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ScienceDirect Procedia Engineering 87 (2014) 1433 – 1436
EUROSENSORS 2014, the XXVIII edition of the conference series
Selective and sensitive detection of C3 molecules with Cu-BTC Metal-organic Framework by means of mass sensitive and work function based read-out P. Davydovskayaa*, A. Ranftb,c, B. V. Lotschb,c, R. Pohlea a Siemens AG, Corporate Technology, Munich, Germany Max Planck Institute for Solid State Research, Stuttgart, Germany c Chemistry Department, Ludwig Maximilian University, Munich, Germany b
Abstract Metal-organic frameworks (MOFs) are porous crystalline materials that are supposed to be good candidates for sensitive and selective gas sensing as they feature wide geometrical and chemical variety and are known for the capability of reversible gas adsorption. In this work Cu-BTC MOF was investigated with mass sensitive and work function based read-out focusing on the gas sensing performance of organic vapors with the same length of their organic carbon chain but different type and position of their functional group for the first time. Based on the simultaneous evaluation of both read-outs a differentiation between C3 molecules is proposed. © by by Elsevier Ltd.Ltd. This is an open access article under the CC BY-NC-ND license © 2014 2014The TheAuthors. Authors.Published Published Elsevier (http://creativecommons.org/licenses/by-nc-nd/3.0/). Peer-review under responsibility of the scientific committee of Eurosensors 2014. Peer-review under responsibility of the scientific committee of Eurosensors 2014 Keywords: Gas Sensor; Metal-organic framework (MOF); Cu-BTC; HKUST-1; Work function read-out; Kelvin Probe; Quartz Crystal Microballance (QCM); Volatail Organic Compound (VOC); C3 Molecules.
1. Introduction Metal-organic frameworks (MOFs) are porous crystalline materials featuring a wide range of framework topologies [1]. Their geometrical and chemical variety and the capability of reversible gas adsorption are of high interest with regards to sensitive and selective detection of different gases and vapors [2, 3]. Cu-BTC (also known as
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1877-7058 © 2014 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license
(http://creativecommons.org/licenses/by-nc-nd/3.0/). Peer-review under responsibility of the scientific committee of Eurosensors 2014 doi:10.1016/j.proeng.2014.11.715
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HKUST-1) consists of Cu ions and 1,3,5-benzenetricarboxylate (BTC) as a linker. Beside pores with different sizes, shapes and polarities [4], Cu-BTC features open metal sites and is promising for selective adsorption and separation of different gases [5]. Gas sensing with Cu-BTC MOF has already been reported [6], but the determining factors for its gas selectivity – especially in humid air – are not completely understood yet. In our previous studies we showed that Cu-BTC MOF is suitable for selective detection of pentanal from other aldehydes by means of work function based read-out [7], while other BTC-linked MOFs (constructed from other metal ions) are good for selective detection of alcohols over alkanes owing to the different polarity of the target gases [8]. As not each kind of MOFanalyte interaction contributes to measurable changes in work function, complementary analytical techniques such as Quartz Crystal Microbalance QCM are desirable. Our previous study of alcohol adsorption on Cu-BTC [9] shows the influence of the length of the carbon chain of alcohols on their adsorption properties and highlights that the combination of Kelvin Probe with the QCM technique furnishes new insights into the adsorption mechanism. In the current work organic vapors with the same length of the organic chain (C3) but different type (carbonyl, hydroxyl) and position (on C1 and C2 atom) of their functional group as well as propane were used as target gases in order to understand the influence of polarity and geometry on the adsorption behavior. Mass sensitive read-out with a QCM and work function based read-out with a Kelvin Probe technique were applied and the measurements were performed in dry and humid synthetic air at room temperature. 2. Materials and Methods Cu-BTC colloid solutions were prepared according to the procedure described in Ref. [9, 10]. QCMs with gold electrodes and a nominal resonance frequency of about 10MHz were used for the preparation of the QCM samples, while for the Kelvin Probe measurements the same Kelvin Probe substrates with TiN back electrode as described in Ref. [7] were used. Sensing layers were prepared by drop coating using 2 μL of the Cu-BTC suspension for the coating of each sample. Photographs of QCM and Kelvin Probe samples and the respective scanning electron micrographs are shown in Fig. 1. Cu-BTC layers on the gold electrodes of QCM samples are 1.0-1.7 μm thick and show cracking. The Cu-BTC layers on the Kelvin Probe samples are homogenous and their layer thickness varies between 1.2-2.5 μm. The particle size of the Cu-BTC nanocrystals is in the range between 50-80 nm.
Fig. 1. Photographs and SEM images of Cu-BTC thin films on Kelvin Probe (a, b) and QCM (c, d) samples.
3. Results and Discussion Detection of 2-propanol, propanal and propane with Kelvin Probe and QCM is shown in Fig. 2. Two cycles of exposure to 2-propanol and propanal were measured to eliminate the possible influence of the pre-adsorption of one gas to the adsorption of another. It can be observed that the exposure to propane in humid air (40 % relative humidity, r.h.) does not lead to remarkable changes in work function and resonance frequency. Only small responses in work function and resonance frequency can be seen during exposure to 50 ppm of propane in dry air, while 10 ppm can be hardly detected with both read-outs. By comparing the responses for propanal and 2-propanol, it can be observed that the signal intensities obtained with both read-outs are bigger in dry air compared to humid air. Moreover, the work function response of Cu-BTC during exposure to propanal in dry air is smaller than that to 2propanol, while the mass sensitive response to propanal is higher. In humid air mass sensitive response to propanal is smaller than that to 2-propanol, and the work function response to pronanal is near the limit of detection.
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Fig. 2. Comparison of work function and mass sensitive responses to propanal, 2-propanol and propane in dry (a) and humid (b) synthetic air.
Similar sensing behavior is observed with acetone, 2-propanol and 1-propanol as target gases (Fig 3). In dry air the sequence of the work function response increases in the order of acetone
