碳修飾與石墨烯量子點於光催化分解水產氫之研究

Application of Carbon Decoration and Graphene Quantum Dots in Hydrogen Generation through Photocatalytic Water Splitting

趙康如(Kang-Ju Chao)

指導教授 ： 呂世源

國立清華大學/工學院/化學工程學系/碩士(2013年)

https://doi.org/10.6843/NTHU.2013.00174

摘要

光催化分解水產氫反應利用入射光的能量，光觸媒產生電子電洞對，遷移至表面，分別產生氧化與還原反應，使水分解為氫氣與氧氣，此反應具環境親和性，但效率不高是主要問題。若是能將電子電洞對有效的分離，將可以改善反應速率。
一般而言，可以使用白金等金屬複合在光觸媒表面，因其具有電子的收集作用，可促使電子電洞對分離，進而提升光觸媒效率達一個數量級，但是貴重金屬地球存量低、價格昂貴。本研究，期望使用碳材達到電子電洞對分離的效果。
本研究中，使用商用二氧化鈦光觸媒P25作為主要材料，加入少量的己二酸，共同高溫氮氣煅燒，將己二酸根分子碳化附著於二氧化鈦光觸媒表面，形成碳包覆之二氧化鈦(P25/C)。首先利用元素分析儀發現碳含量與加入之碳量大致相同，以X光粉末繞射儀分析產物晶相，發現製程並沒有對晶型造成顯著影響，藉由紫外光/可見光吸收光譜儀、X射線光電子能譜儀發現碳並沒有摻雜進入P25晶格內部，由螢光放光圖譜中可觀察到電子電洞對分離的增加。
光觸媒產氫研究為內照式反應系統，其中使用甲醇為系統中的犧牲劑，於400W 高壓汞燈照射下進行反應。發現碳修飾可作為助觸媒的角色，效能達P25光觸媒之3-4倍，產氫效率最高可達24.8 mmol/h‧g，且碳含量有一最佳值為1 wt%。另外在沒有加入甲醇犧牲試劑的情況下，P25/C光觸媒效率可達到479.8
μmol/h‧g。
另一方面，自從石墨烯材料問世之後，隨即掀起研究熱潮，石墨烯2D結構具備高導電度、高比表面積、高電子移動率等優點。石墨烯氧化物大小約為10 nm以下，測量出可吸收波長約300 nm，為紫外光波段，並具有螢光放光之特性，此為具有半導體特性之石墨烯量子點。本研究首先將石墨烯氧化物量子點用於光催化產氫。使用碳纖維為起始材料在120℃溫度條件下，製備出石墨烯氧化物量子點之均勻分散水溶液，由穿透式電子顯微鏡分析尺寸約在3-6 nm之間，原子力顯微鏡結果顯示石墨烯氧化物量子點具有1-2個原子厚度，在不含有犧牲試劑的情況下，於400 W高壓汞燈照射下進行反應，發現具有產氫能力，光照時產氫速率約為11 μmol/hr，在還原完成之後，石墨烯量子點產氫之活性漸漸下降，可產氫照光時間約為14小時。且在照光之後，溶液的顏色明顯從橘黃色變為透明。高解析電子能譜儀分析顯示，在照光的過程之中，含氧官能基會大幅減少，顯示有光還原的作用，與溶液顏色的變化相符。由紫外光/可見光吸收光譜儀以及光激發螢光頻譜分析，發現石墨烯氧化物而在不具有含氧化官能基情況下，半導體特性大幅減少，故不能持續產生電子電洞對用以還原水產生氫氣。

關鍵字

光催化產氫 ； 二氧化鈦 ； 碳修飾 ； 石墨烯量子點

並列摘要

Abstract
Semiconductor photocatalysts, possessing suitable band gaps, are able to absorb solar energy and generate electron/hole pairs. When the electron/hole pairs successfully migrate to the surfaces of the photocatalyst, they may reduce and oxidize water to generate hydrogen and oxygen, respectively. This is a green process but it suffers from low efficiencies. If we are able to reduce the electron-hole recombination, the photocatalytic efficiency would be improved.
A simple carbon decoration, involving only immersion in adipic acid followed by calcination in N2 atmosphere, was developed to prepare thin carbon layer decorated TiO2 nanoparticles. The thin carbon layer was in tight contact with the TiO2 domain and served as an electron trapping centre to improve charge separations necessary for enhancement in photocatalytic water splitting performance of the TiO2 nanoparticles. With an optimal carbon loading of 0.3 wt%, a four-fold improvement was achieved for hydrogen production as compared with that achieved by pristine TiO2 nanoparticles. This simple carbon decoration provides a promising low-cost alternative to traditional Pt-decoration approaches for enhancing hydrogen productions from photocatalytic water splitting.
The second part of the thesis concerns the preparation of graphene oxide quantum dots and their application in photocatalytic water splitting. Graphene, a single layer of graphite, has the characteristics of high conductivities, high surface areas, and high electron mobilities. It has been shown that graphene oxides with a size below 10 nm can absorb UV lights shorter than 300 nm and exhibit significant photoluminescent emissions, implying the semiconductor features of the graphene oxide quantum dots.
In this work, graphene oxide quantum dots were applied for photocatalytic hydrogen production for the first time. Well dispersed graphene oxide quantum dots in aqueous solution were prepared from carbon fibers under 120℃ heat treatment. According to HRTEM analyses, the size of the graphene oxide quantum dots was about 3 to 6 nm. From the AFM analyses, these graphene oxide quantum dots were with a thickness of 1 to 2 atomic layers. Without use of any sacrificial reagent, the graphene oxide quantum dots showed the ability of hydrogen production with a rate of 11μmol/hr under irradiation of a 400 W high-pressure mercury lamp. During the reaction, the oxygen-containing functional groups of the graphene oxide quantum dots were gradually reduced, leading to formation of graphene quantum dots. The activity of the graphene oxide quantum dots gradually decreased with increasing irradiation time. The hydrogen evolution ceased after 14 hours. The color of the graphene oxide quantum dot suspension turned transparent from orange during the reaction. According to the high resolution X-ray photoelectron spectroscopy, the oxygen-containing functional groups of the graphene oxide quantum dots were significantly reduced after exposing the suspension to the light source, which indicates the photo-reduction of the graphene oxide quantum dots. From the UV-visible and photoluminescence spectra, the reduced graphene quantum dots lost their semiconductor features.

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