Pickering Emulsions Prepared by Layered Niobate K₄Nb₆O₁₇ Intercalated with Organic Cations and Photocatalytic Dye Decomposition in the Emulsions

Abstract

We investigated emulsions stabilized with particles of layered hexaniobate, known as a semiconductor photocatalyst, and photocatalytic degradation of dyes in the emulsions. Hydrophobicity of the niobate particles was adjusted with the intercalation of alkylammonium ions into the interlayer spaces to enable emulsification in a toluene-water system. After the modification of interlayer space with hexylammonium ions, the niobate stabilized water-in-oil (w/o) emulsions in a broad composition range. Optical microscopy showed that the niobate particles covered the surfaces of emulsion droplets and played a role of emulsifying agents. The niobate particles also enabled the generation of oil-in-water (o/w) emulsions in a limited composition range. Modification with dodecylammonium ions, which turned the niobate particles more hydrophobic, only gave w/o emulsions, and the particles were located not only at the toluene-water interface but also inside the toluene continuous phase. On the other hand, interlayer modification with butylammonium ions led to the formation of o/w emulsions. When porphyrin dyes were added to the system, the cationic dye was adsorbed on niobate particles at the emulsion droplets whereas the lipophilic dye was dissolved in toluene. Upon UV irradiation, both of the dyes were degraded photocatalytically. When the cationic and lipophilic porphyrin molecules were simultaneously added to the emulsions, both of the dyes were photodecomposed nonselectively.

Exploring optoelectronic properties and mechanisms of layered ferroelectric K4Nb6O17 nanocrystalline films and nanolaminas

“…As an effective and typical photocatalytic material 6, 18–20 , KN has been the subject of extensive studies on its photochemical and ion exchange properties 9, 21–25 . On the one hand, for global energy crisis, hydrogen is widely recognized as the fuel of the future, and photocatalytic water splitting into hydrogen has received significant attention 26–30 .…”

Synthesis of Photocatalytic Niobate Nanosheet/Polymer Composite Microgel Particles through Microfluidic Approach

“…It is plausible that part of the nanosheets are adsorbed on the water/oil interface of the water-in-oil droplet, inducing the alignment of the nanosheets liquid crystal inside the droplets. It was reported that nanosheets tend to adsorb on the interface and act as an emulsifying agent to form stable pickering emulsion [24]. Although the surfactant used here, Span80, is not a non-ionic one and the interaction with nanosheet is not very strong, the adsorption is possible.…”

Preparation of Titanium Dioxide Particles Modified by Salicylic Acid and Arginine and Their Photocatalytic Reaction on Oil-Water Interface

“…Key words: surface modification; surface structure; photocatalysis; Pickering emulsion 纳米 TiO 2 具有化学稳定性好、催化活性强、廉 价无毒以及矿化程度较高等特点, 其光催化性能优 于 ZnO、WO、CdS 等光催化剂 [1][2][3] , 在紫外光下可以 迅速降解 3000 多种难以降解的有机化合物 [4] , 在处 理高浓度、难降解有机废水方面具有独特优势。研 究发现, 高浓度有机废水常以油/水分层形式存在, 纳米 TiO 2 具有表面强亲水特性, 易分散于水相中, 导致污染物与催化剂接触不充分, 传质效率受到一 定限制 [5] 。 20 世纪初, Ramsden [6] 和 Pickering [7] 发现微米级 尺寸的固体粒子可以吸附于油-水界面形成稳定的 乳液, 这种乳液被称为 Pickering 乳液。固体粒子有 效地稳定 Pickering 乳液的前提条件是能够被油/水相 部分润湿。其稳定机制主要有机械阻隔机制、桥连 机理以及三维粘弹粒子网络机理, 即固体粒子形成 单层或多层的紧密固体膜, 阻止液滴聚并, 并形成 三维网状结构导致连续相粘度增加, 降低乳液液滴 迁移速率和程度, 阻止乳液液滴的聚结 [8][9] 。 Pickering 乳液乳化剂的用量较少、 毒性小、 稳定性好, 在化工、 材料、造纸等领域均有着广泛的应用 [10][11] 。将光催化 技术与 Pickering 乳液联合运用, 构建的界面催化体 系可增大污染物与催化剂的接触面积, 在处理有机 污染物特别是高浓度有机物方面显现出独特优势。 改性 ZnO [12] 、层状六铌酸 [13] 、改性 TiO 2 [5] 、负载磷 酸银的碳纳米管 [14][15] 等固体粒子稳定的乳液已用于 光催化降解有机废水, 并取得较好效果。但大部分光 催化剂具有强亲水特性, 不能有效吸附于油/水界面 形成 Pickering 乳液体系, 需要进行表面亲油改性。 近年来, 采用表面活性剂 [16] 、硅烷偶联剂 [17] 、高 分子聚合物 [18] 等有机物对光催化剂进行表面改性可 以降低表面亲水性、扩展吸收光波长、增强对目标 物吸附能力以及降低电子复合率等 [15] , 但由于光生 电子-空穴对产生的强氧化性的自由基会直接攻击 表面改性基团, 导致其发生自降解 [19] , 如酒石酸、柠 檬酸、 乳酸 [20] 等均为牺牲性改性剂, 催化剂无法重复 利用。部分有机酸, 如乙酸 [21] 、水杨酸(SA)及其衍生 物 [22][23] 、精氨酸(Arg) [24] 、天冬氨酸 [25] 可与 TiO 2 结合 形成紫外光下稳定的螯合或桥连结构, 并显著提升 光催化效率, 但这类物质分子链较短且吸附量低, 对光催化剂表面亲油性改善程度较小。研究发现, 二 氧 化 钛 表 面 具 有 多 种 酸 位 点 (Brønsted 酸 位 点 和 Lewis 酸位点) [19] , 可进行多酸类物质共改性 [26] , 进 一步提升催化剂在油-水界面的吸附能力。此外, 学 者还发现 Arg 改性可以增强光催化剂的光致还原能 力, 在处理难以氧化降解的污染物上具有良好应用 前景 [23] 。基于此, 本研究采用 SA 和 Arg 同时对纳米 NH - [23] ; 1331 和1441 cm -1 对应 COO-的对称与非对称伸缩振动峰; (b) 1464 和 1483 cm -1 对应苯环; 1443 和 1611 cm -1 对应 COO-的 对称振动峰和不对称振动峰; (c)1629 和 3380 cm −1 对 应 O-H; 1402 cm −1 对 应 Ti-OH; 669 cm −1 对 应 Ti-O-Ti [5]…”