在水相中以硫摻雜氧化石墨烯量子點做為產氫光催化劑

Abstract

現今我們幾乎都以非再生能源當作電力的來源，其中以不久的將來會耗盡的石化燃料為主。但有效用的可再生能源仍然不足以滿足我們現今不斷增長的能源需求。其中有一種能源方案是通過太陽光光能使水中產生氫氣，並將其用作氫燃料電池中的能量載體，僅產生沒有任何污染物的水。因此，尋找新型的非貴金屬或無金屬特別是碳材料的光催化劑已經引起了工業界和學術界的廣泛關注。 石墨烯量子點 (GQDs),尺寸範圍為2至20 nm的小石墨烯碎片 與其他半導體量子點有著不同改有趣的現像，而引起了相當大的關注。近年來，GQDs具有化學惰性，低毒性，在水中分散性好，且相對穩定的光致發光等特點受到越來越多的關注。本研究闡述摻有雜原子的GQDs能夠有效地調控其能隙和電子密度，從而提高化學活性，提高光學性能和選擇性。 本研究中，我們專注於水相中使用硫摻雜氧化石墨烯量子點(S-GOQDs)作為光催化的產氫材料，S-GOQDs可以利用水熱法(bottom-up)簡單且經濟效益的合成。藉由 原子力顯微鏡(AFM)的觀察S-GOQDs具有雙層和參層石墨烯厚度、透射電子顯微鏡(TEM)分析顯示S-GOQDs有著極高的結晶度，其晶體大小在3-10 nm、透過X射線光電子能譜(XPS)和電子色散譜(EDS)證明了石墨烯量子點晶格中S原子的成功摻雜。S-GOQDs在333、395和524 nm處表現出三個吸收帶， C = S和C-S伸縮振動信號在1075 cm-1和690 cm -1，激發波長最大值在451和520 nm，也證實了硫原子成功摻入GOQDs中。電子結構分析中S-GOQDs 的導帶(CBM)與價帶(VBM) 適合用於水分解。在500 W的Xe燈照射下，S-GOQDs在純水中的氫生成效率達351 μmol·h-1·g-1，當使用80％乙醇作為電子供應者時可達到了1471 μmol·h-1·g -1是純水系統中的4.2倍，在直接日光照射下，純水中的初始速率為18,166 μmol·h-1·g-1，在80％的乙醇水溶液中的初始速率為30,519 μmol·h-1·g-1。因此，無金屬、廉價且環境友好的光催化劑S-GOQDs在從水中有效率的產生氫氣，具有巨大的開發潛力。

Nowadays our electricity production is mostly based on a non-renewable energy resources, mainly fossil fuels which are projected to deplete in near future. Renewable energy resources available today are still not efficient enough to satisfy our constantly growing energy demand. One of the proposed scenarios is to produce hydrogen from water by solar light, and use it as the energy carrier in a hydrogen fuel cells, producing only water without any pollutants. Hence, searching for novel photocatalysts based on nonprecious metals or metal-free, especially carbon materials, which are abundant and environmentally friendly, has attracted considerable attention from both industrial and academic researchers. Graphene quantum dots (GQDs), small graphene fragments of size ranging from 2 to 20 nm, have received considerable attention due to its interesting phenomena different from those in quantum dots of any other semiconductors. In recent years, GQDs receives increasing attention owing to their properties like chemical inertness, low cytotoxicity, excellent dispersibility in water and relatively stable photoluminescence. Further researches showed that GQDs doped with heteroatoms can effectively modulate their band gap and electronic density leading to enhanced chemical activity, new optical properties, and selectivity. In this work we focus on the photocatalytic hydrogen production activity in aqueous media by using sulfur-doped graphene oxide quantum dots (S-GOQDs). S-GOQDs have been synthesized by hydrothermal method with “bottom-up” approach. As investigated by atomic force microscopy (AFM), the synthesized S-GOQDs possessed bi- and tri-layer graphene thickness. As illustrated by transmission electron microscopy (TEM), the synthesized S-GOQDs exhibited high crystallinity and size ranging from 3-10nm. Successful doping of S atoms in graphene quantum dot lattices was proven by X-ray photoelectron spectroscopy (XPS) and electron dispersive spectroscopy (EDS) characterization. The UV‒vis, FT‒IR, and photoluminescent spectra of the synthesized S-GOQDs exhibited three absorption bands at 333, 395, and 524 nm, characteristic of C=S and C-S stretching vibration signals at 1075 cm-1 and 690 cm-1, and two excitation wavelength independent emission signals with maxima centered at 451 and 520 nm, respectively, confirming the successful doping of S atom into the GOQDs. Electronic structural analysis suggested that the S-GOQDs exhibited conduction band minimum (CBM) and valence band maximum (VBM) levels suitable for water splitting. Under a 500 W Xe-lamp irradiation, the S-GOQDs exhibited a high hydrogen generation efficiency of 351 μmol∙h-1∙g-1 in pure water, which is enhanced 4.2-fold to 1471 μmol∙h-1∙g-1 when the use of 80% ethanol as an electron donor. Under direct sunlight irradiation, an initial rate of 18,166 μmol∙h-1∙g-1 in pure water and 30,519 μmol∙h-1∙g-1 in 80% EtOH aqueous solution were obtained. Therefore, metal-free and inexpensive S-GOQDs hold great potential in the development of sustainable and environmental-friendly photocatalysts for efficient hydrogen generation from water-splitting.