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.

Caution: The chemicals and reagents used in this report were handled with care using the appropriate safety protections (lab coats, gloves, safety goggles, fume hoods). This study deals with the manipulation of microporous inorganic materials (zeolites with sizes ranging from 1 to 5 microns), therefore special attention was directed to the use of adequate dust protection (dust masks). We recommend the consultation of the relevant material safety data (MSDS) of the chemicals and reagents employed in this work before use for proper manipulation and/or waste disposal.
1. Zeolite Pre-treatment
<ol>
	<li>Heat pre-treatment 
	NOTE: Pre-treat the zeolite materials before use to remove impurities, such as organic impurities, that could hinder the silver cluster formation and luminescence.
	<ol>
		<li>Weigh 10 g of commercial LTA(Na) zeolites (commercial LTA zeolites contain sodium as counter-balancing ions in their frameworks) and deposit it homogeneously on a porcelain tray.</li>
		<li>Heat the zeolite powder overnight in a muffle oven at 450 &#176;C using a temperature ramp of 5 &#176;C/min with intervals of 1 hr at 80 &#176;C and 110 &#176;C to avoid zeolite structure damage.</li>
		<li>Remove the zeolite material from the oven and let it cool down to room temperature under ambient conditions.</li>
	</ol>
	</li>
	<li>Size selection of zeolite particles 
	NOTE: This will generate a more uniform grain size distribution of the starting zeolite materials, necessary for the creation of a homogenous film. This step also removes large amorphous particles, which are often present in industrially produced zeolites.
	<ol>
		<li>Weigh 10 g of commercial LTA(Na) and suspend it into 1 L of deionized water.</li>
		<li>Sonicate the suspension for 1 hr, vigorously shaking the suspension by hand every 10 min.</li>
		<li>Pour the suspension into an Atterberg cylinder (1 L) for 30 min. Particles smaller than 10 &#181;m in size remain in suspension, but larger particles precipitate.</li>
		<li>Decant the suspension and recover the powder by filtration using a B&#252;chner funnel. Wash the recovered powder three times with deionized water.</li>
		<li>Heat treat the powder as described in step 1.1.2.</li>
	</ol>
	</li>
</ol>
2. Preparation of Luminescent LTA(Na)-Ag Zeolite Composites
<ol>
	<li>Synthesis of luminescent silver exchanged LTA zeolite [LTA(Na)-Ag] as reference material
	<ol>
		<li>Dissolve 74.8 mg of silver nitrate in 200 ml deionized water in a 250 ml high density polypropylene (HDPE) bottle.</li>
		<li>Weigh 1 g of the pre-treated LTA(Na) sample and suspend it in the silver nitrate solution.</li>
		<li>Leave the HDPE flask agitating overnight into an end-over-end shaker oven at room temperature.</li>
		<li>Filter the suspension using a B&#252;chner funnel and wash the zeolite powder 3 times with deionized water.</li>
		<li>Heat the recovered powder in a muffle oven at 450 &#176;C, using the same procedure as described in step 1.1.2.</li>
		<li>Cool down the sample and place it in a desiccator with controlled humidity (98% relative humidity). Control the relative humidity by placing a saturated potassium sulfate solution inside the desiccator.15</li>
		<li>Measure the excitation and emission spectra of the samples (at different wavelengths) using a spectrofluorometer as well as their external quantum efficiencies.
		<ol>
			<li>Measure two dimensional excitation-emission plots by placing the sample in a 1 mm path quartz cuvette. Collect emission spectra starting 30 nm above the excitation wavelength up to 800 nm using 5 nm steps and a dwell time of 0.1 sec.</li>
			<li>Apply corrections using the instrument software for the lamp intensity and the wavelength dependent detection of the emission path to the raw data. Additionally, use a long pass filter to avoid second order peaks in the two dimensional plots.</li>
			<li>Perform quantum efficiency measurements by using an integrating sphere attached to the spectrofluorometer.16 Record the emission scan from 240 nm to 600 nm for both the zeolite sample and BaSO4 reference using 260 nm as excitation wavelength, and then calculate the quantum efficiency using the instrument software.</li>
		</ol>
		</li>
	</ol>
	</li>
</ol>
3. Preparation of Luminescent [LTA(Li)-Ag] Zeolite Composites
<ol>
	<li>Synthesis of partially exchanged lithium LTA zeolite [LTA(Li)] 
	Note: The procedure followed for the fabrication of partially exchanged LTA(Li) zeolites was adapted from the report by Yahiro and collaborators.17
	<ol>
		<li>Dissolve 17.2 g lithium nitrate in 2.5 L deionized water.</li>
		<li>Pour 0.5 L of the lithium nitrate solution into a 1 L HDPE flask.</li>
		<li>Weigh 3 g of pre-treated LTA(Na) zeolite and suspend it in the HDPE flask containing the lithium nitrate solution.</li>
		<li>Agitate the flask using an end-over-end shaker oven at room temperature overnight.</li>
		<li>Filter the suspension using a B&#252;chner funnel and wash the recovered powder 3 times with deionized water.</li>
		<li>Perform lithium exchange
		<ol>
			<li>Add 0.5 L of fresh lithium nitrate solution (3.1.1) to a 1 L HDPE flask containing the recovered powder from the filtration step (3.1.5).</li>
			<li>Repeat steps 3.1.4 and 3.1.5.</li>
			<li>Repeat steps 3.1.6.1 and 3.1.6.2 another 4 times.</li>
		</ol>
		</li>
		<li>Recover the zeolite powder and heat it in a muffle oven at 450 &#176;C overnight using a temperature ramp of 5 &#176;C/min with intervals of 1 hr at 80 &#176;C and 110 &#176;C.</li>
	</ol>
	</li>
	<li>Synthesis of luminescent [LTA(Li)-Ag] zeolites
	<ol>
		<li>Dissolve 74.8 mg silver nitrate in 200 ml deionized water using a 250 ml HDPE bottle.</li>
		<li>Weigh 1 g of the partially exchanged lithium LTA zeolite [LTA(Li)] and suspend it in the silver nitrate solution (3.2.1).</li>
		<li>Agitate the HDPE flask using an end-over-end shaker oven at room temperature overnight.</li>
		<li>Filter the suspension using a B&#252;chner funnel and wash the recovered zeolite powder 3 times with deionized water.</li>
		<li>Heat-treat the powder in a muffle oven at 450 &#176;C overnight using a temperature ramp of 5 &#176;C/min with intervals of 1 hr at 80 &#176;C and 110 &#176;C.</li>
		<li>Cool down the sample under controlled humidity conditions using a desiccator containing a saturated potassium sulfate solution inside (98% relative humidity).15</li>
		<li>Measure the excitation and emission spectra of the samples as well as their external quantum efficiencies following the procedure described in step 2.1.7.</li>
		<li>Perform thermogravimetric analysis (TGA) to determine the water content in the sample at different temperatures.1 Briefly, place 30 to 50 mg of the as-prepared sample on a platinum sample holder and load it into the TGA device. Measure the weight loss from 50 &#176;C until 600 &#176;C using a heat rate of 5 &#176;C/min under a nitrogen flow (90 ml/min).</li>
	</ol>
	</li>
</ol>
4. Fabrication of a LTA(Li)-Ag / Polyethylenimine (PEI) Composite Deposited Film for Humidity Sensing Applications
Note: The deposition procedure used in this report was modified and adapted from reference 18.
<ol>
	<li>LTA(Li)-Ag colloidal suspension preparation.
	<ol>
		<li>Dilute 1 ml of the commercial 50 wt% PEI solution to 100 ml with deionized water.</li>
		<li>Weigh 250 mg of the luminescent LTA(Li)-Ag material.</li>
		<li>Mix the zeolite and PEI solution together in a 125 ml HDPE bottle and shake the suspension vigorously.</li>
		<li>Place the bottle in a 40 kHz sonicator bath at room temperature overnight, to obtain a homogeneous suspension.</li>
		<li>Pour the LTA(Li)-Ag / PEI suspension into a spray bottle.</li>
	</ol>
	</li>
	<li>Deposition of a LTA(Li)-Ag / PEI film onto a quartz plate for sensor prototype production.
	<ol>
		<li>Clean a quartz plate by rinsing it with deionized water and acetone consecutively, prior to the film deposition. Dry the clean plates in an oven at 80 &#176;C for 1 hr.</li>
		<li>Spray coat the quartz plate on one side, by placing the quartz plate horizontally on a clean sheet of aluminum foil and spraying three times (3 sec each time) from a distance of about 20 cm. Place the coated plate inside a drying oven at 50 &#176;C for 30 min.</li>
		<li>Repeat step 4.2.2 another 4 times until the film is uniform.</li>
	</ol>
	</li>
	<li>Hydration/dehydration of the sensor prototype.
	<ol>
		<li>Place the coated quartz plate into the sample compartment of an in-house heating/vacuum cell.5 </li>
		<li>Close the sample chamber of the cell by placing a clean quartz plate in combination with a rubber ring on top of the coated plate and seal the cell using a Teflon stopper and screws as depicted in Figure 2.</li>
		<li>Apply high vacuum, using a pressure below 10-3 mbar, to the cell overnight in order to dehydrate the sample.</li>
		<li>Visually monitor the emission color changes (in the visible region) of the deposited film by using a UV lamp.</li>
		<li>Open the sample chamber to monitor the emission color changes, in the visible region, upon rehydration of the film using a UV lamp.</li>
		<li>Repeat the cycle multiple times starting from step 4.3.2 to 4.3.5 to test the reversibility of the LTA(Li)-Ag/PEI film.</li>
	</ol>
	</li>
</ol>

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Commun. 50, 1350-1352 (2014).</li><li>De Cremer, G., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=In+Situ+Observation+of+the+Emission+Characteristics+of+Zeolite-Hosted+Silver+Species+During+Heat+Treatment.">In Situ Observation of the Emission Characteristics of Zeolite-Hosted Silver Species During Heat Treatment.</a> ChemPhysChem. 11, 1627-1631 (2010).</li><li>Coutino-Gonzalez, E., et al. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Thermally+Activated+LTA(Li)-Ag+Zeolites+with+Water-Responsive+Photoluminescence+Properties.">Thermally Activated LTA(Li)-Ag Zeolites with Water-Responsive Photoluminescence Properties.</a> J. Mater. Chem. 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Comm. 14, 7157-7160 (2012).</li><li>Qi, H., Mader, E., Liu, J. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Unique+Water+Sensors+Based+on+.">Unique Water Sensors Based on .</a> Sensor. Actuat. B-Chem. 185, 225-230 (2013).</li><li>Basabe-Desmonts, L., Reinhoudt, D. N., Crego-Calama, M. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Design+of+Fluorescent+Materials+for+Chemical+Sensing.">Design of Fluorescent Materials for Chemical Sensing.</a> Chem. Soc. Rev. 36, 993-1017 (2007).</li><li>Yamazoe, N., Shimzu, Y. <a target="_blank" href="http://www.ncbi.nlm.nih.gov/entrez/query.fcgi?db=PubMed&cmd=Search&doptcmdl=Citation&defaultField=Title+Word&term=Humidity+Sensors+-+Principles+and+Applications.">Humidity Sensors - Principles and Applications.</a> Sensor. 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The authors have nothing to disclose.

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 &#176;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 &#176;C. The use of heat-resistant polymers, such as polyvinyl alcohol, could expand the temperature range up to 200 &#176;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.

Small sub-nanometer oligoatomic silver clusters formed by self-assembly in confined zeolite matrices display unique optical properties.1-5 Such silver-zeolite composites have high chemical and photo-stability. However, their photoluminescence properties are highly dependent on the local environment of the silver clusters. The environmental conditions that influence the optical features in silver-zeolite composites can be divided into intrinsic and extrinsic properties. Intrinsic properties are related to the zeolite topology, the type of counter-balancing ions, and the silver loading.1 On the other hand, extrinsic properties are associated to the post-synthetic changes, such as the presence of adsorbates or water molecules in the zeolite cavities.3,4 The latter properties confer to silver-zeolite composites the ability to optically respond to external stimuli, such as variations of moisture within the zeolite scaffold6-8 or the presence of determined gases; therefore their use as water vapor and gas sensors has been suggested.9,10
In a recent study we have demonstrated that the optical response of Ag-zeolites to moisture is not only correlated to changes in the absorption or quenching of their emission but also to the appearance of different emission colors with respect to their water content.5 The stabilization of silver clusters in partially Li exchanged LTA zeolites led to the formation of a moisture-responsive material in which changes in the relative low humidity scale were reflected in a dynamic color change from a blue to green/yellow emission in dehydrated and hydrated samples, respectively. Therefore the use of these materials as luminescence-based humidity sensors was proposed. To date, different types of materials such as electrolytes, ceramics, polymers, and nanostructured composites have been proposed for monitoring changes in humidity based on electrical and optical responses.11,12 In this detailed protocol we aim to demonstrate a proof-of-concept for the application of LTA(Li)-Ag zeolites as humidity sensors and for further prototype developments. Due to the versatility of LTA(Li)-Ag zeolites to be incorporated into different substrates, their potential scalability and cost-effective fabrication, the prototype design might be facilitated.13 Such sensors could have potential applicability in different industrial sectors, such as in agriculture, as well as the automobile and paper industry.14

<table><tbody><tr><td>LTA(Na) zeolite</td><td>UOP</td><td>Molsiv adsorbent 4A</td><td></td></tr><tr><td>Silver nitrate</td><td>Sigma Aldrich</td><td>209139</td><td>&amp;ge;99.0%</td></tr><tr><td>Lithium nitrate</td><td>Sigma Aldrich</td><td>62574</td><td>&amp;ge;99.0%, calc. on dry substances</td></tr><tr><td>Polyethyleneimine solution</td><td>Sigma Aldrich</td><td>3880</td><td>~50% H&lt;sub&gt;2&lt;/sub&gt;O</td></tr><tr><td>Scanning electron microscope (SEM)</td><td>JEOL</td><td>JSM-6010LV</td><td></td></tr><tr><td>Thermogravimetric analyzer</td><td>TA instruments</td><td>Q500</td><td></td></tr><tr><td>Spectrofluorimeter</td><td>Edinburgh instruments</td><td>FLS980-s</td><td></td></tr><tr><td>Integrating sphere</td><td>Labsphere</td><td>4P-GPS-033-SL</td><td></td></tr></tbody></table>

nanostructured ag-zeolite composites as luminescence-based humidity sensors

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.

SEM micrographs of the LTA-Ag zeolite were recorded after the cation exchange and heat-treatment step. Subsequently the photoluminescence two-dimensional (2D) excitation/emission plots were measured for both the hydrated LTA(Na)-Ag and LTA(Li)-Ag zeolites (Figure 1). Elemental analysis was performed by XPS on the Ag exchanged zeolites to determine their chemical composition. The analysis shows that silver exchange on LTA(Na) and LTA(Li) zeolites is very close with a silver weight percentage of 19.6 wt% and 21.5 wt%, respectively. The difference in weight percentage could be ascribed to the lower atomic weight of Li atoms. Furthermore the elemental analysis also showed that after Li exchange 33% of Na is replaced. The cation exchange and subsequent heat-treatment step performed on the samples do not seem to affect the structure of the LTA crystals, as demonstrated by SEM. Moreover, the formation of larger silver nanoparticles on the surface of the zeolite crystals was not visualized. The luminescent properties largely differ between both the LTA(Li)-Ag and LTA(Na)-Ag samples in their hydrated state. By incorporating lithium into the zeolite framework as a counter-balancing cation, a blue shift in the excitation maximum occurs from 370 nm to 260 nm, for LTA(Na)-Ag and LTA(Li)-Ag, respectively. In contrast the emission maximum undergoes a small red shift from 550 to 565 nm by adding Li into the system. The largest difference between these samples is observed in their external quantum efficiencies (EQE). LTA(Na)-Ag zeolites possess an EQE of about 4% at its excitation maximum (370 nm), whereas the EQE for LTA(Li)-Ag zeolites reaches 62% (when excited at 260 nm). This results in a bright yellow emitting powder under 254 nm UV- illumination.
The luminescent properties of the LTA(Li)-Ag sample are also dependent on the water content of the system. This was shown by a combination of TGA and temperature dependent luminescence experiments, TGA correlates temperature to hydration level of the zeolite. Additionally, the temperature was indirectly related to the emission color displayed by the LTA(Li)-Ag sample by using an in-house heating cell (Figure 2). The emission color shifts from yellow over green to blue, when removing water from the LTA(Li)-Ag system. The EQE steadily drops from 62% (hydrated state) to 21% (dehydrated state).
Because of the water-responsive behavior of LTA(Li)-Ag, this material was used to fabricate a luminescence based humidity sensor prototype by suspending the powder into a PEI solution and subsequently spray coating the composite onto a quartz plate. Pictures (under daylight and UV illumination) and SEM micrographs of the spray-coated LTA(Li)-Ag/PEI film are displayed in Figure 3. We observed that by using this coating procedure, a relatively homogenous layer of the polymer-zeolite composite in terms of luminescence was obtained. The SEM micrograph shows that the zeolite crystals are not altered by the coating procedure. By using an in-house heating/vacuum cell it was also demonstrated that the polymer-zeolite film retains the water-responsive properties which were observed in the zeolite in powder form.
<img alt="Figure 1" src="/files/ftp_upload/54674/54674fig1.jpg" /> 
Figure 1: SEM pictures and luminescent properties of silver exchanged LTA zeolites. SEM micrographs and 2D excitation-emission plots of LTA(Na)-Ag (a,b) and LTA(Li)-Ag (c,d). The insets in the 2D excitation-emission plots display the simulated emission colors of the samples under different excitation wavelengths (254, 300, 366 and 450 nm). <a href="https://cloudflare-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/54674/54674fig1large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 2" src="/files/ftp_upload/54674/54674fig2.jpg" /> 
Figure 2: Effect of hydration level on the luminescent properties of LTA(Li)-Ag. a) Schematic representation of the in-house heating/vacuum cell employed in this study. b) TGA plot for the LTA(Li)-Ag sample. c) Normalized emission spectra (upon 260 nm excitation) of the LTA(Li)-Ag sample measured at different temperatures. d) Scheme displaying the emission color change of real samples with respect to the water content. <a href="https://cloudflare-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/54674/54674fig2large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>
<img alt="Figure 3" src="/files/ftp_upload/54674/54674fig3.jpg" /> 
Figure 3: Luminescent humidity sensor based on a LTA(Li)-Ag/PEI composite. a) Photographs of the coated plate under daylight illumination. (b,c) Pictures of the dehydrated and hydrated coated plate under 254 nm UV-light irradiation, respectively. d) SEM micrograph of the deposited film showing the distribution of LTA(Li)-Ag crystals on the quartz surface. The inset displays an enlargement of a selected area of the original SEM micrograph. e) Emission maxima of the hydrated and dehydrated PEI/LTA(Li)-Ag zeolite composite, during 10 hydration/dehydration cycles using 260 nm as excitation wavelength. f) Plot displaying the emission maxima profiles behavior of the PEI/LTA(Li)-Ag zeolite composite after 10 hydration/dehydration cycles. <a href="https://cloudflare-jove-com.remotexs.ntu.edu.sg/files/ftp_upload/54674/54674fig3large.jpg" target="_blank">Please click here to view a larger version of this figure.</a>

Watch this Scientific Journal Video about Nanostructured Ag-zeolite Composites as Luminescence-based Humidity Sensors at JoVE.com

Nanostructured Ag-zeolite Composites as Luminescence-based Humidity Sensors

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 ...

nanostructured-ag-zeolite-composites-as-luminescence-based-humidity

Nanyang Technological University

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JoVE Journal

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10.1K Views.  KU Leuven. A protocol for the synthesis of moisture-responsive luminescent Ag-zeolite composites is described in this report.

Video: Nanostructured Ag-zeolite Composites as Luminescence-based Humidity Sensors

Preparation and Use of Photocatalytically Active Segmented Ag|ZnO and Coaxial TiO2-Ag Nanowires Made by Templated Electrodeposition

Photocatalytically active nanostructures require a large specific surface area with the presence of many catalytically active sites for the oxidation and reduction half reactions, and fast electron (hole) diffusion and charge separation. Nanowires present suitable architectures to meet these requirements. Axially segmented Ag|ZnO and radially segmented (coaxial) TiO2-Ag nanowires with a diameter of 200 nm and a length of 6-20 &#181;m were made by templated electrodeposition within the pores of polycarbonate track-etched (PCTE) or anodized aluminum oxide (AAO) membranes, respectively. In the photocatalytic experiments, the ZnO and TiO2 phases acted as photoanodes, and Ag as cathode. No external circuit is needed to connect both electrodes, which is a key advantage over conventional photo-electrochemical cells. For making segmented Ag|ZnO nanowires, the Ag salt electrolyte was replaced after formation of the Ag segment to form a ZnO segment attached to the Ag segment. For making coaxial TiO2-Ag nanowires, a TiO2 gel was first formed by the electrochemically induced sol-gel method. Drying and thermal annealing of the as-formed TiO2 gel resulted in the formation of crystalline TiO2 nanotubes. A subsequent Ag electrodeposition step inside the TiO2 nanotubes resulted in formation of coaxial TiO2-Ag nanowires. Due to the combination of an n-type semiconductor (ZnO or TiO2) and a metal (Ag) within the same nanowire, a Schottky barrier was created at the interface between the phases. To demonstrate the photocatalytic activity of these nanowires, the Ag|ZnO nanowires were used in a photocatalytic experiment in which H2 gas was detected upon UV illumination of the nanowires dispersed in a methanol/water mixture. After 17 min&#160;of illumination, approximately 0.2 vol% H2 gas was detected from a suspension of ~0.1 g of Ag|ZnO nanowires in a 50 ml 80 vol% aqueous methanol solution.

Preparation and Use of Photocatalytically Active Segmented Ag|ZnO and Coaxial TiO2-Ag Nanowires Made by Templated Electrodeposition

Procedures are outlined to prepare segmented and coaxial nanowires via templated electrodeposition in nanopores. As examples, segmented nanowires consisting of Ag and ZnO segments, and coaxial nanowires consisting of a TiO2 shell and a Ag core were made. The nanowires were used in photocatalytic hydrogen formation experiments.

Photocatalytically active nanostructures require a large specific surface area with the presence of many catalytically active sites for the oxidation and ...

Synthesis of Zeolites Using the ADOR (Assembly-Disassembly-Organization-Reassembly) Route

Zeolites are an important class of materials that have wide ranging applications such as heterogeneous catalysts and adsorbents which are dependent on their framework topology. For new applications or improvements to existing ones, new zeolites with novel pore systems are desirable. We demonstrate a method for the synthesis of novel zeolites using the ADOR route. ADOR is an acronym for Assembly, Disassembly, Organization and Reassembly. This synthetic route takes advantage of the assembly of a relatively poorly stable that which can be selectively disassembled into a layered material. The resulting layered intermediate can then be organized in different manners by careful chemical manipulation and then reassembled into zeolites with new topologies. By carefully controlling the organization step of the synthetic pathway, new zeolites with never before seen topologies are capable of being synthesized. The structures of these new zeolites are confirmed using powder X-ray diffraction and further characterized by nitrogen adsorption and scanning electron microscopy. This new synthetic pathway for zeolites demonstrates its capability to produce novel frameworks that have never been prepared by traditional zeolite synthesis techniques.

Synthesis of Zeolites Using the ADOR Assembly-Disassembly-Organization-Reassembly Route

A protocol for the preparation of novel zeolites by the ADOR (Assembly-Disassembly-Organization-Reassembly) synthetic route is presented.

Zeolites are an important class of materials that have wide ranging applications such as heterogeneous catalysts and adsorbents which are dependent on ...

Chemistry

Detection and Recovery of Palladium, Gold and Cobalt Metals from the Urban Mine Using Novel Sensors/Adsorbents Designated with Nanoscale Wagon-wheel-shaped Pores

Developing low-cost, efficient processes for recovering and recycling palladium, gold and cobalt metals from urban mine remains a significant challenge in industrialized countries. Here, the development of optical mesosensors/adsorbents (MSAs) for efficient recognition and selective recovery of Pd(II), Au(III), and Co(II) from urban mine was achieved. A simple, general method for preparing MSAs based on using high-order mesoporous monolithic scaffolds was described. Hierarchical cubic Ia3d wagon-wheel-shaped MSAs were fabricated by anchoring chelating agents (colorants) into three-dimensional pores and micrometric particle surfaces of the mesoporous monolithic scaffolds. Findings show, for the first time, evidence of controlled optical recognition of Pd(II), Au(III), and Co(II) ions and a highly selective system for recovery of Pd(II) ions (up to ~95%) in ores and industrial wastes. Furthermore, the controlled assessment processes described herein involve evaluation of intrinsic properties (e.g., visual signal change, long-term stability, adsorption efficiency, extraordinary sensitivity, selectivity, and reusability); thus, expensive, sophisticated instruments are not required. Results show evidence that MSAs will attract worldwide attention as a promising technological means of recovering and recycling palladium, gold and cobalt metals.

Because of the importance and extensive use of palladium, gold and cobalt metals in high-technology equipment, their recovery and recycling constitute an important industrial challenge. The metal recovery system described herein is a simple, low-cost means for the effective detection, removal, and recovery of these metals from the urban mine.

Developing low-cost, efficient processes for recovering and recycling palladium, gold and cobalt metals from urban mine remains a significant challenge in ...

Iridium Oxide-reduced Graphene Oxide Nanohybrid Thin Film Modified Screen-printed Electrodes as Disposable Electrochemical Paper Microfluidic pH Sensors

A facile, controllable, inexpensive and green electrochemical synthesis of IrO2-graphene nanohybrid thin films is developed to fabricate an easy-to-use integrated paper microfluidic electrochemical pH sensor for resource-limited settings. Taking advantages from both pH meters and strips, the pH sensing platform is composed of hydrophobic barrier-patterned paper micropad (&#181;PAD) using polydimethylsiloxane (PDMS), screen-printed electrode (SPE) modified with IrO2-graphene films and molded acrylonitrile butadiene styrene (ABS) plastic holder. Repetitive cathodic potential cycling was employed for graphene oxide (GO) reduction which can completely remove electrochemically unstable oxygenated groups and generate a 2D defect-free homogeneous graphene thin film with excellent stability and electronic properties. A uniform and smooth IrO2 film in nanoscale grain size is anodically electrodeposited onto the graphene film, without any observable cracks. The resulting IrO2-RGO electrode showed slightly super-Nernstian responses from pH 2-12 in Britton-Robinson (B-R) buffers with good linearity, small hysteresis, low response time and reproducibility in different buffers, as well as low sensitivities to different interfering ionic species and dissolved oxygen. A simple portable digital pH meter is fabricated, whose signal is measured with a multimeter, using high input-impedance operational amplifier and consumer batteries. The pH values measured with the portable electrochemical paper-microfluidic pH sensors were consistent with those measured using a commercial laboratory pH meter with a glass electrode.

The study demonstrates the growth of iridium oxide-reduced graphene oxide (IrO2-RGO) nanohybrid thin films on irregular and rough screen-printed carbon substrate through a green electrochemical synthesis, and their implementation as a pH sensor with a patterned paper-fluidic platform.

A facile, controllable, inexpensive and green electrochemical synthesis of IrO2-graphene nanohybrid thin films is developed to fabricate an easy-to-use ...

Adsorption Device Based on a Langatate Crystal Microbalance for High Temperature High Pressure Gas Adsorption in Zeolite H-ZSM-5

We present a high-temperature and high-pressure gas adsorption measurement device based on a high-frequency oscillating microbalance (5 MHz langatate crystal microbalance, LCM) and its use for gas adsorption measurements in zeolite H-ZSM-5. Prior to the adsorption measurements, zeolite H-ZSM-5 crystals were synthesized on the gold electrode in the center of the LCM, without covering the connection points of the gold electrodes to the oscillator, by the steam-assisted crystallization (SAC) method, so that the zeolite crystals remain attached to the oscillating microbalance while keeping good electroconductivity of the LCM during the adsorption measurements. Compared to a conventional quartz crystal microbalance (QCM) which is limited to temperatures below 80 &#176;C, the LCM can realize the adsorption measurements in principle at temperatures as high as 200-300 &#176;C (i.e., at or close to the reaction temperature of the target application of one-stage DME synthesis from the synthesis gas), owing to the absence of crystalline-phase transitions up to its melting point (1,470 &#176;C). The system was applied to investigate the adsorption of CO2, H2O, methanol and dimethyl ether (DME), each in the gas phase, on zeolite H-ZSM-5 in the temperature and pressure range of 50-150 &#176;C and 0-18 bar, respectively. The results showed that the adsorption isotherms of these gases in H-ZSM-5 can be well fitted by Langmuir-type adsorption isotherms. Furthermore, the determined adsorption parameters, i.e., adsorption capacities, adsorption enthalpies, and adsorption entropies, compare well to literature data. In this work, the results for CO2 are shown as an example.

A protocol for high-temperature and high-pressure gas adsorption measurements on zeolite H-ZSM-5 using an adsorption measurement device based on a langatate crystal microbalance is presented. Prior to the adsorption measurements, the synthesis of zeolite H-ZSM-5 on the langatate crystal microbalance sensor by the steam-assisted crystallization (SAC) method is demonstrated.

We present a high-temperature and high-pressure gas adsorption measurement device based on a high-frequency oscillating microbalance (5 MHz langatate ...

TiO2-coated Hollow Glass Microspheres with Superhydrophobic and High IR-reflective Properties Synthesized by a Soft-chemistry Method

This manuscript proposes a soft-chemistry method to develop superhydrophobic and highly IR-reflective hollow glass microspheres (HGM). The anatase TiO2 and a superhydrophobic agent were coated on the HGM surface in one step. TBT and PFOTES were selected as the Ti source and the superhydrophobic agent, respectively. They were both coated on the HGM, and after the hydrothermal process, the TBT turned to anatase TiO2. In this way, a PFOTES/TiO2-coated HGM (MCHGM) was prepared. For comparison, PFOTES single-coated HGM (F-SCHGM) and TiO2 single-coated HGM (Ti-SCHGM) were synthesized as well. The PFOTES and TiO2 coatings on the HGM surface were demonstrated through X-ray diffraction (XRD), scanning electron microscopy (SEM), and energy-dispersive detector (EDS) characterizations. The MCHGM showed a higher contact angle (153&#176;) but a lower sliding angle (16&#176;) than F-SCHGM, with a contact angle of 141.2&#176; and a sliding angle of 67&#176;. In addition, both Ti-SCHGM and MCHGM displayed similar IR reflectivity values, which were about 5.8% higher than the original HGM and F-SCHGM. Also, the PFOTES coating barely changed the thermal conductivity. Therefore, F-SCHGM, with a thermal conductivity of 0.0479 W/(m&#183;K), was quite like the original HGM, which was 0.0475 W/(m&#183;K). MCHGM and Ti-SCHGM were also similar. Their thermal conductivity values were 0.0543 W/(m&#183;K) and 0.0543 W/(m&#183;K), respectively. The TiO2 coating slightly increased the thermal conductivity, but with the increase in reflectivity, the overall heat-insulation property was enhanced. Finally, since the IR-reflecting property is provided by the HGM coating, if the coating is fouled, the reflectivity decreases. Therefore, with the superhydrophobic coating, the surface is protected from fouling, and its lifetime is also prolonged.

TiO2-coated Hollow Glass Microspheres with Superhydrophobic and High IR-reflective Properties Synthesized by a Soft-chemistry Method

This manuscript proposes a soft-chemistry method to synthesize superhydrophobic, TiO2-coated hollow glass microspheres (HGM) with high IR-reflective properties.

This manuscript proposes a soft-chemistry method to develop superhydrophobic and highly IR-reflective hollow glass microspheres (HGM). The anatase TiO2 ...

Aerosol-assisted Chemical Vapor Deposition of Metal Oxide Structures: Zinc Oxide Rods

Whilst columnar zinc oxide (ZnO) structures in the form of rods or wires have been synthesized previously by different liquid- or vapor-phase routes, their high cost production and/or incompatibility with microfabrication technologies, due to the use of pre-deposited catalyst-seeds and/or high processing temperatures exceeding 900 &#176;C, represent a drawback for a widespread use of these methods. Here, however, we report the synthesis of ZnO rods via a non-catalyzed vapor-solid mechanism enabled by using an aerosol-assisted chemical vapor deposition (CVD) method at 400 &#176;C with zinc chloride (ZnCl2) as the precursor and ethanol as the carrier solvent. This method provides both single-step formation of ZnO rods and the possibility of their direct integration with various substrate types, including silicon, silicon-based micromachined platforms, quartz, or high heat resistant polymers. This potentially facilitates the use of this method at a large-scale, due to its compatibility with state-of-the-art microfabrication processes for device manufacture. This report also describes the properties of these structures (e.g., morphology, crystalline phase, optical band gap, chemical composition, electrical resistance) and validates its gas sensing functionality towards carbon monoxide.

Columnar zinc oxide structures in the form of rods are synthesized via aerosol-assisted chemical vapor deposition without the use of pre-deposited catalyst-seed particles. This method is scalable and compatible with various substrates based on either silicon, quartz or polymers.

Whilst columnar zinc oxide (ZnO) structures in the form of rods or wires have been synthesized previously by different liquid- or vapor-phase routes, ...

Resource Recycling of Red Soil to Synthesize Fe2O3/FAU-type Zeolite Composite Material for Heavy Metal Removal

Heavy metal-polluted water is of great concern to human health and the eco-environment. In situ water remediation techniques enabled by highly efficient adsorption materials are of great importance in these circumstances. Among all the materials used in water remediation, iron-based nanomaterials and porous materials are of great interest, benefiting from their rich redox reactivity and adsorption function. Here, we developed a facile protocol to directly convert the widely spread red soil in south China to fabricate the Fe2O3/faujasite (FAU)-type zeolite composite material. 
The detailed synthesis procedure and synthesis parameters, such as reaction temperature, reaction time, and the Si/Al ratio in the raw materials, have been carefully tuned. The as-synthesized composite materials show good adsorption capacity for typical heavy metal(loid) ions. With 0.001 g/mL Fe2O3/FAU-type zeolite composite material added to different heavy metal(loid)-polluted aqueous solutions (single type of heavy metal(loid) concentration: 1,000 mg/L [ppm]), the adsorption capacity was shown to be 172, 45, 170, 40, 429, 693, 94, and 133 mg/g for Cu (II), Cr (III), Cr (VI), As (III), Cd (II), Pb (II), Zn (II), and Ni (II) removal, respectively, which can be further expanded for heavy metal-polluted water and soil remediation.

Resource Recycling of Red Soil to Synthesize Fe2O3/FAU-type Zeolite Composite Material for Heavy Metal Removal

This article presents a novel and convenient route to synthesize Fe2O3/faujasite (FAU)-type zeolite composite material from red soil. The detailed synthesis parameters have been finely tuned. The obtained composite material can be used for efficient heavy metal-contaminated water remediation, indicating its potential applications in environmental engineering.

Heavy metal-polluted water is of great concern to human health and the eco-environment. In situ water remediation techniques enabled by highly efficient ...

Environment

Luminescence Lifetime Imaging of O2 with a Frequency-Domain-Based Camera System

We describe a method to image dissolved oxygen (O2), in 2D at high spatial (&#60; 50-100 &#181;m) and temporal (&#60; 10 s) resolution. The method employs O2 sensitive luminescent sensor foils (planar optodes) in combination with a specialized camera system for imaging luminescence lifetime in the frequency-domain. Planar optodes are prepared by dissolving the O2-sensitive indicator dye in a polymer and spreading the mixture on a solid support in a defined thickness via knife coating. After evaporation of the solvent, the planar optode is placed in close contact with the sample of interest - here demonstrated with the roots of the aquatic plant Littorella uniflora. The O2 concentration-dependent change in the luminescence lifetime of the indicator dye within the planar optode is imaged via the backside of the transparent carrier foil and aquarium wall using a special camera. This camera measures the luminescence lifetime (&#181;s) via a shift in phase angle between a modulated excitation signal and emission signal. This method is superior to luminescence intensity imaging methods, as the signal is independent of the dye concentration or intensity of the excitation source, and solely relies on the luminescence decay time, which is an intrinsically referenced parameter. Consequently, an additional reference dye or other means of referencing are not needed. We demonstrate the use of the system for macroscopic O2 imaging of plant rhizospheres, but the camera system can also easily be coupled to a microscope.

Luminescence Lifetime Imaging of O2 with a Frequency-Domain-Based Camera System

We describe the use of a novel, frequency-domain luminescence lifetime camera for mapping 2D O2 distributions with optical sensor foils. The camera system and image analysis procedures are described along with the preparation, calibration and application of sensor foils for visualizing the O2 microenvironment in the rhizosphere of aquatic plants.

We describe a method to image dissolved oxygen (O2), in 2D at high spatial (&#60; 50-100 &#181;m) and temporal (&#60; 10 s) resolution. The method employs ...

Encapsulating Cytochrome c in Silica Aerogel Nanoarchitectures without Metal Nanoparticles while Retaining Gas-phase Bioactivity

Applications such as sensors, batteries, and fuel cells have been improved through the use of highly porous aerogels when functional compounds are encapsulated within the aerogels. However, few reports on encapsulating proteins within sol&#8211;gels that are processed to form aerogels exist. A procedure for encapsulating cytochrome c (cyt. c) in silica (SiO2) sol-gels that are supercritically processed to form bioaerogels with gas-phase activity for nitric oxide (NO) is presented. Cyt. c is added to a mixed silica sol under controlled protein concentration and buffer strength conditions. The sol mixture is then gelled and the liquid filling the gel pores is replaced through a series of solvent exchanges with liquid carbon dioxide. The carbon dioxide is brought to its critical point and vented off to form dry aerogels with cyt. c encapsulated inside. These bioaerogels are characterized with UV-visible spectroscopy and circular dichroism spectroscopy and can be used to detect the presence of gas-phase nitric oxide. The success of this procedure depends on regulating the cyt. c concentration and the buffer concentration and does not require other components such as metal nanoparticles. It may be possible to encapsulate other proteins using a similar approach making this procedure important for potential future bioanalytical device development.

Encapsulating Cytochrome c in Silica Aerogel Nanoarchitectures without Metal Nanoparticles while Retaining Gas-phase Bioactivity

This procedure describes how to encapsulate cytochrome c (cyt. c) in silica (SiO2) sol-gels, process these gels to form bioaerogels, and use these bioaerogels to rapidly recognize nitric oxide (NO) through a gas-phase reaction. This type of protocol may aid in the future development of biosensors or other bioanalytical devices.

Nanostructured Ag-zeolite Composites as Luminescence-based Humidity Sensors

Summary

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Chapters in this video

Preparation and Use of Photocatalytically Active Segmented Ag|ZnO and Coaxial TiO₂-Ag Nanowires Made by Templated Electrodeposition

Synthesis of Zeolites Using the ADOR (Assembly-Disassembly-Organization-Reassembly) Route

Detection and Recovery of Palladium, Gold and Cobalt Metals from the Urban Mine Using Novel Sensors/Adsorbents Designated with Nanoscale Wagon-wheel-shaped Pores

Iridium Oxide-reduced Graphene Oxide Nanohybrid Thin Film Modified Screen-printed Electrodes as Disposable Electrochemical Paper Microfluidic pH Sensors

Adsorption Device Based on a Langatate Crystal Microbalance for High Temperature High Pressure Gas Adsorption in Zeolite H-ZSM-5

TiO₂-coated Hollow Glass Microspheres with Superhydrophobic and High IR-reflective Properties Synthesized by a Soft-chemistry Method

Aerosol-assisted Chemical Vapor Deposition of Metal Oxide Structures: Zinc Oxide Rods

Resource Recycling of Red Soil to Synthesize Fe₂O₃/FAU-type Zeolite Composite Material for Heavy Metal Removal

Luminescence Lifetime Imaging of O₂ with a Frequency-Domain-Based Camera System

Encapsulating Cytochrome c in Silica Aerogel Nanoarchitectures without Metal Nanoparticles while Retaining Gas-phase Bioactivity