàn)樗谋砻娣e最小, 即熟知的Kelvin問(wèn)題。1887年, Thomson提出了Kelvin結(jié)構(gòu); 1993年, Weaire和Phelan構(gòu)建了Weaire-Phelan結(jié)構(gòu)[47], 其表面積比Kelvin結(jié)構(gòu)的表面積減少了0.3%, 如圖3(a,b)所示。堆砌問(wèn)題的最優(yōu)方案迄今仍無(wú)確切結(jié)論。

圖2 發(fā)泡過(guò)程示意圖

1.3 發(fā)泡動(dòng)力學(xué)與動(dòng)態(tài)學(xué)

發(fā)泡動(dòng)力學(xué)主要包括: ①氣泡的均/異相成核; ②氣泡的長(zhǎng)大; ③氣泡結(jié)構(gòu)的動(dòng)態(tài)演變; ④穩(wěn)定化過(guò)程, 如圖3(c)所示。氣泡成核與晶體成核有共通之處, 一旦初生氣泡大于臨界尺寸, 則傾向于長(zhǎng)大; 反之則趨于消失。進(jìn)一步, 內(nèi)/外部驅(qū)動(dòng)力驅(qū)使氣源持續(xù)擴(kuò)散進(jìn)入氣泡, 使氣泡長(zhǎng)大。

在形核和長(zhǎng)大的過(guò)程中, 流體泡沫也進(jìn)行著動(dòng)態(tài)學(xué)演化, 包括滲流、熟化、破裂、重排等行為。

(1)滲流是流體泡沫中的流體在表面張力或重力驅(qū)動(dòng)下, 在泡壁和筋內(nèi)部流動(dòng)的過(guò)程[48]。前者由于不同位置的曲率半徑不同, 導(dǎo)致Lapalce壓力差, 驅(qū)使流體自泡壁流向Plateau通道。后者由于流體泡沫的質(zhì)心要降低, 則流體沿著Plateau通道自上而下流動(dòng)。

(2)熟化/粗化是小氣泡內(nèi)的氣體向相鄰大氣泡中擴(kuò)散的過(guò)程, 這是由于小氣泡的半徑小, 內(nèi)壓大, 與大氣泡之間有壓差。

(3)液膜破裂。伴隨著滲流或流體的揮發(fā), 液膜不斷減薄, 當(dāng)其厚度減薄至黑膜狀態(tài)時(shí), 楔壓會(huì)削弱表面張力, 造成液膜破裂, 使兩個(gè)氣泡發(fā)生合并。

(4)流體泡沫具有流變特性, 例如典型的T1重排。

氣泡的成核和生長(zhǎng)在很大程度上決定氣泡的直徑分布, 動(dòng)態(tài)演變亦影響了氣泡尺寸、泡壁厚度等。實(shí)踐發(fā)現(xiàn), 發(fā)泡的關(guān)鍵在于發(fā)泡劑產(chǎn)氣過(guò)程與聚合物固化過(guò)程之間的匹配。兩者的匹配影響了發(fā)泡動(dòng)力學(xué)與動(dòng)態(tài)學(xué)、流體泡沫的結(jié)構(gòu), 最終形成固體泡沫。

2 采用發(fā)泡法制備二維材料泡沫體

傳統(tǒng)發(fā)泡工業(yè)制造了泡沫塑料[49]、泡沫金屬[50]、泡沫陶瓷[51]和泡沫炭[52-54]等民用和特種泡沫。近年來(lái), 學(xué)者們拓展了用發(fā)泡法制備二維材料泡沫體[24,29]。二維材料泡沫體不僅具有固體泡沫的性質(zhì), 又可以呈現(xiàn)二維材料的本征優(yōu)異特性[28]?；贐、C、N和O的聚合物種類(lèi)豐富, 易發(fā)泡, 而且在發(fā)泡過(guò)程中易進(jìn)行摻雜、包裹/擔(dān)載金屬元素, 發(fā)展了多種多樣的B–C–N–O體系二維材料泡沫體, 包括石墨烯、摻雜和擔(dān)載型石墨烯、氮化碳和氮化硼等, 如圖4所示。

圖3 堆砌的(a)Kelvin結(jié)構(gòu)和(b)Weaire-Phelan結(jié)構(gòu); (c)發(fā)泡動(dòng)力學(xué)示意圖

2.1 石墨烯類(lèi)泡沫體

2.1.1 石墨烯泡沫體

2013年, 王學(xué)斌等[24]開(kāi)拓了化學(xué)發(fā)泡法制備一種新型三維石墨烯——筋撐石墨烯(strutted graphene, SG), 如圖5(a~d)所示?；瘜W(xué)發(fā)泡法以糖為碳源, NH4Cl為發(fā)泡劑, 在加熱過(guò)程中誘發(fā)美拉德反應(yīng), 同時(shí)NH4Cl分解釋放出NH3和HCl, 使類(lèi)黑精聚合物發(fā)泡。隨著氣泡膨脹、表面張力滲流和熱解反應(yīng), 類(lèi)黑精發(fā)泡體的泡壁逐漸減薄, 最終其聚合物泡壁厚度可薄至約20 nm[55], 如圖5(e~f)所示。類(lèi)黑精泡沫體經(jīng)1350 ℃退火, 轉(zhuǎn)變?yōu)镾G。SG的泡壁為單層或寡層石墨烯薄膜, 如圖5(g~i), 它們附著在石墨筋上。這種筋耦合膜的結(jié)構(gòu)提升了機(jī)械彈性, 經(jīng)80%的壓應(yīng)變后依然能恢復(fù)原狀; 同時(shí)消除了石墨烯堆疊問(wèn)題, 實(shí)現(xiàn)了1005 m2/g的比表面積。

為了匹配發(fā)泡劑產(chǎn)氣與聚合物固化兩過(guò)程, 選擇適當(dāng)發(fā)泡劑是控制泡沫結(jié)構(gòu)的重要手段。無(wú)殘留的發(fā)泡劑除了NH4Cl (產(chǎn)氣溫度可持續(xù)在200~270 ℃, 下同), 還可以選擇(NH4)2CO3(50~80 ℃)、NH4NO3(150~200 ℃)、(NH4)2SO4(330~380 ℃)、尿素(170~ 220 ℃)、草酸(190~220 ℃)和三聚氰胺(270~320 ℃) 等。若產(chǎn)氣溫度高于或低于聚合物固化溫度, 則不易發(fā)泡。加熱速度可以影響產(chǎn)氣速度和持續(xù)時(shí)間, 高速加熱時(shí)氣泡成核多, 則最終泡孔較小; 反之則泡孔較大, 如圖5(j~m)。使用銨鹽等無(wú)殘留發(fā)泡劑的報(bào)道歸納于表1中。

圖4 適合發(fā)泡的輕元素B-C-N-O體系

圖5 (a)化學(xué)發(fā)泡法示意圖; (b, c)SG實(shí)物和光學(xué)圖像; (d)SG中石墨烯壁和石墨筋的光學(xué)圖像; (e, f)中間體聚合物泡沫及其泡壁的SEM照片; (g)純化后的石墨烯片層的光學(xué)照片; (h)石墨烯片層的AFM圖像; (i)SG的HRTEM照片; (j~m)分別以1、4、20、100 ℃/min加熱發(fā)泡得到的不同SG的SEM照片[24,55]

含金屬的鹽類(lèi)也可以作為發(fā)泡劑。例如, Fe(NO3)3[30]、ZnCl2[56]、KHCO3[57]和Ni(NO3)2[58]分別使葡萄糖、水解淀粉、纖維素、麥芽糖發(fā)泡, 用來(lái)制備石墨烯泡沫體。從金屬元素的角度而言, Na、K和Zn等低沸點(diǎn)金屬在發(fā)泡過(guò)程中對(duì)碳有插層/刻蝕作用, 產(chǎn)生活化[59]、分層[22]等效應(yīng), 有利于提高表面積。過(guò)渡金屬元素Fe、Ni等, 在加熱時(shí)可以提高碳的石墨化程度[30,58,60-62], 但需要用強(qiáng)酸洗去這些高沸點(diǎn)金屬以獲得純碳泡沫體。

2.1.2 摻雜石墨烯泡沫體

發(fā)泡過(guò)程要經(jīng)歷聚合物流體狀態(tài), 其聚合反應(yīng)步驟中易于摻入雜原子。N是常用摻雜元素, 例如吡啶氮可以激活其相鄰碳原子成為氧還原催化活性中心[63]。前述銨鹽發(fā)泡劑就可以在SG中摻入N元素, N含量在300 ℃時(shí)為18at%, 在1400 ℃時(shí)為0.4at%[55]。由于雜原子在高溫下容易流失, 需要提供額外的雜原子源以補(bǔ)償此損失[64-66], 提高摻雜量。在葡萄糖-NH4Cl發(fā)泡體系中加入三聚氰胺, 其發(fā)泡體在1100 ℃時(shí)含N為8.36at%[65]。此外, 也可進(jìn)行多元素共摻雜[67-70], 如三聚氰胺分解使季戊四醇三聚氰胺磷酸鹽(PMP)發(fā)泡, 得到N–P–O共摻雜石墨烯泡沫體, 如圖6所示[67]。

2.1.3 擔(dān)載型石墨烯泡沫體

在發(fā)泡過(guò)程中, 若采用含過(guò)渡金屬鹽類(lèi)作為發(fā)泡劑, 則其金屬元素可以保留在最終產(chǎn)品中, 構(gòu)成擔(dān)載型石墨烯泡沫體。聚乙烯吡咯烷酮(PVP)-Fe(NO3)3體系經(jīng)歷發(fā)泡, 可以制得擔(dān)載Fe2O3的碳泡沫體[35], 如圖7(a,b)。在葡萄糖-NH4Cl體系中分別加入CuCl2、CrCl3和Co(NO3)2, 則最終得到擔(dān)載Cu[71]、CrN[72]和Co2P[36]的碳泡沫體, 如圖7(c,d)。此外, 也可以利用水熱法等手段在石墨烯泡沫體上擔(dān)載功能物質(zhì), 例如SG擔(dān)載NiFeP超薄納米片[73], 如圖7(e,g)。使用含金屬鹽發(fā)泡劑的報(bào)道匯總于表2中。

2.2 類(lèi)石墨氮化碳泡沫體

LU[42]、GUO[87]、WANG[88]和TALAPANENI[89]等利用發(fā)泡法制備了類(lèi)石墨氮化碳g-C3N4納米片的泡沫體。將三聚氰胺或雙氰胺與NH4Cl混合[42,87-88], 在550~600 ℃下共熱, 則NH4Cl分解產(chǎn)氣可使Melem等中間體發(fā)泡, 最終得到g-C3N4泡沫體, 如圖8所示。g-C3N4泡沫體可以增強(qiáng)光催化產(chǎn)氫[42,88]和降解有機(jī)污染物[87-88]。

表1 采用發(fā)泡法制備的石墨烯泡沫體和摻雜石墨烯泡沫體(使用無(wú)殘留的銨鹽發(fā)泡劑)

圖6 (a)以三聚氰胺發(fā)泡PMP過(guò)程示意圖; (b)N-P-O共摻雜石墨烯泡沫體的SEM照片[67]

圖7 擔(dān)載型石墨烯泡沫體制備示意圖及其SEM/HRTEM照片

(a, b) Fe2O3[35]; (c, d) Co2P[36]; (e, g) NiFeP[73]

表2 采用發(fā)泡法制備石墨烯泡沫體和擔(dān)載型石墨烯泡沫體(使用含金屬元素的鹽類(lèi)作為發(fā)泡劑)

圖8 g-C3N4泡沫體的(a)合成示意圖及(b~d)不同加熱速率下發(fā)泡所獲得樣品的TEM照片[87]

2.3 氮化硼泡沫體及硼碳氮雜化泡沫體

六方氮化硼(BN)具有與石墨類(lèi)似的強(qiáng)層內(nèi)σ鍵和弱層間范德華力, 與石墨有類(lèi)似的熱學(xué)和力學(xué)性質(zhì)。由于異原子成鍵, BN具有部分離子性, 其光學(xué)、電學(xué)性質(zhì)與石墨截然相反, 如寬帶隙、絕緣。BN有獨(dú)特的用途, 例如導(dǎo)熱、潤(rùn)滑、絕緣、深紫外發(fā)光、中子吸收和抗癌等[90-94]。

王學(xué)斌等[29]通過(guò)加熱氨硼烷(AB)使之自發(fā)泡, 制備了BN泡沫體, 如圖9(a~e)所示。在加熱過(guò)程中, AB聚合為聚氨硼烷(PAB)、聚亞氨硼烷(PIB), 并釋放出H2, H2恰能使PAB和PIB發(fā)泡, 最終在1200 ℃下得到BN泡沫體, 經(jīng)進(jìn)一步純化可以獲得BN納米片。作為改良, 對(duì)AB進(jìn)行預(yù)處理[95-96], 可以提高產(chǎn)品表面積。外加發(fā)泡劑可以使發(fā)泡過(guò)程更加可控, 例如在AB中添加硫脲或氨基硫脲發(fā)泡劑[43]。考慮到AB成本較高, WENG等[97]探索了更低成本的前驅(qū)體。將硼酸–聚環(huán)氧乙烷(PEO)在NH3氣氛下加熱, PEO為發(fā)泡劑, 最終得到多孔BN, 如圖9(f~h)。其它相關(guān)體系為氧化硼–鹽酸胍[98]、氟硼酸銨–疊氮化鈉[99]、硼酸–氰胺類(lèi)物質(zhì)[100-101]、硼酸–甲醛–雙氰胺[102]和硼酸–尿素[103-104]等。

鑒于BN和石墨互補(bǔ)的電學(xué)和光學(xué)性質(zhì), BCN有望實(shí)現(xiàn)帶隙調(diào)變[105]。在上述AB自發(fā)泡過(guò)程中, 通入乙醇?xì)夥? 可以獲得C-BN雜化納米片的發(fā)泡體,可在0.3~0.7之間調(diào)變, 具有半導(dǎo)體特性[29]。

2.4 氧化物納米片泡沫體

利用金屬鹽類(lèi)發(fā)泡劑進(jìn)行發(fā)泡時(shí), 如果將惰性氣氛調(diào)整為氧化性氣氛, 則在合適條件下可得到氧化物納米片泡沫體[32,35,106]。在陶瓷漿料的傳統(tǒng)物理發(fā)泡過(guò)程中, 往往要加入表面活性劑降低發(fā)泡難度[107]。使用金屬鹽類(lèi)發(fā)泡劑使碳基聚合物發(fā)泡, 則聚合物泡沫起到模板的作用, 引導(dǎo)金屬元素的分布, 最后經(jīng)氧化轉(zhuǎn)化為氧化物的筋、膜, 構(gòu)成氧化物泡沫體, 例如Mn3O4[32]、Fe2O3[32,35]、MgO[106]、MnO2[107]、V2O5[107]和MoO3[107]等。

3 二維材料泡沫體的應(yīng)用

二維材料泡沫體兼?zhèn)涠S材料和泡沫結(jié)構(gòu)的優(yōu)點(diǎn), 是一種有潛力的新材料, 不僅可以用作結(jié)構(gòu)支撐材料, 也可用作多領(lǐng)域功能材料, 如圖10所示。

在力學(xué)方面, 泡沫體是一種輕質(zhì)支撐材料, 如同自然界演化出的骨頭和樹(shù)木, 可用作包裝、填充和漂浮等[29,96,108-109]。石墨烯泡沫體可以支撐自身重量50000倍的物體[17]。三維網(wǎng)絡(luò)結(jié)構(gòu)可以經(jīng)99%應(yīng)變后表現(xiàn)出良好的回彈性, 可循環(huán)上千次[110]。石墨烯泡沫體還可以吸收沖擊能量[17,110]。低楊氏模量的泡沫體可用于壓敏傳感[111]。

在熱學(xué)方面, 石墨烯泡沫體和BN泡沫體在熱控領(lǐng)域應(yīng)用廣泛。它們可以作為三維化填料用于導(dǎo)熱增強(qiáng)復(fù)合材料, 以減小填料-填料的界面熱阻。石墨烯泡沫體還可以用作相變蓄熱材料的填料[112]。BN泡沫體可以用于增強(qiáng)聚合物基復(fù)合材料的熱導(dǎo)率[113-118], 用于電子封裝等方面。

在吸附相關(guān)方面, 石墨烯泡沫體和BN泡沫體可以捕獲CO2[119,120]、儲(chǔ)氫[120,121]、儲(chǔ)甲烷[120]、吸附有機(jī)污染物[122-125]、油水分離[126-128]、吸附重金屬離子[99,102]和氣敏傳感[21]。BN泡沫體吸附能力高, 而且化學(xué)穩(wěn)定性強(qiáng), 循環(huán)性能優(yōu)異。

圖9 氨硼烷(AB)自發(fā)泡法制備BN泡沫體: (a)示意圖, (b)SEM照片, (c)光學(xué)照片, (d)AFM和HRTEM圖像[29]; 硼酸-PEO體系發(fā)泡制備BN泡沫體: (f)示意圖, (g, h)SEM照片[97]

在電化學(xué)方面, 石墨烯泡沫體可用作先進(jìn)的電化學(xué)多孔電極。石墨烯泡沫體適用于超級(jí)電容器, 其高表面積有助于實(shí)現(xiàn)高容量; 同時(shí)高電導(dǎo)、內(nèi)部孔腔網(wǎng)絡(luò)有助于實(shí)現(xiàn)高功率[129-132]。摻雜和擔(dān)載型石墨烯泡沫體可以用于電化學(xué)儲(chǔ)能和催化, 包括贗電容器[133]、鋰離子電池[35,79,83]、鈉離子電池[37]、鉀離子電池[39-41],、燃料電池[33,64,75,84]、微生物燃料電池[58]和電催化分解水[36,73,84]等方面。

在電學(xué)方面, 石墨烯泡沫體可用作多孔集流體, 代替泡沫鎳等, 以改善電流分布[134]。它具有憎水性, 可以用作防水透氣氣體電極。石墨烯基泡沫體還可以吸聲、吸波、屏蔽電磁波和吸收散亂電子[135]。

4 結(jié)束語(yǔ)

二維材料的三維化設(shè)計(jì)是二維材料宏觀塊體材料發(fā)展的必由之路。二維材料泡沫體具有優(yōu)良的機(jī)械特性, 同時(shí)提供了雙聯(lián)通(固相、空腔)的網(wǎng)絡(luò)結(jié)構(gòu)、高比表面積, 對(duì)于界面相關(guān)的應(yīng)用意義重大。發(fā)泡法是一種低成本、可工業(yè)化的工藝, 可以制備先進(jìn)的二維材料泡沫體, 但本領(lǐng)域仍有一些亟需研究的難點(diǎn)和發(fā)展前沿:

1)可控性是發(fā)泡制備工藝的難點(diǎn)。由于發(fā)泡體系的復(fù)雜性, 目前尚難以精確控制泡孔的尺寸和均勻性。此外, 開(kāi)發(fā)混合發(fā)泡劑也是一個(gè)新的研究方向。

圖10 基于二維材料泡沫體的應(yīng)用

2)發(fā)泡法目前集中于B–C–N–O輕元素體系, 極少用于其它二維材料泡沫體。有待大力發(fā)展過(guò)渡金屬硫化物、層狀雙氫氧化物和過(guò)渡金屬碳氮化物的泡沫體。

3)設(shè)計(jì)制備新型結(jié)構(gòu)的泡沫體是新材料的發(fā)展重點(diǎn), 例如, 負(fù)泊松比的擠縮泡沫體、分等級(jí)多級(jí)孔泡沫體和負(fù)曲率曲面的泡沫體等。

期待通過(guò)完善發(fā)泡理論, 探索發(fā)泡體系并提高工藝可控性, 發(fā)展多種二維材料泡沫體新材料; 解析二維材料泡沫體的構(gòu)效關(guān)系, 開(kāi)發(fā)其在多學(xué)科多領(lǐng)域的豐富應(yīng)用。

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Blowing Route to Fabricate Foams of 2D Materials

GAO Tian, XIAO Qinglin, XU Chenyang, WANG Xuebin

(National Laboratory of Solid State Microstructures, Collaborative Innovation Center of Advanced Microstructures, Jiangsu Key Laboratory of Artificial Functional Materials, College of Engineering and Applied Sciences, Nanjing University, Nanjing 210093, China)

Graphene, as a representative of two-dimensional (2D) materials, has excellent intrinsic properties such as high specific surface area and conductivity, but its macroscopic bulk behaves poorly owing to severe face-to-face restacking and hand-in-hand contact resistance. Three-dimensional (3D) design of 2D materials can deliver the excellent nanoscaled properties to the macroscopic world, to realize the high surface area, conductivity, interconnected pores, and good mechanics of the bulks. It is necessary and highlighted to develop the porous monolith of 2D materials for applications as electrodes, adsorbents, elastomers,. The blowing route has the advantages of low cost and simple processing, which has been accentually developed to produce the foams of 2D materials for several years. This article introduces the principle of the blowing method, summarizing the recent examples of blown foams of graphene, boron nitride nanosheet, and others. The scientific front about foams of 2D materials is discussed, and the broad applications of the new materials are prospected in energy, environment,.

2D material; blowing method; graphene; boron nitride; foam; review

TB321

A

1000-324X(2020)12-1315-12

10.15541/jim20200096

2020-02-29;

2020-04-09

青年千人計(jì)劃; 國(guó)家自然科學(xué)基金(51972168, 51672124, 21603096); 江蘇雙創(chuàng)計(jì)劃

Thousand Talents Plan for Youth; National Natural Science Foundation of China (51972168, 51672124, 21603096); Program for Innovative Talents and Entrepreneur in Jiangsu

高天(1992–), 男, 博士. E-mail: 1217387634@qq.com

GAO Tian(1992–), male, PhD. E-mail: 1217387634@qq.com

王學(xué)斌, 教授. E-mail: wangxb@nju.edu.cn

WANG Xuebin, professor.E-mail: wangxb@nju.edu.cn