半導體合金與異質結構在光電轉換效率之提升

Abstract

由於組成可調式的能帶結構，三元硫族化物的奈米晶體為吸收太陽能的可見光波段提供了一個新的契機。因為這些具有不同組成的奈米晶體系統化的組合，使得材料可以有效地捕捉並轉換可見光區的光子，進而大幅提高載子的使用率而提升光電轉換效率。另一方面，不同半導體間的相對能帶結構將有助於顯著的載子分離效果，並進而提高光電轉換與光催化應用的效率。 在此博士論文的第一部分，我們首次集合了五種不同組成的Cd1-xZnxSe奈米棒，其中x分別為0、0.35、0.54、0.61和1，使得材料可以吸收整個可見光波段。在這次的研究中，樣品主要是藉由陽離子交換法合成。經由調控Cd2+和Zn2+之間的莫耳比例，我們可以將Ag2Se 奈米棒置換成Cd1-xZnxSe 奈米棒，並且能精準的調控Cd2+和Zn2+的化學計量，也因為這個組成調控的能帶結構，使得不同化學計量的Cd1-xZnxSe 奈米棒可吸收不同波段的光源，進而將材料的吸收波段擴張至整個可見光波段。與單獨組成的樣品相較，各種組成集合的樣品在白光下可以產生更為優越的光催化效果。這個優越的光催化效應主要來自於各種不同化學計量的Cd1-xZnxSe 奈米棒可分別吸收不同波段的可見光，因為當各種不同組成的Cd1-xZnxSe 奈米棒集合時，便可以使光催化反應作用的波段涵蓋整個可見光區，使光催化反應的作用效率大幅提升。在循環測試中，樣品展現出了高度穩定度。而在實際太陽光的照射實驗中，樣品在實際應用中也有很好的光催化反應與光吸收效率。在這次的研究中成功地完成了複雜的半導體集合反應並且大幅提升了樣品的光吸收與光催化效率。 在論文的第二部分，我們成功地以簡便的濕式化學沈積法製備具高均勻度的 GaOOH 奈米棒。為了進一步提升 GaOOH 之載子分離效率，使用化學還原法將 Au 奈米顆粒沈積於 GaOOH 奈米棒表面，以形成 GaOOH/Au 奈米異質結構，藉由調整 HAuCl4 的濃度，可有效控制所沈積於 GaOOH 奈米棒表面 Au顆粒的數量。由於 GaOOH 與 Au 相對能帶結構的關係，光激發電子會由 GaOOH 端轉移至 Au 端，並留下大量電洞於 GaOOH 內，以達成電子電洞對的分離。由穩定態螢光光譜的量測結果確認 Au 顆粒的沈積確實能提升 GaOOH 奈米棒的載子分離特性，其中以 Au 數量為 1.0 mol%之 GaOOH/Au 樣品具有最佳的載子分離效果，利用時間解析螢光光譜技術，則可量化光激發電子由 GaOOH 轉移至 Au 端的速率常數。GaOOH/Au 樣品展現出較優異的光催化甲醇氧化效能，此乃因為 GaOOH/Au 具有顯著的載子分離特性所致。由此研究所製備出的具 Au 顆粒接枝的 GaOOH 奈米棒，預期可在光催化水分解與光激發電子儲存等用途發揮相當高的效能。 最後，GaOOH/Au奈米異質結構成功地與市售的Pt/C結合，在甲醇燃料電池中作為陽極進行光催化反應。其中，Pt/C‒GaOOH/Au-1.0展現了最高的甲醇氧化效率，其效率在太陽的照射下約較純的Pt/C提高了70.8%，展現了金屬/半導體混合奈米晶體在直接甲醇燃料電池中作為光催化陽極的潛力。我們推測直接甲醇燃料電池的作用效率大幅提升的原因有兩個，其一為GaOOH/Au奈米異質結構在照光後可以提供了大量的電洞參與甲醇氧化反應；其二則是因為GaOOH/Au奈米異質結構的表面會產生了許多氫氧基，使CO在鹼性電解液中可被加速移除而降低CO的毒化效應，進而提升直接甲醇燃料電池的作用效率。

Due to the composition-tunable band edge, the ternary chalcogenide nanocrystals offer new opportunities to harvest light energy in the entire visible region of solar spectrum. The photoconversion efficiency is promoted due to the effective carrier utilization given by systematic combination of these nanocrystals constituting gradient alloyed structures, which can provide synergy for capturing and converting a wide array of photons in the visible region. On the other hand, the difference in band structure of semiconductor heterostructures conduces to a remarkable charge separation property which is beneficial to light conversion and photocatalytic applications. In the first part of this dissertation, we demonstrated for the first time that the assembly of Cd1-xZnxSe nanorods (NRs) with five compositions (x=0, 0.35, 0.54, 0.61, 1) may absorb the whole visible spectrum in a gradient fashion for photoconversion applications. The samples were prepared by conducting cation exchange reactions on Ag2Se NRs with excess Cd2+ and Zn2+ ions. By modulating the molar ratio of Cd2+ to Zn2+ employed, the composition of the resulting Cd1-xZnxSe NRs can be delicately controlled. Because of the tunability of band edge with stoichiometry, Cd1-xZnxSe NRs of varying compositions absorbed light at different wavelength regions, which spanned almost the entire visible spectrum. As compared to the individual constituent NRs, the NRs assembly exhibited higher photocurrent generation as well as superior photocatalytic performance under white light illumination. This superiority emanates from the composition-gradient configuration that significantly improves the light harvesting efficiency by absorbing almost the whole visible spectrum of the incident white light. The full visible photon harvesting of the NRs assembly was validated by the photocurrent action spectrum which showed spectral accordance with the absorption spectrum. The recycling test manifests that the NRs assembly displayed substantially high stability during its use as photocatalyst. Furthermore, the result of performance evaluation under natural sunlight reveals that the present NRs assembly can be used as practical rainbow photocatalysts which may effectively harvest energy from sunlight. The demonstration from this work may facilitate the use of sophisticated assembly of semiconductor nanocrystals in relevant photoconversion processes where the effectiveness of photon harvesting is determinant. Secondly, GaOOH NRs were prepared via a facile chemical precipitation approach. In order to increase the charge separation efficiency of GaOOH NRs, Au nanoparticles (NPs) were deposited on the surface of GaOOH NRs through a chemical reduction method. The amount of Au NPs deposited on GaOOH NRs could be readily controlled by adjusting the initial HAuCl4 concentration. Owing to the difference in band structures between GaOOH and Au, the photoexcited electrons of GaOOH would preferentially transfer to Au, leaving photogenerated holes in GaOOH to achieve charge carrier separation. The result of steady-state photoluminescence analysis confirms the enhancement in charge separation efficiency for Au-decorated GaOOH NRs (GaOOH/Au). Among different samples, GaOOH/Au with 1.0 mol% Au showed the most significant charge separation efficiency. Time-resolved photoluminescence spectra were measured to quantitatively analyze the electron transfer between GaOOH and Au. The as-synthesized GaOOH/Au heterostructures exhibited a superior photocatalytic performance toward methanol oxidation, attributable to the effective charge separation that took place at the interface of GaOOH/Au. The results of photocatalytic methanol oxidation were in good accordance with those of charge carrier dynamics. Finally, GaOOH/Au heterostructures were incorporated with commercial Pt/C and used as the anode photocatalysts in half-cell of direct methanol fuel cell (DMFC) for the first time. The composite catalyst of Pt/C‒GaOOH/Au with 1.0 mol% Au showed the best methanol oxidation activity, about 70.8% improvement over pristine Pt/C catalyst under light irradiation, demonstrating the promising potential for metal/semiconductor hybrid nanocrystals as the anode photocatalyst in DMFC. GaOOH/Au heterostructures may enhance the performance of DMFC by providing abundant photogenerated holes for participation in anodic methanol oxidation under light illumination, and minimizing the CO poisoning effects because of the abundant hydroxides formed on the surface of GaOOH/Au, which speed up the removal of the CO-like species in the alkaline electrolyte.