Nanostrukturierte Ag-Zeolith-Komposite als Lumineszenz-basierte Feuchtigkeitssensoren
Summary
A protocol for the synthesis of moisture-responsive luminescent Ag-zeolite composites is described in this report.
Abstract
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.
Introduction
Kleine Sub-Nanometer oligoatomic Silbercluster durch Selbstorganisation in geschlossenen Zeolith Matrizen zeigen einzigartigen optischen Eigenschaften gebildet. 1-5 Solche Silber-Zeolith – Komposite hohe chemische und photoStabilität aufweisen. Jedoch sind ihre Photolumineszenz-Eigenschaften stark abhängig von der lokalen Umgebung der Silbercluster. Die Umweltbedingungen, die die optischen Eigenschaften in Silber-Zeolith-Komposite beeinflussen können in intrinsische und extrinsische Eigenschaften unterteilt werden. Intrinsischen Eigenschaften werden dem Zeolith Topologie bezogen, der Art des Gegenausgleichs Ionen und der Silberbeladung. 1 Auf der anderen Seite, extrinsische Eigenschaften sind auf die post-synthetische Veränderungen verbunden, wie das Vorhandensein von Adsorbaten oder Wassermoleküle in der Zeolith – Hohlräume. 3,4 letztere Eigenschaften verleihen zu Silber-Zeolith – Komposite die Fähigkeit , auf äußere Reize zu reagieren optisch, wie Schwankungen der Feuchtigkeit innerhalb der Zeolith – Gerüst 6-8 </sup> oder das Vorhandensein von bestimmt Gase; daher ihre Verwendung als Wasserdampf und Gassensoren vorgeschlagen. 9,10
In einer neueren Studie zeigten wir , dass die optische Reaktion des Ag-Zeolithe gegenüber Feuchtigkeit nur auf Änderungen in der Absorption oder Quenching ihrer Emission korreliert ist, sondern auch zum Auftreten verschiedener Emissionsfarben hinsichtlich ihres Wassergehalts. 5 ist die Stabilisierungs Silbercluster in teilweise Li ausgetauscht LTA Zeolithen zur Bildung eines feuchtigkeits ansprechendem Material in dem in dem relativ geringen Feuchtigkeitsmaßstabsänderungen führten, wurden in einer dynamischen Farbwechsel von einem blau zu grün / gelbe Emission in dehydriert und hydratisierten Proben reflektiert bzw. . Daher ist die Verwendung dieser Materialien als Lumineszenz basierende Feuchtesensoren vorgeschlagen. Bisher wurden verschiedene Arten von Materialien, wie Elektrolyte, Keramiken, Polymere und Verbundwerkstoffe nanostrukturiertem wurden zur Überwachung von Änderungen in der Feuchtigkeit b vorgeschlagenenased an elektrischen und optischen Reaktionen. 11,12 In dieser detaillierten Protokoll wollen wir einen Proof-of-Concept für die Anwendung von LTA (Li) -Ag Zeolithe als Feuchtigkeitssensoren und für die weitere Prototypen Entwicklungen zu demonstrieren. Aufgrund der Vielseitigkeit der LTA (Li) -Ag Zeolithe in verschiedene Substrate eingearbeitet werden, deren Potential Skalierbarkeit und kostengünstige Fertigung, kann der Prototyp – Design erleichtert werden. Solche Sensoren 13 potentielle Anwendbarkeit in verschiedenen Industriezweigen haben könnte, wie in Landwirtschaft, sowie der Automobil- und Papierindustrie. 14
Protocol
Representative Results
Discussion
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.
Disclosures
The authors have nothing to disclose.
Acknowledgements
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.
Materials
| 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 |
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