本篇論文製備並系統化研究了三種創新、高靈敏度且高效率的氣體感測器。這些感測器是由在藍寶石基板上鍍上氧化鎳薄膜以及鈀、鉑和金等貴金屬奈米顆粒所構成。金屬奈米顆粒的使用可以顯著提升表面的粗糙度和有效感測面積，進而增強元件的感測性能，而奈米顆粒的催化活性也改善了氣體響應並縮短了響應時間。氧化鎳薄膜和貴金屬奈米顆粒(鈀、鉑和金)分別使用射頻磁控濺鍍機與真空熱蒸鍍機所製備而成。感測器的表面形貌和化學成分組成則透過能量色散X光譜儀(EDS)、高解析度掃描式電子顯微鏡(HRSEM)、原子力顯微鏡(AFM)、穿透式電子顯微鏡(TEM)和X射線光電子能譜儀(XPS)進行分析。
第一種感測器是由氧化鎳薄膜與鈀奈米顆粒所製備而成的氫氣感測器，在鈀奈米顆粒的修飾下，表面粗糙度與有效感測面積都有了顯著的提升，使得此感測器對氫氣展現出極佳的感測能力。在300℃的環境下，通入濃度為10000 ppm的氫氣，此感測器的感測響應達到2.18×105。除此之外，此感測器也展現出相對快速的感測速度，在相同條件下，其響應時間和恢復時間分別為206秒和2秒。為了進行比較，我們製備了沒有經過金屬顆粒修飾的裸氧化鎳薄膜感測元件，在相同的情況下(300℃的溫度，1000 ppm的氫氣)，裸氧化鎳薄膜感測器和所研究的Pd NP/NiO感測器的感測響應分別為0.59和91。這個結果也證實了鈀奈米顆粒的修飾可顯著提高感測性能。
第二種感測器是由氧化鎳薄膜與鉑奈米顆粒所製備而成的氨氣感測器，在鉑奈米顆粒的修飾下，表面粗糙度與有效感測區域都有明顯的增加，使得此感測器對氨氣展現出極佳的感測能力。在275℃的環境下，通入濃度為1000 ppm的氨氣，此感測器的感測響應達到9.96。除此之外，此感測器具有能低至0.01 ppm的極限檢測能力以及相對快速的感測速度，在相同條件下，其響應時間和恢復時間分別為162秒和28秒。為了進行比較，我們製備了沒有經過金屬顆粒修飾的裸氧化鎳薄膜感測元件，在相同的情況下(275℃的環境，1000 ppm的氨氣)，裸氧化鎳薄膜感測器和所研究的Pt NP/NiO感測器的感測響應分別為1.75和9.96。這個結果也證實了鉑奈米顆粒的修飾可顯著提升感測性能。
第三種感測器是由氧化鎳薄膜和金奈米顆粒所製備而成的乙醇與甲醛感測器，在金奈米顆粒的修飾下，表面粗糙度與有效感測面積都有明顯的增加，使得此感測器對乙醇與甲醛都展現出極佳的感測能力。對乙醇而言，在300℃的環境下，通入濃度為1000 ppm的乙醇，此感測器的感測響應為3.94。除此之外，此感測器對乙醇具有能低至0.1 ppm的極限檢測能力以及相對快速的感測速度，在1000 ppm乙醇和300℃的條件下，其響應時間和恢復時間分別為106秒和114秒。為了進行比較，我們製備了沒有經過金屬顆粒修飾的裸氧化鎳薄膜感測元件，在相同的情況下(300℃的環境，1000 ppm的乙醇)，裸氧化鎳薄膜感測器和所研究的Au NP/NiO感測器的感測響應分別為2.81和3.94。對甲醛而言，在300℃的環境下，此感測器對20 ppm的甲醛的感測響應為2.96。除此之外，此感測器對甲醛同樣具有能低至0.1 ppm的極限檢測能力以及相對快速的感測速度，在20 ppm甲醛和300℃的條件下，其響應時間和恢復時間分別為200秒和204秒。在相同的情況下，裸氧化鎳薄膜感測器和所研究的Au NP/NiO感測器的感測響應分別為1.45和2.96。這些結果都證實了金奈米顆粒的修飾可提高元件對乙醇及甲醛的感測性能。
本篇論文成功地開發出了三種不同的氣體感測器，每種氣體感測器都展現出優異的感測性能。重複性、選擇性與長期穩定性等特性也在論文中被系統性的檢驗，並證實了其在實際應用中的可靠性。除了前述的感測性能外，這些感測器更具備結構簡單、製造程序便捷及成本低廉等特點，而這些優勢也凸顯了其在實際氣體感測應用中的巨大潛力。

In this study, three innovative, high-sensitivity, and efficient gas sensors were fabricated and systematically investigated. These sensors consist of a nickel oxide (NiO) thin film decorated with noble-metal nanoparticles (NPs), including palladium (Pd), platinum (Pt), and gold (Au), on a sapphire substrate. The incorporation of NPs significantly increases the surface roughness and the effective sensing area, thereby enhancing the sensing performance. In addition, the catalytic activity of the NPs improves the gas response and reduces the response (recovery) time. The NiO thin film and the metal nanoparticles (Pd, Pt, and Au) were fabricated by radio frequency (RF) magnetron sputtering and vacuum thermal evaporation (VTE), respectively. The surface morphology analyses and chemical composition of the studied sensors were characterized using energy-dispersive X-ray spectroscopy (EDS), high-resolution scanning electron microscopy (HRSEM), atomic force microscopy (AFM), transmission electron microscopy (TEM), and X-ray photoelectron spectroscopy (XPS).
The first sensor, consisting of a NiO thin film and Pd nanoparticles (Pd NP/NiO), was applied for hydrogen (H2) sensing. Since the decoration of Pd NPs significantly enhances the surface roughness and effective sensing area, this sensor demonstrates a remarkable sensing response (SR) to hydrogen gas. When exposed to 10000 ppm H2/air at 300℃, the SR of the studied sensor reaches 2.18×105. Furthermore, this sensor exhibits relatively fast sensing behavior, with response time and recovery time of 206 s and 2 s, respectively, under the same conditions. For comparison, a bare NiO thin-film-based sensor was also fabricated. Under the same conditions (1000 ppm H2/air at 300℃), the sensing response of the bare NiO thin-film-based sensor and the studied Pd NP/NiO sensor are 0.59 and 91, respectively. This result confirms that decorating Pd NPs can significantly improve the hydrogen sensing performance.
The second sensor, consisting of a NiO thin film and Pt nanoparticles (Pt NP/NiO), was employed for ammonia (NH3) sensing. The incorporation of Pt NPs significantly increases surface roughness and the number of active sites, leading to an improved sensing performance for ammonia gas. When exposed to 1000 ppm NH3/air at 275℃, the SR of the studied sensor reaches 9.96. Furthermore, at 275℃, this sensor exhibits a limit of detection (LOD) of 0.01 ppm, along with relatively fast response and recovery times of 162 s and 28 s, respectively, under 1000 ppm NH3/air. For comparison, a bare NiO thin-film-based sensor was also fabricated. Under identical conditions (1000 ppm NH3/air at 275℃), the SR value of the bare NiO thin-film-based sensor and the studied Pt NP/NiO sensor are 1.75 and 9.96, respectively, demonstrating the effectiveness of Pt NP decoration.
The third sensor, based on a NiO thin film and Au nanoparticles (Au NP/NiO), was applied for ethanol (C2H5OH) and formaldehyde (HCHO) sensing. Due to the decoration of Au NPs, the surface roughness and the effective sensing area increase, resulting in a high sensing response (SR) to ethanol and formaldehyde. For ethanol detection, the sensor shows an SR of 3.94 with 1000 ppm C2H5OH/air at 300℃. In addition, this sensor exhibits a LOD of 0.1 ppm and the relatively fast sensing behavior, with response and recovery times of 106 s and 114 s, respectively, under 1000 ppm C2H5OH/air. For comparison, a bare NiO thin-film-based sensor was also fabricated. Under the same conditions (1000 ppm C2H5OH/air at 300℃), the sensing responses of the bare NiO thin-film-based sensor and the studied Au NP/NiO sensor are 2.81 and 3.94, respectively. For formaldehyde gas sensing, the sensor exhibits an SR of 2.96 with 20 ppm HCHO/air at 300℃. In addition, this sensor also exhibits a LOD of 0.1 ppm and a relatively fast sensing speed with response and recovery times of 200 s and 204 s, respectively, under 20 ppm HCHO/air. Moreover, under the same conditions (20 ppm HCHO/air at 300℃), the sensing response of the bare NiO thin-film-based sensor and the studied Au NP/NiO sensor are 1.45 and 2.96, respectively. These results confirm that Au NP decoration enhances sensing performance for both ethanol and formaldehyde.
In this work, three distinct gas sensors were successfully developed, each demonstrating superior sensing performance. The repeatability, selectivity, and long-term stability of the sensors were systematically evaluated in the study, confirming their reliability for practical operation. In addition to their high performance, the proposed sensors feature a simple device structure, facile fabrication process, and low production cost. These advantages highlight their strong potential for practical gas-sensing applications.

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