Homogeneous Bimetallic AuPd Nanocatalysts Decorated TiO₂ Nanohelices for Selective Toluene Gas Sensor

Functionalizing metal oxide (MOX)-based gas sensors with nanocatalysts (NCs) can significantly enhance gas sensitivity and reduce operating temperatures. Especially, bimetallic NCs decorated on MOX have garnered great attention due to their unique catalytic properties controlled by tailoring the composition, atomic ordering, and size. However, it is hard to control the atomic ratio of bimetals with typical synthesis methods, leading to the mixture phase of monometallic and bimetallic NCs with random compositions. This heterogeneity obscures the understanding of the catalytic kinetics with well-controlled atomic compositions of bimetals, acting as the bottleneck of designing highly selective and sensitive MOX-based gas sensors decorated with bimetallic NCs.<br/>Herein, we propose a rational strategy to synthesize and decorate bimetallic NCs with homogeneous composition and size on MOX-based gas sensors. AuPd bimetallic NCs were synthesized homogeneously using the Turkevich method, and the NCs were isolated in micelle form through polyethylene glycol (PEG)-conjugation to ensure uniform dispersion. Subsequently, PEG-conjugated AuPd NCs were coated onto vertically aligned TiO₂ nanohelices (NHs) prepared by the oblique angle deposition (OAD) method, followed by rapid calcination for removing the PEG and anchoring the AuPd NCs to the TiO₂ NHs. Owing to the benefits of homogeneous AuPd NCs, the d-band position of the sensing materials was precisely controlled by tailoring the composition ratio of Au and Pd. For Au₅₅Pd₄₅ NCs with a d-band center value of -2.8 eV from the Fermi level (E_F), the adsorption of oxygen (O_ads) on AuPd NCs-decorated TiO₂ NHs was maximized, achieving a significant response value (R_g/R_a ~ 10⁶ at 100 ppm) and a low limit of detection (LoD= 32 ppb) for toluene gas at 200 °C. Further density functional theory calculations confirmed both the selectivity for toluene gas and d-band shift with different composition ratios of Au and Pd. With our homogeneous bimetal NCs decoration method, we believe that sensors with outstanding selectivity and sensitivity can be designed and realized by exploring optimum catalytic combinations, far surpassing traditional gas sensors.