Bulk SnO2 Research Articles

In Situ IR Spectroscopy Studies of Atomic Layer-Deposited SnO2 on Fullerenes for Perovskite Photovoltaics.

In recent years, atomic layer deposition (ALD) has established itself as the state-of-the-art technique for the deposition of SnO2 buffer layers grown between the fullerene electron transport layer (ETL) and the ITO top electrode in metal halide perovskite-based photovoltaics. The SnO2 layer shields the underlying layers, i.e., the fullerene-derivative materials such as C60 and PCBM, as well as the perovskite absorber, from water ingress and damage induced by the sputtering of the transparent front contact. Our study undertakes a comprehensive investigation of the impact of SnO2 ALD processing on fullerenes by means of in situ spectroscopic ellipsometry (SE) and transmission infrared spectroscopy (FTIR). While no difference in SnO2 bulk properties is observed and the perovskite absorber degradation is nearly entirely avoided during exposure to heat and vacuum, when the absorber is introduced beneath the organic ETLs, a SnO2 growth delay of about 50 ALD cycles is measured on PCBM, whereas the delay is limited to 10 cycles in the case of growth on C60. Notably, FTIR measurements show that while C60 remains chemically unaffected during SnO2 ALD growth, PCBM undergoes chemical modification, specifically of its ester groups. The onset of these modifications corresponds with the detection of the onset, after the initial delay, of ALD SnO2 growth. It is expected that the modification that the PCBM layer undergoes upon ALD SnO2 processing is responsible for the systematic lower photovoltaic device performance in the case of PCBM-based devices, with respect to C60-based devices.

Polysaccharide‐Modified SnO2 for Highly Efficient Perovskite Solar Cells

Tin oxide (SnO2) has been demonstrated as a promising electron transport material for perovskite solar cells (PSCs) due to its low‐temperature process and high charge extraction ability. However, the key to improving the internal performance of PSCs toward unity lies in the ability of enhancing bulk SnO2 quality and reducing the energy level mismatch between SnO2 and perovskite layer. Herein, a facile and effective strategy to simultaneously functionalize SnO2 structure by sodium alginate (SA) polysaccharide compound is reported. The investigations reveal that SA leads to the formation of nanosynapse structure of SnO2 films, which allows the aligned energy levels and suppressed charge recombination, significantly enhancing carrier extraction and transport. Consequently, CsMAFAPb(IxBr1−x)3 and CsPbI2Br solar cells achieve a remarkable power conversion efficiency of 24.11% and 16.90%, respectively, based on SnO2‐SA electron transport layers (ETLs). This work offers a facile and effective way to fabricate high‐performance SnO2 ETL in PSCs.

2D nature of magnetic states at SnO2 surfaces: a combined experimental and theoretical study.

For undoped SnO2, room temperature ferromagnetism could be seen uniquely in 2-dimensional configurations, particularly in ultra-thin films (whose thickness is ideally below 100 nm). Both bulk samples and nano-powders of pristine SnO2 are diamagnetic, indicating that a 2D surface is a key point in shaping up the magnetic properties in SnO2. As a complement to our experiments, we have performed a series of quantum-mechanical calculations for the bulk rutile-structure SnO2 as well as its (001) and (101) surfaces. The calculations included several atomic configurations with and without vacancies in/under the studied surfaces. The stability of the non-magnetic ground state of rutile SnO2 bulk was cross-checked and confirmed by its phonon spectrum computed within the harmonic approximation. Regarding the surfaces, the bulk-like (001) surface containing Sn vacancies has turned out to be ferromagnetic, while the shift of Sn vacancies under the surface resulted in a more complex ferrimagnetic state. The bulk-like (001) surface without vacancies and that with the O vacancies are predicted to be non-magnetic. Regarding the (101) surfaces, those terminated by a single layer of oxygen atoms and those terminated by tin atoms are non-magnetic, while a surface terminated by two layers of oxygen has turned out to be ferromagnetic.

Characterization of Nanocrystalline SnO<sub>2</sub>-Y<sub>2</sub>O<sub>3</sub> Thin Films Prepared Using Chemical Bath Deposition

Nanocrystalline SnO2-Y2O3 thin film has been successfully prepared by using chemical bath deposition method at low reaction temperature 72 °C on SiO2/Si substrates. The structural and surface morphology of the annealed sample at 500 °C for 2 h in air were investigated using X-ray diffraction, field emission scanning electron microscopy and Energy-dispersive X-ray spectroscopy. The crystallization of SnO2-Y2O3 film with tetragonal rutile structure was achieved when the film was exposed to annealing at 500 °C. Where several diffraction peaks that correspond to the (110), (101), (200), (211), (220) and (002) planes that agree very well with standard bulk SnO2 having a tetragonal rutile structure. As well as the diffraction peak that correspond to (111) emerged at θ = 29.48o is matched with bulk Y2O3. The surface morphology appeared as polycrystalline with uniform nanoparticle distribution. The EDX spectra of examined film showed the film consists of O, Sn, Y, and Si elements. The cross-section image and the average thickness of the annealed SnO2-Y2O3 film at 500 °C was approximately 330 nm. Additionally, approximately 880 nm thick layer of SiO2 emerges on the top of the silicon substrate. This finding demonstrates the ability to prepare nonocrystalline SnO2-Y2O3 thin film with high quality by using chemical bath deposition method.

Crossover from small polaron tunneling to correlated barrier hopping and its evolution with thermal instability of SnO surface

In this paper, we have studied the surface and bulk stoichiometry and its effect on DC-AC conductivity measurements by x-ray photoelectron spectroscopy (XPS) as well as Raman spectroscopy depth profiling respectively. The SnO surface sheets are readily oxidized to Sn3O4phases and partially SnO2. The temperature dependence of DC resistance shows three-dimensional variable range hopping (VRH) below 200 K and Nearest Neighbor Hopping (NNH) between 200 and 300 K and undergoes a crossover to the thermally activated band conduction above 300 K till it decomposes at 600 K. This crossover temperature increases beyond 300 K during the subsequent heating cycles. In the frequency and temperature dependent conductivity, the bulk SnO throughout 300–600 K shows the frequency exponent of 0.8 which is ascribed to tunneling mechanism. Whereas, the surface progressively changes from correlated barrier hopping, CBH (band conduction) to a crossover of non-overlapping small polaron tunneling to CBH in samples in 300 to 400 K range after a thermal cycling. The oxidized phases bring about the polaronic sites which tunnel below 400 K. The energy of hopping reveals that the pristine sample has band like transport due to shallow Sn vacancies acceptors and the oxidized surface repositions fermi level to midgap due to oxygen vacancy donors.

Temperature and Ambient Band Structure Changes in SnO2 for the Optimization of Hydrogen Response

Tin dioxide (SnO2) is one of the most used materials for sensing applications operating at high temperatures. Commonly, “undoped SnO2” is made by precursors containing elements that can have a deleterious impact on the operation of SnO2 sensors. Here, we employ experimental and theoretical methods to investigate the structural properties and electronic structure of the rutile bulk and surface SnO2, focusing on unintentional doping due to precursors. Unintentional doping from precursors as well as intrinsic doping can play an important role not only on the performance of gas sensors, but also on the properties of SnO2 as a whole. The theoretical calculations were performed using density functional theory (DFT) with hybrid functionals. With DFT we examine the changes in the electronic properties of SnO2 due to intrinsic and unintentional defects and we then discuss how these changes affect the response of a SnO2-based gas sensor. From an experimental point of view, we synthesized low-cost SnO2 thin films via sol–gel and spin-coating processes. To further enhance the performance of SnO2, we coated the surface with a small amount of platinum (Pt). The crystalline structure of the films was analyzed using x-ray diffraction (XRD) and scanning electron microscopy (SEM), while for the determination of the elements contained in the sample, X-ray photoelectron spectroscopy (XPS) measurements were performed. Furthermore, we investigated the effect of temperature on the band structure of SnO2 in air, in a vacuum and in nitrogen and hydrogen chemical environments. To optimize the response, we used current–voltage characterization in varying environments. The aim is to associate the response of SnO2 to various environments with the changes in the band structure of the material in order to gain a better understanding of the response mechanism of metal oxides in different pressure and temperature environments. We found that the resistance of the semiconductor decreases with temperature, while it increases with increasing pressure. Furthermore, the activation energy is highly affected by the environment to which the thin film is exposed, which means that the thin film could respond with lower energy when exposed to an environment different from the air.

Electronic stability study of tin dioxide as a catalyst for lithium-air batteries

The production of unstable, irreversible discharge products which contributes to capacity fading still remains the main challenge that limits the practical use of Li-air batteries. To improve the electrochemical performance of the Li-air batteries, a clear understanding of the catalytic activity of SnO2 towards the production of stable discharge products is essential. In this study, we use the density functional theory calculation to investigate the bulk properties and surfaces stability of SnO2. The SnO2 bulk structure was found to be a stable semiconductor with a band gap of 0.63 eV. The phonon dispersion curves indicate that the structure is vibrationally stable since it shows no soft mode along the brillouin zone direction. The elastic constants show that our structure is mechanically stable since the C’ is positive and obey the mechanical stability criteria for a tetragonal crystal.

Electron and Hole Mobility of SnO2 from Full-Band Electron–Phonon and Ionized Impurity Scattering Computations

Mobility is a key parameter for SnO2, which is extensively studied as a practical transparent oxide n-type semiconductor. In experiments, the mobility of electrons in bulk SnO2 single crystals varies from 70 to 260 cm2V−1s−1 at room temperature. Here, we calculate the mobility as limited by electron–phonon and ionized impurity scattering by coupling the Boltzmann transport equation with density functional theory electronic structures. The linearized Boltzmann transport equation is solved numerically beyond the commonly employed constant relaxation-time approximation by taking into account all energy and momentum dependencies of the scattering rates. Acoustic deformation potential and polar optical phonons are considered for electron–phonon scattering, where polar optical phonon scattering is found to be the main factor which determines the mobility of both electrons and holes at room temperature. The calculated phonon-limited electron mobility is found to be 265 cm2V−1s−1, whereas that of holes is found to be 7.6 cm2V−1s−1. We present the mobility as a function of the carrier concentration, which shows the upper mobility limit. The large difference between the mobilities of n-type and p-type SnO2 is a result of the different effective masses between electrons and holes.

Structural properties of ordered porous SnO2 nanostructure

Three-dimensional porous ordered SnO2 nanostructures have been fabricated by templating a sol–gel pre- cursor solution against the polystyrene nanospheres for the first time. Field emission scanning electron micrography (FESEM) indicates that the surface of the nanostructures is highly regular and the porous structures are perfectly ordered. Besides a broad emission band at 600 nm, the porous SnO2 nanostructures show an additional emission band at 430 nm, which is seldom seen in the bulk SnO2 materials. Spectral examinations and analyses reveal that the 430 nm band is induced by the interfacial effects between the porous frameworks.

Polymorphic Control of Solution-Processed Cu2SnS3 Films with Thiol-Amine Ink Formulation.

There is increasing demand for tailored molecular inks that produce phase-pure solution-processed semiconductor films. Within the Cu–Sn–S phase space, Cu2SnS3 belongs to the I2–IV–VI3 class of semiconductors that crystallizes in several different polymorphs. We report the ability of thiol–amine solvent mixtures to dissolve inexpensive bulk Cu2S and SnO precursors to generate free-flowing molecular inks. Upon mild annealing, polymorphic control over phase-pure tetragonal (I4̅2m) and orthorhombic (Cmc21) Cu2SnS3 films was realized simply by switching the identity of the thiol (i.e., 1,2-ethanedithiol vs 2-mercaptoethanol, respectively). Polymorph control is dictated by differences in the resulting molecular metal–thiolate complexes and their subsequent decomposition profiles, which likely seed distinct Cu2–xS phases that template the ternary sulfide sublattice. The p-type tetragonal and orthorhombic Cu2SnS3 films possess similar experimental direct optical band gaps of 0.94 and 0.88 eV, respectively, and strong photoelectrochemical current responses. Understanding how ink formulation dictates polymorph choice should inform the development of other thiol–amine inks for solution-processed films.

First-principles Density Functional Theory Calculation of Bulk SnO2 and Grain Boundaries

First-principles Density Functional Theory Calculation of Bulk SnO₂ and Grain Boundaries

Creation of three dimensional octahedral tin oxide nanostructure produced by laser ablation in liquid

The laser ablation in liquid is a simple, easy, and less hazardous method available to create nanoparticles without chemical additives. In this paper, nanosecond laser ablation was conducted in various aqueous solvents to create tin oxide (SnO[Formula: see text] nanoparticles from bulk SnO powder. After the laser ablation at 532 nm wavelength, the prepared samples were examined by UV–visible spectroscopy, transmission electron microscopy (TEM), scanning electron microscopy (SEM), and energy-dispersive X-ray spectroscopy (EDS). UV–visible spectra showed that the significantly different absorption characteristics of nanoparticles are dependent on the original SnO amount in the solvent. The SEM observation clearly showed the creation of SnO2 nanoparticles of about 10–20 nm in size with irregular shapes, which were found to be secondary nanoparticles from the TEM images. Interestingly, a three-dimensional octahedral structure with a size of 220–300 nm having the surface made of aggregates of fine nanoparticles was confirmed by TEM analysis. From the EDS spectra, it was clarified that the octahedral agglomeration is made from tin and oxygen, and the electron diffraction image confirmed the formation of the SnO2 crystalline phase.

Effects of potassium treatment on SnO2 electron transport layers for improvements of perovskite solar cells

SnO2 has been studied intensively as an electron transport layer (ETL) for highly efficient metal halide perovskite solar cells. However, SnO2 ETL frequently exhibits defect-related issues associated with the bulk SnO2 and the perovskite/SnO2 interface, and to passivate the defect states potassium ion has been used. In order to investigate the passivation effect of potassium ion, we carried out KCl treatment on the solution-processed SnO2 ETL. It was found that KCl-treatment shifted up conduction band maximum and Fermi energy of SnO2, and reduced the band gap of perovskite absorber by K+ diffusion, which resulted in a better conduction band alignment at perovskite/SnO2. Admittance spectroscopy revealed that diffused K+ ions can passivate defects of the perovskite absorber layer. With the improvement of band alignment and defect passivation via the KCl treatment, the J-V hysteresis was almost eliminated and power conversion efficiency was much enhanced with improved open-circuit voltage and fill factor.

Diverse-shaped tin dioxide nanoparticles within a plastic waste-derived three-dimensional porous carbon framework for super stable lithium-ion storage

Tin dioxides (SnO2) inserted into carbons to serve as anodes for rechargeable lithium-ion batteries are known to improve their cycling stability. However, studies on diverse-shaped SnO2 nanoparticles within a porous carbon matrix for super stable lithium-ion storage are rare. Herein, a hollow carbon sphere/porous carbon flake (HCS/PCF) framework is fabricated through template carbonization of plastic waste. By changing the doping mechanism and tuning the loading content, nano SnO2 spheres and cubes as well as bulk SnO2 flakes and blocks are in-situ grown within the HCS/PCF. Then, the as-prepared hybrids with built-in various morphological SnO2 nanoparticles serve as anodes towards advanced lithium-ion batteries. Notably, HCS/PCF embedded with nano SnO2 spheres and cubes anodes possess superb long-term cycling stability (~0.048% and ~0.05% average capacitance decay per cycle at 1 A/g over 400 cycles) with high reversible specific capacities of 0.45 and 0.498 Ah/g after 1000 cycles at 5 A/g. The ultra-stabilized Li+ storage is attributed to the effective mitigation of nano SnO2 spheres/cubes volume expansion, originating from the compact SnO2 yolk-HCS/PCF shell construction. This study paves a general strategy for disposing of polymeric waste to produce SnO2 core-carbon shell anodes for super stable lithium-ion storage.

Flash-thermochemical engineering of phase and surface activity on metal oxides

<h2>Summary</h2> Momentary thermal annealing techniques based on the generation of photo-excited electrons in materials have been applied for ultrafast optical sintering in carbon, metal, and narrow-band-gap black-colored oxides. However, activating the photothermal effect for white-colored oxides is a grand challenge due to their low photothermal conversion efficiencies. Here, we report remarkably enhanced photothermal effects on binary, white-colored tin oxide (SnO₂) via a flash-thermal shock (FTS) lamping, which are triggered by low thermal conductivity of polycrystalline nanosheets with abundant defects. These features enable momentary high-temperature annealing (temperature >1,800°C and duration <20 ms) in SnO₂ under ambient air. Importantly, the FTS process facilitates thermochemical tuning of crystal structures in bulk SnO₂ lattice and the simultaneous ultrafast synthesis of multi-elemental nanoparticles (<10 nm) and/or single-atom catalyst on the host tin oxides. To prove their practical utility, we demonstrated the exceptional chemiresistive gas-sensing capabilities with microelectromechanical systems.

Palladium Embedded in SnO2 Enhances the Sensitivity of Flame-Made Chemoresistive Sensor

Introduction Palladium is commonly used to enhance the performance of chemoresistive metal-oxide gas sensors [1]. Typically, this enhancement is attributed to the presence of Pd clusters on the surface of their metal oxide support (e.g. SnO2). More specific, small Pd clusters on SnO2 were associated to enhanced sensitivity to H2 due to fermi-level control [2] or spillover effects [1]. However, Pd may not be present in metallic but oxidic form, that is catalytically active as well [3]. E.g. PdO can form during fabrication or upon the typical annealing [4] at 400 - 700 °C [3] for a few hours prior to gas sensing. Furthermore, possible Pd incorporation or embedding into the support rarely has been considered [5].Here, SnO2 particles with different Pd contents were prepared by flame spray pyrolysis (FSP). Their surface Pd was removed through leaching with HNO3 to guaranty the surface and embedded Pd content was measured. Lastly, the influence of surface and embedded Pd on SnO2 gas sensors was evaluated with acetone, ethanol and CO at 350 °C and 50% relative humidity. Method Pristine and Pd-containing (0 - 3 mol%) SnO2 nanoparticles were produced by FSP and annealed for 5 h at 500 °C. To remove all Pd from the surface, the particles were reduced in 5% H2 in Ar at 150 °C for 30 min and then leached in 10% HNO3 in water at 60 °C for 4 h (Figure 1a). Inductively coupled plasma-optical emission spectrometry was used to determine the Pd content in the leached solution, that corresponds to the surface amount. Sensing films were prepared by doctor-blading the particles onto 15 × 13 × 0.8 mm Al2O3 sensor substrates followed by annealing at 300 °C for 30 min. These sensors were then heated to 350 °C and tested with different concentrations (5 - 1000 ppb) of acetone, ethanol and CO in a gas sensing setup described elsewhere [6]. Results and Conclusions Figure 1b displays the fraction of surface (or leachable) Pd over the nominal content of flame-made and annealed particles. Only a fraction of Pd content is on the surface whereas the rest is embedded in the bulk SnO2. For instance, at nominal 0.5 mol% Pd, only 30% of it is leached while at 3 mol% Pd it increased to 45%. Apparently, most Pd is embedded and thereby not exposed to the analytes during sensing.Figure 1c shows the sensor responses at 350 °C to 1 ppm of acetone at 50% RH of annealed Pd-containing SnO2 as a function of the nominal Pd content. Before leaching (triangles), the addition of small amounts of Pd (up to 0.2 mol%) slightly increases the responses to acetone (within the error bar of reproducibility). At higher Pd contents, the responses continuously deteriorate until they are hardly detectable anymore at 3 mol% Pd. The circles in Figure 1c show the analyte responses after leaching the surface Pd as a function of the nominal Pd content. While the responses of pristine SnO2 were hardly affected by leaching, surprisingly, adding only 0.2 mol% Pd almost doubled the acetone response from 4 to 7. Overall, SnO2 containing both surface & embedded Pd (i.e. prior to leaching, triangles) results in lower responses than after leaching (circles).We reveal that flame-made Pd-containing SnO2 contains a significant fraction of Pd embedded in the bulk of (and/or strongly surface-bonded to) SnO2. Furthermore, small amounts of embedded Pd nearly double the responses of SnO2 to acetone at 350 °C. In contrast, the presence of surface Pd deteriorates the sensor performance below that of pure SnO2. This might point out, that small amounts of noble metals embedded in metal oxides can be more effective than on their surface.

First principles study of electronic, optical and transport properties of bulk and monolayer SnO

In this study, we used first principles calculations to investigate the influence of extracting the SnO (ML) monolayer, through electronic, optical and transport properties. Our simulation results of high optical transmission of SnO-ML comprehend the previously reported theoretical results. It shows around 96% optical transmission. These characteristics suggest that the SnO-ML is a potential material for optoelectronic and solar cell applications. In terms of thermoelectric properties, the electrical conductivity has been significantly improved at low temperature, the power factor and figure of merit are enhanced in SnO-ML. The materials’ specifications considered that two-dimensional materials SnO-ML are better thermoelectric materials and may be efficient for thermoelectric energy conversion. • Applied the exact exchange-correlation potentials for treating different properties of SnO-Bulk and SnO-ML. • TB-mBJ potential is better for SnO-ML.Band gap is in reasonable agreement with experimentally reported. • The structural optimization and electronic properties of SnO-Bulk and SnO-ML are investigated. • The optical and thermoelectric properties for SnO-ML are significantly enhanced and the transparent properties can be still maintained and attained 95%.

The investigation and DFT calculation on the gas sensing properties of nanostructured SnO2

The performance of SnO2 as a sensor of gaseous ethanol was experimentally and theoretically investigated. SnO2 particles were synthesized via a hydrothermal method. A side-heated sensor was fabricated and its sensing properties for various vapors were experimentally tested. The influence of the nanostructure and morphology of SnO2 particles on their sensing ability was also investigated. The results suggest that hollow-sphere structured SnO2 has a better sensing performance than other morphologies. Under an operating temperature of 330 °C, the response of SnO2 to ethanol gas was found to be ~68 times higher than that of the other tested gases. Additionally, a general mechanism describing the performance of the SnO2 sensor in the presence of gaseous ethanol is presented, and supported by density functional theory (DFT) calculations to explore the electrical properties of bulk SnO2 and its (110) surface. It is suggested that SnO2-based nanostructured materials can be employed in selective and efficient sensors of gaseous ethanol.

Synthesis of SnO2 nanoparticles for formaldehyde detection with high sensitivity and good selectivity.

During the detection of industrial hazardous gases, like formaldehyde (HCHO), the selectivity is still a challenging issue. Herein, an alternative HCHO chemosensor that based on the tin oxide nanoparticles is proposed, which was obtained through a facile hydrothermal method. Gas sensing performances showed that the optimal working temperature located at only 180 °C, the response value of 79 via 50 ppm HCHO was much higher than that of 35 at 230 °C. However, the compromised test temperature was selected as 230 °C, taking into account the faster response/recovery speeds than 180 °C, named 20/23versus 53/60 s, respectively. The response (35) of the SnO2 nanoparticles-based sensor to 50 ppm of HCHO is about 400% higher than that of bulk SnO2 sensor (9), especially when the gas concentration is 1 ppm, SnO2 nanoparticles also has a higher sensitivity which may possibly result from more exposed active sites and small size effect for nanoparticles than for bulk ones. The gas sensor based on SnO2 nanoparticles can be utilized as a promising candidate for practical low-temperature detectors of HCHO due to its higher gas response, excellent response–recovery properties, and perfect selectivity.

Surface Leaching of Pd-Containing SnO2 Gas Sensors Enhances Sensitivity

Introduction Palladium is commonly used to enhance the performance of chemoresistive metal-oxide gas sensors [1]. Typically, this enhancement is attributed to the presence of Pd clusters on the surface of their metal oxide support (e.g. SnO2). More specific, small Pd clusters on SnO2 were associated to enhanced sensitivity to H2 due to fermi-level control [2] or spillover effects [1]. However, Pd may not be present in metallic but oxidic form, that is catalytically active as well [3]. E.g. PdO can form during fabrication or upon the typical annealing [4] at 400 - 700 °C [3] for a few hours prior to gas sensing. Furthermore, possible Pd incorporation or embedding into the support rarely has been considered [5].Here, SnO2 particles with different Pd contents were prepared by flame spray pyrolysis (FSP). Their surface Pd was removed through leaching with HNO3 to guaranty the surface and embedded Pd content was measured. Lastly, the influence of surface and embedded Pd on SnO2 gas sensors was evaluated with acetone, ethanol and CO at 350 °C and 50% relative humidity. Method Pristine and Pd-containing (0 - 3 mol%) SnO2 nanoparticles were produced by FSP and annealed for 5 h at 500 °C. To remove all Pd from the surface, the particles were reduced in 5% H2 in Ar at 150 °C for 30 min and then leached in 10% HNO3 in water at 60 °C for 4 h (Figure 1a). Inductively coupled plasma-optical emission spectrometry was used to determine the Pd content in the leached solution, that corresponds to the surface amount. Sensing films were prepared by doctor-blading the particles onto 15 × 13 × 0.8 mm Al2O3 sensor substrates followed by annealing at 300 °C for 30 min. These sensors were then heated to 350 °C and tested with different concentrations (5 - 1000 ppb) of acetone, ethanol and CO in a gas sensing setup described elsewhere [6]. Results and Conclusions Figure 1b displays the fraction of surface (or leachable) Pd over the nominal content of flame-made and annealed particles. Only a fraction of Pd content is on the surface whereas the rest is embedded in the bulk SnO2. For instance, at nominal 0.5 mol% Pd, only 30% of it is leached while at 3 mol% Pd it increased to 45%. Apparently, most Pd is embedded and thereby not exposed to the analytes during sensing.Figure 1c shows the sensor responses at 350 °C to 1 ppm of acetone at 50% RH of annealed Pd-containing SnO2 as a function of the nominal Pd content. Before leaching (triangles), the addition of small amounts of Pd (up to 0.2 mol%) slightly increases the responses to acetone (within the error bar of reproducibility). At higher Pd contents, the responses continuously deteriorate until they are hardly detectable anymore at 3 mol% Pd. The circles in Figure 1c show the analyte responses after leaching the surface Pd as a function of the nominal Pd content. While the responses of pristine SnO2 were hardly affected by leaching, surprisingly, adding only 0.2 mol% Pd almost doubled the acetone response from 4 to 7. Overall, SnO2 containing both surface & embedded Pd (i.e. prior to leaching, triangles) results in lower responses than after leaching (circles).We reveal that flame-made Pd-containing SnO2 contains a significant fraction of Pd embedded in the bulk of (and/or strongly surface-bonded to) SnO2. Furthermore, small amounts of embedded Pd nearly double the responses of SnO2 to acetone at 350 °C. In contrast, the presence of surface Pd deteriorates the sensor performance below that of pure SnO2. This might point out, that small amounts of noble metals embedded in metal oxides can be more effective than on their surface.