用於分解水反應之二維異質結構光催化劑的開發

Abstract

氫氣作為一種潔淨且具高能量密度的能量載體，正日益廣泛地應用於石油、化學、能源及軍事等多個工業領域，成為替代化石燃料的可行方案。透過熱化學、光化學及電解水等再生方式生產的「綠氫」，為能源永續發展提供了極具前景的途徑。儘管目前尚缺乏普及且具成本效益的綠氫技術，但光催化分解水技術仍是一個引人注目的選擇。雖然其轉化效率目前低於電解水，但其系統設計更為簡單經濟，且具備擴展潛力，使其成為綠氫生產中值得關注的技術。因此，光催化分解水在推動綠色轉型與實現碳中和目標中扮演著關鍵角色。 鈦酸銫（SrTiO3）因具備無毒、成本效益高以及在化學與光化學環境下表現穩定等優點，是光催化分解水的常用催化劑。儘管其在光催化領域具有先驅地位，但仍面臨能隙較寬、氫超電位較高以及電子-電洞對復合速度過快等挑戰。本研究探討了利用新興的非金屬半導體聚合物類石墨相氮化碳（g-C3N4）來克服上述限制的潛力。g-C3N4 具有 2.7 eV 的能隙及顯著的化學與熱穩定性，雖展現出應用前景，但目前受限於光生電子-電洞對復合率高及比表面積有限等缺點。 研究進一步透過原位光還原法，在 SrTiO3/g-C3N4 型異質結中引入金屬鉍（Bi）修飾。在光照下，Bi3+ 被光生電子還原為 Bi0，使其在模擬太陽光下作為非貴金屬助催化劑促進光催化全分解水產氫。在最佳鉍添加量（1 mol%）下，該催化劑展現出最卓越的產氫速率，在模擬太陽光（AM 1.5G）照射 ~360 μmol/g/h，是未添加鉍複合催化劑的 46 倍。在去離子水中的光催化產氫實驗也取得了 27.7 μmol/g/h的顯著成果。本研究提出了 1BiGCNSTO光催化全分解水反應的可能機制，該機制符合 II 型異質結模型，並與 XPS 測得的界面電子相互作用預測一致。

其次，我們介紹了透過簡易混合煅燒法合成的二維（2D）SrTiO3/g-C3N4異質結構。其中 2D SrTiO3奈米片是透過 Bi4Ti3O12 的獨特拓撲化學轉化獲得，而 2D g-C3N4奈米片則透過剝離技術製備。在含有 5 wt% 2DSrTiO3且以鉑（Pt）為助催化劑的情況下，該 2D 異質結構在模擬太陽光下產氫量約為 ~460μmol/g/h，是純 2D g-C3N4奈米片的 4 倍。此外，亦合成了由立方體 SrTiO3和塊狀 g-C3N4組成的異質結催化劑進行對比，結果顯示具有更大接觸面積的 2D/2D 異質結展現出更優異的光催化性能。為了進一步提升效能，對 2D g-C3N4進行剝離以獲得極高的表面積，提供豐富的活性位點並改善光吸收。由高表面積剝離 2D g-C3N4合成的異質結構催化劑，在相同實驗條件下展現了約~2060μmol/g/h的驚人產氫速率。 最後，研究引入了另一種半導體材料——氧化鎢WO3, 其具有優異的化學穩定性和強烈的可見光吸收能力。本研究提出利用金屬改性 g-C3N4來降低能隙並提升光化學性能。金屬添加至 g-C3N4結構中可降低電荷復合率並擴展可見光響應範圍。此外，g-C3N4 的庚嗪環（Heptazine ring）結構與其空腔構造易於形成金屬配位中心。結果顯示，添加鎳（Ni）的 g-C3N4奈米片與 WO3奈米片構成的 2D 異質結構催化劑，在有、無犧牲劑的情況下均展現出優異的產氫速率。在含 10 vol% 三乙醇胺溶液及 1 wt% Pt 助催化劑的模擬太陽光照射下，該催化劑達到了 1310 μmol/g/h 的產氫率；而在無犧牲劑的去離子水中，其產氫與產氧速率分別達到 60.9 與 30.4 μmol/g/h。這些數據使該 2D/2D 鎳改性 g-C3N4/WO3異質結構在同類催化劑中脫穎而出。本研究亦針對此系統提出了直接 Z-scheme 異質結機制，以進一步探討其增強全分解水反應的原理。

Hydrogen, a clean and high-energy-density energy carrier, is increasingly employed across various industries, including petroleum, chemistry, energy, and military, as a viable substitute for fossil fuels. The emergence of green hydrogen, produced through renewable methods such as water thermolysis, photolysis, and electrolysis, presents a promising avenue for sustainable energy production. Despite the absence of readily available and cost-effective green hydrogen technologies, photocatalytic water splitting is a compelling option. Therefore, photocatalytic water splitting plays a pivotal role in advancing the goals of green transition and achieving carbon neutrality. Strontium titanate (SrTiO3) is a photocatalyst for water splitting due to its advantageous properties, including non-toxicity, cost-effectiveness, and stability in chemical and photochemical environments. Additionally, this work will explore the potential of g-C3N4, an emerging metal-free semiconducting polymer, to address these limitations. Characterized by a 2.7 eV bandgap and notable chemical and thermal stability, g-C3N4 offers promise. Herein, metallic bismuth decorated in g-C3N4/SrTiO3 type II heterojunction by in-situ photoreduction is introduced. At the optimal amount of bismuth-added (1 mol%), the g-C3N4/SrTiO3-based photocatalyst had the most remarkable hydrogen evolution rate, approximately 360 μmol/g/h under simulated sunlight AM 1.5G, 46-fold higher than the composite catalyst without Bi-decorated. The photocatalytic hydrogen production from deionized water was also conducted, which achieved a noteworthy value of 27.7 μmol/g/h under simulated sunlight. The possible mechanism for the hydrogen production from the photocatalytic whole water splitting reaction of 1BiGCNSTO was proposed, which was appropriate with a type II heterojunction, consistent with the interfacial electronic interaction prediction by X-ray photoelectron spectroscopy. Secondly, we introduce the two-dimensional (2D) heterostructure SrTiO3/g-C3N4, which was synthesized using a straightforward method involving mixing and calcination. The 2D SrTiO3 nanoplatelet was obtained through a unique topochemical conversion from Bi4Ti3O12, while the 2D g-C3N4 nanosheet was prepared via an exfoliation technique. With 5 wt% of 2D SrTiO3, the 2D heterostructure SrTiO3/g-C3N4 with Pt as co-catalyst exhibited excellent photocatalytic hydrogen, approximately 460 μmol/g/h under simulated sunlight, which was 4-fold higher than the pure 2D g-C3N4 nanosheet. The results indicated that the composite heterojunction 2D/2D SrTiO3/g-C3N4 with a greater contact area generated exceptional photocatalytic performance. To enhance the photocatalytic ability, the 2D g-C3N4 was exfoliated to achieve a significantly higher surface area, providing abundant active sites for easier access of reactants and improving light absorption. The heterostructural catalyst was synthesized from exfoliated 2D g-C3N4 with a high surface area, demonstrating an extraordinary hydrogen production rate of approximately 2060 μmol/g/h under identical experimental conditions. The photocatalytic overall water splitting reaction of our best photocatalysts was conducted as well, which achieved a remarkable gas evolution rate. Finally, two possible mechanisms for the photocatalytic hydrogen evolution reaction in sacrificial solution and deionized water are proposed, respectively. Finally, another semiconductor introduced for further research is tungsten oxide (WO3), which has excellent chemical stability, strong visible-light absorption, and has also been regarded as an ideal photocatalytic material. Therefore, owing to the suitable band positions between g-C3N4 and WO3, the photocatalytic activity could be significantly enhanced by constructing a 2D/2D metal-modified-g-C3N4/WO3 heterostructure. With its suitable band position, the 2D/2D heterostructure catalyst would form a direct Z-scheme heterostructure, which is considered an effective strategy to hinder the recombination of electron-hole pairs and improve redox activity. Primarily, the results spoiled that the 2D heterostructural catalyst of Ni-added g-C3N4 nanosheet with WO3 nanoplate indicated a superior hydrogen production rate in conditions with and without a sacrificial agent. The Ni(OH)2 clusters coordinated to exfoliated g-C3N4 are synthesized via calcination, which is then partially reduced to Ni0 atoms, ultimately leading to the formation of the Ni clusters cocatalyst. Initially, 0.15 wt% Ni-added g-C3N4 nanosheet combined with 5 wt% WO3 nanoplates to generate the excellent 2D heterostructural catalyst, which released an outstanding hydrogen evolution rate of 1310 μmol/g/h in the condition of 10 vol% triethanolamine solution, 1 wt% of Pt as co-catalyst, and simulated sunlight irradiation. The overall water splitting reaction was conducted in DI water without any sacrificial agent, indicating a remarkable hydrogen and oxygen evolution rate of 60.9 and 30.4 μmol/g/h, respectively. These results make this 2D/2D heterostructured Ni-modified-g-C3N4/WO3 stand out higher than most of the similar heterojunction catalysts. The possible mechanism for the optimal catalyst is proposed to further investigate the 2D heterostructural direct Z-scheme heterojunction system in enhancing the overall water splitting reaction.