掃描電化學顯微鏡於二維材料奈米介面電子轉移動力學之研究

Abstract

本文之目的為利用掃描電化學顯微鏡對二維電催化材料在固液相介面之奈米級結構活性之研究。利用原子力顯微鏡結合掃描電化學顯微鏡，進行定量原位之介面電子轉移動力學探測，並達到高解析度之空間解析。本研究中運用並討論掃描電化學顯微鏡之兩種主要模式：回饋模式用於探測外層(弱耦合、非催化)電子轉移，基材生成與尖端收集模式應用於檢測內層(強耦合、催化)電子轉移，特別是可用於析氫反應之研究。

本文將其分為七個章節，其中包含兩章緒論與五章研究結果之深入探討。第一章闡述了研究背景、動機與目標，強調掃描電化學顯微鏡在能源相關奈米材料表徵鑑定之應用。第二章將闡述實驗與計算方法，從二維材料生長到掃描電化學顯微鏡量測細節。 第三章揭示了在析氫反應過程中氫奈米氣泡之成核、生長及脫附於石墨烯表面之觀察結果。本文視覺化了氣泡動態於不同石墨烯基材，例如高定向熱解石墨與於石墨烯覆蓋碳化矽。基材生成與尖端收集模式展示了氫氣氣泡之析出是受局域性活性位點與表面性質(邊緣、褶皺與親水性)影響。高定向熱解石墨烯之疏水性將使氣泡大小、覆蓋面積與生命週期提升。反之，親水性的石墨烯覆蓋碳化矽則會產生較小氣泡且容易從表面脫離。結果進一步揭示了石墨烯覆蓋碳化矽較高定向熱解石墨烯對於產氫反應具較優之電化學活性。

第四章揭示二硒化鎢作為電催化劑其基面活性與介面電子轉移之火山型行為。本研究發現最佳厚度為四層，由於穿隧勢壘與特定電子態密度之間的交互作用，有利於非絕熱電子轉移。奈米級電子映射和微米級電化學裝置展示了少層較單/雙層二硒化鎢能產生更快之介面電子轉移與更高的析氫反應活性。其更快的動力學與更低的析氫反應過電位歸因於費米能階附近的更高電子態密度，此外並提高電極-電解質界面處平面外電子轉移之可能性與改善平面內電荷傳輸。本研究表明透過調整二硒化鎢厚度，其基面可以被電化學活化，以用於非催化與催化電子轉移。

第五章討論了半導體二硒化鎢與金屬二硫化鈮異質結構之功能化，二硫化鈮之高功函數提高了二硒化鎢之基面電化學活性。此結果可以歸因於豐富的局域性電子狀態密度與強電子耦合，從而增強界面電子轉移動力學與電催化析氫反應。原子力顯微鏡之力-體積映射揭示二硒化鎢與金屬二硫化鈮之間的強黏附性，確保二硒化鎢在酸性環境中之穩定性。此外，在本實驗中觀察到二維二硒化鎢之層間電化學屏蔽行為，該行為選擇性地鈍化二硫化鈮表面的自然氧化。在此異質結構中實現了較低的半導體/金屬能帶偏移，這意味著肖特基勢壘高度降低，接觸電阻降低，從而提高了電荷注入效率。結果表明，二硒化鎢/二硫化鈮異質結構具有更高的電化學析氫性能和更長的穩定性。並證明利用二硫化鈮作為金屬載體，二硒化鎢基面既可用於非催化電子轉移，也可用於電催化析氫。

第六章和第七章簡要探討了層間堆疊結構和合金化分別如何影響各種二維過渡金屬二硫屬化物(如二硫化鉬、二硒化鉬、二硫化鎢、二硒化鎢與硫硒化鎢合金)之非絕熱電子轉移動力學速率。我們觀察到對應3R相之AB堆疊較對應2H相之AA'堆疊具有更高的電化學活性；此種結果很可歸因於更強的層間電子耦合，從而改善了平面內和平面外的電荷傳輸特性。同時，單層硫硒化鎢合金比二硫化鎢和二硒化鎢具有更好的電化學性能，這是由於豐富的活性位點和提高的電導率。

本論文涵蓋了二維材料的各種功能化，透過調控其形貌、異質結構、扭轉角和合金化，以及掃描電化學顯微鏡如何作為多功能原位電分析工具來全面研究界面物理和奈米級電化學，從而實現表面結構與局部電化學性質之間的直接關聯，且實用於各種應用。

The purpose of this thesis is to present the progress made in the nanoscale structure–electrochemical activity studies of two-dimensional (2D) electrocatalytic materials across solid–liquid interfaces with scanning electrochemical microscopy (SECM). Utilizing atomic force microscopy (AFM) coupled with SECM, the interfacial electron transfer dynamics is quantitatively probed in situ and spatially resolved at the nanoscale with high resolution. Here, the two main modes of SECM are thoroughly employed and discussed: SECM feedback mode is used to probe the outer-sphere/weak-coupling/non-adiabatic/non-catalytic electron transfer, while SECM substrate generation and tip collection (SG/TC) mode is used to probe the inner-sphere/strong-coupling/adiabatic/catalytic electron transfer, particularly for the hydrogen evolution reaction (HER).

This thesis is divided into eight chapters, which consists of two introductory chapters, followed by five chapters discussing the main findings, and one concluding chapter. Specifically, Chapter 1 describes the background, motivation, and objectives of the study, emphasizing the state of the art of the applications of SECM in the characterizations of nanomaterials for energy-related applications. The technical sections covering the experimental and computational methods are presented in Chapter 2, from the growth of 2D materials to the details of SECM measurements.

Chapter 3 reveals the observation of hydrogen nanobubbles nucleation, growth, and departure from the surface of graphene during HER. We visualize the gas bubble dynamics on different graphene substrates, i.e., highly oriented pyrolytic graphite (HOPG) and few-layer graphene on SiC (FLG/SiC). SECM SG/TC mode shows that the hydrogen bubble evolution is influenced by the local active sites and surface properties (edges, wrinkles, and wettability). Hydrophobic HOPG increases the size, coverage, and the lifetime of the bubbles. On the other hand, hydrophilic FLG/SiC generates smaller bubbles which tend to easily depart from the surface. The results further show that FLG/SiC is electrochemically more active than HOPG for hydrogen generation.

Chapter 4 unravels the basal plane activity and volcano-type behavior of interfacial electron transfer at WSe2 electrocatalysts. We find the optimal thickness, which is four-layers, at which the nonadiabatic electron transfer becomes favorable due to the interplay between the tunneling barrier and the layer-specific electronic density of states (DOS). Nanoscale electrochemical mapping and microscale electrochemical devices show that the few-layer WSe2 generates faster interfacial electron transfer and higher HER activity than monolayer/bilayer WSe2. Faster kinetics and lower HER overpotential is assigned to the higher DOS near the Fermi level that promotes the probability of out-of-plane electron transfer at the electrode–electrolyte interface and improved in-plane charge transport. We show that WSe2 basal plane can be electrochemically activated for both non-catalytic and catalytic electron transfer by tailoring the thickness.

Chapter 5 discusses the functionalization of semiconductor/metal WSe2/NbS2 heterostructures, in which the high work function metallic NbS2 improves the basal plane electrochemical activity of WSe2. This outcome is likely assigned to the enriched local DOS and strong electronic coupling, resulting in an enhancement of interfacial electron transfer kinetics and electrocatalytic hydrogen production. AFM force–volume mapping reveals a strong adhesion between NbS2 and WSe2, ensuring the stability of atomically thin WSe2 in a harsh acidic environment. Further, we observe a layer-dependent electrochemical screening behavior of 2D WSe2 which selectively passivates the surface of NbS2 from native oxidation. A lower semiconductor/metal band offset is realized in WSe2/NbS2 heterostructures, implying a reduced Schottky barrier height and lower contact resistance, resulting in an improved charge injection efficiency. The results show that WSe2/NbS2 heterostructures achieve higher electrochemical HER performance with prolonged stability. We demonstrate that the basal plane of WSe2 can be activated for both non-catalytic electron transfer and electrocatalytic HER by utilizing NbS2 as the metal support.

Chapter 6 and 7 briefly explores how interlayer stacking configurations and alloying, respectively, influences the rate of nonadiabatic electron transfer kinetics at various 2D transition metal dichalcogenides (TMDs), i.e., MoS2, MoSe2, WS2, WSe2, and WSSe alloy. We observe that the AB stacking, which corresponds to the 3R phase, achieves higher electrochemical activity than the AA’ stacking, which corresponds to the 2H phase; this outcome is likely assigned to the stronger interlayer electronic coupling that improves the in-plane and out-of-plane charge transport properties. Meanwhile, monolayer WSSe alloy performs better electrochemical activity than WS2 and WSe2 possibly due to the enriched active sites and increased conductivity.

In summary, this thesis covers various structural functionalization of 2D materials—i.e., by tailoring their morphology, thickness, heterostructures, twist-angle, and alloying—to modulate the electronic structures, resulting in a modified local electrochemical activity. AFM-SECM is carefully utilized to probe the in situ nanoscale electrochemistry of these modified 2D materials, enabling direct correlation between the nanoscale features and local electrochemical properties, which is useful for a wide variety of applications.