ZnS的低溫相轉移與Ag-In-Zn-S固態溶液的光化學性質

Low-temperature phase transformation of ZnS and photochemical properties of Ag-In-Zn-S solid solution

林伯璋(Po-Chang Lin)

指導教授 ： 李岱洲 ； 華繼中

國立中正大學/工學院/化學工程學系/博士(2013年)

摘要

氫能源與再生能源皆被視為未來能源的重點項目，其中太陽能為具發展空間之產業，結合這兩個領域的光觸媒科技亦成為熱門研究領域之一，如光分解水產氫、二氧化碳轉化成甲醇與甲烷等。其中Ag-In-Zn-S (AIZS)、Cu-In-Zn-S (CIZS)和Ag-Cu-In-Zn-S (ACIZS)等系列之固態溶液光觸媒，皆為可調整能隙值、具高產氫活性、無毒之半導體材料，適當組成之硫化物粉體，能於可見光下與無外加偏壓的粉體系統中運作。然而製備時所需的高熱處理溫度(~ 850 °C)，將破壞導電玻璃基材，限制薄膜製程的開發。此外，光觸媒的能隙值影響陽光照射下的產氫效率，一般認為此類光觸媒最佳能隙值約為2.3 eV。
本文以與AIZS同晶相的wurtzite ZnS (WZ-ZnS)為出發，期望在ZnS不需要相轉移的情況下，降低AIZS的燒結溫度。但ZnS相轉移溫度高達1020 °C，即使WZ-ZnS低溫製程有眾多的文獻報導，於低溫下獲得純相的WZ-ZnS仍相當困難。本研究利用NaOH調控ZnO的含量，可於350 °C ~ 700 °C燒結後，獲得WZ-ZnS晶相。相較於由小粒徑誘發WZ-ZnS的傳統觀點，本研究找到了更關鍵的因素－ZnO存在的關鍵角色。使用WZ-ZnS為起始晶種，對Ag-In-Zn-S形成溫度的改變僅約50 °C，如此高的處理溫度仍不適合直接鍍膜，最終採電泳鍍膜的方式，將已燒結的粉體沉積於基材表面，並於K2SO3與Na2S所組成的犧牲試劑中，測得光電流，電流值為0.137 mA/cm2。
持續地研究發現，調整AIZS系列的元素比例，隨著In含量的提高，不僅仍保持單一相－並未造成AgxInxZnyS2x+y和AgIn5S8分相，更可在能隙值不變動的情形下，大幅提升裝載Pt之光觸媒產氫速率達3.75 L/m2∙h，其產氫速率為In/Ag = 1的5.86倍。與能隙值效應相比，In與共觸媒Pt之間的交互作用更顯著地提升了光觸媒的產氫活性。
為進一步地比較AIZS、CIZS和ACIZS等系列光觸媒的差異，再改用了更簡化與穩定的製程－固相反應法，在同樣的條件下(犧牲試劑、共觸媒、入射光強度與溫度)，進行產氫速率的測試。發現除了能隙值外，導帶與價帶的位置也相當重要。使用紫外光光電子能譜 (UPS) 結合UV-Vis吸收圖譜，鑑定出費米能階、價帶與導帶之絕對位置，提供更多訊息以解釋高產氫速率的原因。
上述研究為基礎，以AIZS、CIZS與ACIZS等硫化物可以調整能隙值的特性，再結合電泳法鍍膜技術。能進一步設計能帶，調整主要光觸媒層與次要隔離層之間的能帶相對位置，未來可望達到抑制光腐蝕與提高光電流的目的。

關鍵字

導帶 ； 價帶 ； 紫外光光電子能譜 ； 相轉移 ； 硫化鋅 ； 纖鋅礦 ； 光觸媒 ； 產氫 ； 水分解

並列摘要

Renewable and hydrogen energy are considered solutions for future energy. Solar energy has attracted increasing attentions. Combining these two fields, photocatalyst technology has become an important research area; for example, hydrogen generation from water splitting, methane synthesis and methanol synthesis etc.
The solid solutions of Ag-In-Zn-S (AIZS), Cu-In-Zn-S (CIZS) and Ag-Cu-In-Zn-S (ACIZS) with adjustable band gaps possess high hydrogen production activities. However, thin film fabrication process was restricted to the high-temperature thermal treatment (850 °C) leaded to the damage of ITO glass substrate. The hydrogen production rate is related to the band gap of photocatalyst. Generally, the optimal band gap was believed approximated 2.3 eV for these materials.
Our investigation stared with wurtzite ZnS, the same phase type with AIZS. The formation temperature of AIZS is expected to be lower than 850 °C when the phase transformation of ZnS is absence. The transition temperature of wurtzite type for pure ZnS is rather high, as 1020 °C. Although there were many articles reporting synthetizing WZ-ZnS at lower temperatures, it was very difficult to obtain WZ-ZnS high-purity. This study controlled the levels of ZnO content in the final products by different amounts of NaOH, WZ-ZnS can be obtained after annealing at 350 °C ~ 700 °C. As opposed to the temperature reduction induced by a smaller particle size, this research pointed out that the existence of ZnO is another critical factor to reduce the processing temperature. Unfortunately, the temperature reduction of AIZS formation is only 50 °C when we utilize WZ-ZnS as a seed to start the reaction. This result drove us to modify our strategy from a chemical bath deposition to the electrophoretic deposition. The annealed powders were deposited on subtracts directly, and the photocurrent of 0.137 mA/cm2 can be obtained successfully in a sacrificial reagent with Na2S and K2SO3.
We also varied the compositions of AIZS by increasing the indium content. The indium-rich samples did not induce phase separation between AgxInxZnyS2x+y and AgIn5S8, instead forming a single-phase solid solution. Compared to the steady H2 evolution rate of Pt-loaded photocatalyst obtained with equal moles of indium and silver, that obtained with In-rich photocatalyst is over 5.86 times higher even though the band gaps were fixed. This study demonstrates the interaction, Pt loading and In2S3, played more critical role than band gap on the hydrogen production rate.
We further synthesized the crystalline powders of AIZS, CIZS, and ACIZS by solid-state reaction. The H2 evolution rates were compared with the same condition (sacrificial reagent, co-catalyst, luminescence intensity and temperature). UV-Vis spectra and Ultraviolet Photoelectron Spectroscopy (UPS) were employed to identify the Fermi level, valence band and conduction band edges of each samples. These data provide additional information for the physical origin of high-efficiency photocatalyst.
Based on the aforementioned findings, the property of band gap adjustable in AIZS, CIZS and ACIZS can be used to tune the relative levels of energy band between the photocatalyst layer and the barrier layer. This strategy can be applied to restrain the photocorrosion and enhance the photocurrent in the future.

並列關鍵字

Conduction band ； Nanoparticle ； Ultraviolet photoelectron spectra ； Thermodynamic stability ； Phase transformation ； Hexagonal ZnS ； Wurtzite ZnS ； Solar energy ； Water splitting ； Photocatalyst ； Hydrogen evolution ； Valence band

參考文獻

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