SnO2 Nanowires Research Articles

Influence of oxygen vacancies on the performance of SnO2 gas sensing by near-ambient pressure XPS studies

Modulation of oxygen vacancy (VO··) concentrations in SnO2 is an efficient strategy for improving its NO2 sensing performance. However, due to the complex reaction process between NO2 and adsorbed oxygen species on the surface of metal oxide, the role of VO·· is still unclear without in-situ characterization. Here, SnO2 nanowires (NWs) were synthesized via chemical vapor deposition method, and the VO·· concentration in SnO2 NWs was enhanced by removing adsorbed H2O on the surface of precursor during the growth process. The VO··-abundant SnO2 NWs exhibited large improvement in NO2 sensing performance with ultralow detection limit (2 ppb NO2) and high response value towards 1 ppm NO2 (49.5 at 50 °C). Near-ambient pressure X-ray photoelectron spectroscopy was used to reveal the in-situ sensing reactions occurred on the surface of SnO2 NWs. The results revealed that VO·· provided active sites for adsorption of oxygen species and confirmed the formation of NO. This work demonstrates an easy route to grow SnO2 NW with tunable VO·· concentration, and provides a direct evidence for deeper understanding of the effect of VO·· in sensing process.

Synthesis of SnO2 nanowires using thermal chemical vapor deposition with SnO powder and their application as self-powered ultraviolet photodetectors

We report on the performance of self-powered ultraviolet (UV) photodetectors composed of SnO2 nanowire (NW) networks. SnO2 NWs with a length of several hundred micrometers can be synthesized by thermal chemical vapor deposition (CVD) using SnO powder as the raw material, which has the advantages of a low process temperature and no requirement for a reducing agent. Based on the characterization results obtained through synchrotron X-ray diffraction (XRD), scanning electron microscopy, and transmission electron microscopy (TEM), we determined the growth behavior of SnO2 NWs via a vapor-liquid-solid mechanism with Au nanoparticles. The NW growth switched from an initial in-plane growth to subsequent vertical growth, forming NW cotton. This was started at a process temperature of 600 ℃ and optimized at 800 ℃. Moreover, the XRD and TEM results indicate that the NWs mainly grew in the form of SnO2, although the formation of SnO NWs was also possible. Metal−SnO2 NW−metal type photodetectors were fabricated, and their photoresponsivity to UV light in the range from 200 to 400 nm was investigated. The device exhibited a photo-to-dark current ratio of ∼2.17 × 106 at an applied bias of 10 V and 254 nm UV exposure. A maximum responsivity of about 1100 A/W was estimated at a wavelength of 270 nm, and the cutoff edge wavelength appeared around 350 nm. In particular, the self-powered photoresponse at nominal zero bias was ∼1.23 nA. The results of this study support the idea that SnO2 NWs are promising candidates for self-powered deep-UV photodetectors and that thermal CVD using SnO powder is suitable for synthesizing SnO2 NWs at temperatures as low as 700 °C.

Influence of single-ionized oxygen vacancies on the generation of ferromagnetism in SnO2 and SnO2:Cr nanowires

In this work, we report the influence of single-ionized oxygen vacancies (V^{prime}_{{text{O}}}) as a spin ½ system in the ferromagnetic response of undoped and Cr-doped SnO2 nanowires. For this study, undoped and Cr-doped SnO2 nanowires were synthesized by a thermal evaporation method. Raman, Auger, and X-ray photoelectron spectroscopies confirmed the incorporation of Cr3+ ions in the SnO2 lattice. Electron paramagnetic resonance measurements demonstrated the presence of single-ionized oxygen vacancies (V^{prime}_{{text{O}}}) in undoped and Cr-doped nanowires. Complementarily, cathodoluminescence measurements confirmed the presence of VO defects in the samples. Magnetic measurements revealed FM behavior from the undoped SnO2 and Cr-doped SnO2 nanowires, showing magnetization saturation values (MS) of ± 1 × 10–3 and ± 1.6 × 10–3 emu/g, respectively, and magnetic coercivity values (HC) of 180 and 200 Oe. We assign the FM response of nanowires to the presence of single ionized V^{prime}_{{text{O}}} acting as a spin ½ system and to the alignment of magnetic moments of Cr3+ ions, finding that V^{prime}_{{text{O}}} defects dominate in the FM generation.

Operando photoelectron spectromicroscopy of nanodevices: Correlating the surface chemistry and transport in SnO2 nanowire chemiresistors

With size reduction of active elements in microelectronics to tens of nanometers and below, the effect of surface and interface properties on overall device performance becomes crucial. High resolution spectroscopic and imaging techniques provide a metrological route for characterization of these properties relevant to device diagnostics and failure analysis. With its roughly 100 nm spatial resolution, superior surface sensitivity, and approximately 200 meV spectral resolution, scanning photoelectron microscopy (SPEM) stands out as a comprehensive tool to access the surface/interface composition of nanodevices, as well to provide chemical state designations and materials property evolutions upon treatment by thermal, electrical, chemical, radiative and other stimuli. Here we present a SPEM-on-device setup that combines X-ray spectromicroscopy with advanced NIST microhotplate technology to demonstrate new combined analytical and electrical measurements capabilities of this metrology platform for operando nanodevice characterization. Using model integrated SnO2 nanowire (NW) chemiresistor devices, the chemically induced alterations in the chemical state of the nanowire surface are correlated to the observed conductance changes, thus directly testing the receptor and transduction mechanisms for SnO2 NW conductometric chemical sensors.

Xe+ ion irradiation to boost NO2 sensing characteristics of SnO2 nanowires

In this paper, pristine and Xe+ ion-irradiated sensors are studied for NO2 sensing purposes. SnO2 nanowires (NWs) were synthesized via a simple growth method and irradiated with Xe+ ions at different doses (1014, 1015, and 1016 ions/cm2). The gas sensors were fabricated after different characterizations of morphology, phase, and chemical composition. The NO2 sensing experiments revealed that there was an optimal dose (1014 ions/cm2) for which the highest response to NO2 was achieved. At 200 °C, the optimal sensor had a high response of 27.32–10 ppm of NO2. Furthermore, the sensor demonstrated high selectivity to NO2 gas. The boosted gas response of the optimized sensor was due to the formation of n-n homojunctions between the SnO2-SnO2 NWs and the irradiation-induced formation of defects. This study confirms the usefulness of irradiation in boosting the sensing features of metal oxides.

Defect Location in Laterally Aligned Tin Oxide Nanowires: The Role of Growth Direction, Interface Dimensionality, and Catalyst

Laterally aligned SnO2 nanowires, which exhibit planar growth along crystallographically defined directions of a sapphire substrate surface, are superior for bio- and gas-sensing applications. Little is known about their cross-sectional geometry and the defect distribution within the nanowire cross section, although this substantially determines the electronic properties of the nanowires. In this study, SnO2 nanowires were grown on r-plane sapphire substrates. Scanning electron microscopy (SEM) and atomic force microscopy (AFM) reveal the highly oriented growth of the SnO2 nanowires toward the substrate edges. High-resolution transmission electron microscopy (HRTEM) of the NW cross section and strain mapping allowed for analyzing the crystallographic alignment of the SnO2 NW to the sapphire substrate and the defect distribution within the SnO2 nanowire cross section. These techniques revealed a defect-free SnO2–Al2O3 interface and a high alignment of the SnO2 NW lattice toward the sapphire substrate along the NW width. The determined high defect density close to the nanowire surface will be discussed in comparison to freestanding SnO2 nanowires and SnO2 thin films on sapphire substrates considering the differences in the growth direction and the interface dimensionality.

High Gas Response Performance Based on Reduced Graphene Oxide/SnO2 Nanowires Heterostructure for Triethylamine Detection

SnO2 nanowires are locally synthesized by a simple thermal evaporation method and its growth mechanism is confirmed. Here, we present a simple strategy for realizing reduced graphene oxide (RGO)/SnO2 nanowires heterostructure. As expected, the heterostructure gas-sensing response is up to 63.3 when the gas concentration of trimethylamine (TEA) is 50 ppm, and it exhibits an excellent dynamic response with high stability at 180 °C. A low detection limit of 50 ppb level is fully realized. Compared to SnO2 nanowires, the sensing performance of the RGO/SnO2 heterostructure-based sensor is greatly enhanced, which can be ascribed to the RGO and the heterostructure. The RGO/SnO2 composite engineering poses an easy way to make full use of the advantages originating from RGO and heterostructure.

Sequential coating of Sb-doped SnO2 nanowire for photoelectrochemical cells of panchromatic light absorption and low thermalization loss

A photoelectrochemical (PEC) cell is a device of enormous potential in producing hydrogen via water splitting using solar energy. However, the low charge collection efficiency and thermalization loss of a photoelectrode limit the performance of the PEC cell. In this study, these problems are addressed by sequentially coating CdS and CdSe along the longitudinal direction of antimony-doped tin oxide and tin oxide nanowire (NW) arrays. Because of the vertical alignment of CdS and CdSe, larger band gap material (CdS) and smaller band gap material (CdSe) absorb blue and red light, respectively. Such selective absorption reduces the thermalization loss and increases the photocurrent density. In addition, antimony-doped tin oxide NWs increase the collection efficiency of photogenerated carriers and decrease the onset voltage by increasing the electric conductivity and decreasing the mismatch in the Fermi energy level. The photocurrent of the vertically aligned CdS and CdSe on antimony-doped tin oxide NWs is 17.6 mA cm−2, which is 50% higher than that of the traditional cascade-type CdSe/CdS photoelectrode.

Resistive switching properties of SnO2 nanowires fabricated by chemical vapor deposition

Resistive switching (RS) devices have great application prospects in the emerging memory field and neuromorphic field, but their stability and unclear RS mechanism limit their relevant applications. In this work, we construct a hydrogenated Au/SnO2 nanowire (NW)/Au device with two back-to-back Schottky diodes and investigate the RS characteristics in air and vacuum. We find that the I on/I off ratio increases from 20 to 104 when the read voltage decreases from 3.1 V to −1 V under the condition of electric field. Moreover, the rectification ratio can reach as high as 104 owing to oxygen ion migration modulated by the electric field. The nanodevice also shows non-volatile resistive memory characteristic. The RS mechanism is clarified based on the changes of the Schottky barrier width and height at the interface of Au/SnO2 NW/Au device. Our results provide a strategy for designing high-performance memristive devices based on SnO2 NWs.

Highly sensitive SnO2 nanowire network gas sensors

In this work we present a methodology for the localized growth of nanowires on prespecified areas of microhotplates that allows to independently adjust the device’s resistance and its response to the gas. This is achieved through the fabrication stripes containing the nanowires, with or without the presence of a gap in the stripe, giving rise that the nanowires bridge the current. The methodology is demonstrated growing SnO2 nanowire-based chemoresistors and the fabricated sensors have been characterized against CO and NO2. The results show the capability of tailoring nanowire stripe sizes from 1 to 100 µm, including empty areas of the same sizes along the sensing material, and a response increase by a factor of up to 500. We attribute the response enhancement to the absence of nucleation seeds in the gap area, where only arching nanowires can allow the current to flow between electrodes. In this way, the current flow along the bridge of nanowires is restricted principally to the surface conduction, which is controlled by the interaction of the nanowires with gases.

Enhanced sensitivity towards hydrogen by a TiN interlayer in Pd-decorated SnO2 nanowires

In this study, we designed a new structure based on Pd-decorated TiN-coated SnO2 nanowires (NWs) for the selective detection of H2 gas.

Pt nanoparticle decoration on femtosecond laser-irradiated SnO2 nanowires for enhancing C7H8 gas sensing

In this study, we have fabricated a sensitive and selective toluene gas sensor based on SnO2 nanowires (NWs) using the femtosecond (FS) laser irradiation and Pt nanoparticle (NPs) decoration. The effects of different laser parameters, including wavelength, laser pulse energy, hatch distance, and scan speed, are investigated on the gas-sensing properties. The FS laser irradiation-induced embossing surface is found to be a dominant factor in determining the gas-sensing behavior, which provides more oxygen absorption sites, resulting in an increase in the width of the electron depletion layer (EDL) and the sensor response. The Pt decoration on the FS laser-irradiated SnO2 NWs developed additional EDL, which further enhances the sensor response. The Pt-decorated and FS laser-irradiated SnO2 NWs gas sensor shows a high selectivity toward C7H8 owing to the catalytic effect of Pt toward toluene, where the sensor response to 50 ppm C7H8 is as high as 53 at 300 °C. Based on X-ray photoelectron spectroscopy and transmission electron microscopy analyses, a gas-sensing mechanism of the Pt-decorated and FS laser-irradiated SnO2 NWs gas sensor is proposed. This study highlights a promising combination of Pt decoration and FS laser irradiation for the realization of reliable toluene gas sensors.

Neuromorphic Gustatory System with Salt-Taste Perception, Information Processing, and Excessive-Intake Warning Capabilities.

Emulation of the process of a biological gustatory system could benefit the reconstruction of sense of taste. Here we demonstrate the first neuromorphic gustatory system that emulates the ability of taste perception, information processing, and excessive-intake warning functions. The system integrates a chitosan-derived ion-gel sensor, SnO2 nanowire artificial synapses, and an effect-executive unit. The system accomplish perception and encoding behaviors for taste stimulation without using complex circuits and multivariate analysis, showing short response delay (<1 s), long taste memory duration (>2 h), and a wide perceptive concentration range (0.02-6 wt % salt solution). Especially, SnO2 NW artificial synapses have extremely small response voltage (1 mV), exceeding the biological level by orders of magnitude, representing so-far the highest sensitivity record. This work provides a promising strategy to develop bioinspired and biointegrated electronics with the intention of mimicking and restoring the functions of biological sensory systems.

High-Performance Room-Temperature NO2 Gas Sensor Based on Au-Loaded SnO2 Nanowires under UV Light Activation.

Optical excitation is widely acknowledged as one of the most effective means of balancing sensor responses and response/recovery properties at room temperature (RT, 25 °C). Moreover, noble metals have been proven to be suitable as photosensitizers for optical excitation. Localized surface plasmon resonance (LSPR) determines the liberalization of quasi-free electrons in noble metals under light irradiation, and numerous injected electrons in semiconductors will greatly promote the generation of chemisorbed oxygen, thus elevating the sensor response. In this study, pure SnO2 and Au/SnO2 nanowires (NWs) were successfully synthesized through the electrospinning method and validated using XRD, EDS, HRTEM, and XPS. Although a Schottky barrier led to a much higher initial resistance of the Au/SnO2 composite compared with pure SnO2 at RT in the dark, the photoinduced resistance of the Au/SnO2 composite became lower than that of pure SnO2 under UV irradiation with the same intensity, which confirmed the effect of LSPR. Furthermore, when used as sensing materials, a detailed comparison between the sensing properties of pure SnO2 and Au/SnO2 composite toward NO2 in the dark and under UV irradiation highlighted the crucial role of the LSPR effects. In particular, the response of Au/SnO2 NWs toward 5 ppm NO2 could reach 65 at RT under UV irradiation, and the response/recovery time was only 82/42 s, which far exceeded those under Au modification-only or optical excitation-only. Finally, the gas-sensing mechanism corresponding to the change in sensor performance in each case was systematically proposed.

Implementation of binder-free SnO2 NWs@C electrode and LiTFSI-based electrolyte for high-performance lithium-ion battery

This study has investigated the effect of carbon coating on the electrochemical performance of SnO2 Nanowires (NWs) as an electrode along with a bis (trifluoromethane) sulfonimide lithium (LiTFSI)-based electrolyte in a lithium-ion battery (LIB). The vapor–liquid–solid approach has been used to grow SnO2 NWs on the stainless steel mesh current collector. The obtained results have demonstrated that the utilization of the LiTFSI-based electrolyte improved the battery performance with the SnO2 NWs electrode over the LiPF6-based electrolyte. This may be due to the formation of a stable and thin solid electrolyte interphase layer. Since bare SnO2 NWs exhibit inferior cycling stability due to their high volumetric expansion and poor conductivity, incorporating carbon coating could improve performance. Although the initial discharge specific capacity obtained for the carbon-coated SnO2 NWs (SnO2 NWs@C) electrode (1328.4 mAh g−1) was in the order of the first discharge capacity for the SnO2 NWs, the SnO2 NWs@C electrode possessed capacity retention with six times improvement as compared to the pristine SnO2 NWs after 120 cycles. The Coulombic efficiency of 99% has been achieved for the SnO2 NWs@C electrode over 120 cycles. Field emission scanning electron microscopy images revealed that the carbon coating could keep the overall structure of the electrode upon cycling. It has been evidenced that the SnO2 NWs@C electrode with LiTFSI-based electrolyte has an excellent potential to implement in high-performance LIBs.

Patterning Design of Electrode to Improve the Interfacial Stability and Rate Capability for Fast Rechargeable Solid-State Lithium-Ion Batteries.

Patterned electrodes were developed for use in solid-state lithium-ion batteries, with the ultimate goal to promote fast-charging attributes through improving electrochemically activated surfaces within electrodes. By a conventional photolithography, patterned arrays of SnO2 nanowires were fabricated directly on the current collector, and empty channel structures formed between the resulting arrays were customized through modifying the size and interval of the SnO2 patterns. The composite electrolyte comprising Li7La3Zr2O12 and poly(ethylene oxide) was exploited to secure intimate interfacial contact at the electrode/electrolyte junction while preserving ionic conductivity in the bulk electrolyte. The potential and limitation of the electrode patterning approach were then explored experimentally. For example, the electrochemical behaviors of patterned electrodes were investigated as a function of variations in microchannel structures, and compared with those of conventional film-type electrodes. The findings show promise to improve electrode dynamics when electrochemical reaction kinetics could be hindered by poor interfacial characteristics on electrodes.

Chemical vapor deposition growth of tin oxide nanowires for electrochemical sensor development

AbstractThis paper reports a growth of tin oxide nanowires on gold patterns by chemical vapor deposition. A 90 nm gold layer was deposited on a silicon wafer using a sputtering system and pre‐pattern by lift‐off techniques. Tin oxide nanowires were selectively grown on the gold pattern by chemical vapor deposition technique with 750°C and 5 × 10‐2 torr. High resolution transmission electron microscopy measurements showed that nanowire possessed a single crystal nature with tetragonal crystal system and the calculated lattice constants (a = 0.474 nm; c = 0.318 nm) were closed to those of standard pattern of SnO2. The ready SnO2 nanowire grown on the gold patterns were investigated for electrochemical sensor development.

Size Effect in SnO2/Al2O3 Core/Shell Nanowires after Battery Cycling

Full utilization of the high storage capacity of conversion electrode materials as tin oxide (SnO2) in lithium‐ion batteries is hindered by the high volumetric expansion due to the high lithium storability which can lead to major cell damage and consequent safety issues. To overcome this issue, two promising approaches, nanostructures and buffer layers, are combined and evaluated. SnO2 nanowires (NWs) are coated with an aluminum oxide (Al2O3) buffer layer to investigate the combination SnO2–Al2O3. Strong differences in the crystallinity after cycling between the SnO2/Al2O3 core/shell NW‐based heterostructure and uncoated SnO2 NWs based on detailed structural analysis are shown via transmission electron microscopy (TEM) and determination of the elemental distribution of tin, oxygen, lithium, and aluminum via energy‐dispersive X‐Ray spectroscopy and energy‐filtered TEM in the as‐prepared and postmortem nanostructures. The core/shell NWs exhibit two different states after charge/discharge cycling, amorphous or crystalline, depending on the NW diameter; for the uncoated SnO2 NWs, only an amorphous postmortem structure is found. Additionally, differences in the elemental distribution for the amorphous and crystalline postmortem SnO2/Al2O3 core/shell NWs, especially for tin, are measured. Consequently, the structures and effects of the Al2O3 coating on the lithiation behavior of SnO2 NW‐based heterostructures are discussed.

Facile strategy for advanced selectivity and sensitivity of SnO2 nanowire-based gas sensor using chemical affinity and femtosecond laser irradiation

This study proposes an effective strategy to improve the selectivity and response of SnO2 nanowire (NW)-based gas sensors. Self-assembled monolayer (SAM) functionalization of SnO2 NWs enhances the gas selectivity by tuning the surface chemical states. Different chemical moieties with alkyl and fluoroalkyl of the SAM molecules are adopted for the chemical affinities toward the corresponding target gas molecules. The alkyl and fluoroalkyl moieties show excellent selectivity toward CH4 and C3F8, respectively, owing to the strong chemical affinity between the moieties and gases. Post-treatment femtosecond (FS) laser irradiation with different laser fluences is applied to the SAM-functionalized SnO2 NW-based gas sensor. The FS laser-irradiated SnO2 NWs show an enhanced sensor response with higher resistance values and shorter response and recovery times compared to those of the pristine SnO2 NWs. Transmission electron microscopy analysis reveals that the FS laser irradiation induces the formation of embossing surface on the SnO2 NW, creating additional gas adsorption sites. Moreover, twin structures develop on the SnO2 NWs with increasing laser fluences. Electron energy loss spectroscopy analysis supports that the FS laser irradiation creates non-stoichiometric SnO and SnOx phases, providing O vacancies and improved response.

Modelling electronic transport in monocrystalline metal oxide gas sensors: from the surface kinetics to the experimental response

Gas sensing systems and devices based on metal oxides are widely spreading due to their high performance in terms of sensor response and relatively low costs. Despite several experimental studies and molecular simulations are available in the literature, a tool that can quickly predict the macroscopic sensor response, and potentially be used for predictive purposes, is still missing. In this work, we present a modelling approach based on finite-element simulations, using material electrical properties available in the literature. In a first approach, we derive the surface electron trap concentration from fitting the global sensor response. Then, we improve the model by considering the actual time-dependent experimental response. We consider sensors based on single SnO2 nanowires and show how our model predicts with a good agreement the experimental response vs. NO2, as a function of the working temperature and gas concentration, and additionally provides many other physical quantities of interest, such as the conduction band edge bending, the space charge and the width of the depletion layer. We further discuss ideas for improving the model and thus increasing its predictive potential with an engineering perspective.