Nanowires Utilized to Improve Room Temperature Gas Sensing

Due to their strong chemical stability and sensitivity, metal oxide semiconductors have been utilized in chemiresistive gas sensors, although their poor intrinsic conductivity causes high working temperatures (above 100 °C – 200 °C).

Image Credit: Love Employee/Shutterstock.com

As a result, optimizing the reaction sites for carrier transfer and concentration is critical to achieving low-temperature functioning.

Two-dimensional semiconductors, particularly transition metal dichalcogenides (TMDs) such as MoS₂, WSe₂, and SnSe₂, have been considered for use as active sensing materials. Several issues remain, however, including irreversible sensing behavior, sluggish reaction and recovery time, and high oxidation rate at high working temperatures.

To increase sensing properties at low temperatures, a nanostructure with a wide effective reaction area (i.e., specific surface, crystal defects, etc.) and optimized carrier concentration is extremely preferred.

Heterostructures composed of 2D semiconducting TMDs and metal oxide semiconductors have recently been studied as possible active sensing materials for chemiresistive gas sensors.

The effective charge concentration can be adjusted by charge transfer between the two atomic layers due to their differing electronic band structures. These materials offer a high specific surface area and a quick reaction time.

Researchers analyzed the gas sensing properties of a 1D–2D hybrid network structure containing SnO₂ nanowires and SnSe₂ layers. After selenization, the hybridized nanowire networks demonstrated improved chemisorption of oxygen ions and stable NO₂ detection capability at ambient temperature. The study was published in the journal iScience.

Results and Discussion

A vapor-liquid-solid (VLS) approach was used to make pristine SnO₂ nanowires, which were then selenized utilizing a CVD-based direct selenization procedure, as seen in Figure 1A.

First, interdigitated Au (3 nm)/Pt (300 nm)/Ti (50 nm) electrodes with a 20 μm width and spacing were fabricated, and then SnO₂ nanowires were selectively grown at 900 °C for 30 minutes using the VLS growth technique.

Using Se powder as the Se precursor, the ultrathin SnSe₂ was produced on the surface of the SnO₂ nanowires network at 550 °C for 30 minutes.

The Au layer was employed to selectively develop SnO₂ nanowires, resulting in a network topology of SnO₂ nanowires growing on the interdigitated electrodes. This networked structure of SnO₂ nanowires is retained following the CVD-based selenization process, as seen in Figure 1B.

Low and high magnification field emission scanning electron microscopy (FESEM) images of the 1D SnO₂–2D SnSe₂ hybrid nanowire network are shown in Figures 1C and 1D. Highly dense nanowires with a diameter of 40 to 50 nm are orientated randomly.

A field emission transmission electron microscopy (FETEM) picture of the 1D SnO₂–2D SnSe₂ hybrid nanowire network is shown in Figure 1E. On the surface of grown SnO₂ nanowires, an ultrathin SnSe₂ layer with a thickness of a few nanometers was created evenly.

Figure 1. Preparation and structural characterization of 1D SnO₂–2D SnSe₂ hybrid nanowire network(A) Illustration showing the CVD-based selenization process of pristine SnO₂ nanowires.(B) FESEM image of 1D SnO₂–2D SnSe₂ nanowire network selectively grown on Au/Pt/Ti interdigitated electrodes.(C and D) FESEM images of SnO₂ nanowires with ultrathin SnSe₂ under (C) low and (D) high magnification.(E) FETEM image of SnO₂ nanowire after the selenization process showing ultrathin shell of SnSe₂ formed on SnO₂ nanowire.(F) STEM-HAADF image with corresponding EDS mapping images of Sn (green), O (red), and Se (blue). Image Credit: Seo, et al., 2022

X-ray diffraction (XRD) was used to analyze the crystal structure of the SnO₂-SnSe₂ hybrid nanowires. The (002), (011), (003), and (110) planes of SnSe₂ are attributed to the peaks at roughly 29.6°, 30.6°, 43.5°, and 47.5°, respectively.

The XRD spectra of as-grown SnO₂ nanowires before and after CVD-based surface selenization are shown in Figure 2A. This implies that the ultrathin SnSe₂ nanowire networks were effectively produced on the surface of SnO₂.

Raman spectroscopy of virgin SnO₂ nanowires reveals three distinct peaks at 474, 633, and 776 cm^-1, respectively.

The in-plane A_1g and out-of-plane E_g modes are attributed to the Raman peaks, respectively. The few scattering centers in the few-layered SnSe₂ are responsible for the E_g mode’s low intensity. The frequency of these two distinctive Raman peaks is determined by the number of SnSe₂ layers present.

As a consequence, crystalline SnSe₂ is well produced on the surface of SnO₂ nanowires with less than 5 layers, as shown by FETEM in Figure 1E and XRD findings.

SnO₂’s in-plane A_1g vibrational mode similarly has a low peak intensity, implying the presence of SnO₂ nanowires under the selenized surface. However, the A_1g mode has shifted from 633 to 631 cm^-1, indicating that oxygen has been incorporated into the crystal lattice.

This can alter the planar lattice’s interaction, influencing the in-plane vibrational mode. Furthermore, a modest Raman peak of about 107 cm^-1 is found, which correlates to the B_3g form of SnSe. This suggests that the as-prepared nanowires have a partly selenized area.

The valence state of every element on the surface of the selenized SnO₂ nanowires was further studied using X-ray photoelectron spectroscopy (XPS), as illustrated in Figures 2C–E.

Sn-O bonding in SnO₂ (Sn 3d_5/2: 487.19 eV, Sn 3d_3/2: 495.6 eV) and Sn-Se bonding in SnSe₂ (Sn 3d_5/2: 485.63 eV, Sn 3d_3/2: 494.16 eV) are seen in the high-resolution Sn 3d spectrum in Figure 2C.

The occurrence of the ultrathin SnSe₂ layer on the surface of SnO₂ nanowires is indicated by the modest intensity of the Sn-Se bonding peaks.

Figure 2. Structural and chemical characterizations of 1D SnO₂–2D SnSe₂ hybrid nanowire network (A) XRD spectra of pristine SnO₂ nanowire (top) and SnO₂ nanowires with ultrathin SnSe₂ (bottom). (B) Raman spectra of SnO₂ nanowires before (top) and after (bottom) selenization process. The inset shows the magnified Raman spectra in the range of 100–250 cm⁻¹, in which two characteristic Raman peaks of 2D SnSe₂ indicate the successful formation of ultrathin SnSe₂ layer on the surface of the SnO₂ nanowires. (C–E) XPS spectra of (C) Sn 3d, (D) O 1s, and (E) Se 3d for the1D SnO₂–2D SnSe₂ heterostructure. Image Credit: Seo, et al., 2022

Based on these findings, it can be inferred that the CVD procedure properly selenized the surface of the rutile SnO₂ nanowires with ultra-thin SnSe₂ with excellent crystallinity. The development of a 2D SnSe₂ layer on the surface of 1D SnO₂ nano-wires boosts the interface with the environment, which should improve gas sensing capability.

Figure 3A shows a schematic illustration of the 1D SnO₂ nanowire–2D SnSe₂-based gas sensor. The dynamic resistance responses of the SnO₂ nanowires were examined before and after the surface selenization at varying NO₂ concentrations and temperatures to study the characteristics of the ultrathin 2D SnSe₂ layers on 1D SnO₂ nanowires.

Figure 3. Gas sensing characteristics of 1D SnO₂–2D SnSe₂ hybrid nanowire network-based gas sensor forNO₂ gas(A) Illustration of NO₂ gas sensor based on SnO₂ nanowires with ultrathin SnSe₂.(B) Dynamic resistance curves of pristine SnO₂ nanowires (right) and 1D SnO₂–2D SnSe₂ heterostructure (left)-based gas sensor to different NO₂ gas concentrations of 10–50 ppm at different temperatures (room temperature – 200 °C).(C) Comparative analysis of NO₂ gas sensitivity at different operating temperatures and gas concentrations of 10–50 ppm.(D) Comparison of gas sensing response of the 1D SnO₂ nanowire–2D SnSe₂ heterostructure-based gas sensor against various gases (NO₂,C₃H₆O, C₆H₆,and C₇H₈) at different gas concentration from 10 to 50 ppm. Image Credit: Seo, et al., 2022

At ambient temperature, a hybrid gas sensor based on a network of 1D SnO₂ and 2D SnSe₂ nanowires demonstrated acceptable responsiveness, although resistance dropped as temperature increased. 

The sensor’s sensitivity is related to the gas concentration and decreases as the temperature rises. The buildup of electrons transmitted from the as-prepared SnO₂ nanowires on the surface of the SnSe₂ layer improves the sensing capabilities at ambient temperature.

At room temperature, the ultrathin SnSe₂ layer on SnO₂ nanowire networks greatly improved gas sensing capability. Due to the higher concentration of chemisorbed oxygen ions, the reactions between NO₂ molecules and adsorbed oxygen ions proliferate, resulting in a better gas response.

By testing the detecting behavior in various gases at ambient temperature, the selectivity of the 1D SnO₂–2D SnSe₂ heterostructure-based gas sensor was examined. The sensor features NO₂ gas selectivity.

Stimulating adsorbed oxygen molecules and lattice ionic oxygen to create active O^2-, O^-, and mobile O^2- species determines the pace of chemical processes. This effect lasts until the operational temperature reaches a particular point.

Furthermore, the exothermic gas molecule is difficult to adsorb by thermal energy, and the gas molecules are rapidly desorbed, lowering the gas response.

At ambient temperature, ultrathin SnSe₂ layers atop SnO₂ nanowire networks greatly improved gas sensing capability.

Even at low NO₂ concentrations, a 1D SnO₂ nanowire–2D SnSe₂ heterostructure-based gas sensor for NO₂ gas demonstrates good responsiveness. The back-gate bias reduces the energy barrier and increases gas sensing.

These findings show that the reaction to S_NO2/S_gas is nearly equal to the response to NO₂, showing that the 2D SnSe₂-based gas sensor possesses NO₂ selectivity.

The energy gap between the Fermi level of metal oxide and the valence band of the gas molecule determines the usual gas molecule adsorption systems on the surface of the sensing material.

At room temperature, the transient resistance to various NO₂ concentrations was measured when gate bias (V_g = 0.5, 1.5, and 2.5 V) was applied. Even at the lowest NO₂ concentration, the gas sensor provides a very good reaction (more than 50% greater) compared to the response without gate voltage.

The gate voltage enhanced the responsiveness and recovery behavior, as demonstrated in Figure 4B (1.5 V). The recovery time was reduced from 373 seconds to 346 seconds, while the response time was reduced from 375 s to 110 s, using a gate voltage of 1.5 V.

This means that the back-gate voltage lowers the energy barrier for adsorption on reactive sites, resulting in a more efficient interaction between NO₂ gas molecules and the nanowire surface at ambient temperature.

However, the recovery period was still lethargic, necessitating more study to enhance it.

Figure 4. Enhancement in gas sensing performance of 1D SnO₂–2D SnSe₂ heterostructure-based gas sensor for NO₂ gas by introducing the back-gate bias (A) Gate-bias dependent dynamic resistance curves of the 1D SnO₂–2D SnSe₂ hybrid nanowire networks-based gas sensor to different NO₂ gas concentrations of 10–50 ppm at room temperature. Electrical characteristics measured with V_DS=1 V and V_g= 0, 0.5, 1.0, 1.5, 2.0, 2.5 V.(B) Response and recovery times as a function of the gate voltage under the exposure to 50 ppm of NO₂ gas at room temperature.(C) Comparison of NO₂ gas sensing response at different back-gate bias and gas concentrations of 10–50 ppm. Image Credit: Seo, et al., 2022

At room temperature, Figure 4C shows the reaction to various NO₂ gas concentrations as a function of back-gate bias. At ambient temperature, the best gas reaction was 2.2 (R_a/R_g) when exposed to 50 ppm NO₂ gas at V_g = 1.5 V.

This result was nearly twice as high as the gas response of 1.13 (R_a/R_g) when the gate bias was not applied. With rising gas concentration, the response also increased, which might be due to the increased number of reactive gas molecules.

The energy barrier for the adsorption of analyte gas molecules on the surface of 1D–2D hybrid nanowires is controlled by the back-gate voltage, which contributes to electron flow.

As a result of using the back-gate voltage, the sensing performance was increased.

Conclusion

A 1D–2D hybrid nanowire network made up of SnSe₂ layers and SnO₂ nanowires were used to create a gas sensor. The gas sensor showed enhanced sensing performance, including reaction time and sensitivity, when back-gate bias (V_g = 1.5 V) was applied.

The energy barrier for NO₂ gas adsorption on the reactive sites was reduced by the back-gate voltage, resulting in improved sensing performance. Compared to a previous sensor that works at room temperature, the sensor displayed improved gas detection qualities for NO₂ gas at ambient temperature.

The findings might lead to improvements in sensing performance for low-temperature gas sensors, nanostructure design, and a variety of applications for future sensing platforms, such as portable diagnostic tools and smart sensing devices.

Journal Reference:

Seo, J., Nam, S. H., Lee, M., Kim, J.-Y., Kim, S. G., Park, C., Seo, D.-W., Kim, Y. L., Kim, S. S., Kim, U. J., and Hahm, M. G. (2022) Gate-controlled gas sensor utilizing 1D–2D hybrid nanowires network. iScience. 25, 103660 Available Online: https://www.sciencedirect.com/science/article/pii/S2589004221016308

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