A protocol for the synthesis of moisture-responsive luminescent Ag-zeolite composites is described in this report.
Small silver clusters confined inside zeolite matrices have recently emerged as a novel type of highly luminescent materials. Their emission has high external quantum efficiencies (EQE) and spans the whole visible spectrum. It has been recently reported that the UV excited luminescence of partially Li-exchanged sodium Linde type A zeolites [LTA(Na)] containing luminescent silver clusters can be controlled by adjusting the water content of the zeolite. These samples showed a dynamic change in their emission color from blue to green and yellow upon an increase of the hydration level of the zeolite, showing the great potential that these materials can have as luminescence-based humidity sensors at the macro and micro scale. Here, we describe the detailed procedure to fabricate a humidity sensor prototype using silver-exchanged zeolite composites. The sensor is produced by suspending the luminescent Ag-zeolites in an aqueous solution of polyethylenimine (PEI) to subsequently deposit a film of the material onto a quartz plate. The coated plate is subjected to several hydration/dehydration cycles to show the functionality of the sensing film.
通过自组装在密闭沸石基质形成小亚纳米oligoatomic银团簇显示独特的光学性质。1-5这种银沸石复合材料具有高的化学和光学稳定性。然而,它们的光致发光特性是高度依赖于银簇的局部环境。在银 – 沸石复合影响光学特性的环境条件,可分为内在和外在属性。内在特性都涉及到沸石拓扑,抗衡离子的类型和银负载1。另一方面,外在属性关联到合成后的变化,如吸附或水分子的存在沸石空腔。3,4-后者属性赋予对银-沸石复合,以光学地对外部刺激,例如在沸石骨架6-8内的水分的变化作出反应的能力</suP>或确定气体的存在;因此它们的水蒸气和气体传感器的使用已被建议。9,10-
在最近的研究中,我们已经表明,银-沸石的湿气光学响应不仅相关,在其发射的吸收或骤冷的变化,但也对不同的发射颜色外观相对于它们的含水量。5的稳定化在部分锂交换的LTA沸石分别导致其改变在相对低湿度的比例反映在从蓝色动态颜色变化至绿色/黄色发射脱水和水合样品在透湿响应材料的形成中,银金属簇的。因此,使用这些材料作为基于发光的湿度传感器中提出的。迄今为止,不同类型的材料,如电解质,陶瓷,聚合物和纳米复合材料已经提出了用于在湿度b监控的变化ASED上电和光反应。11,12在此详细协议中,我们的目标是展示一个证明的概念为LTA(李)-Ag沸石的应用湿度传感器和用于进一步原型发展。由于LTA(李)-Ag沸石的通用性被纳入不同的衬底,其潜在的可扩展性和成本效益的制造,原型设计可能来促进。13这样的传感器可以具有在不同的工业部门,例如在潜在适用性农业,以及汽车和造纸工业。14
A simple device to demonstrate the proof of concept of using LTA(Li)-Ag as a luminescence based humidity sensor was produced by spray coating the LTA(Li)-Ag powder suspended in a PEI solution onto a quartz plate. The PEI solution produces a polymer layer with homogenous thickness when the water is evaporated. The polymer-zeolite composite layer displays similar luminescent properties as that of the zeolite in powder form. The PEI/LTA(Li)-Ag zeolite composite displays the expected water-responsive luminescent properties, whose emission color changes upon variations in the water content present in the composite at relatively low humidity scale.
Replacing Na with Li ions in LTA zeolites (calculated exchange rate 33%) has a notable impact on the self-assembly and stabilization of luminescent silver clusters in the LTA(Li) scaffolds leading to unique optical properties. The EQE of LTA(Li)-Ag as compared to LTA(Na)-Ag samples is enhanced by more than one order of magnitude. Moreover, the emission colors displayed by the LTA(Li)-Ag samples have a water-dependence, providing a potential application of the samples as luminescence based humidity sensors.
We have thus demonstrated an easy method to fabricate a luminescent film-like humidity sensor through which changes in hydration levels can be visually monitored simply by using a UV lamp. The availability of the raw materials, the direct visualization of the color changes correlated with humidity content, the photo-stability of the films, and the relative ease of fabricating cost-effective devices make these luminescent materials potential candidates to compete with state-of-the-art humidity sensors based upon electrical responses. The procedure described in this report could also be applied and extended to different substrates, at different micro and macro scales, to make the sensor more flexible. Additionally, several critical steps during the fabrication of Ag-zeolites, which play an important role in determining the final optical properties of such materials, were discussed in this protocol. For instance, the pre-cleaning of the raw zeolite material leads to the removal of optical and chemical impurities, as well as to homogenous zeolite crystal size distribution. This is crucial for the incorporation of zeolites into functional devices. One limitation of the present methodology is the restriction on the use of thin film sensors beyond 75 °C. This is mainly due to the decomposition of the PEI polymer, rather than to the degradation of the LTA(Li)-Ag zeolites, which can withstand up to 500 °C. The use of heat-resistant polymers, such as polyvinyl alcohol, could expand the temperature range up to 200 °C. We expect that further investigations will be directed to the development of methodologies for the synthesis of nanostructured Ag-zeolite composites with (multi)functional properties and finally to the design of advanced sensor prototypes.
The authors have nothing to disclose.
The authors gratefully acknowledge financial support from the Belgian Federal government (Belspo through the IAP VI/27 and IAP-7/05 programs), the European Union’s Seventh Framework Programme (FP7/2007-2013 under grant agreement no. 310651 SACS), the Flemish government in the form of long-term structural funding “Methusalem” grant METH/08/04 CASAS, the “Strategisch Initiatief Materialen” SoPPoM program, and the Fund for Scientific Research Flanders (FWO) grant G.0349.12. W.B. gratefully acknowledge the chemistry department of the KU Leuven for a FLOF-scholarship. The authors thank UOP Antwerp for the kind donation of zeolite samples and the mechanical workshop of the KU Leuven for helping with the design and construction of the heating/vacuum cell used in this study.
LTA(Na) zeolite | UOP | Molsiv adsorbent 4A | |
Silver nitrate | Sigma Aldrich | 209139 | ≥99,0% |
Lithium nitrate | Sigma Aldrich | 62574 | ≥99,0%, calc. on dry substances |
Polyethyleneimine solution | Sigma Aldrich | 3880 | ~50% H2O |
Scanning electron microscope (SEM) | JEOL | JSM-6010LV | |
Thermogravimetric analyzer | TA instruments | Q500 | |
Spectrofluorimeter | Edinburgh instruments | FLS980-s | |
Integrating sphere | Labsphere | 4P-GPS-033-SL |