摻硼奈米級鑽石電極-特性、表面活化之探討及其於染料廢水脫色及重金屬量測之應用

Abstract

鑽石薄膜電極為一具寬廣電位視窗之材料，也因具備此特性故常用於許多電化學程序中。現階段，鑽石電極之製備多利用氣相沉積法合成多晶相之鑽石，相較於前述之高溫高壓技術，本研究所使用之方法可於較低壓力以及低成本下操作；於環境領域之應用，摻硼鑽石薄膜電極因具較高之電位視窗，除可做為水中重金屬之檢測，偵測相對較高的陽極電位氧化之化合物；其在降解污染物的過程中伴隨水的放電而產生羥基自由基，亦可作為廢水處理之應用。然而應用效果不僅取決於薄膜本質，也會因製備後表面活化程序而有所不同。本研究以熱絲化學氣相沉積法合成之摻硼鑽石薄膜電極進行特性分析，其表面活化為經由電化學極化程序，並探討不同前處理程序之摻硼鑽石薄膜電極其應用於染料之脫色及其在水中檢測鉛、銅、汞之能力。 本研究之摻硼鑽石薄膜電極為經由HF-CVD程序製備出具類似於金屬電阻特性之摻硼奈米級鑽石電極(BD-NCD)；2 µm厚度之非面體結節BD-NCD薄膜有80.68%的粒徑分布範圍界於10-30 nm之間；ID/IG值為1.36-1.73之典型NCD圖譜顯示出拉曼光譜之偏移為多個激發波長而造成；sp3之碳含量隨深度而增加，其表面約有40%而在9 nm處則達到60%。此外，許多研究顯示BD-NCD薄膜電極可藉由電化學極化活化之電極表面而達到脫色效果。相較於陰極，陽極極化更適合用於電極的預處理程序。經陽極極化後，可將BD-NCD電極表面上之氫端轉為氧端以提升其親水性。在極化過程中，BD-NCD電極表面官能基及部分sp2碳發生氧化，可使電極成功活化，其相較於未經極化的電極有較小的背景電流、較寬的電位視窗以及較低的表面活性。因此，BD-NCD電極降解染料溶液的電氧化能力經陽極極化後可顯著提升，甚至可藉由重複循環數次來完全去除溶液中的色度及COD。BD-NCD薄膜電極降解染料的能力於酸性條件下較佳，且溶液中氯離子的存在比硫酸鹽類活性物種更加有利。當施加10、20及30 mA cm-2等不同的電流密度時，可發現電流密度與脫色速率成正比。經批次循環反應槽可達到近完全脫色及約80%左右之COD去除率，其一般電流效率約為74%、單位能耗為4.4 kWh m-3(處理溶液之體積)或145 kWh kg-1(處理COD之公斤數)。 此外，施加於BD-NCD電極上之動能極化被證明可提升其表面性能，由XPS的分析結果顯示，極化後之電極表面上之Csp2及不定型碳含量減少，使得電極動能增加並提升以線性掃描伏安法(LSV)測量水溶液中鉛、銅及汞的能力，且不需預先濃縮之步驟。在醋酸緩衝溶液中，電流回饋會隨重金屬濃度而增加，其於鉛及銅離子之線性測量範圍為1至22.5 µM、汞離子則是1至10 µM；而鉛離子、銅離子、及汞離子之偵測極限及敏感度分別為1.339、0.102、和0.666µM，以及0.014、0.104、和0.057 µA µM-1 mm-2。雖然銅離子的存在被視為系統干擾，BD-NCD仍展現出同時檢測多種金屬的能力。另一方面，天然水樣中的高有機物含量如湖水等亦會阻礙重金屬的測量。最後，本研究證實，BD-NCD經動能極化後，無論於醋酸緩衝溶液或自然水系統中，皆可單獨或同時測定水體中之鉛離子、銅離子以及汞離子至µM等級。

Diamond film electrode has been known as a material with very wide potential window for water electrolysis which leads to its applicability in numerous electrochemical processes. Its expansive application is even more driven by the possibility of producing synthetic polycrystalline diamond via chemical vapor deposition (CVD), a technique that is operated under low pressure and cost considerably lower compared to previously known High Pressure High Temperature (HPHT) technology. Especially in environmental application, boron-doped diamond (BDD) film electrode finds suitable use in wastewater treatment and heavy metal measurements. High potential window allows BDD film electrode to detect compounds that is oxidized in relatively high anodic potential. A wide potential window for oxygen evolution also permits electrogeneration of oxidants such as hydroxyl radicals from water discharge for pollutants degradation. Nevertheless, the effectiveness of its application is not only determined by intrinsic quality of the film, but also the surface activation process following the fabrication. In this study, after the fabricated boron-doped nanocrystalline diamond (BD-NCD) via hot filament CVD (HFCVD) being thoroughly characterized, its surface activation was conducted by electrochemical polarization. The effect of this treatment on the BD-NCD characteristics and its correlation to the performance in decolorization and Pb(II), Cu(II) and Hg(II) measurement in aqueous solutions were investigated. BD-NCD film with metal-like resistivity characteristics are successfully fabricated via HF-CVD. The grain size distribution of non-faceted nodular 2 µm-thick BD-NCD film is 80.68% within the range of 10–30 nm. Raman shift acquired by multiple excitation wavelength show typical NCD spectra with ID/IG of around 1.36–1.73. Carbon sp3 content is higher in the deeper film; 40% on the surface and 60% at 9 nm-deep. The performance of boron-doped nanocrystalline diamond (BD-NCD) film electrode for decolorization with respect to the surface activation by electrochemical polarization was studied. Anodic polarization found to be more suitable as electrode pretreatment compared to cathodic one. After anodic polarization, the originally H terminated surface of BD-NCD was changed into O terminated, making it more hydrophilic. Due to the oxidation of surface functional groups and some portion of sp2 carbon in the BD-NCD film during anodic polarization, the electrode was successfully being activated showing lower background current, wider potential window and considerably less surface activity compared to the non-polarized one. Consequently, electrooxidation capability of the anodically-polarized BD-NCD to degrade the dye solution was significantly enhanced; capable of nearly total decolorization and COD removal even after several times of re-using. The BD-NCD film electrode favored acidic condition for the dye degradation, and the presence of chloride ion in the solution was found to be more advantageous than sulphate active species. Applying different current density of 10, 20 and 30 mA cm-2, it is found that the higher the current density the faster the decolorization rate. General current efficiency achieved after nearly total decolorization and ~80% COD removal in batch recirculating reactor was around 74% , with specific power consumption of 4.4 kWh m-3 (in terms of volume of solution treated) or 145 kWh kg-1 (in terms of kg COD treated). Dynamic polarization applied on the BD-NCD electrode was proven to enhance its surface properties. The results of XPS analysis show that polarized electrode exhibits diminution of Csp2 and possess minimum adventitious carbon content on its surface. This leads to an increase of electrode kinetic and promotes its capability for Pb, Cu and Hg ions measurements in the aqueous solution using linear sweep voltammetry (LSV) technique without pre concentration step. The linearity of current response with increasing heavy metals concentration in an acetate buffer is ranging from 1 to 22.5 µM for Pb and Cu ions and 1 to 10 µM for Hg ion. The detection limit and sensitivity are 1.339, 0.102 and 0.666 µM, and 0.014, 0.104 and 0.057 µA µM-1 mm-2 for Pb2+, Cu2+ and Hg2+, respectively. The BD-NCD electrode also exhibits capability in simultaneous multi-metals determination although the presence of Cu ion was found to be a disturbance in the system as well. On the other hand, the high content of organics in the real water sample such as lake water also demonstrated to hinder heavy metals measurement. Finally, this study confirms that the BD NCD after undergoing dynamic polarization are capable of direct determination of Pb2+, Cu2+ and Hg2+ ions both individually and simultaneously either in the acetate buffer solution or natural water system, within µM level.