Temperature-dependent selectivity of bead-like TeO₂ nanostructured gas sensors

Abstract

Bead-like p-TeO₂ nanowires (NWs) were obtained directly from Te powder by thermal evaporation along with assisting 40 sccm of O₂ gas. Scanning and transmission electron microscopy showed that the two different formation origins for the two types of bead-like TeO₂ NWs take place at the vicinity of the surface and are characterized by (1) the presence of locally higher concentration and saturation of O₂, and (2) the surface state (terraces, ledges, and kinks (TLK)) of Te reacting with adsorbed oxygen atoms. In the present work, the gas-sensing performances of bead-like p-TeO₂ NW gas sensors fabricated using a facile and low-temperature route has been investigated for the detection of nitric dioxide (NO₂), ethanol (C₂H₅OH), and hydrogen sulfide (H₂S) gases contained in human breath. Specifically, this work systematically investigates the gas response of bead-like p-TeO₂ NW sensors in dependence of temperature in the range from 200 °C to 400 °C. The sensing capabilities of bead-like p-TeO₂ NWs are investigated with respect to C₂H₅OH, NO₂, and H₂S without any artificial handling, such as surface modification, n- and/or p-type doping, and heterostructure formation. The selectivity of bead-like p-TeO₂ NWs by adjusting the operating temperature is dependent on the target gas, which is attributed to an interaction between p-TeO₂ NWs and gas molecules at a specific operating temperature. In particular, the bead-like p-TeO₂ NWs show quite superior sensitivity and selectivity to C₂H₅OH, NO₂, and H₂S at 250, 350, and 400 °C, respectively. Thus, bead-like TeO₂ NWs are promising for the detections of C₂H₅OH, NO₂, and H₂S with high selectivity at the ppm level and may contribute to the realization of more selective NW sensors.

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Introduction

Since gas sensors require a strong chemical reaction combined with thermal stability, practical gas-sensing devices based on the large surface-to-volume ratios of 1D nanostructures are important from the viewpoint of industrial and environmental safety: (1) ensuring precise low-density limits of dangerous gases, (2) protection from carcinogenic gases, (3) protection from pollutant gases, and (4) detection of poisonous or toxic substances. Considering the merits such as high sensitivity/selectivity, quick response/recovery times, low-cost, and simplicity of fabrication, all kind of 1D nanostructures including rods, wires, belts, and tubes are ideally suited for nanoscale chemical gas sensors. In addition, the 1D nanomaterials could also enhance the sensing capability by adjusting the grain size, surface chemical properties, microstructures, addition of noble metals, as well as concentration-, thickness-, and temperature-dependent process variables. Over the past few decades, in the family of 1D nanostructured semiconducting oxides, most research for gas-sensing applications has been performed on n-type semiconductors, such as ZnO,¹ In₂O₃,² SnO₂,³ TiO₂,⁴ Ga₂O₃,⁵ WO₃,⁶ and Fe₂O₃,⁷ in comparison with restricted p-type semiconductors because the mobility of negative electrons is much faster than that of positive holes to induce fast response between gas molecules and solid interactions.⁸ Furthermore, for improving the gas-sensor performance, various surface modifications have been used, such as noble-metal (Pt, Pd, and Au) decoration,^9,10 controlled impurity doping,^11,12 ion implantations,^13,14 and heterostructure constructions.^15,16 Among these methods, the surface modification with protuberances is probably the easiest and most efficient route for enhancing the gas-sensor performance. Semiconducting p-type oxides, such as CuO,¹⁷ CeO₂,¹⁸ NiO,¹⁹ Co₃O₄,²⁰ and Bi₂O₃,²¹ have often been used as gas-sensor materials for the detection of toxic/hazardous gases. Particularly, nanostructured p-type TeO₂ is one of most important semiconducting materials that has been investigated for various potential applications, such as optical storage,²² laser devices,²³ and gas sensors.^24–27

Meanwhile, to the best of our knowledge, there are some reports on the synthesis and sensing properties of surface-functionalized TeO₂ nanostructures in comparison to pure TeO₂ NWs.^28,29 In this regard, systematic investigations of p-type TeO₂ NWs with respect to device fabrication and sensing properties are essential for the actual implementation of p-type NWs to sensor devices.

In this study, multiple networked bead-like p-TeO₂ NW sensors were fabricated by the spray method, and their microstructure and sensing performance were examined for a wide range of operating temperatures. It has been found that the gas-selectivity of bead-like p-TeO₂ NWs is highly dependent on the operating temperature and that this material is suitable for chemical gas-sensing applications.

Experimental details

Synthesis of bead-like p-TeO₂ NWs

The bead-like p-TeO₂ NWs were fabricated on p-type Si (100) substrates after deposition of the ∼3 nm thick Au catalyst. About 2 g of Te metal powder in an alumina boat along with 40 sccm of assisting O₂ gas were used in the center of a horizontal tube furnace at 400 °C for 1 h. The substrate was placed with 20 cm distance downstream from the centered boat containing the Te source. After carrying out the evaporation/condensation processes, the tube furnace was subsequently cooled to room temperature.

Materials characterization

The morphologies of two different types of bead-like p-TeO₂ NWs were analyzed using scanning electron microscopy (SEM, Hitachi S-4200) and transmission electron microscopy (TEM, Phillips CM-200). In addition, an energy-dispersive X-ray (EDX) spectrometer attached to the SEM was used to determine the elemental compositions of the two types of TeO₂ NWs.

Sensor fabrication and sensing measurement

The bead-like p-TeO₂ NWs were dispersed in ethanol and the colloid was coated on SiO₂/Si substrate using the spray method. Ti (∼50 nm)/Pt (∼200 nm) double-layer electrodes were sequentially sputtered onto the SiO₂/Si substrate coated by the randomly dispersed, multiply networked TeO₂ NWs by using an interdigital shadow mask. The sensing properties of the bead-like p-TeO₂ NWs against the gases C₂H₅OH, NO₂, and H₂S were investigated using a home-made gas-sensing system. To determine the optimal temperature for best sensing performance toward the respective gases, all sensing measurements were carried out in the range of 200–400 °C. It must be note that, in spite of NO₂ response at room temperature, all the sensing measurements were performed in the range of 200–400 °C for C₂H₅OH, H₂S, and NO₂ gases. This was due to insufficient response of p-TeO₂ NWs for C₂H₅OH and H₂S at room temperature (see ESI, Fig. S1†). The target gas concentration was regulated by mixing the air-balanced target gas and dry air using mass flow controllers (MFCs). The total flow rate was set to 500 sccm to exclude possible variation in the sensing measurements. The gas flow rate has a big influence on the sensor's gas response. Therefore, the fabricated sensors were tested the response for C₂H₅OH, NO₂, and H₂S with different gas flow rate at 250, 350, and 400 °C, respectively. The best gas response was obtained at a 500 sccm flow rate for all gases. Based on these results, all the other sensing measurements were performed at this flow rate. The content of water vapor was negligible, according to the specifications provided by the manufacturer. The gas response R of the fabricated sensors against reducing gas was calculated by the ratio of R_g/R_a, where R_g and R_a are the resistances in the presence and absence of the target gas, respectively, For an oxidizing gas, R_a/R_g was used as gas response. The response/recovery times were defined as the times needed to reach 90% changes in resistance upon supply or removal of the target gas, respectively.

Results and discussion

Fig. 1 illustrates schematically the sensor-fabrication procedure used in this work. The first step involved is the growth of bead-like TeO₂ NWs on Si (100) substrate by thermal evaporation.²⁸ In the second step, the TeO₂ NWs were dispersed in ethanol and coated on a 200 nm thick SiO₂ layer formed on top of the Si substrate for fabricating the sensor platform by using an airbrush spray gun that can coat uniform thin layers on irregular-shaped substrates. In the third step, for the sensing measurements, layers of Ti (∼50 nm) and Pt (∼200 nm) were sequentially deposited via sputtering on the networked TeO₂ NWs using an interdigital shadow mask.

Fig. 1 Schematic illustration of the fabrication process of bead-like p-TeO₂ NW sensors.

Fig. 2a and b shows SEM images of the 1D bead-like TeO₂ NWs synthesized using evaporation/condensation of Te powder with 40 sccm of assisting O₂ gas. The average diameter and length of the TeO₂ NWs were ∼100 nm and a few tens of micrometers, respectively. As can be seen from Fig. 2a, the bead-like TeO₂ NWs were randomly distributed on the substrate. At first glance, TeO₂ nanoparticles are attached to all surfaces of the NWs. The existence of many protuberant nanocrystals shown in Fig. 2b means two different possible types of nucleation formation: (1) nucleation of bead-like TeO₂ occurred readily at local regions where the O₂ concentration is higher than at other parts, (2) nucleation of bead-like TeO₂ happened quickly at saturated regions where the density of Te is enhanced via flat regions (terraces), atomic steps (ledges), and corners (kinks).^30,31 These morphologies of the bead-like TeO₂ NWs would increase the number of adsorption/desorption sites for various target gases, inducing reactions between solid and gas. The EDX spectrum presented in Fig. 2c shows that the bead-like TeO₂ NWs consist of Te and O and additional Au and Si originating from the catalyst and substrate, respectively.

Fig. 2 SEM images showing the morphologies of the bead-like TeO₂ nanowires (NWs) synthesized at 400 °C: (a) SEM image of the NWs formed at the distance of 20 cm downstream from the source powder, (b) high-magnification image of (a), (c) EDX spectrum of the bead-like TeO₂ NWs.

Fig. 3 shows images of TEM (Fig. 3a and b), HRTEM (Fig. 3c), and the corresponding SAED pattern (Fig. 3d) of the bead-like TeO₂ NWs obtained from thermal evaporation. Fig. 3a indicates even thickness at diameters below ∼100 nm with uneven beads along the length direction. As mentioned above, the origin of this morphology comes from the reaction of relatively O₂-rich regions and Te at constant concentration. However, Fig. 3b shows the uniform thickness of the NW attached with a nanoparticle at protruding surface regions, such as ledges and kinks. In contrast to Fig. 3a, the source of this morphology stems from the reaction of relatively Te-rich regions at constant O₂ concentration. The local high-resolution TEM (HRTEM) image (Fig. 3c) reveals a clear fringe pattern, suggesting 0.29 nm interplanar spacing, in accordance with the (102) lattice plane of tetragonal TeO₂. The SAED pattern (Fig. 3d) supports that the bead-like TeO₂ NWs consist of single crystals, corresponding to the (010), (102), and (112) planes.

Fig. 3 TEM images of the bead-like TeO₂ NWs obtained by thermal evaporation: (a) TEM image originating from the region of high concentration and saturation of Te, (b) TEM image originating from the area of anisotropically distributed Te reacting with adsorped oxygen, (c) HRTEM micrograph of the bead-like TeO₂ NWs, (d) corresponding SAED pattern.

The C₂H₅OH gas-sensing properties of the bead-like p-TeO₂ NW sensors were investigated at various operating temperatures in the range of 200–400 °C and concentrations ranging from 50 to 200 ppm shown in Fig. 4. As shown in Fig. 4a, all the fabricated sensors showed p-type sensing behavior. That is, the resistance increases in the presence of the reducing gas C₂H₅OH, and maintained its original value after removal of C₂H₅OH. The C₂H₅OH gas responses of the fabricated sensors at various operating temperatures are shown Fig. 4b. The best C₂H₅OH response was obtained at 250 °C. The response and recovery times are summarized in Fig. 4c. As evident, the response and recovery times of TeO₂ NWs to C₂H₅OH gas at 250 °C are of the order of some tens of seconds. Regarding the response/recovery times at different temperatures, it is sluggish in case of C₂H₅OH. Also, the C₂H₅OH response of p-TeO₂ NWs is higher than the sensors fabricated with other p-type nanostructured materials (see ESI, Table S1†).

Fig. 4 (a) Change in resistance of the bead-like p-TeO₂ NW sensors for various C₂H₅OH concentrations at various operating temperature. (b) Sensor responses and (c) response and recovery times of the bead-like p-TeO₂ NW sensors.

The H₂S-sensing properties of the bead-like p-TeO₂ NW sensors have been also investigated under the same experimental conditions. The change in resistance at various operating temperatures in the range of 200–400 °C is displayed in Fig. 5a. It is evident that the resistances of all the fabricated sensors track the supply and stoppage of H₂S at different temperatures. It is clear that the H₂S-responses of the sensors improved with increasing operating temperature. Fig. 5b summarizes the sensor responses obtained at various temperatures. The sensors showed the best H₂S-response at 400 °C, the temperature that was chosen to compare the H₂S-selectivity of the bead-like TeO₂ NWs. The response and recovery times for H₂S are summarized in Fig. 5c. As evident, both the response and recovery times of the bead-like p-TeO₂ NWs toward H₂S amount to hundreds of seconds.

Fig. 5 (a) Change in resistance of the bead-like p-TeO₂ NW sensors for various H₂S concentrations at various operating temperature. (b) Sensor responses and (c) response and recovery times of the bead-like p-TeO₂ NW sensors.

In order to confirm the selectivity of the bead-like p-TeO₂ NW sensors toward oxidizing gases, we have tested the sensing properties of these sensors against the representative oxidizing gas NO₂. Fig. 6a shows the dynamic response curves obtained in case of exposure to 50, 100, and 200 ppm of NO₂. The NO₂-sensing properties of the fabricated sensors are dependent on the operating temperature. Fig. 6b summarized the gas response of the p-TeO₂ NWs toward NO₂. The responses of the sensors operating at different temperatures are also included in Fig. 6b for comparison. For better description, Fig. 6c summarizes the response and recovery times of the p-TeO₂ NWs as a function of the NO₂ concentration. It is of note that the bead-like p-TeO₂ NWs showed best sensing properties with respect to gas response and response/recovery times at 350 °C.

Fig. 6 (a) Change in resistance of bead-like p-TeO₂ NW sensors for various NO₂ concentrations at various operating temperature. (b) Sensor responses and (c) response and recovery times of bead-like p-TeO₂ NW sensors.

For the selectivity characterization, the sensors were exposed to 200 ppm of C₂H₅OH, H₂S, CO (not shown in this work), SO₂ (not shown in this work), and NO₂ at various operating temperatures, as shown in Fig. 7. Fig. 7a shows the typical bell-shape gas response as a function of the operating temperature. The best gas response against C₂H₅OH, H₂S, and NO₂ is obtained at 250, 350, and 400 °C, respectively. The bell- or volcano-shaped gas-response behavior is quite common among semiconductor-based gas sensors.^32–34 In semiconducting oxide gas sensors, the response usually increases and reaches its maximum at a certain temperature, and then decreases with further increase in temperature. The competition between slow kinetics of gas molecules at low temperature and their enhanced desorption at high temperatures is likely to be responsible for this behavior. Accordingly, the use of bead-like p-TeO₂ NWs for the selective detection of gas species is rather promising. In general, pristine oxide NWs do not show enough sensing ability that they could be applied to actual sensor applications. Therefore, conventional approaches to enhancing the sensing properties of pristine NWs including decorating, doping, and surface modification were attempted.^9–12 To improve the sensing properties of p-TeO₂ NWs, similar methodologies have been carried out to realize actual applications. In particular, one important feature of the fabricated sensors in this work is the surface modification by protuberances. Basically, the tendency in surface modification is the same as observed in case of n-SnO₂ NWs reported earlier.³⁵ The geometrical factors, such as NW diameter and length, and their density for best sensing performances were specifically discussed in an earlier report.³⁵

Fig. 7 (a) Responses to 200 ppm of C₂H₅OH, H₂S, and NO₂ as a function of operating temperature. (b) Responses to 200 ppm of C₂H₅OH, H₂S, and NO₂ at optimized temperatures.

Conclusions

The bead-like p-TeO₂ NWs originated from two different growth mechanisms were fabricated by evaporation/condensation of Te powder with 40 sccm of assisting O₂ gas. Many fine nanoparticles were formed at the local surfaces of the TeO₂ NWs. The existence of many rough nanocrystals helped the formation of bead-like TeO₂ nanostructures, implying suitable adsorption and desorption sites for NO₂, C₂H₅OH, and H₂S gas molecules, in turn resulting in chemical gas sensing. Especially, the optimum sensitivity of the sensors against the three investigated individual gases was clearly different and was affected only the process variable temperature without any functionalizing control, such as doping, surface modification, and heterostructures. This means that in case of the bead-like TeO₂ NWs, the optimal sensitivity and selectivity against various chemical gases could be simply adjusted by using only temperature as process variable.

Acknowledgements

This study was supported by the KU Research Professor Program of Konkuk University.

Notes and references

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Footnote

† Electronic supplementary information (ESI) available. See DOI: 10.1039/c5ra12263e

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