電鍍電位與鍍液離子濃度和pH值對電鍍碲化鉍的影響

Abstract

碲化鉍(Bi2Te3)是目前在中低溫環境中最為廣泛應用的熱電材料，本實驗將探討電鍍參數對鍍層的影響，分別為電鍍電位、鍍液離子濃度、鍍液pH值，與鍍液中陰離子的種類。在硝酸系統中，電鍍電位在-0.475 V < E < -0.1 V (vs. SCE)，只有在-0.1 V下可獲得較緻密的鍍層，其餘電位下則為樹枝狀，而所有電位下鍍層碲含量皆低於劑量比。固定鍍液中Bi3+為11 mM，改變HTeO2+濃度由8~20 mM，發現鍍層中碲含量與碲離子濃度呈正相關，故可藉由鍍液離子濃度的調配，獲得劑量比(Bi2Te3)碲化鉍鍍層。針對最佳劑量比鍍層之鍍液，探討鍍液pH值與電鍍電位(-0.1 V < E < +0.02 V)對鍍層的影響，發現電鍍電位主要改變鍍層顆粒大小，並不影響鍍層成分，而鍍液pH值也不影響成分，但對鍍層顏色、表面形貌以及附著性均有很大的影響。在-0.1 V以及0.7 M硝酸條件下，鍍層外觀為灰色，表面形貌為針狀，附著性較佳；而相同電位，在1.5 M硝酸下鍍層則為黑色、顆粒狀，附著性較差。使用鹽酸來取代硝酸可提高TeO2的溶解速率，使鍍液配製更為便利。在較高的電鍍電位，鹽酸系統中獲得的鍍層成分會偏離劑量比，意即其電位操作區間較硝酸系統小，但在-0.02 V與0.35 M鹽酸下可獲得平坦光亮緻密的碲化鉍鍍層。 由循環伏安掃描結果得知，當溶液中只有其中一種離子時，鉍離子的還原峰會隨著硝酸與鹽酸濃度增加往負方向偏移，碲離子的還原峰則往正方向偏移。而兩種離子的混合溶液還原峰則隨著硝酸濃度增加往正偏移，但會隨著鹽酸濃度增加往負偏移。在鹽酸系統中，鉍離子會與氯離子錯合形成結構穩定的BiCl4 使得鉍離子的還原比在硝酸系統中困難，而碲離子則會與氯離子形成TeCl62-，並形成離子橋促進電化學反應，使碲離子的還原比在硝酸系統中容易。無論在硝酸還是鹽酸系統中，碲化鉍的還原起始電位皆比單獨成分的鉍和碲來的正，可知有pure underpotential deposition (PUD)的發生。

Bi2Te3 is the thermoelectric material most widely used at low temperature ranges. The effects of electrodeposition parameters on deposited layer, such as potential, electrolyte concentration, electrolyte pH, and anion species, is discussed in this study. The Bi-Te film plated at -0.1 V presents a dense morphology; however, the others show a dendrite structure within the potential range of -0.475 V < E < -0.1 V (vs. SCE) in the nitric acid system. By varying the electrolyte HTeO2+ concentration from 8 to 20 mM at a constant Bi3+ concentration of 11 mM, the deposits with Te contents ranging from 45 to 67 at% can be made, specifically the deposit Te content varies linearly with the electrolyte HTeO2+ concentration. The electrolyte, in which the deposit close to stoichiometric Bi2Te3 was electroplated, was further subjected to study the effect of the potential (-0.1 V to +0.02 V) and electrolyte pH. It is found that the potential markedly influences the grain size but hardly affects the composition of the Bi-Te deposits. In contrast, the solution pH strongly influences the color, morphology, and adhesion of the deposits. The Bi2Te3 plated at -0.1 V in the solution containing 0.7 M HNO3 shows a gray color, needle-like morphology with sufficient adhesion. In contrast, the deposit plated at -0.1 V in 1.5 M HNO3 displays a black color, loose granular morphology with inferior adhesion. The preparation of electrolyte becomes easier due to the dissolution rate of TeO2 can be increased using hydrochloric acid instead of nitric acid. The deposit composition becomes Te-rich at higher applied potentials in the HCl system. The applicable potential range of the HCl system is smaller than that of the HNO3 system although a flat, dense Bi2Te3 film can be obtained at -0.02 V in 0.35 M HCl. Cyclic voltammetry results show that with increasing HNO3 and HCl concentration the reduction potential of BiIII shifts toward negative direction, but the reduction potential of TeIV shifts along positive direction in the solution solely composed of BiIII or TeIV. With increasing HNO3 concentration and decreasing HCl concentration in the solution containing of BiIII and TeIV, the reduction peak shifts along positive direction. In the HCl system, the reduction of BiIII becomes more difficult than in the HNO3 system because stable complexant ion BiCl4- forms but the reduction of TeIV becomes easier due to the fact that Cl- ion bridge promotes electrochemical reaction. Owing to the pure underpotential deposition (PUD), the reduction potential of the solution containing BiIII and TeIV is more positive than the solution only containing BiIII or TeIV.