在超重力系統中製備碘化銀奈米粉體

Abstract

奈米碘化銀具有特殊的光、電及催化性質使其在工業上具有很大的用途。奈米碘化銀常見的製備方法有:逆微乳膠法和反應沉澱法。逆微乳膠法能藉由調控油、水及界面活性劑的比例來調控乳胞的大小，然而逆微乳膠法受限於產量及成本，因此放大仍有困難。反應沉澱法則因傳統反應器混合效率不佳而不易得到均勻且單分布的粒子。為了改善微觀混合，超重力反應沉澱技術已經被使用來製備奈米粒子，其是利用強大的離心力來增進質傳速率和微觀混合。 本研究使用超重力系統製備碘化銀，在嘗試了不同的保護劑後挑選出PVP、PEG、BPEG進行系統式的實驗，以反應物濃度、保護劑濃度、超重力旋轉盤轉速為實驗參數製備碘化銀，並以動態光散射儀、電子顯微鏡分析碘化銀之粒徑。 實驗結果發現當反應物濃度為0.20 M，保護劑PVP濃度 10.00 g/L，將所得之碘化銀再分散於去離子中可得到數目平均粒徑47.6 nm，體積平均粒徑66.3 nm的粒子，SEM圖則顯示出粒子乾燥後容易聚集；當反應物濃度為0.20 M保護劑PEG 5.00 g/L時，若將所得之碘化銀再分散以懸浮液方式儲存，可得到數目平均粒徑約134.8 nm的粒子，然體積粒徑成雙峰值，且有大型團聚物出現。以BPEG為保護劑，雖可得到比PEG為保護劑略小的粒子，但在超重力系統的作用下，BPEG非常容易產生泡沫，對產物後續分離純化有不良影響。 在轉速效應方面，當反應物濃度為0.20 M 保護劑PVP濃度為10.00 g/L時，在500 rpm 、2000 rpm和 4000 rpm得到相似的粒徑分布;若使用反應物濃度0.20 M保護劑濃度5.00 g/L PEG或5.00 g/L BPEG探討轉速對粒徑的影響則可看出隨著轉速的提高，大型團聚物的比例則逐漸下降，顯示高轉速有助於提升混合效率。 在常壓下當溫度高於147℃時β/γ-AgI會轉變成α態，然而本研究使用DSC量測碘化銀一階相變化溫度，發現使用PVP製備的碘化銀相變化溫度Tβ/γ→α往高溫方向位移，而Tα→β/γ往低溫方向位移，形成一個大的遲滯迴圈，且隨著粒徑大小與保護劑量而改變。α-AgI是廣為人知的典型超離子導體，其電導度可高達1Ω-1cm-1，若能使α-AgI穩定存在室溫下，將可擴大其在電子產品上的運用，如染料敏化太陽能電池和感應器。

The AgI nanoparticles have unique optical, electrical and catalytic properties that make it useful in industrial applications. The common methods for preparing AgI nanoparticles include reverse microemulsion and reactive precipitation. The size of particle synthesized by reverse microemulsion is tunable by controlling the ratio of the water, oil and surfactant. However, this method is difficult to scale up due to low production rate and high cost. It is also difficult to obtain uniform particles through the reactive precipitation method using a stirred tank reactor because of the poor mixing efficiency. To improve mixing efficiency, a high-gravity (Higee) reactive precipitation method has been introduced, which is the technique using centrifugation force to enhance the mass transfer rate and micromixing. In this research we used the Higee technique to prepared AgI. After trying various protecting agents, we chose PVP, PEG and BPEG as protecting agent to proceed a systematic study. The effects of operation variables, including reactant concentration, protecting agent concentration, and rotation speed were investigated. Finally we used particle size analyzer and an electron microscope to analyze the size of prepared AgI. Our results showed that when the reactant concentration is 0.20 M using 10.00 g/L PVP as protecting agent, the number mean particle size of re-dispersed slurry was 47.6 nm and the volume mean size was 66.3 nm, but the SEM image showed that agglomerates formed after drying. When the reactant concentration is 0.20 M using 5.00 g/L PEG as protecting agent, the number mean particle size of re-dispersed slurry was about 134.8 nm, and the volume particle size distribution had two peaks due to agglomeration. When using BPEG as protecting agent the particle size was small than the one using PEG, but high rotation speed result in foaming, which would influence the consequent separation and purification steps. As to the rotation speed effect is concerned, no matter the rotation was 500, 2000 or 4000 rpm, we obtained similar volume particle size distribution(PSD) and number particle size distribution when the reactant concentration was 0.20 M using 10.00 g/L PVP as protecting agent. When the reactant concentration was 0.20 M using 5.00 g/L PEG or 5.00 g/L BPEG as protecting agent, the rotation speed had a remarkable influence on the volume PSD. As the rotation speed was increasing, the percentage of agglomerated particles was decreasing. This showed that higher rotation speed enhanced the mixing efficiency. It is well known that β/γ-AgI undergoes a phase transition into α-phase when the temperature is above 147℃ at normal pressure. In our research, when using DSC to study the first order phase transition temperature of the prepared AgI, we found that the phase transition temperature Tβ/γ→α shifts to higher temperature and Tα→β/γ shifts to lower temperature when using PVP as protecting agent. The two temperatures form a large hysteresis loop which was related with particle size and protecting agent. It is well known that α-AgI is a typical superionic conductor with conductivity up to 1 Ω-1cm-1. If we can stabilize α-AgI at room temperature, we can broaden its applications to many electronic products such as solar cell and sensor.