以濺鍍法製備具可見光吸收之雙層光電極並應用於光催化水分解產氫

Abstract

氫氣是未來地球上具發展淺力的能源之一，因為它有潔淨、高燃燒效率、元素蘊藏量豐富等特性。由於利用太陽光催化進行水分解反應之產氫技術具有低成本及對環境友善等優勢，該技術近年來逐漸受到各國研發單位的重視。以H-type反應器系統進行光催化水分解反應是提升產氫效率的方式之一，因為該系統可以達到氫氧分離的目的，避免水分解逆反應的發生。H-type反應器系統主要包含反應器、光電極及質子交換膜，本研究以射頻磁控濺鍍法在高溫下製備具可見光吸收之二氧化鈦(TiO2)及三氧化鎢(WO3)光觸媒薄膜，並將該薄膜作為光電極材料，應用於光催化水分解反應，接著利用X射線繞射儀(XRD)、場發射掃描電子顯微鏡(FESEM)、原子力顯微鏡(AFM)、能量散佈分析儀(EDS)、X射線光電子能譜儀(XPS)、紫外光-可見光吸收能譜儀(UV-Vis)等設備來探討光觸媒薄膜製備後之物理及化學特性，包括晶相、表面形態、化學組成、光吸收特性等。其中，光觸媒薄膜之可見光吸收現象主要來自於高溫濺鍍過程中產生之氧空缺，進而改變了薄膜之光吸收能隙。為了進一步提升光電極之光吸收範圍，本研究另提出一新型雙層光觸媒(Dual-Layer Photocatalyst)薄膜結構，由具可見光吸收之二氧化鈦及三氧化鎢光觸媒薄膜組成，並透過電流電壓量測儀與光催化水分解反應實驗了解單層及雙層光觸媒薄膜之光電化學性質與光催化活性，由實驗結果證實，光催化水分解反應之氫氣及氧氣產率趨勢與光觸媒薄膜之光電效率趨勢吻合，雙層光觸媒薄膜之活性高於單層光觸媒薄膜，相較於單層光觸媒薄膜，我們認為雙層光觸媒薄膜結構可以有效分離光激發觸媒材料後所產生之電子與電洞，增加其還原與氧化能力。另外，本研究也探討了氧化鎂(MgO)材料修飾雙層光觸媒薄膜表面之影響，由光電流量測結果發現，以少量的氧化鎂修飾TiO2表面有利於提升TiO2的光催化活性。最後，本研究嘗試將氧化鎂改質後的雙層光觸媒薄膜製備成光電極，並搭配H-type光反應器系統進行光催化水分解反應實驗。實驗結果證明，氧化鎂的確可以有效地增加雙層光觸媒電極的產氫及產氧效率，我們認為效率的提升主要是來自於氧化鎂親水特性，因此，以該材料修飾雙層光觸媒薄膜可提升其表面與水分子之作用能力，即提升水分子傳送至觸媒材料表面之能力，進而增加水分子被觸媒材料氧化及還原之效率，除此之外，氧化鎂也可以有效的抑制光電極的漏電現象。

Hydrogen is the ideal fuel for the future because it is clean, energy efficient and abundant in nature. Recently, solar hydrogen via photocatalytic water splitting has attracted tremendous attention due to its great potential for low-cost and clean hydrogen production. One of the technologies for improving the efficiency of photocatalytic water splitting is the adoption of an H-type reactor system that allows separate evolution of hydrogen and oxygen during the reaction, preventing the backward reaction to form water. The key components of an H-type reactor system are reactor, photoelectrode and proton exchange membrane. In this study, visible light-absorbing TiO2 (vis-TiO2) and WO3 (vis-WO3) thin films as the photoelectrode materials were prepared by radio-frequency magnetron sputtering at high temperature and used to carry out photocatalytic water splitting in an H-type reactor system. Instrumental analyses such as XRD, FESEM, AFM, EDS, XPS, and UV-Vis were performed to reveal the crystallinity, surface morphology, chemical composition, and light absorption of the prepared photocatalytic thin films. It is believed that the shift of the absorption spectra towards the visible-light region for the prepared thin films is resulted from the increase of their metal-to-oxygen ratio due to high-temperature sputtering. To further extend the light absorption spectrum of the photoelectrode, a dual-layer photocatalyst (DLP) that consists of both vis-TiO2 and vis-WO3 was prepared. The photoactivities of single-layer and dual-layer photocatalysts were examined first by photovoltammetry followed by conducting water-splitting reactions in an H-type reactor under both UV and visible-light irradiations. It was demonstrated that H2 and O2 yields obtained from water-splitting reaction are consistent with the photocurrent results (from photovoltammetry); showing that DLP is more active than single-layer photocatalyst. The enhanced performance of DLP comparing with single-layer photocatalyst is mainly attributed to the improved charge separation of the dual-layer structure. Later on, the effect of adding a thin layer of MgO on TiO2 was investigated. From the photocurrent measurement results, it was found that coating a very thin layer of MgO on TiO2 may have the benefit to improve TiO2’s photoactivity. Finally, MgO modified DLP was prepared and its photoactivity was demonstrated by carrying out water-splitting reaction in the H-type reactor system under visible-light irradiation. Comparing with DLP, the MgO modified DLP show improved H2 and O2 yield. It is believed that the improvement is resulted from the hygroscopic nature of MgO, which can increase the concentration of water molecules on the photocatalyst surface to perform water oxidation, consequently, reducing the probability of electron-hole recombination. Moreover, the inclusion of MgO may also suppress current leakage of the photoelectrode by preventing electrons from diffusing toward the TiO2-water interface.