Silica Nanocomposites for Selective and Stable H2S

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Cite This: ACS Appl. Nano Mater. 2019, 2, 3335−3338

Copper Oxide/Silica Nanocomposites for Selective and Stable H2S Gas Detection Andrej Paul, Christian Weinberger, Michael Tiemann,* and Thorsten Wagner Department of Chemistry, University of Paderborn, Warburger Strasse 100, D-33098 Paderborn, Germany

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S Supporting Information *

ABSTRACT: A composite material of copper oxide (CuO) dispersed in the nanopores of KIT-6 silica (SiO2) is used as a dosimetric sensor for the detection of hydrogen sulfide (H2S) gas in low parts per milion concentrations. The sensor principle is based on the reversible chemical conversion of CuO to CuS, which guarantees a high selectivity, and on the corresponding percolation-induced change in electronic conductance. KEYWORDS: CuO, nanocrystals, nanoporous materials, host−guest materials, H2S, sensor ydrogen sulfide (H2S) is a highly toxic1 and flammable gas that occurs, e.g., in biogas plants, and there is a strong demand for H2S monitoring for matters of work safety and process reliability. Hence, mobile and easy-to-operate H2S sensors with high sensitivities and long lifetimea are required. Nanostructured semiconducting metal oxides are frequently used as chemiresistive gas-sensing materials.2,3 Upon exposure to oxidizing or reducing gases, their electronic properties change because of adsorption and surface-chemical reaction of the gas molecules. However, despite their robustness and high sensitivity, metal oxide-based chemiresistors often lack selectivity to particular analyte gases. An alternative approach for the highly selective detection of critical threshold quantities of H2S is based on using copper(II) oxide (CuO):4−7 CuO is converted to copper(II) sulfide (CuS) upon exposure to H2Scontaining air at 160 °C (eq 1). Raising the temperature to 350 °C leads to the regeneration of CuO due to reaction with oxygen (O2; eq 2), including in the presence of H2S:

H

160 °C:

CuO + H 2S → CuS + H 2O

(1)

350 °C:

CuS + 1.5O2 → CuO + SO2

(2)

that accumulates the analyte (although by strict terms, the analyte, H2S, is not exactly ’accumulated’, but consumed by chemical reaction). CuO was created in situ inside the nanopores of KIT-6 silica8 by impregnating the pores with copper nitrate [Cu(NO3)2] and subsequent calcination, as described in the Supporting Information. Figure 1 shows the powder X-ray diffraction (XRD) patterns of KIT-6 silica before and after loading with increasing amounts of CuO. The low-angle reflections result from the periodic arrangement of the nanopores in the KIT-6 silica host matrix. The decrease in peak intensity upon repeated loading with CuO reflects the reduction of the scattering contrast due to gradual (partial) filling of the pores with the guest species.9 The wide-angle peaks correspond to crystalline CuO (JCPS card # 05−0661). Figure 2 displays the N2 physisorption isotherms and corresponding pore-size-distribution curves of the same materials. The specific pore volume decreases by ca. 73% (from 1.44 to 0.39 mL/g; see Table S1) upon loading with CuO, while the average pore size of ca. 8.5 nm remains approximately constant. These findings further confirm gradual filling of the pores with the CuO guest species. It also indicates that CuO is formed as individual particles in the pores rather than as a continuous layer at the pore-wall surface; in the latter case, the average pore width would decrease. Characterization of the CuO@SiO2 composite materials by electron microscopy has turned out to be difficult. In the transmission electron microscopy (TEM) image, empty silica pores can hardly be distinguished from the pores that contain CuO; Figure 3 shows TEM images of the CuO-loaded material; the periodically ordered silica pore structure can be clearly seen; CuO nanocrystals can be identified by their lattice fringes at high magnification.

Since the electronic conductivity of CuS is much higher than that of CuO, the system can be used for conductometric H2S detection. The detection is discontinuous because hightemperature regeneration of the CuO is required once the CuS has been formed (cycle mode). This approach poses some challenges due to the fact that significant volume expansion and contraction occur during each cycle, since the densities of CuO and CuS differ drastically from each other. This usually leads to morphological changes in the sensing material which affect the long-term signal stability, as observed in typical filmtype sensing layers. Here we present a concept (Scheme 1) that not only solves the stability issue by utilizing nanoporous KIT-6 silica as a stabilizing matrix, but even turns the volume expansion into an actual advantage, as will be shown below. The sensor resembles a ’dosimeter-type’ sensor,5,6 i.e., a sensor © 2019 American Chemical Society

Received: May 28, 2019 Accepted: June 6, 2019 Published: June 6, 2019 3335

DOI: 10.1021/acsanm.9b01004 ACS Appl. Nano Mater. 2019, 2, 3335−3338

Letter

ACS Applied Nano Materials

Scheme 1. (Top) Synthesis of CuO Particles in the Pores of KIT-6 Silica by Infiltration of Cu(NO3)2 and Subsequent Thermal Conversion (Calcination) and (Bottom) Conversion of CuO to CuS in Each Sensing Cycle by Reaction with H2S at 160 °Ca

The volume expansion leads to conductive percolation paths; CuO is regenerated by heating to 350 °C.

a

Figure 1. Powder XRD diagrams of nanoporous KIT-6 silica without and with increasing amounts of CuO in the pores (1−4).

Figure 2. N2 physisorption isotherms and pore-size-distribution curves (vertically shifted for clarity) of the same samples as those in Figure 1 (for the color code, see Figure 1).

For the H2S sensor, the CuO@SiO2 composite material was deposited on a ceramic substrate that contains platinum electrodes and a heating element; sensor tests were carried out in a custom-made apparatus (see the Supporting Information). The overall amount of CuO in the silica pores was deliberately chosen to be low (62 wt %) in order to provide enough empty space for the CuO particles to expand. As a consequence, many particles may not be in contact with adjacent ones, and conducting paths will likely be discontinuous to quite some degree. Hence, the overall electronic conductance of the composite material may be lower than that if the amount of CuO in the silica pores was higher. However, this will not compromise the sensor performance, as will be shown below.

Figure 4 shows the temporal evolution of the measured conductance. The sensor was initially kept at 350 °C for 30 min and then at 160 °C for another 30 min under synthetic air to assess its conductance at both temperatures in the absence of H2S. (The conductance drops by a factor of ca. 100, as expected for thermally activated semiconducting CuO.) The sensor was then exposed to 10 ppm of H2S in synthetic air at 160 °C with 30% relative humidity. Immediately after H2S exposure, the conductance does not yet increase. (It actually decreases slightly because of a change in the electronic band structure of CuO by surface interaction with H2S; this is the 3336

DOI: 10.1021/acsanm.9b01004 ACS Appl. Nano Mater. 2019, 2, 3335−3338

Letter

ACS Applied Nano Materials

Figure 3. TEM images of mesoporous CuO-loaded KIT-6 silica. The CuO nanocrystals can be identified by their lattice fringes in the highresolution TEM image (right).

respectively). This can be explained by a percolation transition caused by the chemical conversion of CuO to CuS (eq 1): (i) A continuous conducting path is formed once the amount of highly conducting CuS reaches a certain critical value, the socalled “percolation threshold”. The conductance will then suddenly increase steeply.6 (ii) In addition, the percolation effect may be enhanced by the fact that during conversion of CuO to CuS the individual particles inside the silica pores expand by a factor of ca. 1.59, as mentioned above (densities:12 ρCuS = 4.76 g/cm3 = 49.8 mmol/cm3; ρCuO = 6.31 g/cm3 = 79.3 mmol/cm3). Because of this expansion, more adjacent CuS particles come into contact with each other, which leads to an increase in the number of continuous conducting pathways through the silica pores, as depicted in Scheme 1. The resulting increase in conductance is therefore stronger than the difference in the specific conductivities alone because, prior to H2S exposure, the conductance of CuO was below its intrinsic value because of the low level of conducting pathways. This is what is observed after the above-mentioned accumulation period. (A more detailed theoretical description of the percolation effect is in progress and will be published in a separate paper.) Upon continued H2S exposure, the conductance finally saturates when the gas-accessible surface near the region of CuO has been converted to CuS. Heating to 350 °C then leads to regeneration of CuO (eq 2), and the conductance is observed to decrease to its original value at this

Figure 4. Initialization (without H2S exposure) and the first cycle of CuO-to-CuS conversion (160 °C) and CuO regeneration (350 °C).

above-mentioned chemiresistive effect and will be disregarded in the following.) The most prominent feature, however, is a sudden and steep increase in conductance after an accumulation period of several minutes of H2S exposure by a factor of more than 107. The increase in conductance is significantly stronger than one might expect from the specific conductivities of “metallic conducting” CuS and CuO (0.3710 and 1.5 × 10−5 S/cm,11

Figure 5. Conductance of the sensor during repeated cycles of CuO-to-CuS conversion (160 °C; blue) and CuO regeneration (350 °C; red) under continuous H2S exposure (10 ppm of H2S in air, 30% relative humidity). 3337

DOI: 10.1021/acsanm.9b01004 ACS Appl. Nano Mater. 2019, 2, 3335−3338

ACS Applied Nano Materials

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ACKNOWLEDGMENTS We thank Pascal Vöpel (University of Gießen, Germany) for help with the TEM studies. A.P. and T.W. acknowledge support from the German Federal Ministry of Education and Research (Grant 13N12969) and the German Research Foundation (Grant WA 2977/3-1).

temperature. The conversion of CuO to CuS in the silica pores has only a small impact on the scattering contrast in the lowangle XRD as well as on the specific pore volume observed by N2 physisorption, despite the swelling. The accuracy of these techniques did not allow further characterization of the conversion. The time delay between the onset of H2S exposure (at 160 °C) and the percolation-related steep increase in conductance turns out to correlate with the H2S concentration in air, as would be expected for the described conversion-type sensing mechanism. The higher the concentration, the sooner the percolation threshold is reached, as will be elucidated in detail in a separate paper. Hence, measuring this time delay will allow for assessment of the H2S concentration (after calibration); this will expand the versatility of the sensor from a merely “dosimetric” system to a semiquantitative system. Eventually the cycling stability of the above-described sensor material was tested. The sensor was tested for ca. 900 cycles (over 8 days) and showed a stable performance. Figure 5 shows 25 cycles of CuO-to-CuS conversion (at 160 °C) and subsequent regeneration of CuO (at 350 °C), during which the conductance of the sensor shows a very reproducible behavior, which confirms that the percolating pathways are indeed formed and disrupted reproducibly during each cycle. The porous matrix prevents the CuO/CuS particles from undergoing irreversible morphological changes, such as the displacement of clustering due to mechanical stress (upon expansion) or diffusion. The void pore volume in the initial composite material (CuO@SiO2) compensates for and directs the swelling (CuO → CuS), as depicted in Scheme 1. A stable and selective H2S gas-sensing material based on the reversible chemical conversion of CuO-to-CuS particles inside the pores of nanoporous KIT-6 silica has been developed. The reaction goes along with the strong volume expansion of the particles, which leads to electronically conductive percolation paths. Long-term structural deterioration and loss of performance are hindered by the pore structure of the silica matrix. The system shows high sensitivity to H2S in low parts per million concentration.

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REFERENCES

(1) Toxicological Profile for Hydrogen Sulfide and Carbonyl Sulfide; Agency for Toxic Substances and Disease Registry, U.S. Department of Health and Human Services, Public Health Service: Atlanta, GA, 2016. (2) Tiemann, M. Porous Metal Oxides as Gas Sensors. Chem. - Eur. J. 2007, 13, 8376−8388. (3) Wagner, T.; Haffer, S.; Weinberger, C.; Klaus, D.; Tiemann, M. Mesoporous Materials as Gas Sensors. Chem. Soc. Rev. 2013, 42, 4036−4053. (4) Ramgir, N. S.; Ganapathi, S. K.; Kaur, M.; Datta, N.; Muthe, K. P.; Aswal, D. K.; Gupta, S. K.; Yakhmi, J. V. Sub-ppm H2S Sensing at Room Temperature Using CuO Thin Films. Sens. Actuators, B 2010, 151, 90−96. (5) Hennemann, J.; Kohl, C.-D.; Smarsly, B. M.; Sauerwald, T.; Teissier, J.-M.; Russ, S.; Wagner, T. CuO Thin Films for the Detection of H2S Doses - Investigation and Application. Phys. Status Solidi A 2015, 212, 1281−1288. (6) Hennemann, J.; Kohl, C.-D.; Smarsly, B. M.; Metelmann, H.; Rohnke, M.; Janek, J.; Reppin, D.; Meyer, B. K.; Russ, S.; Wagner, T. Copper Oxide based H2S Dosimeters - Modeling of Percolation and Diffusion Processes. Sens. Actuators, B 2015, 217, 41−50. (7) Kneer, J.; Knobelspies, S.; Bierer, B.; Wöllenstein, J.; Palzer, S. New Method to Selectively Determine Hydrogen Sulfide Concentrations Using CuO Layers. Sens. Actuators, B 2016, 222, 625−631. (8) Kleitz, F.; Hei Choi, S.; Ryoo, R. Cubic Ia3d Large Mesoporous Silica: Synthesis and Replication to Platinum Nanowires, Carbon Nanorods and Carbon Nanotubes. Chem. Commun. 2003, 2136− 2137. (9) Haffer, S.; Lüder, C.; Walther, T.; Köferstein, R.; Ebbinghaus, S. G.; Tiemann, M. A Synthesis Concept for a Nanostructured CoFe2O4/BaTiO3 Composite: Towards Multiferroics. Microporous Mesoporous Mater. 2014, 196, 300−304. (10) Mukherjee, N.; Sinha, A.; Khan, G. G.; Chandra, D.; Bhaumik, A.; Mondal, A. A Study on the Structural and Mechanical Properties of Nanocrystalline CuS Thin Films Grown by Chemical Bath Deposition Technique. Mater. Res. Bull. 2011, 46, 6−11. (11) de Los Santos Valladares, L.; Salinas, D. H.; Dominguez, A. B.; Najarro, D. A.; Khondaker, S. I.; Mitrelias, Z.; Barnes, C. H. W.; Aguiar, J. A.; Majima, Y. Crystallization and Electrical Resistivity of Cu2O and CuO Obtained by Thermal Oxidation of Cu Thin Films on SiO2/Si Substrates. Thin Solid Films 2012, 520, 6368−6374. (12) Lide, D. R. CRC Handbook of Chemistry and Physics, 85th ed.; Taylor & Francis, 2004.

ASSOCIATED CONTENT

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsanm.9b01004. Experimental details (synthesis, characterization, sensor preparation, and sensing measurements) and structural properties of the materials (Table S1) (PDF)

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Letter

AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. ORCID

Christian Weinberger: 0000-0002-2222-7527 Michael Tiemann: 0000-0003-1711-2722 Thorsten Wagner: 0000-0002-4014-0185 Author Contributions

The manuscript was written through contributions of all authors. All authors have given approval to the final version of the manuscript. Notes

The authors declare no competing financial interest. 3338

DOI: 10.1021/acsanm.9b01004 ACS Appl. Nano Mater. 2019, 2, 3335−3338