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Elucidating different $NO_{2}$ sensing mechanisms in oxidized PbS nanocrystals

Fernando M. Fernandes, Fouad El Haj Hassan, Sophie Hermans, Benoît Hackens

Abstract

In this work we provide an in-depth analysis of the sensing mechanisms of $NO_{2}$ by lead-sulfide nanocrystals (PbS-NCs). A detailed model for the sorption mechanism is proposed, and the correlation is established between experimental sensing characteristics and the surface composition, based on both experimental characterization and ab initio (DFT) simulations. We demonstrated how the sensitivity and the sensing dynamic response can be tuned by a post-deposition multistep dry-thermal process at mild temperature, that alternates vacuum-assisted annealing and heating in open-air. Sensors with different surface compositions were fabricated, and their dynamic response was characterized at low concentration of $NO_{2}$ (0.5 ppm) in air, at ambient temperature. DFT simulations indicate that both surface stoichiometry and oxidation critically govern $NO_{2}$ interaction on PbS, with sulfur-rich terminations favoring weaker binding and faster desorption, while intermediate oxidation enhances interaction and overoxidation leads to surface passivation, in agreement with the measured experimental sensing dynamics. By linking surface composition, adsorption chemistry, and resistance transduction within a single framework, this work provides clear indications to design room-temperature, low-ppm $NO_{2}$ microsensors fabricated through a simple and scalable processes.

Elucidating different $NO_{2}$ sensing mechanisms in oxidized PbS nanocrystals

Abstract

In this work we provide an in-depth analysis of the sensing mechanisms of by lead-sulfide nanocrystals (PbS-NCs). A detailed model for the sorption mechanism is proposed, and the correlation is established between experimental sensing characteristics and the surface composition, based on both experimental characterization and ab initio (DFT) simulations. We demonstrated how the sensitivity and the sensing dynamic response can be tuned by a post-deposition multistep dry-thermal process at mild temperature, that alternates vacuum-assisted annealing and heating in open-air. Sensors with different surface compositions were fabricated, and their dynamic response was characterized at low concentration of (0.5 ppm) in air, at ambient temperature. DFT simulations indicate that both surface stoichiometry and oxidation critically govern interaction on PbS, with sulfur-rich terminations favoring weaker binding and faster desorption, while intermediate oxidation enhances interaction and overoxidation leads to surface passivation, in agreement with the measured experimental sensing dynamics. By linking surface composition, adsorption chemistry, and resistance transduction within a single framework, this work provides clear indications to design room-temperature, low-ppm microsensors fabricated through a simple and scalable processes.
Paper Structure (2 sections, 2 equations, 4 figures, 3 tables)

This paper contains 2 sections, 2 equations, 4 figures, 3 tables.

Table of Contents

  1. Introduction
  2. Results and discussion

Figures (4)

  • Figure 1: Fabrication: (a) On the left, the PbS-NPs dispersed in water (ink) obtained after ultra-sonication at 4-different powers (from left-to-right: 100 %, 50 %, 40 % and 30 %) and filtering to discard largest agglomerates. On the right, in panel (a), the simple experimental setup used for the synthesis of PbS-NPs, at room temperature and using water as the main solvent Nabiyouni2012. (b) SEM-image showing the morphology of the sensing layer of PbS-NPs after the heating step in vacuum at 180 $\celsius$ for 30 minutes. In the inset, the high-resolution (TEM) image showing a single PbS nanoparticles with its crystalline core. (c) The scheme of the fabrication process, with the deposition of the sensing layer by drop coating, the geometry of the IDEs-array chip, the wafer-printing layout, and the illustration of the final device.
  • Figure 2: (a) XPS data used in the composition analysis, of lead-compounds (Pb: 4f$_{7/2}$ and 4f$_{5/2}$) on the left column, and for sulfur-compounds (S: 2s) on the right column (the gray curves are the raw experimental data). (b) Surface composition, in percentage of lead-compounds (%) detected per sample. The amounts were quantified from the respective areas of the peaks shown in panel a. (c) Evolution of the crystallite size, on the PbS-NCs layer, after each thermal step, as determined from the XRD spectra (shown in the supporting information). (d) Composition (in %) of the crystalline phases detected after each step indicated in (c), as determined from the Rietveld analysis of XRD spectra (the PbS phase was omitted to highlight the evolution of the composition).
  • Figure 3: Sensing characterization: (a) Variation of the normalized resistances of sensors sv and sa, at room-temperature (RT) and variable relative humidity (RH), during the measurement under a constant flow of synthetic air with introduction of step-like concentrations of NO2 gas (the target curve in panel b, black dashed line). (b) In gray (shaded area), the NO2 concentration calculated from the resistances variations (in panel a), obtained after fitting of equation \ref{['eq:dconv']}, using the parameters in Table \ref{['tab.Calib']}.
  • Figure 4: DFT: Results of theoretical analysis, based on DFT calculations, exploring the binding energy of NO2 molecule on the surface of a model particle as a function of the surface composition, i.e. combining different amounts of lead-to-sulfur, and decoration with oxygen atoms (each value of binding energy corresponds to an averaged value taken from 6-different interaction sites, randomly distributed on the surface). This result allowed to clarify the role of the ratio Pb-to-S and the oxidation level, in balancing the surface reactivity and the sorption kinetics.