Influence Of Binary Alloys On Gas Sensing Properties Of Semiconductor Oxides

As the front-end of the Internet of Things,sensors are widely used in various fields such as environmental protection,safety and security,healthcare,energy management,smart manufacturing,and virtual reality.To achieve rapid and accurate detection of low-concentration volatile organic compounds（VOCs）or non-polar gases（H₂,CH₄,etc.）,it is necessary to significantly improve the sensitivity and selectivity of oxide semiconductor gas sensors.The use of noble metals as surface modifiers is an effective way to enhance the gas sensing performance of sensors due to their unique physical and chemical properties.Binary alloys,as a new type of catalyst,often exhibit excellent selective catalytic activity due to the synergistic effect between the two metals,thus having great potential as gas sensitizers.This paper focuses on the modification and sensitization of oxide semiconductor(W₁₈O₄₉,Sn O₂)gas sensors,using a co-reduction method to synthesize small-sized,monodisperse,and compositionally controllable binary alloy nanocrystals in suitable reaction systems.By uniformly loading them onto the surface of oxide semiconductors through an impregnation and annealing process,the relationship between the gas sensing performance and the loading ratio,composition,and crystal structure of the alloy nanocrystals was systematically investigated,leading to high-performance sensors for butanol,acetone,and hydrogen.The specific research contents are as follows:（1）Component-tunable monodisperse Au_xRh1-x alloy nanocrystals were synthesized in the oil-amine system and utilized for modifying the surface of sea urchin-shaped W₁₈O₄₉ to investigate their effect on the gas-sensing performance of oxide semiconductor from the perspective of"chemical sensitization".The structures,morphologies,and gas sensitivity of a series of Au_xRh1-x-W₁₈O₄₉ were studied,and the results showed that the optimal loading ratio of Au_xRh1-x nanocrystals was 0.3 wt%and the optimal atomic ratio was 39:61.Based on the gas sensor with 0.3 wt%Au₃₉Rh₆₁-W₁₈O₄₉,a response of 10.6 was achieved for 50 ppm n-butanol,which was4 times that of Au-W₁₈O₄₉,and 2 times that of Rh-W₁₈O₄₉,with a response time of only 2 seconds.In addition,it exhibited good selectivity,excellent reproducibility,and stability.The excellent catalytic ability of Au_xRh1-x alloy nanocrystals was the main factor in enhancing n-butanol detection,which could effectively dissociate oxygen molecules.This chapter demonstrates the huge potential of noble metal alloy nanocrystals in enhancing gas sensitivity of oxide semiconductors and indicates that gas-sensing performance is closely related to the alloy composition.It can be anticipated that a series of alloy nanocrystal gas sensitizers can be obtained by combining different metals.（2）Compared to noble metals,the introduction of common metal alloying can significantly reduce costs and save resources.In this study,the sensitivity enhancement of noble metal and common metal alloys was investigated by choosing common metal Cu and Pt for alloying.Monodisperse octahedral Pt_xCu_1-xnanoparticles with adjustable composition were prepared using appropriate capping agents.The particle size,around 5 nm,was not affected by the alloy composition.Alloying Pt and Cu not only reduced the loading amount of Pt in the sensor,but also obtained better catalytic activity due to the synergistic effect between multiphase atoms.By quickly annealing Pt_xCu_1-x on Sn O₂ nanoclusters with numerous mesopores and macropores on the cluster surface,gas molecules diffused inside the sensitive film more easily.By adjusting the composition of the nanocrystals,a sensing material with optimal acetone sensitivity,0.3 wt%Pt₄₀Cu₆₀-Sn O₂,was obtained.Under conditions of similar exhaled humidity,the response to 5 ppm acetone could reach 22.0.Compared with pure Sn O₂,the sensitivity of Pt₄₀Cu₆₀ nanocrystals to acetone increased by a factor of 10 after modification,making it an effective acetone sensitizing agent.More importantly,Pt₄₀Cu₆₀-Sn O₂ can overcome the influence of high humidity in exhaled gases and interference from multiple gases,and can distinguish between exhaled gases from 4 healthy individuals and 4 patients with diabetic ketoacidosis,making it suitable for auxiliary testing of diabetic ketoacidosis.This work confirms that common metals,especially Cu,alloyed with noble metals can obtain gas sensitizers with excellent performance.（3）The performance of alloy nanocrystals is not only related to their composition,but also to their crystal structure,which is an important factor affecting the sensitization properties of binary alloys.In this study,we investigated the effect of twin defects on the gas sensitization performance of Pt Cu nanocrystals.By controlling the type of solvent in the precursor solution and regulating the reduction rate of the metal precursor,we synthesized both multiply twinned Pt Cu icosahedra and single-crystal Pt Cu octahedra with similar chemical composition,particle size,work function,and exposed crystal facets,and with electron coupling strength similar to that of Sn O₂.Gas sensitivity tests showed that under high humidity conditions,using the same Sn O₂ as the main material,the twin Pt₅₇Cu₄₃ icosahedra-modified sensor exhibited higher response and better selectivity to acetone than the single-crystal Pt₅₆Cu₄₄octahedra-modified sensor,with a 112%increase in response to 5 ppm acetone and a detection limit as low as 5 ppb.The superior acetone sensitization performance of twin Pt₅₇Cu₄₃ is attributed to its higher density of twin defects and surface strain.This work highlights the uniqueness of twin defects in enhancing the gas sensitization properties of binary alloy nanocrystals and has important research value.（4）Palladium（Pd）is one of the most commonly used metal catalysts,which can form Pd H_x compounds with hydrogen and thus exhibits specificity towards H,making it highly attractive for use in hydrogen sensors.However,the amount of Pd required for detecting H gas using a single Pd-based sensor is typically high.Here,we designed Pd Cu alloy nanocrystals and one-dimensional W₁₈O₄₉ nanowire arrays to lower the working temperature and improve the sensitivity and selectivity of hydrogen sensing.Considering the trend towards miniaturization and integration of sensor structures,the hydrogen sensor was fabricated using micro-electro-mechanical systems（MEMS）technology with a micro-flat structure,which has a heating wire insulated by an oxide layer,thus reducing the risk of hydrogen explosions.Compared with pure W₁₈O₄₉ and Pd-W₁₈O₄₉,the 0.3 wt%Pd Cu-W₁₈O₄₉ hydrogen sensor exhibits high response（response to 0.5%hydrogen is 22.0）,fast response and recovery（6 s/130 s）at a lower operating temperature（125°C）and excellent selectivity.The key factors contributing to the excellent hydrogen sensitivity are the large specific surface area of ultrafine one-dimensional W₁₈O₄₉ nanowires,the increased Pd active sites introduced by Cu,the electronic sensitization of Pd O（Pd O（?）Pd）,the Pd-specific reaction with H to generate Pd H_x compounds,and the Pd⁴⁺（?）Pd²⁺catalytic redox cycle.This work is of significant importance for hydrogen detection and has promising applications in monitoring hydrogen leaks.In conclusion,the performance of binary alloy nanocrystals is closely related to their composition,structure,and atomic ratio,and the catalytic performance and selectivity of alloy nanocrystals can be greatly enhanced by optimizing the synthesis conditions,which has important research significance and promising applications in the field of gas sensors.