Mixture Of Au Research Articles

Optimal experiment design for element specific atom counting using multiple annular dark field scanning transmission electron microscopy detectors

This paper investigates the possible benefits for counting atoms of different chemical nature when analysing multiple 2D scanning transmission electron microscopy (STEM) images resulting from independent annular dark field (ADF) detector regimes. To reach this goal, the principles of statistical detection theory are used to quantify the probability of error when determining the number of atoms in atomic columns consisting of multiple types of elements. In order to apply this theory, atom-counting is formulated as a statistical hypothesis test, where each hypothesis corresponds to a specific number of atoms of each atom type in an atomic column. The probability of error, which is limited by the unavoidable presence of electron counting noise, can then be computed from scattering-cross sections extracted from multiple ADF STEM images. Minimisation of the probability of error as a function of the inner and outer angles of a specified number of independent ADF collection regimes results in optimal experimental designs. Based on simulations of spherical Au@Ag and Au@Pt core–shell nanoparticles, we investigate how the combination of two non-overlapping detector regimes helps to improve the probability of error when unscrambling two types of atoms. In particular, the combination of a narrow low angle ADF detector with a detector formed by the remaining annular collection regime is found to be optimal. The benefit is more significant if the atomic number Z difference becomes larger. In addition, we show the benefit of subdividing the detector regime into three collection areas for heterogeneous nanostructures based on a structure consisting of three types of elements, e.g., a mixture of Au, Ag and Al atoms. Finally, these results are compared with the probability of error resulting when one would ultimately use a pixelated 4D STEM detector and how this could help to further reduce the incident electron dose.

The efficient recovery of Au(III) ions from acidic solutions by a novel scavenger based on functionalized poly(styrene-co-maleimide) nanoparticles

Tertiary amine functionalized poly(styrene-co-maleimide) nanoparticles were readily synthesized by an organic solvent-free thermal imidization reaction. The obtained spherical nanoparticles, with average diameters of about 50 nm, were shown to be highly effective as a unique scavenger for the recovery of Au(III) ions from strong acidic solution. High equilibrium Au loading capacities (>570 mg/g) were obtained at 25 °C and the rate of metal ion uptake was remarkably fast. The Au(III) ions in the feed solutions were almost quantitatively recovered within 3–10 min, depending on the shaking speed during batch sorption. Elution of the adsorbed Au species was however incomplete with a maximum elution efficiency of 75% using acidified thiourea as eluant. XPS analysis showed that the adsorbed Au species were present as a mixture of Au(0), Au(I), and Au(III), which explained the inadequate elution. An alternative and innovative method to recover the adsorbed Au was developed consisting of digestion of the Au-loaded scavenger followed by reductive precipitation with water as reducing agent. The Au formerly loaded on the scavenger was successfully and completely recuperated as pure metallic Au. This is a step-change in the way Au is recovered from solution and eliminates the need for difficult, partial, and environmentally unfavorable elution processes of Au species strongly adsorbed on the scavenger particles.

Deposition of Au and ZnO nanoparticles from concentrated colloidal dispersions in ethanol on glass, polyethylene terephthalate, polystyrene and silicone substrates for manufacturing simple and combined coatings

The deposition of coatings of Au and ZnO nanoparticles (NPs) on glass, semiconductor silicon, polyethylene terephthalate, and polystyrene substrates was studied. Deposition was carried out using relatively concentrated dispersions of particles in ethanol (approximately 5–10 mM Au, ZnO), which showed limited time resistance to coagulation and sedimentation, but returned to their original dispersed state by ultrasonic treatment before each cycle of deposition of NPs. The NPs of Au and ZnO were stabilized by abietate and acetate ions, respectively. Simple coatings from Au or ZnO NPs and a combined coating from a mixture of Au and ZnO NPs, as well as two-layer combined coatings with alternating layers of Au/ZnO and ZnO/Au NPs, were obtained by drop casting and the proposed method of deposition from the capillary layer of the dispersion with the use of a linker. The formation of coatings on transparent substrates was studied by means of UV–Vis spectroscopy. Coatings were characterized by SEM, EDX, XRD, PL, UV–Vis and surface resistance methods. The effect of heat treatment on coatings characteristics was tested.

Size-Controlled Au–Cu2Se Core–Shell Nanoparticles and Their Thermoelectric Properties

One promising approach to improving thermoelectric energy conversion is to use nanostructured interfaces that enhance Seebeck coefficient while reducing thermal conductivity. Here, we synthesized Au-Cu2Se core-shell nanoparticles with different shell thicknesses by controlling the precursor concentration in solution. The Au-Cu2Se core-shell nanoparticles are about 37-53 nm in size, and the cores of the nanostructures are composed of Au nanoparticles with sizes of ∼11 nm. The effect of shell thickness on the thermoelectric properties of core-shell nanocomposites is investigated after sintering the core-shell nanoparticles into pellets using the spark plasma sintering (SPS) technique. The power factor was optimized by the synergetic effect of the improvement of Seebeck coefficient by energy filtering in the Au/Cu2Se interface and the effective tuning of carrier concentration by Ohmic contact in the interface. The lattice thermal conductivity of core-shell nanocomposites is reduced by coherent phonon scattering, which is caused by the wavelike interference of phonons due to the phase shift in the core-shell interface. The highest ZT value of 0.61 is obtained at 723 K for Au-Cu2Se core-shell nanocomposite with a shell thickness of 21 nm, which is higher than that of pure Cu2Se nanocomposite or a mixture of Au and Cu2Se particles.

Determination of Hg(II) based on the inhibited catalytic growth of surface-enhanced Raman scattering-active gold nanoparticles on a patterned hydrophobic paper substrate

Anthropogenic mercury (Hg) presents serious risks to the surroundings and public health due to its toxicity and bioaccumulation. In this study, a new assay was developed to combat mercury-related issues, and it is based on surface-enhanced Raman scattering (SERS) on an active paper substrate for rapid Hg(II) determination. The sensing principle is based on the hindering effect of Hg(II) on the catalytic growth of SERS-active gold nanoparticles (AuNPs) on a hydrophobically patterned paper substrate. 4-mercaptobenzonic acid (4-MBA) was chosen as an effective Raman reporter molecule, and its Raman signal was enhanced by surface plasmonic 2-[4-(2-hydroxyethel) piperazine-1-yl]ethanesulfonic acid-stabilized gold nanostars (HEPES-AuNSs) on a piece of paper with a hydrophobic surface. After adding a Hg(II) analyte solution to this SERS substrate, an amalgam formed at the solid–liquid interface of the HEPES-AuNSs. The formation of the AuHg amalgam resulted in the dissolution of the branches of the HEPES-AuNSs. In the presence of a growth solution (mixture of Au(III), HCl, and H2O2), the remaining HEPES-AuNSs and the AuHg amalgams acted as seeds to form small AuNPs, resulting in a weak SERS signal of 4-MBA (Raman shift at 1590 cm−1). Solution concentration, temperature, and treatment time were optimized to realize a significant decrease in the Raman intensity of 4-MBA when the paper sensor was exposed to Hg(II). The Raman intensity decreased with the increasing concentration of Hg(II) ions in the range of 0.1 nM–1.0 μM (R2 = 0.98), with a limit of detection (S/N = 3.0) of 0.03 nM. For practicality, the proposed paper substrate was examined by estimating Hg(II) concentrations in environmental water samples (i.e., seawater and pond water) and the certified standard of SRM 1641d through standard addition and a recovery study. The promising selectivity, sensitivity, and reproducibility of the proposed paper sensor constitute substantial progress toward the portable detecting mercury in real water samples, which can facilitate crucial surrounding monitoring.

Effect of irradiation time in the synthesis of Au-Ag nanoalloys by femtosecond laser

The synthesis of metallic nanoparticles can be performed by femtosecond laser-induced photoreduction of the metallic ion solution. Due to hydrogen radical and the solvated electron generated through the interaction of ultrafast laser and water medium, the dissolved metallic ion in the medium can be converted to their respective metallic atom and then precipitated into nanoparticles. When there is more than one metallic ion available in the solution, alloy nanoparticles can be generated. One parameter that affects this laser-based synthesis was the duration of laser irradiation time that has an advantage for size modification of nanoparticles. In this work, we have synthesized Au-Ag nanoalloys from the mixture of Au and Ag ion in various volume fraction in water medium added with 0.01wt% polyvinylpyrrolidone (PVP) as a capping agent and irradiated for 5 and 10 minutes. The result showed that there was linear shifting in surface plasmon resonance (SPR) of the nanoalloys in respect with their volume fraction of the ions. The results also revealed that the nanoparticle size was indeed reduced as the laser irradiation was prolonged, i.e., Au50Ag50 has a particle size of 7.98 nm and 5.18 nm for 5 and 10 minutes irradiation time, respectively.

Determining the Composite Structure of Au-Fe-Based Submicrometre Spherical Particles Fabricated by Pulsed-Laser Melting in Liquid.

Submicrometre spherical particles made of Au and Fe can be fabricated by pulsed-laser melting in liquid (PLML) using a mixture of Au and iron oxide nanoparticles as the raw particles dispersed in ethanol, although the detailed formation mechanism has not yet been clarified. Using a 355 nm pulsed laser to avoid extreme temperature difference between two different raw particles during laser irradiation and an Fe2O3 raw nanoparticle colloidal solution as an iron source to promote the aggregation of Au and Fe2O3 nanoparticles, we performed intensive characterization of the products and clarified the formation mechanism of Au-Fe composite submicrometre spherical particles. Because of the above two measures (Fe2O3 raw nanoparticle and 355 nm pulsed laser), the products—whether the particles are phase-separated or homogeneous alloys—basically follow the phase diagram. In Fe-rich range, the phase-separated Au-core/Fe-shell particles were formed, because quenching induces an earlier solidification of the Fe-rich component as a result of cooling from the surrounding ethanol. If the particle size is small, the quenching rate becomes very rapid and particles were less phase-separated. For high Au contents exceeding 70% in weight, crystalline Au-rich alloys were formed without phase separation. Thus, this aggregation control is required to selectively form homogeneous or phase-separated larger submicrometre-sized particles by PLML.

One‐step Synthesis of AuAg Alloy Nanodots and its Electrochemical Studies towards Nitrobenzene Reduction and Sensing

AbstractEfficient, stable, and low‐cost electrocatalysts for the degradation and sensing of environment pollutants are essential components of clean environment monitoring. Here we report, one‐step synthesis and characterization of 1–3 nm diameter sized bi‐metallic AuAg nanodots (NDs) embedded in amine functionalized silicate sol‐gel matrix (SSG) and its electrochemical studies toward nitrobenzene. The SSG was used as a reducing agent as well as stabilizer for the prepared mono‐ and bi‐metallic nanoparticles (NPs). From the HRTEM, STEM‐EDS and XPS analyses, the bi‐metallic AuAg NDs were identified as an alloy and not the mixtures of Au and Ag NPs. Characteristic surface plasmon resonance (SPR) band between the Au and Ag NPs SPR absorption region was noticed for the prepared AuAg NDs. The AuAg alloy NDs with different concentrations of Au and Ag (Au25Ag75, Au50Ag50 and Au75Ag25 NDs) modified electrodes exhibited synergistic electrocatalytic effect than did the Au and Ag NPs towards nitrobenzene reduction and detection. Together with ultra‐small size and exceptional colloidal stability features within these SSG‐AuAg NDs pave convenient way for nanotechnology‐based catalysts development and sensor applications.

Broadband optical absorption enhancement of N719 dye in ethanol by gold–silver alloy nanoparticles fabricated under laser ablation technique

The formation of gold–silver alloy nanoparticles (Au–Ag alloy NPs) by a two-step process with a pulsed Nd:YAG laser without any additives is presented. Mixtures of Au and Ag colloidal suspensions were separately obtained by 1064-nm laser ablation of metallic targets immersed in ethanol. Subsequently, the as-mixed colloidal suspensions were reirradiated by laser-induced heating at the second-harmonic generation (532 nm) for different irradiation periods of time. The absorption spectra and morphology of the colloidal alloys were studied as a function of exposure time to laser irradiation. Transmission electron microscopy revealed the formation of monodispersed spherical nanoparticles with a homogeneous size distribution in all the synthesized samples. UV–vis and photoluminescence spectroscopy measurements were also employed to characterize the changes in the light absorption and emission of N719 dye solution with different concentrations of Au–Ag colloidal alloys, respectively. The localized surface plasmon resonance (LSPR) of Au–Ag alloy NPs enhanced the absorption and fluorescence peak of the dye solution. The mixture of dye molecules with a higher concentration of alloy NPs exhibited an additional coupling of dipole moments with the LSPR, thereby contributing to the improvement of the optical properties of the mixture.

Photochemical Synthesis of Au@Pd Core-Shell Nanoparticles for Methanol Oxidation Reaction: the Promotional Effect of the Au Core

A novel method for synthesizing Au@Pd core-shell nanoparticles was proposed based on photochemistry. By irradiating the mixture of Au (III) and Pd (II) ions using ultraviolet light, the Au@Pd core-shell nanoparticles were prepared. The size of the nanoparticles and the thickness of the Pd shell could be efficiently adjusted by changing the molar ratio of Au (III) to Pd (II) ion. In this way, nanoparticles with diameter in the range of 5.6~4.6 nm were obtained. The core-shell structure of the synthesized nanoparticles was showed by the characterization using UV-Vis, TEM/HR-TEM and XPS. The paper investigated the electrocatalysis performance of Au@Pd nanoparticles in the methanol catalytic oxidation reaction, as well as the electron donating effect of Au core to Pd shell and the promotion of this effect on the catalytic activity of Pd shell. The experimental results provided reference for the development of non-platinum catalysts of low-temperature fuel cell anode.

Injectable and Self-Healing Dynamic Hydrogels Based on Metal(I)-Thiolate/Disulfide Exchange as Biomaterials with Tunable Mechanical Properties.

Despite numerous strategies involving dynamic covalent bonds to produce self-healing hydrogels with similar frequency-dependent stiffness to native tissues, it remains challenging to use biologically relevant thiol/disulfide exchange to confer such properties to polymeric networks. Herein, we report a new method based on Metal(I) [Au(I) or Ag(I)] capping to protect thiolates from aerial oxidation without preventing thiolate/disulfide exchange. Dynamic hydrogels were readily prepared by injecting simultaneously aqueous solutions of commercially available HAuCl4 and 4-arm thiol-terminated polyethylene glycol [(PEGSH)4], resulting in a network containing a mixture of Au(I)-thiolate (Au-S) and disulfide bonds (SS). While the dynamic properties of the hydrogel were closely dependent on the pH, the mechanical properties could be easily tuned by adjusting (PEGSH)4 concentration and amount of Au-S, as judged by dynamic rheology studies. Permanent Au-S/SS exchange at physiological pH conferred self-healing behavior and frequency-dependent stiffness to the hydrogel. In addition, in vitro studies confirmed that Au-based dynamic material was not cytotoxic to human dermal fibroblasts, demonstrating its potential use as a medical device. Dynamic hydrogels obtained using Ag(I) ions demonstrated that the exchange reaction was not affected by the nature of the Metal(I) capping. Finally, this efficient thiolate capping strategy offers a simple way to produce injectable and self-healing dynamic hydrogels from virtually any thiol-containing polymers.

Preparation of gold-coated sand for ultratrace mercury analysis

In this study, the gold-coated sand was prepared to make gold-trap for mercury analysis at ultra-trace levels. The preparation process was conducted with the etching of sand surface to form silanol groups using a mixture of H2O2/H2SO4 followed by the replacement of the silanol groups by amino groups with aminopropyltriethylsilane. The amino-modified surface is embedded with Au atoms by soaking the amino modified sand in nano gold solution then the Au film is thickened with a mixture of Au(III)+NH2OH.HCl. The gold-coated sand can capture up to 21 μg Hg per 1 g of sorbent. The interferences for the efficiency of mercury sorption such as HNO3, HCl, H2SO4 and temperatureare were investigated. The goldcoated sand was successfully applied for mercury analysis in certified reference materials (DORM-4) by CV-AAS and combustion/thermal decompostion methods.

Ag-Fe2O3 Heterostructured Nanoparticles as Photocatalysts under Visible Light Irradiation

Due to the synergistic effect between different composites, an increasing number of functional semiconductor-metal hybrid nanomaterials with core-shell or heterostructure geometry has been fabricated and proposed as the preferred form of catalysts. For example, Au@Co3O4 core-shell nanostructures showed higher oxygen evolution reaction activity than a mixture of Au and Co3O4 nanoparticles, or Co3O4 and Au nanoparticles alone. In addition, PbS-Ag heterostructure nanotubes exhibited significantly enhanced photocatalytic activity. Thanks to the controlled syntheses of semiconductor-metal nanocomposites with desired morphologies, it has been feasible to study the nanostructure tuning capacities. Herein, Ag-Fe2O3 core-shell and heteromer nanocomposites were synthesized by a seed-mediated growth method reported previously, and hollow Fe2O3 nanoparticles were obtained by the transformation of Ag-Fe2O3 core-shell nanocomposites. Subsequently, the photocatalytic activity of Ag-Fe2O3 core-shell nanocomposites was directly compared with that of Ag-Fe2O3 heteromer and hollow Fe2O3 nanoparticles, respectively. Finally, we could demonstrate experimentally whether the enhanced photocatalytic activity of Ag-Fe2O3 core-shell arise from its wide absorbance range of visible light and favorable charge separation. This investigation would help us to further understand nanostructure tuning capacity of nanocomposites and control their syntheses for a given application.

Laser-Induced Synthesis of Au–Ag Alloy Nanoparticles in Polyvinylpyrrolidone (C6H9NO)n Solution

Au–Ag alloy nanoparticles (NPs) were prepared in Polyvinylpyrrolidone (C6H9NO)n (PVP) solution by laser irradiation method. Au and Ag NPs were separately prepared in PVP solution by laser ablation. The mixture of Au and Ag NP colloids was irradiated by the second harmonic (532 nm) of pulsed Nd:YAG laser with different laser fluencies and exposure times. The Plasmon resonance absorption spectrum, morphology and structure of Au–Ag alloy NPs were observed by a UV-2450 spectrometer (Shimadzu), a high resolution transmission electron microscope (TEM Tecnai G2 20 S-TWIN/FEI). The results show that the resulting Au–Ag alloy NPs are homogeneous alloyed particles and dispersed in PVP solution with average diameter of 4 nm without sintered structure. Au–Ag alloy NPs were produced by 532 nm laser with different Au/Ag molar ratio of the initial mixed solution to give the plasmon resonance absorption peak in the intermediate positions between about 420 and 520 nm.

Fabrication and evaluation of Au–Pd core-shell nanocomposites for dechlorination of diclofenac in water

Nanocomposites with core-shell structure usually exhibit excellent catalytic properties due to unique interfaces and synergistic effect among composites. In this study, Au–Pd bimetallic nanoparticles (NPs) with core-shell structure (Au–Pd cs) by using Au NPs as core and Pd as shell were successfully fabricated and, for the first time, were used to investigate the dechlorination of diclofenac (DCF) at H2 atmosphere in water at room temperature. The degradation products were studied as well by using HPLC/Q-ToF MS/MS. The operational factors such as pH and composition of the Au–Pd cs were also studied. The results showed that nearly 100% of DCF (30 mg L−1, 50 mL, pH = 7) was dechlorinated in 4.5 h by 10 mL of 56 mg L−1 of Au–Pd cs. Ninety per cent of DCF was degraded in 6.5 h by the mixture of Au and Pd NPs. However, the individual Au NPs had no obvious effect in degrading DCF and the monometallic Pd NPs with comparable concentration only degraded less than 20% of DCF. Furthermore, the reaction mechanism of this catalytic process was studied in detail. It was found that the degradation was a second-order exponential reaction. The two main degradation products were obtained by cleaving the carbon–halogen bond of DCF and this made the degradation products more environmentally friendly.

Chiral monolayer-protected Au–Pd bimetallic nanoclusters: Effect of palladium doping on their chiroptical responses

We present an effect of palladium doping on the chiroptical responses of optically active gold nanoclusters. Monolayer-protected Au–Pd bimetallic nanoclusters of 1.5–2.0nm in diameter are synthesized by the reduction in mixtures of Au and Pd salts in the presence of chiral thiol, glutathione (GSH), and isolated by electrophoretic separation. For synthetic feed mole ratios of Au/Pd=3/1 and 1/1, the isolated nanocluster compounds have much higher Au/Pd ratios of ∼8.4 (7.9–8.8) and ∼5.1 (4.7–5.4), respectively. We find that circular dichroism (CD) offers a greater advantage in detecting a foreign atom doping in the nanocluster as compared to absorption spectroscopy; namely, there is a significant difference in Cotton effects between the Au–Pd bimetallic nanoclusters and the monometallic counterparts (Au or Pd nanoclusters), whereas almost no difference can be seen between them in the UV–vis absorption spectra. In the bimetallic nanoclusters, interestingly, their CD profiles are almost independent on the Pd dopant content. The X-ray photoelectron spectroscopy (XPS) reveals that Au atoms are predominated in the surface (including staples) layer of these Au–Pd nanoclusters, indicating a Pdcore–Aushell (inverted core–shell) structure, so the similarity in the CD responses observed would be due both to the predominance of Au in the outermost chiral surface structure and possession of their similar metal core configuration.

Local structures of ionic liquids in the presence of gold under high pressures

The interactions between ionic liquid ([EMI][TFS]) and gold surfaces have been investigated via the application of pressures up to ca. 2 GPa. Comparing the spectral features of [EMI][TFS]/gold with those of pure [EMI][TFS], no appreciable changes of C-H bands in the presence of gold powders were observed under ambient pressure. Nevertheless, the imidazolium C-H bands display red shifts in frequency as the [EMI][TFS] / Au mixture was compressed to the pressure above 1.4 GPa and a new alkyl C-H band at ca. 3016 cm−1 was also revealed. These spectral changes, being related to the addition of gold powders and pressure elevation, should be attributed to the local structural changes of C-H groups caused by pressure-enhanced interfacial interactions between [EMI][TFS] and Au. Gold powders tend to induce the changes in hydrogen bonding structures of imidazolium C2-H group under high pressures. The pressure-dependent spectral features in the asymmetric SO3 stretching region display band-narrowing and minor local structural changes induced by the presence of gold particles under high pressures. These observations suggest that Au powders perturb structural equilibrium of C-H groups of cations under high pressures.

Fabrication of solid-solution gold–platinum nanoparticles with controllable compositions by high-intensity laser irradiation of solution

In the present study, solid-solution gold–platinum (Au–Pt) nanoparticles with controllable compositions were fabricated by high-intensity femtosecond laser irradiation of an aqueous solution of gold and platinum ions without any chemicals and complicated processes. Transmittance electron microscopy revealed that the single nanometer-sized particles were fabricated by femtosecond laser irradiation of mixed aqueous solutions of gold and platinum ions. The crystalline structure of nanoparticles was characterized by electron and X-ray diffractions. Contrary to the bulk Au–Pt binary systems, which commonly contain a pair of diffraction peaks between pure gold and platinum peaks because of its large miscibility gap in phase diagram, or mixture of Au and Pt, the diffraction peaks of Au–Pt nanoparticles fabricated in the experiment showed a characteristic of the fcc-type lattice. Moreover, the diffraction patterns shifted monotonically from the peak position of pure gold to that of pure platinum as the fractions of platinum ions in the solution were increased. These observations strongly imply that the Au–Pt nanoparticles were solid solution with intended compositions. This technique is not only simple and environmentally friendly, but also applicable to other binary and ternary systems.

Recovery of high-purity Pt from Pt-Au bimetallic nanoparticles using organic aqua regia

Using organic aqua regia, a recently discovered powerful organic leaching agent, an effective process of recovering Pt directly from Pt-Au bimetallic nanoparticles was demonstrated. The purities of the Pt recovered from a mixture of Au and Pt nanoparticles and from Pt-Au core-shell nanoparticle catalyst are as high as (99.49±0.22) %, and (95.02±0.08) %, respectively. The novel recovery process promises applications in catalysis industry.

Effects of Calcination Temperature and Acid-Base Properties on Mixed Potential Ammonia Sensors Modified by Metal Oxides

Mixed potential sensors were fabriated using yttria-stabilized zirconia (YSZ) as a solid electrolyte and a mixture of Au and various metal oxides as a sensing electrode. The effects of calcination temperature ranging from 600 to 1,000 °C and acid-base properties of the metal oxides on the sensing properties were examined. The selective sensing of ammonia was achieved by modification of the sensing electrode using MoO3, Bi2O3 and V2O5, while the use of WO3, Nb2O5 and MgO was not effective. The melting points of the former group were below 820 °C, while those of the latter group were higher than 1,000 °C. Among the former group, the selective sensing of ammonia was strongly dependent on the calcination temperature, which was optimum around melting point of the corresponding metal oxides. The good spreading of the metal oxides on the electrode is suggested to be one of the important factors. In the former group, the relative response of ammonia to propene was in the order of MoO3 > Bi2O3 > V2O5, which agreed well with the acidity of the metal oxides. The importance of the acidic properties of metal oxides for ammonia sensing was clarified.