具奈米結構氧化鋅之合成、鑑定及在化學感測器及染料敏化太陽能電池之應用

Abstract

本論文主要目的是要探討具奈米結構氧化鋅修飾電極在化學感測器以及染料敏化太陽能電池的應用。在化學感測器方面，針對具不同長寬比之氧化鋅奈米柱於一氧化氮氣體感測器之應先作探討。氧化鋅奈米柱係由化學水浴沉積法製備，並且藉由調整化學水浴沉積法反應次數控制氧化鋅奈米柱之長寬比。由於具有較高的比表面積，氧化鋅奈米柱對一氧化氮的氣敏性較氧化鋅薄膜高。然而，氧化鋅奈米柱之長寬比有最適值；由於氧化鋅奈米柱的長寬比及氧化鋅奈米柱數目隨著反應次數的增加，一氧化氮之擴散至氧化鋅奈米柱之路徑及阻力也隨之增加，因此部分氧化鋅奈米柱表面積沒有被利用到。在最佳條件下，氧化鋅奈米柱在240度下之增測下限可達2 ppb。另一方面，氧化鋅可當作氧化銦微結構之微修飾物。氧化鋅在氧化銦-氧化鋅複合膜之含量可藉由調整在共沉澱步驟之Zn2+/In3+莫耳比來控制。由結果得知，當Zn2+/In3+莫耳比為0.33時，氧化銦-氧化鋅複合膜對一氧化氮在溫度低於200度的氣敏性大為提高。氣敏性的提高主要歸因於氧化銦奈米顆粒之晶粒由32.3降低至15.4 nm。然而，當Zn2+/In3+莫耳比高於0.33時，氧化銦晶粒大小提高且微結構受到改變。因此，複合膜對一氧化氮之氣敏性降低。在最佳條件下，複合膜在工作溫度150度下的偵測下限可至12 ppb。此外，複合膜對一氧化碳不具氣敏性。另一方面，氧化鋅薄膜也可當作二氧化鈦導電媒介，以增進二氧化鈦對一氧化氮的感測性能；雖然單純二氧化鈦對一氧化氮的氣敏性極小，但TiO2NP/ZnO 雙層膜對一氧化氮的氣敏性大為提高。此外，氣敏性的提升與退火時間有很大的關聯；當退火時間由半小時提高到兩小時時，最大氣敏性可提高大約6.2倍。TiO2/ZnO 雙層膜修飾電極之偵測下限可達 8.8 ppb，並且，相對於單純氧化鋅膜，TiO2/ZnO雙層膜修飾電極對二氧化氮及一氧化碳有較好的選擇性。除了電阻式氣體感測應用之外，氧化鋅在電化學感測器應用上也是個良好的材料。在紫外光照射下，銀離子將化被還原且以奈米粒子的形式沉積於氧化鋅表面上。因此，若結合氧化鋅奈米柱表面積可控制性的優點，銀奈米粒子的大小及分布皆可輕易地調整。實驗結果顯示，固定在氧化鋅奈米柱表面的銀奈米粒子對過氧化氫的還原具有很好的電催化性。藉由對銀奈米粒子沉積條件最佳化，此複合膜在操作電位為-0.55 (vs. Ag/AgCl/sat’d KCl)下，偵測下限、靈敏度及反應時間(t95)分別可達0.9 mM、152.1 mA M-1 cm-2及30~40 s。此外，維生素C及尿酸對複合膜感測過氧化氫的干擾極小。 另一方面，我們也可利用化學水浴沉積法製備氧化鋅奈米片修飾電極；首先藉由化學水浴沉積法在FTO電極上沉積聚層狀鋅耐酸鹽氫氧化物，接著再藉由在300度下的 退火程序，將層狀鋅耐酸鹽氫氧化物轉換成氧化鋅奈米片。在最佳化條件下，以氧化鋅奈米片修飾電極當工作電極之染料敏化太陽能電池效率可達6.06%，遠高於以氧化鋅奈米顆粒修飾電極當工作電極之染料敏化太陽能電池之效率(2.92%)。以氧化鋅奈米片修飾電極當工作電極之染料敏化太陽能電池具有較高的光轉換效率主要來自於(i)電子在氧化鋅奈米片具有較高的有效擴散係數(2.15 × 10-2 cm2 s-1 vs. 0.87 × 10-2 cm2 s-1)及(ii)氧化鋅奈米片修飾電極具有高的染料吸附量(2.66 × 10-7 mole cm-2 vs.1.99 × 10-7 mole cm-2)。此外，若利用電泳法將二氧化鈦奈米顆粒沉積在氧化鋅奈米片膜上，染料敏化太陽能電池之效能可再進一步提高(Jsc=19.53 mA cm-2, Voc= 576 mV, FF=0.628, and efficiency=7.07%)。太陽能電池性能提高之原因來自於二氧化鈦奈米粒子所提供的額外表面積，使得染料吸附量提高所造成。

The main purpose of this dissertation is to discuss the applications of nanostructured ZnO modified electrodes in chemical sensors and dye-sensitized solar cells (DSSCs). In the aspect of chemical sensors, ZnO nanorods (ZnONR) were first applied to the detection of NO gas. ZnONR with various aspect ratios (AR) were synthesized using chemical bath deposition (CBD) method; the AR was controlled by adjusting the number of the CBD growth cycle. As compared with ZnO thin film, ZnONR showed higher sensing response to NO gas, which can be attributed to the higher surface-area-to-volume ratio of ZnONR. However, there is an optimal AR value for ZnONR. It is likely that higher density and higher aspect ratio of ZnONR prevent NO gas from penetrating into the ZnONR zone, thus leaving some dead zone. Under optimal conditions, the limit of detection (LOD) for the ZnONR based NO gas sensor achieved about 2 ppb at the working temperature of 240 oC. On the other hand, ZnO has been also found as an effective micro-structure modifier for In2O3 film. In order to control the amount of ZnO incorporated into In2O3 film, the molar ratio (r) of Zn2+/In3+ during the co-precipitation step was adjusted. It was found that with optimal r value (r=0.33), the sensor responses at temperatures lower than 200 oC were significantly improved. The enhancement in sensor response could be ascribed to the reduced grain size, from 32.3 to 15.4 nm, of In2O3 nanoparticles (In2O3NP). However, as the r value was further increased, an increase in the grain size of In2O3NP along with the change of the microstructure was noticed. As a resut, the sensing response of the composite film decreased. Under optimal conditions, the LOD for the In2O3-ZnO composite based NO gas sensor achieved 8.8 ppb at the working temperature of 150 oC. In addition, the In2O3-ZnO composite film showed no response to CO gas. ZnO can also be used as a current conducting medium to improve the sensor performance of TiO2 based NO gas sensor. Although the response of the TiO2-nanoparticles’ layer itself towards NO gas was minute, the TiO2NP/ZnO double-layer film showed significant enhanced response. Besides, the enhancement in the sensor response is strongly related to the annealing time; the maximum response to NO was enhanced about 6.2 times as the annealing time was increased from 30 min to 2 hr. With the high sensitivity, the limit of detection (S/N=3) for TiO2/ZnO/Al2O3 electrode can be achieved at 8.8 ppb. The double-layer also showed improved selectivities with respect to NO2 and CO. In addition to the chemiresistive gas sensor, ZnO is also an excellent material for electrochemical sensor application. It has been found that the Ag+ ions can be photochemically reduced and deposited onto the surface of ZnO in the form of AgNP under UV irradiation. Taking advantages of controllable surface area of ZnONR, the distribution and loading amount of AgNP can be easily tailored. Our results showed that the immobilized AgNP exhibited excellent electrocatalytic response to the reduction of hydrogen peroxide. By optimizing the conditions for the photo-deposition conditions ofAgNP, the amperometric determination of H2O2 at -0.55 V (vs. Ag/AgCl/sat’d KCl) gave a limit of detection of 0.9 µM (S/N=3) and a sensitivity of 152.1 mA M-1 cm-2 up to 0.983 mM, with a response time ( t95) of 30 ~ 40 s. The interfering effects from ascorbic acid (AA) and uric acid (UA) were minute. In the DSSC application, the ZnO nanosheets (ZnONS) photoanode was prepared by the direct growth of the layered hydroxide zinc carbonate (LHZC) onto the FTO substrate using CBD methods followed by the pyrolytic transformation of LHZC into ZnONS at 300 oC. Under optimal conditions, the conversion efficiency of ZnONS based DSSC achieved 6.06 % under 100 mW cm-2 illumination, which is much higher than that of ZnO nanoparticles (ZnONP) based DSSCs (2.92%). The better performance of ZnONS based DSSCs can be attributed to the (i) higher effective electron diffusion coefficient of ZnONS (2.15×10-2 cm2 s-1 ) than that of ZnONP (0.87 × 10-2 cm2 s-1), and (ii) higher dye loading on ZnONS (2.66× 10-7 mole cm-2) than that on ZnONP (1.99 × 10-7 mole cm-2). In addition, the performance of ZnONS based DSSC was further improved by depositing a layer of TiO2NP (d~14 nm) using EPD method. Although the effective electron diffusion coefficient (1.81 × 10-2 cm2 s-1) decreased in the TiO2NP/ZnONS hybrid photoande, an enhancement in cell performance (Jsc=19.53 mA cm-2, Voc= 576 mV, FF=0.628, and efficiency=7.07%) was obtained. The enhancement in cell performance can be ascribed to the increased dye loading, which is resulted from an increase in the surface area.