Experimental and Simulation Studies of Acetone Detection By Pt-Au Nanofilm Sensors

Abstract

Introduction Acetone in exhaled breath is a biomarker of diabetes. However, since it is a very stable substance, its detection by chemical sensors has been a challenging task. Alloy nanoparticles have attracted great attention because of their great catalytic performance, and it has been reported that the response to acetone can be improved by functionalizing oxide semiconductors (In2O3) with PtAu nanoparticles [1]. However, the mechanism of better response to acetone has not been fully understood, which is partly because the effects of alloying and supporting materials cannot be separated in the composite material systems. In this study, we demonstrate that PtAu-alloy nanofilms, where the effect of supporting materials is eliminated, show a greater response to acetone than Pt nanofilms. Furthermore, the impact of alloying on sensing performance was studied by high-throughput atomistic simulation with neural network potentials. Method First, we fabricated Pt and PtAu nanofilm sensors. As the Pt and PtAu nanofilms, Pt and Pt/Au were deposited, respectively, by electron beam evaporation method. The thickness of Pt film is 3 nm, whereas Pt/Au thicknesses are 2.5 nm/0.5 nm. The electrodes consisted of 40-nm Pt and 40-nm Au layers, and they were annealed in N2 ambient for 1 hour at 300 oC. XRD measurements were performed to evaluate the alloy states for three samples: Pt (20nm), Au (20 nm), and PtAu (18 nm). The PtAu film for XRD was fabricated by repeating sequential depositions of 5-nm Pt and 1-nm Au three times. All the films were annealed for 1 hour at 300 oC in N2 ambient. The sensor responses to acetone were evaluated by measuring the resistance of Pt and PtAu at 260 oC under the following sequences.i) Dry air (15 min)ii) 1-ppm acetone in dry air (2 min)iii) Dry air (3 min)We applied DC current of 100 µA and the voltage difference between the terminals were measured. To examine the factors that contribute to the difference in sensing performance of PtAu and Pt nanofilms, we calculated the reaction pathway using the nudged elastic band method. All calculations were carried out with PFP version 2.0.0 on Matlantis [2]. DFT-D3 was used to describe the van der Waals interaction [3]. Results Pt, Au, and PtAu samples showed (111) peaks in the XRD analysis. The position of (111) peak of PtAu located between those of Pt and Au peaks, suggesting that PtAu was successfully alloyed. By applying the Vegard’s law, the composition of PtAu was determined to be 0.823:0.177, which is consistent with the thickness ratio. When acetone was exposed, the resistance of the devices decreased. The response of PtAu to acetone was greater than that of Pt. In addition, the responses of Pt and PtAu devices were proportional to the square root of the concentration of acetone. The limit of detections (LOD) determined by 3σ method were 76 ppb for Pt and 14 ppb for PtAu. Discussion We considered that H atoms introduced by CH dissociation of acetone prevent the adsorption of oxygens from the atmosphere and reduce the surface scattering of electrons, leading to a decrease in the resistance of Pt nanofilms. The Nudged Elastic Band method was used to calculate the activation energy of CH dissociation of acetone on the surface of Pt and PtAu. PtAu slab was prepared by replacing Pt atoms partly with Au atoms. The calculations showed that the activation energy for PtAu was 64-meV lower than that of Pt. In the transition state, the dissociated H atom stayed at the bridge site of Pt. When Pt atoms are substituted with Au atoms, the distance between Pt’s at the bridge site expands due to the large lattice constant of Au. As a result, the transition state became more stable on PtAu surface than on Pt surface. In fact, we confirmed that the activation energy was lowered as the Pt-Pt distance increased. Conclusion We demonstrated, for the first time, that PtAu nanofilm sensors show a greater response to acetone than Pt nanofilm. The atomic simulation demonstrated that the dehydrogenation of methyl group of acetone is the fundamental mechanism of the acetone sensing, and that the lattice expansion induced by replacing Pt with Au contributed to stabilize the transition state and to reduce the activation energy. Further optimization of the alloy composition should be essential to achieve a more highly sensitive acetone sensor.