奈米孔洞金屬薄膜合成與表面改質對於其觸媒、感測應用之評估

Abstract

自工業革命起，溫室氣體逐年增加的排放量與隨之影響的劇烈氣候變遷，造成地球生態系統的浩劫之外，亦壓縮著人們生存空間與品質。鑑於此，各國紛紛訂定出相關減碳政策與目標，我國政府亦在2022年發布「2050淨零碳排」以12大政策推動相關產業與研究加速進行，如何解決既有碳排放於大氣中的總含量，同時發展相關零污染之綠色經濟成了發展目標。除了上述環境相關議題之外，近年來新聞報導上層出不窮的食安問題造成人心惶惶，隨著食品加工技術與各式添加物的發展，接觸更複雜、更多來源的食品風險亦隨之提高，預防性的食品檢測成了把關食安的關鍵流程。 如何解決上述兩種重要議題成了本文章的研究重點，當中減碳技術藉由電催化進行二氧化碳轉換成不同化合物或是有機物之反應較其他技術低耗能且有效率，同時還原之產物可進一步投入工業使用，進而發展為環保且安全之綠色循環經濟；另一方面，透過表面增強拉曼技術進行食安檢測與分析，可以快速、有效率且較低成本的進行微量食品添加物的辨識，隨著可攜式拉曼光譜儀的發展，行動檢測食品安全亦不再是夢想。為有效的將二氧化碳進行轉換與提升表面增強拉曼訊號，開發具有高比表面積、表面粗糙、開放式多孔性等特性的材料作為催化用觸媒與光譜基板為首要目標，而本研究開發不同雙金屬系統以合成奈米孔洞金屬薄膜之製程，當中分別透過製程參數的調整與結合不同的表面改質法進行材料優化，同時亦針對上述的電催化二氧化碳還原反應與拉曼檢測食品安全進行測試與分析，以此拓展奈米孔洞金屬薄膜之應用面，並期望解決環境與食安相關問題。 本研究使用真空共濺鍍法與後續的化學去合金法製備出具有奈米孔洞結構之銅薄膜作為電催化反應之觸媒，其中亦藉由改變去合金時間以合成不同形貌與尺寸的奈米孔洞銅薄膜，當中包含支架尺寸為37 nm至98 nm的奈米多孔銅薄膜以及分層的奈米片狀銅薄膜；同時也藉由後退火熱處理改變奈米孔洞尺寸與評估材料耐熱性，經高溫500°C處理之薄膜其支架粗化至63 nm但仍保有雙連續之多孔結構；最後，透過常溫常壓氦氣噴射電漿進行奈米孔洞銅薄膜表面處理以改變其表面特性，成功的改變奈米多孔銅薄膜表面之鍵結組成並提升其整體潤濕性。在電催化二氧化碳還原反應之結果可以得知不同形貌與表面氧化價態皆對於還原產物選擇性有所影響，具有奈米孔洞結構之銅觸媒較平坦的銅箔提供更多的反應位點；因此，可有效地增加其整體反應速率，同時亦提升其有效還原產物之轉換率。而典型奈米多孔結構之銅觸媒具備的隨機表面缺陷更能夠有效地將還原產物單一轉換至甲酸，如氦氣噴射電漿之銅薄膜其甲酸選擇率為32%；另外，經過後退火處理之奈米多孔銅薄膜亦保有奈米多孔結構與適量的氧化亞銅在其表面，其甲酸轉換率則提升為45%。 於表面增強拉曼技術進行食安檢測應用中，所選用之基板為具有奈米孔洞、表面粗糙等特性的奈米多孔銀薄膜，設計不同孔洞分布之奈米多孔銀基板以釐清孔洞尺寸與增強訊號之關係。藉由改變濺鍍時的參數與去合金時間以合成出不同孔洞尺寸之奈米銀薄膜，其中孔洞尺寸分布從51 nm至246 nm，當中去合金90分鐘合成的奈米多孔銀薄膜具有最小的孔洞尺寸為51 nm。於光學檢測中以10-6 M的R6G進行測試，其響應的增強拉曼訊號較純銀薄膜的30 counts/s提升至4041 counts/s，後續也透過時域有限差分法模擬得知具有較小孔洞分布的奈米基板能有較多熱點分佈以增加其表面電場，進而提升表面增強拉曼訊號。最後，透過奈米銀薄膜檢測養殖業禁用之孔雀石綠其最低偵測濃度可達10-5M，增強因子為1.34x105。 本研究藉由真空濺鍍法與化學去合金法成功製備出不同系統之奈米孔洞金屬薄膜，並藉由去合金時間參數的調整了解其孔洞結構演變之過程，此外，結合不同的表面處理(後退火處理、氦氣噴射電漿)以製備出不同特性與形貌之奈米孔洞薄膜。於電催化二氧化碳還原中，調整觸媒之結構與表面特性可有效提升單一還原產物之選擇性；於食品安全檢測中，合成較小奈米孔洞之基板可有效提升其偵測能力。鑒於上述研究成果，展示奈米孔洞金屬薄膜於催化、感測應用中具有其發展性。

Since the Industrial Revolution, greenhouse gas emissions have increased year by year and the ensuing severe climate change has made a serious impact on the world. In addition to the disaster of the global ecosystem, it also compresses the living space and quality of people. In view of this, countries have formulated relevant carbon emission reduction policies and targets. Our government also announced “2050 Net Zero Carbon Emissions” in 2022 to promote the development of related industries and research with 12 major policies. How to solve the existing total carbon emissions in the atmosphere while developing a related zero-pollution green economy has become an important goal. In addition to the above-mentioned environmental issues, food safety issues that have frequently appeared in news in recent years have also caused public panic. With the development of food processing technology and various additives, the risk of exposure to more complex and more food sources has also increased. Preventive food inspection has become a critical process for food safety. How to solve the above two important problems becomes the research focus of this paper. Among them, carbon reduction technology uses electrocatalysis to convert carbon dioxide (CO2) into different compounds or organics, which consumes less energy and is more efficient than other technologies. At the same time, the reduction products can be further put into industrial use, and then develop into an environmentally safe green circular economy. On the other hand, food safety detection and analysis by surface-enhanced Raman (SERS) technology can quickly, efficiently and economically identify food additives. With the development of portable Raman spectrometers, mobile food safety detection is no longer a dream. To efficiently convert CO2 and promote surface-enhanced Raman signals, the primary goal is to develop materials with high specific surface area, rough surface, open porosity, etc. as catalysts and spectroscopic substrates. In this study, different bimetallic systems were developed to synthesize nanoporous metal films, and films were optimized by adjusting process parameters and combining different surface modifications. At the same time, the above-mentioned electrocatalytic CO2 reduction reaction (CO2RR) and food safety Raman detection were also tested and analyzed to expand the application range of nanoporous metal films. It is expected to solve environmental and food safety related issues. Firstly, different systems and ratios of bimetallic precursor films were prepared by vacuum co-sputtering method. Metal films with nanoporous structures were subsequently synthesized by selectively etching in chemical dealloying process. In the copper-aluminum system, the experiment attempted to change the dealloying time to synthesize nanoporous copper (NPC) films with different shapes and sizes. These films include nanoprous copper films with ligament sizes ranging from 37 nm to 98 nm, and nanosheet copper cluster films. At the same time, post-annealing treatment was also applied to change the nanopores and evaluate the heat resistance of the films. The ligament size of NPC films after 500°C treatment increased to 63nm. But the film still maintained a bicontinuous porous structure. Finally, the surface of NPC film was modified by atmospheric-pressure helium plasma. It successfully changed the bonding composition of the NPC film and further improve the surface wettability. On the other hand, in the silver-aluminum system, nano-silver films with different pore sizes were also fabricated by changing the sputtering parameters and dealloying time. The pore size distribution ranges from 51nm to 246nm. In the electrocatalytic CO2RR, a catalyst with high activity and selectivity is necessary. In this study, NPC films synthesized by different preparations were utilized as reduction catalysis. The experimental results showed that different morphologies and surface oxidation states could affect the reaction rate and selectivity of reduced products. The copper films with nanoporous structure could effectively increase the reaction rate and the conversion rate of the effective reduction products compared with the copper foil；The copper films with typical nanoprous structure could convert the CO2 into formic acid efficiently, and the helium plasma treated copper film had the formic acid faradaic efficiency of 32%. In addition, the NPC film after post-annealing also retained the nanoporous structure and appropriate amount of cuprous oxide, and it formic acid conversion rate increased to 45%. In the application of SERS technology for food safety detection, the nanoporous silver (NPS) films with different nanopores were used as the substrates. Among these films, the nanoporous silver films after 90 minute dealloying process, NPS46_t90 film, had the smallest pore with the size of 51 nm. In the optical inspection, the enhanced Raman intensity of NPS46_t90 film is 4041 counts/s, which is relatively high compared with 30 counts/s for pure silver film. Then, finite-difference time-domain simulations also illustrated nanoprous films with smaller pore size could provide the higher hot-spot area to increase the surface electric field, thereby enhancing the SERS performance. Finally, the limit concentration of malachite green detection via NPS film substrates would be 10-5 M, while the enhancing factor is 1.34 x 105.