CoN/g-C3N4 0D/2D復合結構及其光催化制氫性能研究

陳瀚翔, 周敏, 莫曌, 宜堅堅, 李華明, 許暉

CoN/g-C3N40D/2D復合結構及其光催化制氫性能研究

陳瀚翔1, 周敏1, 莫曌2, 宜堅堅3, 李華明2, 許暉2

(1. 江蘇大學 環境與安全工程學院, 鎮江 212013; 2. 江蘇大學 能源研究院, 鎮江 212013; 3. 揚州大學 環境科學與工程學院, 揚州 225127)

在光催化產氫反應中引入助催化劑可促進光生電子快速轉移, 是提高光催化活性的有效方法。而目前, 高效助催化劑主要仍然是貴金屬, 其高昂的價格極大限制了實際應用。本研究探討了構筑非貴金屬助催化劑CoN與g-C3N40D/2D緊密界面對光催化制氫性能的影響。負載非貴金屬助催化劑CoN可以有效提高2D g-C3N4的光催化制氫活性, 負載量對其活性也有影響。構筑的0D/2D緊密界面有利于光生電子快速傳輸。兩者的共同作用使得10% CoN/2D g-C3N4復合物光催化制氫效率達到403.6??μmol·g–1·h–1, 是2D g-C3N4單體的20倍。在CoN/2D g-C3N4復合材料中, 負載CoN作為析氫助催化劑可以顯著促進電荷轉移過程, 從而大幅提高光催化析氫活性。

非貴金屬; CoN; 助催化劑; 光催化; 產氫

氫能作為清潔能源已被廣泛應用于原油精煉、合成氨和甲醇、燃料電池等領域[1-4]。通過水煤氣轉化和甲烷熱解得到氫氣是傳統的制氫方法之一, 但生產過程需要消耗大量不可再生能源。相比而言, 利用太陽光將水分解產生氫氣, 能源消耗小, 無二次污染等問題, 是眾多學者的研究目標。將太陽能直接轉化為清潔的氫能已成為水分解制氫的研究熱點[5-6]。

氮化碳(C3N4)于1834年被發現, 是最古老的有機共軛聚合物之一。氮化碳具有可見光響應、熱穩定性和化學穩定性好等優點[7-10]。然而, g-C3N4存在氧化還原能力低、光激發載流子復合率高等問題, 單體g-C3N4的光催化活性較低, 限制了其廣泛應用[11-13]?？紤]到光催化產氫主要利用光催化劑所產生的電子, 提高電子的利用效率即可提高光催化產氫的速率。二維薄層結構的g-C3N4具有良好的電子傳遞路徑和高比表面積, 是一類很有應用前景的催化劑。與之對應, 貴金屬具有較大的功函數和合適的氫吸附脫附自由能, 是最合適的助催化劑。但貴金屬昂貴的價格限制了其工業化應用, 因此, 需要探索合適的替代材料。研究表明, 過渡金屬氮化物因為優秀的導電性及電催化性能, 在提升光催化活性助催化劑領域具有巨大的應用潛能。過渡金屬鈷基材料中若引入氮元素, 可使過渡金屬d帶電子密度增大并收縮, 使其電子結構類似貴金屬且反應能壘更低[14-16], 有利于降低光催化反應的能量。而且引入氮元素會占據過渡金屬鈷基材料的層間隙, 從而提升耐腐蝕性[17-18]。其次, 材料的結構也會在一定程度上對催化反應產生影響, 優質的0D/2D結構可以在界面上與反應物接觸良好和優化活性位點分布[19-22]。實驗通過選擇研究CoN的助催化性能, 旨在探索一種低成本、高效的可替代貴金屬基助催化劑的材料。在光催化反應中, CoN能夠有效地提取氮化碳產生的電荷, 提高e–-h+對的分離效率, 最終達到提高光催化性能的目的。

為了構筑具有高效HER反應活性的光催化體系, 達到提高催化劑光催化性能的目的, 本研究利用簡單的水熱法和氨氣煅燒法, 構筑了由超小尺寸CoN納米點修飾2D g-C3N4納米片的0D/2D光催化復合材料。并深入分析了CoN提升光催化性能的機制, 通過多種表征測試方法探究了構效關系。

1 實驗方法

1.1 材料合成

2D g-C3N4的制備:將2 g三聚氰胺置于坩堝, 在馬弗爐中以2 ℃/min升溫到550 ℃, 保溫4 h, 獲得黃色C3N4。研磨后取400 mg C3N4置于方舟, 在馬弗爐中, 以10 ℃/min升溫到550 ℃, 保溫1 h, 獲得白色2D g-C3N4。

Co(OH)2/2D g-C3N4的制備:將50 mg 2D g-C3N4、22.8 mg Co(NO3)2?6H2O和22.1 mg 六亞甲基四胺(Hexamethylenetetramine, HMT)溶解在12 mL去離子水中并攪拌0.5 h, 將混合溶液轉移到25 mL不銹鋼高壓釜中, 在120 ℃烘箱中加熱12 h。反應結束后冷卻至室溫, 離心得到的混合液用蒸餾水和乙醇各洗滌3次, 在55 ℃真空干燥箱下干燥12 h, 得到Co(OH)2/2D g-C3N4。

Co(OH)2的制備:將0.5036 g Co(NO3)2?6H2O和0.488 g六次甲基四胺(Hexamethylenetetramine, HMT)溶解在12 mL去離子水中并攪拌0.5 h, 而后轉移到25 mL不銹鋼高壓釜并放入烘箱中, 在120 ℃下加熱12 h。冷卻至室溫, 對得到的混合液進行離心處理, 用蒸餾水和乙醇各洗滌3次, 在55 ℃真空干燥箱內干燥12 h, 可得Co(OH)2。

CoN的制備:稱取50 mg Co(OH)2置于方舟, 在管式爐中通入100 mL/min的氨氣, 以5 ℃/min速率加熱到380 ℃, 保持3 h, 獲得氮化鈷。

CoN/2D g-C3N4的制備:稱取50 mg Co(OH)2/ 2D g-C3N4置于方舟, 在管式爐中通入100 mL/min氨氣, 以5 ℃/min速率升溫到380 ℃, 保持3 h, 獲得10% CoN/2D g-C3N4。再按比例調整得到5% CoN/2D g-C3N4和15% CoN/2D g-C3N4復合物。

1.2 光催化制氫性能測試

光催化制氫速率測試:向反應器中加入10 mg光催化劑、10 mL三乙醇胺(Triethanolamine, TEOA)以及90 mL水, 混合超聲30 min。將反應器與光催化制氫反應儀相連接, 混合液持續攪拌。在反應開始前對測試系統抽真空, 去除系統內的空氣, 并在配備有10 ℃的恒溫冷卻水循環的條件下進行測試, 避免光致熱效應。模擬太陽光的光源為配備400 nm濾光片的300 W氙燈, 光催化實際制氫量通過氣相色譜進行在線分析(Ar載氣、TCD檢測器、0.5 nm分子篩)。

光催化循環實驗:光催化循環實驗的測試步驟類似于制氫速率測試步驟。在每輪循環反應結束后重新對裝置抽真空, 然后開始新一輪的測試。

1.3 光電化學測試

將2 mg催化劑溶解在1 mL乙二醇和1 mL乙醇的混合液中并超聲0.5 h。移取20 μL混合液, 懸滴在氧化錫銦(ITO)玻璃(1 cm× 0.5 cm)上,紅外燈下烘干, 并在80 ℃烘箱中加熱2 h確保接觸緊密。光電流循環測試時, 照射和遮光各20 s為一個循環, 阻抗測試時保持遮光。

2 結果與討論

2.1 結構及形貌分析

通過X射線衍射(X-ray diffraction, XRD)和紅外光譜(Fourier transform infrared spectroscopy, FT-IR)表征2D g-C3N4和CoN/2D g-C3N4的晶體結構和化學結構。如圖1(a)所示, 2D g-C3N4在2=27.7°處存在單獨的衍射峰, 對應于2D g-C3N4的(002)晶面, 這是層間堆疊造成的[23-24]。位于2=36.2°、42.2°、61.3°、73.3°和76.8°處的衍射峰, 分別對應于CoN的(111)、(200)、(220)、(311)和(222)晶面, 這表明CoN制備成功, 且具有立方晶體結構。與此同時, 隨著CoN含量增大, 2D g-C3N4的主峰強度逐漸減小, (111)、(200)和(220)晶面的特征峰逐漸增強, 這表明2D g-C3N4的結構未發生改變, 并與CoN成功復合。如圖1(b)所示, FT-IR光譜圖中, 復合材料的特征峰與2D g-C3N4相似, 與XRD結果一致。800 cm–1處的吸收峰歸屬于-三嗪單元的振動拉伸[25];1000～1800 cm–1之間的吸收峰歸屬于CN雜環的振動[26-27];其他在3000～3500 cm–1之間的吸收峰歸屬于O–H和N–H的振動[28-29]。以上結果表明成功制備了復合物, 并且引入CoN沒有破壞2D g-C3N4的結構。由紫外–可見漫反射光譜(UV-visible diffuse refle-ctance spectroscopy, DRS)(圖1(c))可見, CoN可以吸收可見光, 但無法被激發, 說明它的主要作用是充當捕獲電子的角色。但引入CoN導致復合物由白色變為灰色, 在一定程度上增強了對太陽光的吸收。測試2D g-C3N4以及10% CoN/2D g-C3N4復合催化劑的比表面積, 如圖1(d)所示。復合材料的比表面積降低, 這可能是由于CoN以及煅燒對氮化碳結構產生了一定的破壞。結合上述結果說明, 引入CoN可以促進對太陽光的吸收。

圖1 (a)2D g-C3N4, CoN/2D g-C3N4復合催化劑和CoN的XRD圖譜, (b)2D g-C3N4和CoN/2D g-C3N4復合催化劑的 FT-IR譜圖, (c)2D g-C3N4, CoN/2D g-C3N4復合催化劑和CoN的紫外-可見光漫反射吸收光譜圖, (d)2D g-C3N4和10% CoN/2D g-C3N4的氮氣吸附–脫附等溫線

Colorfulfigures are available on website

為了研究2D g-C3N4和CoN/2D g-C3N4復合催化劑的形貌結構和組成, 對其進行掃描電鏡(Scan-ning electron microscope, SEM)及元素能譜(Energy dispersive spectrometer mapping, EDS mapping)、透射電鏡(Transmission electron microscope, TEM)和原子吸收分光光度法(Atomic absorption spectrophoto-metry, AAS)測試。圖2(a, b)中, 2D g-C3N4單體呈現卷曲的褶皺結構, CoN單體則是團聚的納米點結構。圖2(c)中復合物的SEM形貌依舊保持2D g-C3N4所具有的褶皺結構, 但出現了明顯的點狀顆粒。TEM照片(圖2(d))中, 10% CoN/2D g-C3N4復合物依舊保持了2D g-C3N4的二維結構, 并且納米點錨定在2D g-C3N4的表面。增加放大倍數, 圖2(e)中, CoN納米點均勻分布在超薄2D g-C3N4上, 成功構建了優質的0D/2D界面。復合物的高分辨率透射電鏡(High resolution transmission electron microscope, HR-TEM)照片(圖2(f))中觀察到明顯的晶格條紋, 間距為0.25 nm, 對應CoN的(111)晶面, 0D的CoN納米點的直徑為5～6 nm。

為了了解材料的元素組成和金屬元素含量, 對復合材料進行掃描電鏡的元素能譜測試和原子吸收分光光度測試, 10% CoN/2D g-C3N4中CoN的實際質量分數為8.6%。圖S1中, 10% CoN/2D g-C3N4含有C、N和Co元素且分布均勻, 證明成功制備了CoN/2D g-C3N4復合材料。

2.2 可見光光催化水分解測試分析

首先, 為了評估引入CoN是否影響2D g-C3N4的光催化活性, 對2D g-C3N4以及不同比例的CoN/2D g-C3N4復合催化劑在可見光照射下光分解水制氫性能進行測定。如圖3(a), 引入CoN納米點有效促進了2D g-C3N4光催化分解水制氫的活性, 而且不同CoN的負載量對制氫活性存在一定影響。引入CoN后, 復合物的制氫活性顯著提升, 其中最優負載量10% CoN/2D g-C3N4的制氫速率達到了403.6 μmol·g–1·h–1, 約為2D g-C3N4制氫速率的20倍。已報道的類似光催化材料的析氫性能如表 S1[45-51]所示, 與同類光催化材料比較, CoN/2D g-C3N4的平均析氫效率較高。然而, 當CoN的負載量超過10%, CoN/2D g-C3N4催化劑的光催化活性發生衰減, 其原因是過多CoN影響了2D g-C3N4對光的吸收, 同時CoN也發生了自團聚現象。其次, 對復合光催化劑10% CoN/2D g-C3N4進行四個周期的循環穩定性測試(圖3(b)), 樣品前三次產氫量較為穩定, 第四次循環的產氫量略微降低, 約為第一次循環產氫量的85%, 說明復合材料的性能整體較穩定[30-32]。

CoN/2D g-C3N4的單波長產氫表觀光合量子效率(Apparent quantum yield, AQY)測試結果顯示其在420 nm單波長條件下的AQY為0.13%, 而在435 nm單波長條件下的AQY為0.09%, 由此可見CoN/2D g-C3N4的單波長產氫表觀光合量子效率隨著材料的光吸收能力減弱而逐漸降低。這說明在CoN/2D g-C3N4催化劑體系中, 只有2D g-C3N4才能受到激發產生光生電子, 而CoN的作用是捕獲電子并且完成質子得到電子生成氫氣的過程。

圖2 (a)2D g-C3N4, (b)CoN和(c)10% CoN/2D g-C3N4的SEM照片, 10% CoN/2D g-C3N4的 (d, e)TEM照片和(f)高倍TEM照片

圖3 (a)2D g-C3N4和CoN/2D g-C3N4復合催化劑的光催化制氫速率測試, (b)10% CoN/2D g-C3N4復合催化劑的制氫穩定性測試(10% TEOA作為犧牲劑, 10 mg催化劑, 氙燈作為光源, λ>400 nm)

Colorfulfigures are available on website

除了CoN, 氮化鐵、氮化鎳等也可有效促進基體催化劑的性能[33-34], 表明金屬氮化物可有效促進電荷分離, 提升光解水活性。

2.3 復合材料的光學及光電性能分析

為進一步研究材料光生載流子的分離與轉移, 進行了穩態熒光光譜(Photoluminescence spectro-scopy, PL)、光電流以及電化學阻抗(Electrochemical impedance spectroscopy, EIS)等測試。如圖4(a)所示, 相比于2D g-C3N4, 10% CoN/2D g-C3N4復合催化劑的熒光強度發生顯著淬滅, 光生載流子復合減少[35-36]。這些結果表明, 負載CoN可以有效抑制2D g-C3N4的電荷復合。與此對應的是圖4(b)中光電流測試結果, 相較于2D g-C3N4, 10% CoN/2D g-C3N4的光電流有一定提升, 說明更多電子被激發到氧化銦錫(Indium tin oxide, ITO)玻璃表面。圖4(c)中阻抗結果也顯示, 引入CoN可以有效促進電荷傳輸。電化學測試結果表明, 在CoN/2D g-C3N4復合材料中, 負載CoN作為析氫助催化劑可以顯著促進電荷轉移過程, 從而大大提高光催化析氫活性。為了研究材料負載CoN前后的導帶(Conduction band, CB)變化, 進行莫特肖特基(Mott Schottky, MS)測試, 從(圖4(d)) Mott Schottky曲線可知, 2D g-C3N4的平帶電位為–0.84 V (. Ag/AgCl, pH6.8), 對應于–0.1 V (NHE, pH0)。而CoN/2D g-C3N4復合材料的平帶電位為–0.82 V (. Ag/AgCl, pH6.8), 這說明CoN與2D g-C3N4復合后, 復合材料的CB沒有明顯改變[42]。

2.4 光催化機理研究

為了進一步探索光催化反應機理, 進行電子自旋共振(Electron spin resonance, ESR)光譜測試[37-39]。測試時在溶液中加入自由基捕獲劑5, 5-二甲基-1-吡咯啉--氧化物(5,5-dimethyl-1-pyrroline--oxide, DMPO)抑制或減緩光氧化過程, 可以穩定超氧自由基(?O2?)和羥基自由基(?OH), 分別在甲醇溶液和水溶液中測試, 得到DMPO-?O2?和DMPO-?OH的ESR信號。如圖5所示, 2D g-C3N4和10% CoN/2D g-C3N4在黑暗條件下均未出現自由基信號。在光照條件下, 2D g-C3N4僅出現自由基?O2?的信號, 而沒有自由基?OH信號。10% CoN/2D g-C3N4的譜圖中兩種自由基的信號均存在, 且?O2?的信號強于純2D g-C3N4。?O2?的信號變化說明引入CoN能夠加速導出光生電子, 促進光生電子和空穴分離?；谏鲜鼋Y果, 本研究提出該體系的光催化制氫機制: 在可見光照射下, 2D g-C3N4受光激發產生光生電子, 電子由價帶躍遷至導帶, 由犧牲劑三乙醇胺(Triethanolamine, TEOA)消耗了價帶上留下的空穴, 而CoN可以捕獲并有效轉移電子。同時, CoN起到了電子陷阱的作用, 電子很難回到2D g-C3N4中,不能與空穴重新結合。并且由于CoN具有優良的導電性, 電子可以快速轉移到催化劑表面并被H+捕獲, 完成光催化HER中H2的生成過程[2,13,30-31,44]。

圖4 2D g-C3N4和10% CoN/2D g-C3N4的(a)穩態熒光光譜圖(激發波長為384 nm), (b)光電流響應圖, (c)電化學阻抗譜圖和(d)2D g-C3N4和10% CoN/2D g-C3N4的莫特肖特基曲線

Colorfulfigures are available on website

圖5 2D g-C3N4和10% CoN/2D g-C3N4在(a, b)可見光照射和(c, d)暗處的(a, c)超氧自由基和(b, d)羥基自由基ESR譜圖

Colorfulfigures are available on website

3 結論

本研究以CoN為助催化劑, 設計了由CoN納米點修飾2D g-C3N4納米片的0D/2D高效光催化劑, 構筑了具有高效HER反應活性的光催化體系。其中最優負載量10% CoN/2D g-C3N4的制氫速率達到403.6 μmol·g–1·h–1, 約為2D g-C3N4的20倍?；钚蕴嵘脑蚩梢詺w為以下2點: (1)該體系的原位合成策略保證了界面緊密接觸; (2)負載小尺寸助催化劑CoN保證了活性單元覆蓋最大化, 而沒有影響光吸收, 良好的導電性有利于抽取光生電荷, 提高光生載流子的分離效率。

本文相關補充材料可登陸 https://doi.org/ 10.15541/jim20210806 查看。

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0D/2D CoN/g-C3N4Composites: Structure and Photocatalytic Performance for Hydrogen Production

CHEN Hanxiang1, ZHOU Min1, MO Zhao2, YI Jianjian3, LI Huaming2, XU Hui2

(1. School of the Environment and Safety Engineering, Jiangsu University, Zhenjiang 212013, China; 2. School of the Institute for Energy Research, Jiangsu University, Zhenjiang 212013, China; 3. College of Environmental Science and Engineering, Yangzhou University, Yangzhou 225127, China)

In the photocatalytic hydrogen production reaction, the introduction of cocatalyst to promote the rapid transfer of photogenerated electrons is an effective way to improve the photocatalytic activity. At present, the most efficient cocatalysts are mainly precious metals, greatly restricting the use of this method. In this study, influence of the close interface between non-noble metal cocatalyst CoN and g-C3N40D/2D on the performance of photocatalytic hydrogen production was explored. The results showed that the support of non-noble metal cocatalyst CoN can effectively improve the photocatalytic hydrogen production of 2D g-C3N4, and the support amount also has an positive effect on its activity. The compact 0D/2D interface is favorable for the rapid transmission of photo-generated electrons. Photocatalytic efficiency of 10% CoN/2D g-C3N4composite reached 403.6?μmol·g–1·h–1, 20 times of that of 2D g-C3N4monomer, which was comparable to that of noble metal cocatalyst. As a hydrogen evolution cocatalyst, loaded CoN can significantly promote the charge transfer process, thus greatly improving the activity of photocatalytic hydrogen evolution.

non-noble metal; CoN; cocatalyst; photocatalysis; hydrogen production

1000-324X(2022)09-1001-08

10.15541/jim20210806

TQ174

A

2021-12-30;

2022-04-27;

2022-05-05

國家自然科學基金(22108110); 江蘇省杰出青年科學家基金(BK20190045); 江蘇省研究生科研創新計劃(KYCX21_ 3391); 中國博士后基金(2021M691305); 江蘇省博士后基金(2021K079A)

National Natural Science Foundation of China (22108110); Jiangsu Funds for Distinguished Young Scientists (BK20190045); Postgraduate Research & Practice Innovation Program of Jiangsu Province (KYCX21_3391); China Postdoctoral Science Foundation (2021M691305); Jiangsu Province Postdoctoral Science Foundation (2021K079A)

陳瀚翔(1993–), 男, 博士研究生. E-mail: 2350505633@qq.com

CHEN Hanxiang (1993–), male, PhD candidate. E-mail: 2350505633@qq.com

許暉, 教授. E-mail: xh@ujs.edu.cn

XU Hui, professor. E-mail: xh@ujs.edu.cn