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Metal oxide semiconductors (MOSs) hold great promise for electronic devices such as gas sensors. The utilization of ZnO as a conductometric gas sensor material can be traced back to its early stages; however, its application has primarily been limited to high-temperature environments. A gas sensor based on highly porous and interconnected 3D networks of ZnO tetrapod (ZnO-T) micro-nano structures was fabricated via an easy chemical vapor deposition (CVD) method. Homemade instruments were utilized to evaluate the gas-sensing of the sample at room temperature. It exhibited good gas-sensing at room temperature, particularly with a response of up to 338.80% toward 1600 ppm ethanol, while also demonstrating remarkable repeatability, stability, and selectivity. Moreover, the unique gas-sensing properties of ZnO-T at room temperature can be reasonably explained by considering the effect of van der Waals forces in physical adsorption and the synergistic effect of carrier concentration and mobility. The aforementioned statement presents an opportunity for the advancement of gas sensors utilizing ZnO at room temperature. [ABSTRACT FROM AUTHOR]

Metal oxide semiconductors (MOSs) hold great promise for electronic devices such as gas sensors. The utilization of ZnO as a conductometric gas sensor material can be traced back to its early stages; however, its application has primarily been limited to high-temperature environments. A gas sensor based on highly porous and interconnected 3D networks of ZnO tetrapod (ZnO-T) micro-nano structures was fabricated via an easy chemical vapor deposition (CVD) method. Homemade instruments were utilized to evaluate the gas-sensing of the sample at room temperature. It exhibited good gas-sensing at room temperature, particularly with a response of up to 338.80% toward 1600 ppm ethanol, while also demonstrating remarkable repeatability, stability, and selectivity. Moreover, the unique gas-sensing properties of ZnO-T at room temperature can be reasonably explained by considering the effect of van der Waals forces in physical adsorption and the synergistic effect of carrier concentration and mobility. The aforementioned statement presents an opportunity for the advancement of gas sensors utilizing ZnO at room temperature.

Metal oxide semiconductors (MOSs) hold great promise for electronic devices such as gas sensors. The utilization of ZnO as a conductometric gas sensor material can be traced back to its early stages; however, its application has primarily been limited to high-temperature environments. A gas sensor based on highly porous and interconnected 3D networks of ZnO tetrapod (ZnO-T) micro-nano structures was fabricated via an easy chemical vapor deposition (CVD) method. Homemade instruments were utilized to evaluate the gas-sensing of the sample at room temperature. It exhibited good gas-sensing at room temperature, particularly with a response of up to 338.80% toward 1600 ppm ethanol, while also demonstrating remarkable repeatability, stability, and selectivity. Moreover, the unique gas-sensing properties of ZnO-T at room temperature can be reasonably explained by considering the effect of van der Waals forces in physical adsorption and the synergistic effect of carrier concentration and mobility. The aforementioned statement presents an opportunity for the advancement of gas sensors utilizing ZnO at room temperature.

In this work, a simple method of ZnAl2O4‐functionalization of ZnO microstructures is presented. The different characterization methods (structural, chemical, and micro‐Raman) demonstrated the presence of only ZnO and ZnAl2O4 crystalline phases. ZnAl2O4 nano‐crystallites grow on the surfaces of ZnO 3D microstructures having diameters of 50–100 nm and with high density. Transmission electron microscopy (TEM) and high‐resolution TEM (HRTEM) results clearly show ZnAl2O4 crystallites functionalizing zinc oxide tetrapod arms. The individual structures (microwires (MWs) and three‐dimensional (3D) tetrapods (Ts)) are integrated into functional devices, suitable for gas sensing applications. All devices show excellent hydrogen gas selectivity at relatively low operating temperature in the range of 25–100 °C. The highest gas sensing performances are obtained based on individual ZnAl2O4‐functionalized ZnO tetrapods (ZnAl2O4/ZnO‐T, with an arm diameter (D) of ≈400 nm) and a response of ≈2 at 25 °C to 100 ppm of hydrogen gas (H­2), while a ZnAl2O4/ZnO‐MW (D ≈ 400 nm) shows only a response of ≈1.1. The Al‐doped ZnO MW (D ≈ 400 nm) without ZnAl2O4 elaborated in another work, chosen only for comparison reason, shows no response up to 800 ppm H2 gas concentration. A gas sensing mechanism is proposed for a single ZnAl2O4/ZnO‐T microstructure based sensor. The obtained results on ZnAl2O4/ZnO‐T‐based devices is superior to many reported performances of other individual metal oxide nanostructures with much lower diameter, showing promising results for room temperature H2 gas sensing applications. [ABSTRACT FROM AUTHOR]

Hybridization of micro- and nanostructures of semiconducting oxides is known to be an efficient way to greatly improve their sensing properties and photocatalytic activity. In this work, zinc oxide (ZnO) tetrapod (T) three-dimensional (3-D) highly porous networks were hybridized with Me x O y and Zn x Me 1-x O y compounds (Me = Sn, Fe, Bi, Cu or Al), and their ultraviolet (UV) sensing properties were investigated. Additionally, individual Al-doped ZnO-T (ZnO-T:Al) with different diameters were integrated into devices using a FIB/SEM system to study the influence of diameter on the UV sensing properties. ZnO-T−CuO hybrid networks demonstrated the highest increase in UV response (with about 2.5 times) and decrease in response and recovery time (τ r1 = τ r2 ≈ 0.03 s and τ d1 = τ d2 ≈ 0.045 s) compared to pristine ZnO-T networks before hybridization, which is quite promising for applications in fast optical communication. The ZnO-T−Zn 2 SnO 4 hybrid networks showed only a slight increase in UV response while other types of hybrid networks showed a considerable decrease in UV response, especially in the case of ZnO-T−Bi 2 O 3 hybrid networks, which could be attributed to the fast recombination of photoexcited electrons and holes in Bi 2 O 3 under the UV light illumination. The results demonstrate that hybridization with p -type materials is more efficient due to higher photogenerated charge separation properties. In the case of individual structures the device based on a microwire with lower diameter showed higher stability and good repeatability with a relatively high UV response of about 5.5. The excellent UV sensing properties combined with ultra-low power consumption make such devices very attractive for real applications like portable UV dosimeters. This work demonstrated the high efficiency of ZnO-T hybridization with p -type metal oxides for improvement of UV sensing properties. [ABSTRACT FROM AUTHOR]

Ag/AgCl is a visible-light plasmonic photocatalyst that has attracted considerable attention because of its high visible-light absorption and activity owing to the surface plasmon resonance of noble-metal nanoparticles. In this study, Ag/AgCl/ZnO tetrapod composite was prepared by introducing ZnO tetrapods into Ag/AgCl prepared by a polydopamine reduction route. Ag/AgCl was densely deposited on the three-dimensional support framework provided by the ZnO tetrapods. The framework possessed a certain degree of porosity, thereby improving the specific surface area of the Ag/AgCl/ZnO composite. The interaction of ZnO with Ag/AgCl further increased the separation and transfer of electron-hole pairs. The Ag/AgCl/ZnO composite showed excellent photocatalytic activity and good stability. Under xenon lamp irradiation for 20 min, degradation of rhodamine B reached 90%. After four recycling tests, degradation remained stable without any sign of reduction. Ag/AgCl/ZnO tetrapod composite is shown to be a kind of green photocatalyst offering high activity, good stability, and recyclability. [ABSTRACT FROM AUTHOR]

A flexible self-powered e-skin has been presented with multi-functions of tactile-perception, atmosphere-detection and self-clean. Piezoelectric PVDF and tetrapod ZnO (T-ZnO) nanostructures are hybridizing on the flexible fabric substrate. The piezoelectric, gas sensing and photocatalytic properties of T-ZnO nanostructures are combined together. The piezoelectric effect of T-ZnO/PVDF leads to the motion-powered tactile-perception behavior, e.g. the e-skin can detect elbow bending or finger pressing. The piezoelectric/gas-sensing coupling effect of T-ZnO nanostructures delivers the motion-powered atmosphere-detection performance, and the piezoelectric output of the e-skin is different upon exposure to different atmosphere conditions, acting as the gas-sensing signal. The piezo-photocatalytic activity of T-ZnO nanostructures results in the distinct self-clean characteristics, and the organic-pollutants/bacteria can be degraded/sterilized on the surface of the e-skin. This novel material system and device architecture can promote the development of flexible self-powered multi-functional e-skin. [ABSTRACT FROM AUTHOR]

Abstract: Tetrapod-shaped ZnO nanocrystals were synthezised via thermal evaporation of Zn powder in air. The Zn powder was oxidized at 930°C for 60min. To investigate the optimal Zn/O molar ratio in such vapor phase synthesis, the Zn content was varied in range of 0.1–0.75g. When the Zn content was 0.1g, no nanocrystals were formed. Above 0.25g Zn, growth of ZnO nanocrystals started without clear tetrapod like morphology. Beyond Zn content of 0.50g, clear tetrapod-shaped ZnO nanocrystals were detected. These results establish that there is a certain required ratio of zinc and oxygen in vapor phase for generation of tetrapod-shaped ZnO nanocrystals. Such Zn:O ratio is also calculated theoretically from the ideal gas law. Clear tetrapod type ZnO nanocrystals prepared with Zn content of 0.5g exhibited the highest intensity of the ultraviolet emission centered at 380nm which also confirms the high crystalline quality of such ZnO nanocrystals. [Copyright &y& Elsevier]

Abstract: Transferable ZnO tetrapods were grown by an aqueous solution method. An individual ZnO tetrapod-based sensor was fabricated by in situ lift-out technique and its ultraviolet (UV) and gas sensing properties were investigated. This single tetrapod-based device responds to the UV light rapidly and showed a recovery time of about 23s. The sensitivity of a single ZnO tetrapod sensor to oxygen concentration was also investigated. We found that when UV illumination is switched off, the oxygen chemisorption process will dominate and assists photoconductivity relaxation. Thus relaxation dynamics is strongly affected by the ambient O2 partial pressure as described. We also studied the response of ZnO tetrapod-based sensor in various gas environments, such as 100ppm H2, CO, i-butane, CH4, CO2, and SO2 at room temperature. It is noted that ZnO tetrapod sensor is much more sensitive to H2, i-butane and CO. It is demonstrated that a ZnO tetrapod exposed to both UV light and hydrogen can provide a unique integrated multiterminal architecture for novel electronic device configurations. [Copyright &y& Elsevier]

Abstract: We present the characterization of the luminescence properties of ZnO nano-tetrapods. Low temperature, power dependence, and temperature dependence of the ZnO nano-tetrapods were investigated. From the low-temperature photoluminescence (PL), strong surface exciton emission (SX, at 3.367eV), two-electron satellite of donor-bound exciton emission (TES, at 3.315eV), and donor–acceptor pair emission (DAP, at 3.226eV) are observed. Temperature dependence of those emission lines shows complex features. Delocalization due to thermal activation was observed from all emission lines, which strongly indicates that evaluation of luminescence properties of nanostructures requires special care about the localization centers for excitons. [Copyright &y& Elsevier]

Highly porous hybrid materials with unique high-performance properties have attracted great interest from the scientific community, especially in the field of gas-sensing applications. In this work, tetrapodal-ZnO (ZnO-T) networks were functionalized with carbon nanotubes (CNTs) to form a highly efficient hybrid sensing material (ZnO-T-CNT) for ultrasensitive, selective, and rapid detection of ammonia (NH 3 ) vapor at room temperature. By functionalizing the ZnO-T networks with 2.0 wt % of CNTs by a simple dripping procedure, an increase of 1 order of magnitude in response (from about 37 to 330) was obtained. Additionally, the response and recovery times were improved (by decreasing them from 58 and 61 s to 18 and 35 s, respectively). The calculated lowest detection limit of 200 ppb shows the excellent potential of the ZnO-T-CNT networks as NH 3 vapor sensors. Room temperature operation of such networked ZnO-CNT hybrid tetrapods shows an excellent long-time stability of the fabricated sensors. Additionally, the gas-sensing mechanism was identified and elaborated based on the high porosity of the used three-dimensional networks and the excellent conductivity of the CNTs. On top of that, several single hybrid microtetrapod-based devices were fabricated (from samples with 2.0 wt % CNTs) with the help of the local metal deposition function of a focused ion beam/scanning electron microscopy instrument. The single microdevices are based on tetrapods with arms having a diameter of around 0.35 μm and show excellent NH 3 sensing performance with a gas response (I gas /I air ) of 6.4. Thus, the fabricated functional networked ZnO-CNT hybrid tetrapods will allow to detect ammonia and to quantify its concentration in automotive, environmental monitoring, chemical industry, and medical diagnostics.

In this work, the exceptionally improved sensing capability of highly porous three-dimensional (3-D) hybrid ceramic networks toward reducing gases is demonstrated for the first time. The 3-D hybrid ceramic networks are based on doped metal oxides (Me x O y and Zn x Me 1-x O y , Me = Fe, Cu, Al) and alloyed zinc oxide tetrapods (ZnO-T) forming numerous junctions and heterojunctions. A change in morphology of the samples and formation of different complex microstructures is achieved by mixing the metallic (Fe, Cu, Al) microparticles with ZnO-T grown by the flame transport synthesis (FTS) in different weight ratios (ZnO-T:Me, e.g., 20:1) followed by subsequent thermal annealing in air. The gas sensing studies reveal the possibility to control and change/tune the selectivity of the materials, depending on the elemental content ratio and the type of added metal oxide in the 3-D ZnO-T hybrid networks. While pristine ZnO-T networks showed a good response to H 2 gas, a change/tune in selectivity to ethanol vapor with a decrease in optimal operating temperature was observed in the networks hybridized with Fe-oxide and Cu-oxide. In the case of hybridization with ZnAl 2 O 4 , an improvement of H 2 gas response (to ∼7.5) was reached at lower doping concentrations (20:1), whereas the increase in concentration of ZnAl 2 O 4 (ZnO-T:Al, 10:1), the selectivity changes to methane CH 4 gas (response is about 28). Selectivity tuning to different gases is attributed to the catalytic properties of the metal oxides after hybridization, while the gas sensitivity improvement is mainly associated with additional modulation of the electrical resistance by the built-in potential barriers between n-n and n-p heterojunctions, during adsorption and desorption of gaseous species. Density functional theory based calculations provided the mechanistic insights into the interactions between different hybrid networks and gas molecules to support the experimentally observed results. The studied networked materials and sensor structures performances would provide particular advantages in the field of fundamental research, applied physics studies, and industrial and ecological applications.