Ordered Monolayer Gold Nano-urchin Structures and Their Size Induced Control for High Gas Sensing Performance

Abstract

The synthesis of ordered monolayers of gold nano-urchin (Au-NU) nanostructures with controlled size, directly on thin films using a simple electrochemical method is reported in this study. In order to demonstrate one of the vast potential applications, the developed Au-NUs were formed on the electrodes of transducers (QCM) to selectively detect low concentrations of elemental mercury (Hg(0)) vapor. It was found that the sensitivity and selectivity of the sensor device is enhanced by increasing the size of the nanospikes on the Au-NUs. The Au-NU-12 min QCM (Au-NUs with nanospikes grown on it for a period of 12 min) had the best performance in terms of transducer based Hg(0) vapor detection. The sensor had 98% accuracy, 92% recovery, 96% precision (repeatability) and significantly, showed the highest sensitivity reported to date, resulting in a limit of detection (LoD) of only 32 μg/m3 at 75 °C. When compared to the control counterpart, the accuracy and sensitivity of the Au-NU-12 min was enhanced by ~2 and ~5 times, respectively. The results demonstrate the excellent activity of the developed materials which can be applied to a range of applications due to their long range order, tunable size and ability to form directly on thin-films.

Figure 1

(a) Schematic representing the QCM transducer modification process a-1) Depositon of Ti electrodes on quartz substrates through e-beam evaporation in order to form Ti QCMs followed by a-2) the transfer of self-assembled PSNS on the Ti electrodes of the QCMs followed by a-3) deposition of gold through e-beam evaporation to develop Au-MNM and finally a-4) electrochemical deposition of nanospikes to form gold nano-urchins (Au-NUs); and SEM images of (b) close-packed Au-MNM, (c) Au-NU-6 min, (d) Au-NU-8 min, (e) Au-NU-10 min, (f) Au-NU-12 min and (g) Au-NU-15 min, all deposited directly on the Ti electrodes of the QCM transducers.

Figure 2

(a) Low magnification SEM image for Au-MNM, the inset shows FFT graph of the SEM image; (b) Side view of packed monolayer of Au-MNM; (c) Low magnification image of Au-NU-12 min sample showing large surface coverage; (d) Side view of packed monolayer of Au-NU-12 min showing the formation of nano-urchins.

Figure 3

Electrochemical characterization data showing (a) cyclic voltammograms (CVs) obtained at an Au electrode for the reduction of gold from an electrolyte containing 2.718 g/L HAuCl₄•3H₂O and 0.177 g/L Pb(CH₃COO)₂•3H₂O recorded at 50 mV s⁻¹; (b) the current stability during the formation of Au-NU with different nanospike sizes; (c) Linear sweep voltammograms (LSVs) for the Au-control and Au-NU-6 min samples obtained in 1 M H₂SO₄ at 100 mV s⁻¹ (d) the electrochemical surface area (ESA) calculate from the reduction of one oxide monolayer formed on the Au surfaces during the reduction phase of the cyclic voltammogram recorded in 1 M H₂SO₄. The geometric surface area of each substrate was 0.196 cm².

Figure 4

Modified (Au-NUs) and Au-control based QCMs’ (a) dynamic response, (b) response magnitudes (The solid fitted lines represent the best fits for the LRC equation) and (c) selectivity performance toward Hg⁰ vapor at 75 °C. The dynamic response was obtained when the QCMs were exposed toward Hg⁰ vapor concentrations of 0.21, 0.31, 0.45, 0.64, 0.93, 1.27, 1.74, 2.38, and 3.26 ± 0.05 mg/m³. The target concentration of 3.26 mg/m³ is represented by the solid magenta lines in each of the panels.