利用原子沉積及化學法製備光觸媒及其應用於光催降解與光催產氫之研究

Fabrication of Photocatalysts by Atomic Layer Deposition and Chemical Methods for Photocatalytic Degradation and Hydrogen Evolution

劉維斯(Wei-Szu Liu)

指導教授 ： 彭宗平

國立清華大學/工學院/材料科學工程學系/博士(2018年)

摘要

本論文係研究光觸媒材料的製備，且根據其光照產生電子電洞可用以進行氧化還原反應的特性，將其應用於光催產氫(photocatalytic H2 evolution )及光催有機物降解 (photocatalytic organic degradation)。本論文根據不同的材料系統區分為三個部分。
第一個部分是研究如何有效提升石墨相氮化碳(g-C3N4)的光催產氫率。g-C3N4為少見的無機光觸媒材料。由於其製備簡單、地球含量多、二維(2D)結構等優點，近年來備受矚目。然而，其快速的電子電洞對的再結合率導致極低的光催效率，所以本論文主要從三個方向進行改質，包含多孔結構、異質接面、助催化劑沉積。首先，多孔的結構可藉由增加反應面積、提高光的吸收，以及抑制電子電洞對的結合來提升光催效率，所以利用氯化銨於高熱會產生大量氣體的方式，在熱縮聚合成過程中，於g-C3N4片狀物上形成大量的孔洞，進而提升48%的太陽光產氫率。而原子沉積法(atomic layer deposition, ALD)是一項薄膜技術，可以將前驅物傳輸至複雜奈米結構內，在其表面形成薄膜，且可藉由ALD層數控制其薄膜厚度。所以本論文採用ALD系統，均勻成長TiO2薄膜於多孔g-C3N4片狀基板上，藉此形成2D/2D異質接面。由於大面積的接觸，所以2D/2D異質接面可以有效傳輸電子，進而提升電子電洞對的分離率，而且，適當的 TiO2薄膜厚度(~ 13 nm, 180 層數之TiO2)可提供更有效電子傳輸。另外，以ALD製程將白金奈米顆粒沉積於TiO2@P-g-C3N4複合材料上，會加快光電子遷移至表面的速率，進而提升光催產氫率。綜合三個方向的改質後，可以大幅度的提升g-C3N4奈米片的太陽光產氫率約1500倍。
第二部分則是研究五氧化二鉭(Ta2O5)材料。因為其傳導帶的最小值(conduction band minimum, CBM)高於TiO2，理論上其產氫量應該大於TiO2；然而其能帶遠大於TiO2，不利於光催產氫。所以第二部分區分為兩個主軸。第一主軸為確認Ta2O5是否在UV光的照射下，產出比TiO2還要高的產氫量。由光催產氫結果發現，在UV光的照射下，Ta2O5微米顆粒(12.4 m2/g)的光催產氫率相似於TiO2奈米顆粒(P25) (39.0 m2/g)，顯示在單位面積上的產氫率，Ta2O5遠大於TiO2。所以為了進一步提升其光催效率，本論文致力於合成出Ta2O5中空纖維內含中孔互連奈米管，實驗結果發現藉由中孔奈米管的結構，其Ta2O5的產氫率比P25高出2.1倍。然而，Ta2O5的能隙太大，不利於光催產氫，所以第二主軸為研究如何在降低其能帶之情況下，同時提高其光催產氫率。本論文利用氮摻雜(N-Ta2O5)及真空退火處理(獲取灰色Ta2O5)降低電子電洞的再結合速率，提高光催產氫率。
利用不同的退火溫度對商用的Ta2O5進行氮化(nitridation)合成氮摻雜的Ta2O5 (N-Ta2O5)。隨著溫度的升高，氮摻雜的濃度也隨之提高，其中摻雜的氮離子會與Ta2O5的氧離子置換，且置換反應主要是發生在Ta2O5表面。而氮摻雜會在Ta2O5內會產生一些空缺，造成能隙窄化，進而提升Ta2O5在太陽光照射下的產氫能力。然而，當表面形成Ta3N5結晶相時，因為電子電洞對容易在此處結合，光催產氫率因而下降。所以，相較於Ta2O5在太陽光照射下無H2產出，在沒有Ta3N5存在的情況下，大量的氮摻雜可以有效提升Ta2O5的太陽光產氫率。
利用後段真空退火，除了使無晶相的Ta2O5結晶化外，還能於表面形成大量的氧空缺，形成灰色的Ta2O5。相比之下，在空氣下退火則會形成白色的Ta2O5。表面氧空缺的存在，除了會在Ta2O5的表面形成無晶相外殼外，同時還會降低電子電洞對再結合率，且增強可見光的吸收能力。因此，灰色的Ta2O5光催產氫率比白色Ta2O5高出48%。
總之，Ta2O5的產氫量高於TiO2，且改質後的Ta2O5能進一步提升光催產氫率，所以Ta2O5可藉由ALD技術，取代TiO2與P-g-C3N4結合，形成另一個有潛力的光產氫複合材料。
第三個部分則是架設連續流的光降解系統。由於現在供水壓力的升高，其中之一的解決方案，就是利用連續流光催系統大量純化雨水成中水，供小家庭使用。利用ALD能夠準確控制鍍膜材料的厚度與均一性的特性，將氧化鋅(ZnO)薄膜鍍在多孔的聚砜中空纖維(PSF hollow fiber)上，製作固定床的奈米反應器。此外，利用光還原法鍍奈米銀顆粒在上面，除了抗菌外還可以提升25%的效率。與此同時，藉由光催染料降解實驗來評估ZnO@PSF奈米反應器應用在水淨化系統的可行性。

關鍵字

光觸媒 ； 原子沉積法 ； 五氧化二鉭 ； 石墨相氮化碳

並列摘要

This dissertation studies the fabrication and the characteristics of several photocatalysts which are applied to photocatalytic H2 evolution and photocatalytic organic degradation. According to different material systems, this dissertation is divided into three parts.
The first part is to study the improvement of photocatalytic H2 production rate by g-C3N4. g-C3N4, a metal-free photocatalyst, has become a popular material because of several advantages, such as easy fabrication, abundance, and 2D structure. However, it exhibits poor photocatalytic efficiency due to its fast recombination of charge carriers. Thus, there are three directions for modification of g-C3N4, including construction of heterojunction, formation of porous structure, and deposition of co-catalyst. First, because of the addition of NH4Cl, numerous pores were formed in P-g-C3N4 sheets by gas release during the thermal treatment. This would enlarge specific surface, trap more light, and reduce recombination rate of charge carriers, resulting in higher photocatalytic efficiency. Atomic layer deposition (ALD) is a thin film process in which the precursors can pass through the complicate nanostructure of substrate and form thin film on it. In addition, the thickness of thin film can be controlled by the cycle number. Second, with porous g-C3N4 (P-g-C3N4) sheet as a substrate to deposit TiO2 by ALD, it resulted in the formation of a 2D/2D heterostructure. It exhibited fast transfer of electrons due to interface contact, leading to higher separation rate of charge carriers. In addition, a suitable thickness (~13 nm，180 ALD cycles of TiO2) provided more efficient transport of electrons. Finally, platinum (Pt) nanoparticles were deposited on TiO2@P-g-C3N4 by ALD as a co-catalyst to enhance the migration of photoelectrons to surface, leading to even higher H2 generation rate. The synergy of the three modifications could greatly enhance the photocatalytic H2 evolution of g-C3N4 by about 1500 times.
The second part is modification of Ta2O5. Because its conduction band minimum (CBM) is higher than that of TiO2, Ta2O5 theoretically is expected to generate more H2. However, its bandgap is much larger than that of TiO2, which is a disadvantage for photocatalysis. Thus, this part is separated into two directions. The first part is to confirm whether Ta2O5 can indeed exhibit higher H2 generate rate than TiO2 under UV light illumination. Based on the photocatalytic test, Ta2O5 microparticles showed similar photocatalytic H2 generation rate to TiO2 nanoparticles (P25), which implies that the H2 generation rate per unit area of Ta2O5 is much higher than that of P25. In order to further improve the efficiency, Ta2O5 hollow fibers with internal interconnected mesoporous nanotubes were fabricated, and its photocatalytic H2 generation rate was formed to be 2.1 times higher than that of P25 due to the existence of mesopores. Although Ta2O5 shows better performance than TiO2 under UV light irradiation, its bandgap is too big for high photocatalysis efficiency. Thus, the second part is to study how to achieve bandgap narrowing and higher photocatalytic H2 evolution rate at the same time. In this study, nitrogen doping and vacuum annealing methods were adopted to modify Ta2O5, both of which could enhance the photocatalytic H2 generation rate by lowering the recombination rate of charge carriers.
N-Ta2O5 powder was prepared by annealing commercial Ta2O5 powder in NH3 at different temperatures. With increasing the nitridation temperature, the amount of nitrogen doping increased. During the nitridation process, some oxygen was substituted by nitrogen, and the substitution occurred preferentially on the surface of Ta2O5. Nitrogen doping in Ta2O5 induced formation of vacancies and narrowing of band gap, resulting in higher hydrogen production from water splitting than pure Ta2O5 under solar light illumination. However, if Ta3N5 forms, it can act as recombination center of charge carriers that leads to lower photocatalytic activity. Therefore, compared to pure Ta2O5 which did not produce H2 under solar light irradiation, N-Ta2O5 treated at 650 oC showed the highest H2 evolution because of more nitrogen doping and no presence of Ta3N5.
Post-vacuum annealing caused not only crystallization of amorphous Ta2O5 but also formation of surface oxygen vacancies, leading to formation of gray Ta2O5. White Ta2O5, formed by annealing in air, was used as a control specimen. The existence of surface oxygen vacancies resulted in formation of a disordered shell, lower recombination rate of charge carriers, and more absorption of visible light. Thus, gray Ta2O5 exhibited 48% higher photocatalytic hydrogen rate than white one.
In summary, pure Ta2O5 exhibited better photocatalytic performance than TiO2, while the modified Ta2O5 could generate more amount of H2 than pure Ta2O5. It is expected, therefore, that using Ta2O5 to replace TiO2 for deposition on P-g-C3N4 by ALD would be another potential composite for improved photocatalytic H2 production.
The third part is to fabricate a continuous-flow photocatalysis system. Because the stress of water supply has become higher, one of the solutions is to purify a large amount of rainwater to recycled water by continuous-flow photocatalysis in each home. Hence, ZnO was deposited on porous polysulfone (PSF) hollow fiber by ALD to construct a fixed-bed nanoreactor because ALD can provide good conformity and precise thickness-control of the film. In addition, Ag nanoparticles, loaded on ZnO@PSF by photoreduction, showed 25% higher efficiency. In the meanwhile, photocatalytic dye-degradation experiment was conducted to evaluate the feasibility of the design of nanoreactor using ZnO@PSF fibers as the photocatalyst for rainwater purification.

並列關鍵字

Photocatalyst ； ALD ； Ta2O5 ； g-C3N4

參考文獻

Chapter 1

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