Roles of (001) and (101) Facets of Anatase TiO₂ in Photocatalytic Reactions

Abstract

Single crystals of anatase TiO2 with exposed (001) and (101) facets were synthesized by a hydrothermal method. We carried out photocatalytic reduction reactions to deposit noble metals (Au, Ag, and Pt) and photocatalytic oxidation reactions to deposit metal oxides (PbO2 and MnOx) on the surface of TiO2. The deposited anatase TiO2 samples were characterized by transmission electron microscopy (TEM), scanning electron microscopy (SEM), energy-dispersive X-ray spectroscopy (EDX), and X-ray photoelectron spectroscopy (XPS) to study the roles of the two facets of anatase TiO2 in photocatalytic reactions. The noble metals were selectively deposited on the exposed (101) facet, while metal oxides were selectively deposited on the exposed (001) facet. This result indicated that photogenerated electrons and holes mainly accumulated on the (101) and (001) facets, and then took part in photocatalytic reduction and oxidation reactions, respectively. These results also suggested that the simultaneous exposure of the two facets could facilitate charge separation. Therefore, it was proposed that the simultaneous exposure of two facets with different functions will be a new strategy to effectively promote photocatalytic reaction.

Unifying the Nitrogen Reduction Activity of Anatase and Rutile TiO2 Surfaces

“…In the first case, the photo-excited electron would flow to the facet whose CBM is the lowest, and hole to the facet with the highest valence band maxima. [46] The co-existence of different facets was shown to enhance the electron-hole separation, and therefore the performance of the TiO 2 . [47] In this case, the photocatalytic NRR activity will be mainly determined by the facet with the most negative CBM with respect to the normal hydrogen electrode (NHE).…”

Preparation of MnS with Different Crystal Phases for Photocatalytic H2 Production from H2S

“…已成为全球性问题。 自从 1972 年日本科学家 Fujishima 和 Honda 利用紫外光照射 n 型半导体 TiO 2 可以实现 分解水产生氢气以来 [1] , 光催化技术引起了广泛关 注 [2][3] 。其中光催化制备氢气是同时解决环境污染和 能源短缺两大问题的有效途径之一, 因此在过去几 十年里, 大量的研究集中在通过光催化分解水制备 氢气 [4][5][6][7] 。然而, 光解水制氢需要克服较大的势垒 (284.7 kJ/mol), 效率较低。 H 2 S 具有相对较低的解离 能(39.4 kJ/mol) [8][9][10] , 作为一种潜在的氢能来源, 逐 渐引起人们的关注。此外, H 2 S 是一种有毒气体, 当 浓度大于 70010 6 时就会导致死亡 [11] 。传统的 H 2 S 分解技术是通过克劳斯工艺(H 2 S+1/2O 2 →H 2 O+S) 实现的 [12] 。克劳斯工艺虽然可以实现 H 2 S 的分解, 但是需要高温处理, 且在处理过程中会产生大量的 副产物, 造成一系列的环境问题。 更重要的是, 储存 在 H 2 S 中的氢能并没有被有效利用。近年来, 光催 化分解 H 2 S 逐步引起人们的关注 [13][14][15] , 然而, 寻找 合适的光催化剂用于 H 2 S 分解制氢依然是一个挑战。 MnS 是一种重要的 VIIB-VIA 族弱磁性宽禁带 (E g =3.7 eV) p 型半导体 [16] , 在光学、电子学和磁学 等方面具有独特的性质 [17] , 因而, MnS 常被用作电 池材料的窗口材料、光电材料、太阳光选择性涂层 和光学存储器等 [18][19] 。近年来, 随着光催化技术的 发展, MnS 作为一种光学半导体又被广泛应用于光 催化领域 [20][21][22][23] 。MnS 具有三种物相, 分别是具有稳 定形态的 α-MnS(立方相)、亚稳态的 β-MnS(闪锌矿 结构)和 γ-MnS(纤锌矿结构) [24] 。其中亚稳态 MnS 只能存在于较低的温度下, 在高温或高压条件下, 很容易转变成稳态相 [25][26] [27][28][29] [36][37] , 而光电流的变化与阻抗的变化是成反比关系, 因此 本工作通过测试样品在不同条件下的阻抗变化, 对 其电子和空穴的分离能力进行了研究, 结果如图 8 所示。 对比可以发现, α, γ-MnS 从暗场到可见光再到 全光谱对应的阻抗都是逐渐变小的, 说明 α, γ-MnS 都可以被可见光激发产生光电流; 紫外光的引入更 有利于激发半导体产生空穴和电子。对比 α, γ-MnS 在相同的光照条件下的阻抗可以发现, 不管是在暗 场、可见光还是全光谱, γ-MnS 比 α-MnS 都具有更 小的阻抗, 因此, γ-MnS 比 α-MnS 具有更好光生载 流子分离能力, 这与 γ-MnS 的晶体结构有直接关 系。γ-MnS 具有典型的六方纤锌矿结构, Mn 2+ 和 S 2 离子均呈四面体配位。由于 Mn 2+ 和 S 2 之间离子半 径的差异使得正负离子之间接触不良, 进而在晶体 内部形成局部正电荷中心和负电荷中心 [38] 。这些正 负电荷中心的存在有利于光催化过程中光生电子和 空穴的分离, 这是 γ-MnS 具有更好光催化活性的主 要原因 [39][40]…”

Strategies for improving the efficiency of semiconductor metal oxide photocatalysis

“…48,49 The facet reactivity may not necessarily be the same for different photocatalytic reactions, with low energy facets in some cases exhibiting higher activity for hydrogen evolution, 48 decomposition of acetaldehyde, 49 and noble metal deposition. 50,51 Furthermore, different facets may have different roles in photocatalytic reactions, such as (101) participating in photocatalytic reduction while (001) participated in photocatalytic oxidation. 51 Methods to synthesize titania nanostructures with a higher proportion of high energy facets have been recently reviewed.…”

“…50,51 Furthermore, different facets may have different roles in photocatalytic reactions, such as (101) participating in photocatalytic reduction while (001) participated in photocatalytic oxidation. 51 Methods to synthesize titania nanostructures with a higher proportion of high energy facets have been recently reviewed. 47 A common synthesis method involves the use of a capping agent to inhibit crystal growth in a certain direction and thus preserve more reactive facets.…”