Construction of 1D/2D α-Fe₂O₃/SnO₂ Hybrid Nanoarrays for Sub-ppm Acetone Detection

Abstract

Exhaled acetone is one of the representative biomarkers for the noninvasive diagnosis of type-1 diabetes. In this work, we have applied a facile two-step chemical bath deposition method for acetone sensors based on α-Fe₂O₃/SnO₂ hybrid nanoarrays (HNAs), where one-dimensional (1D) FeOOH nanorods are in situ grown on the prefabricated 2D SnO₂ nanosheets for on-chip construction of 1D/2D HNAs. After annealing in air, ultrafine α-Fe₂O₃ nanorods are homogenously distributed on the surface of SnO₂ nanosheet arrays (NSAs). Gas sensing results show that the α-Fe₂O₃/SnO₂ HNAs exhibit a greatly enhanced response to acetone (3.25 at 0.4 ppm) at a sub-ppm level compared with those based on pure SnO₂ NSAs (1.16 at 0.4 ppm) and pure α-Fe₂O₃ nanorods (1.03 at 0.4 ppm), at an operating temperature of 340°C. The enhanced acetone sensing performance may be attributed to the formation of α-Fe₂O₃–SnO₂ n-n heterostructure with 1D/2D hybrid architectures. Moreover, the α-Fe₂O₃/SnO₂ HNAs also possess good reproducibility and selectivity toward acetone vapor, suggesting its potential application in breath acetone analysis.

1. Introduction

As a potential alternative for the noninvasive diagnosis of disease, exhaled breath analysis has been proposed and developed over the past decades [1–3]. The exhaled breath of human beings includes not only nitrogen, oxygen, carbon dioxide, nitric oxide, and water vapor but also a mixture of volatile organic compounds (VOCs) and some other nonvolatile molecules. Encouragingly, a few of them have been regarded as biomarkers to diagnose diseases (). For example, formaldehyde (lung cancer) [4], toluene (lung cancer) [5], ammonia (hemodialysis) [6], H₂S (halitosis) [7], isoprene (heart disease) [8], benzene (smoker) [9], and pentane (acute asthma) [10] at few dozens to few thousands of ppb are known as biomarkers for patients. Researchers have also found that exhaled acetone can intuitively correlate with type-1 diabetes, which may exceed 1.8 ppm (only 0.3–0.9 ppm for healthy people) [1, 11]. Therefore, an ultrasensitive acetone sensor is of great importance to detect acetone vapor at a sub-ppm level.

Metal oxide semiconductors (MOXs), such as SnO₂, ZnO, α-Fe₂O₃, CuO, and NiO, have been widely explored in the field of gas detection owing to their simple and cost-effective synthesis, high sensitivity, and good stability. Among these MOXs, α-Fe₂O₃ is a multifunctional n-type semiconductor with a direct bandgap (E_g = 2.2 eV at 300 K) that has been intensively investigated in the field of gas sensing [12–15]. Several effective strategies have been designed to improve the gas sensing properties of these MOXs, such as doping, surface modification, porous/hollow structures, and hierarchical architectures [16–18]. Recently, construction of hybrid nanostructures is rapidly emerging as a fascinating strategy that combines different MOXs with precise control of their morphologies, such as hollow ZnO/ZnFe₂O₄ heterostructures that were synthesized by growing ultrathin ZnFe₂O₄ nanosheets on the outer surface of ZnO hollow microspheres [19], NiO nanoparticle-decorated SnO₂ nanosheets [20], CuO nanosheets/ZnO nanorods (NRs) [21], and Fe₂O₃ nanoparticle-decorated CuO NRs [22]. For this purpose, the rational combination of SnO₂ and α-Fe₂O₃ has been proven to improve their gas sensing performances (). The results show that the α-Fe₂O₃/SnO₂ composites present excellent sensing performances to acetone [23, 24], ethanol [25–30], toluene [31], and LPG [32, 33]. Moreover, their gas sensing properties can be largely affected by the size and shape of nanobuilding blocks (α-Fe₂O₃ and SnO₂). As far as we know, only a few reports about α-Fe₂O₃–SnO₂ system have concerned on the detection of ultralow concentrations of acetone.

In this work, we report a two-step chemical bath deposition (CBD) method to construct the α-Fe₂O₃/SnO₂ hybrid nanoarrays (HNAs) on-chips with subsequent annealing in air. The one-dimensional (1D) α-Fe₂O₃ NRs are distributed homogenously on the surface of the 2D SnO₂ nanosheets to construct novel 1D/2D HNAs. In comparison with pure SnO₂ NSAs and α-Fe₂O₃ NRs, the α-Fe₂O₃/SnO₂ HNAs show a dramatically enhanced response to acetone (down to sub-ppm). Moreover, the α-Fe₂O₃/SnO₂ HNAs also possess a superior selectivity to acetone against other interfering gases (formaldehyde, toluene, benzene, and ammonia). A possible sensing mechanism based on the formation of α-Fe₂O₃–SnO₂ n-n heterostructure is proposed.

2. Results and Discussion

2.1. Morphological Characteristics

As illustrated in Figure 1(a), a chip with interdigital Au electrodes (200 μm lines separated by 200 μm gaps) was fabricated on a (100) silicon substrate with a 2 μm thermally grown SiO₂ layer, and the SnO₂ NSAs were prepared with an on-chip growth method similar to our previous work [34]. The chip was vertically dipped into the mixed solution (containing Sn²⁺ and CO(NH₂)₂) during this process, and then the prefabricated SnO₂ NSAs were immersed in another aqueous solution (containing Fe²⁺ and CO(NH₂)₂) for depositing FeOOH NRs on SnO₂ NSAs. After annealing in air, the as-prepared sensors were placed on a CGS-4TP gas sensing measurement system. A schematic of the gas sensing measurement systems used in this work is illustrated in . Figure 1(b) presents a schematic diagram of the test platform used in this work. A hotplate was used to adjust the operating temperature, and two pins of a sensor were connected with a pair of probes. A digital photograph of the gas sensing chip (3 mm × 6 mm in size) with α-Fe₂O₃/SnO₂ HNAs is given in the inset of Figure 1(b). During the test, four sensors were measured simultaneously, as shown Figure 1(c), and the electrical resistance of each sensor was recorded.

Figure 1.

Schematic diagrams of (a) synthesis process of α-Fe₂O₃/SnO₂ HNAs and (b) gas sensing measurement platform; the inset shows the digital photograph of the α-Fe₂O₃/SnO₂ HNAs gas sensing chips. (c) A photograph of the test platform of CGS-4TPs.

Figure 2 shows the morphologies of as-prepared pure SnO₂ NSAs and α-Fe₂O₃/SnO₂ HNAs. It can be seen from Figure 2(a) that the pure SnO₂ NSAs are composed by oriented growth of nanosheets, where the adjacent SnO₂ nanosheets will interconnect with each other and form a semiopen network. From the cross-sectional scanning electron microscope (SEM) images of SnO₂ NSAs (Figure 2(b)), the flake-like SnO₂ stands vertically on the chip with a uniform film thickness (~100 nm), and the SnO₂ NSAs are robustly adhered to the substrate. After the growth of α-Fe₂O₃ NRs, it is obvious that the surface morphology of α-Fe₂O₃/SnO₂ HNAs is much different from that of the pure SnO₂ NSAs. As shown in Figure 2(c), numerous ultrathin α-Fe₂O₃ NRs are homogenously distributed among the interconnected SnO₂ NSAs. A SEM close-up image of α-Fe₂O₃ NRs (inset Figure 2(c)) reveals that the diameter of NRs ranges from 9 nm to 20 nm, and the average diameter is about 12.7 nm. Otherwise, for the second-step (CBD method), the α-Fe₂O₃ NRs tend to form irregular aggregates without a substrate (). The cross-sectional SEM images of α-Fe₂O₃/SnO₂ HNAs in Figure 2(d) further indicate that the 1D α-Fe₂O₃ NRs are in situ grown on the surface of 2D SnO₂ nanosheets, and novel 1D/2D hybrid nanoarrays can be achieved by a facile two-step CBD method. At the same time, the average film thickness of α-Fe₂O₃/SnO₂ HNAs increases up to 220 nm.

Figure 2.

Top view SEM images of (a) SnO₂ NSAs and (c) α-Fe₂O₃/SnO₂ HNAs. Cross-sectional SEM images of (b) SnO₂ NSAs and (d) α-Fe₂O₃/SnO₂ HNAs. The inset of (c) shows a SEM close-up image of α-Fe₂O₃ NRs. (e) TEM and (f) HRTEM images of the pure SnO₂ NSAs. (g) TEM and (h) HRTEM images of α-Fe₂O₃/SnO₂ HNAs. The inset of (g) shows an individual α-Fe₂O₃ rod grown on the surface of SnO₂ nanosheet.

To get further insight into the definite morphology of pure SnO₂ NSAs and α-Fe₂O₃/SnO₂ HNAs, transmission electron microscope (TEM) images were taken from the scraped-off products. As shown in Figure 2(e), SnO₂ NSAs made up of interconnecting flakes with a thickness of <10 nm are obtained. Because of the vertical direction of growth, the thickness of a SnO₂ nanosheet can be easily measured in Figure 2(e) (marked by arrows, ~8 nm). These 2D nanosheets have an edge length of tens of nanometers, which agree well with the SEM observation (Figure 2(a)). A high-resolution TEM (HRTEM) image (Figure 2(f)) reveals the fringe patterns in SnO₂ NSAs, and the d-spacings of 0.264 and 0.336 nm are assigned to the interplanar distances of (101) and (110) planes of rutile SnO₂, respectively.

For α-Fe₂O₃/SnO₂ HNAs, the overall TEM image (Figure 2(g)) indicates that the hybrid composites are constructed by interconnected 2D nanosheets and some disordered 1D nanorods with respect to their different structural features. The inset of Figure 2(g) shows an individual nanorod grown on the surface of the nanosheet. The diameter of the rod is around 9 nm, and the length is estimated to be 57 nm. Figure 2(h) provides the HRTEM image of the selected region from the inset of Figure 2(g) (marked by a dashed rectangle). The lattice fringes with d-spacings of 0.270 and 0.336 nm can be indexed to the (104) plane of α-Fe₂O₃ and (110) plane of SnO₂, respectively. These results further confirm the construction of the 1D/2D hybrid nanostructure of α-Fe₂O₃/SnO₂ HNAs.

The chemical composition of the samples was identified by EDS and XPS analysis. The EDS mapping and spectrum of α-Fe₂O₃/SnO₂ HNAs are depicted in Figures 3(a)–3(c) and 3(d), respectively. Sn (Figure 3(b)) and Fe (Figure 3(c)) elements were distributed randomly and uniformly on the substrate, which was in agreement with the fact that the hybrid composites were constructed by hybrid α-Fe₂O₃ and SnO₂. The existence of Sn and Fe elements was confirmed by the EDS characterization, and the atomic percentages of Sn and Fe were measured to be 5.38% and 4.17%, respectively.

Figure 3.

(a) SEM image, (b–c) the corresponding EDS mapping images, and (d) EDS spectrum of α-Fe₂O₃/SnO₂ HNAs.

Moreover, XPS analysis was used to obtain more information about the chemical valences of our samples. Figure 4(a) displays the high-resolution Sn 3d spectra of SnO₂ NSAs and α-Fe₂O₃/SnO₂ HNAs. In the pure SnO₂ NSAs, the two peaks centered at 487.4 and 495.8 eV can be ascribed to the peaks of Sn 3d_5/2 and Sn 3d_3/2, respectively, which are in good agreement with Sn⁴⁺. With the modification of α-Fe₂O₃, a slight negative shift of the binding energies is observed in α-Fe₂O₃/SnO₂ HNAs, shifting to 487.3 and 495.7 eV, respectively, as a result of the formation of α-Fe₂O₃/SnO₂ heterojunction interface. In the spectrum of Fe 2p in Figure 4(b), interference peaks are detected at 717.6 eV in SnO₂ NSAs and 716.5 eV in α-Fe₂O₃/SnO₂ HNAs, which come from the Sn 3p peak. The Fe 2p peaks are not found in α-Fe₂O₃/SnO₂ HNAs due to the strong interference peak. In comparison, the two peaks at 711.9 and 725.2 eV detected in pure α-Fe₂O₃ NRs are attributed to Fe 2p_3/2 and Fe 2p_1/2, respectively, corresponding to Fe³⁺ in α-Fe₂O₃.

Figure 4.

The high-resolution XPS spectra of SnO₂ NSAs, α-Fe₂O₃ NRs, and α-Fe₂O₃/SnO₂ HNAs: (a) Sn 3d and (b) Fe 2p.

2.2. Gas Sensing Properties

As is well-known, the gas sensing properties of MOXs are highly dependent on the operating temperature. To confirm it, the as-prepared gas sensors were examined at various temperatures (280-380°C) toward 5 ppm acetone. The sensor response is defined as R_a/R_g, where R_a and R_g are the sensor resistance in air and target gas, respectively. As shown in Figure 5(a), the response of α-Fe₂O₃/SnO₂ HNA-based sensor increases with the increase in operating temperature and reaches its maximum value (13.63) at 340°C, then decreases with the further increase of operating temperature. Therefore, 340°C can be chosen as the optimum operating temperature of α-Fe₂O₃/SnO₂ HNAs. Differently, the pure SnO₂ NSA-based sensor exhibits no obvious variation over the whole temperature range (2.00, at 340°C). The pure α-Fe₂O₃ NR-based sensor shows a monotonic decrease of the response with an increase in operating temperature, and the highest response value is about 4.28 at 280°C. It is clear that the α-Fe₂O₃/SnO₂ HNA-based sensor displays the highest response in the three sensors, revealing that the acetone sensing properties of SnO₂ NSAs can be significantly enhanced by the modification of α-Fe₂O₃ NRs.

Figure 5.

(a) Sensor responses of the pure SnO₂ NSAs, α-Fe₂O₃ NRs, and α-Fe₂O₃/SnO₂ HNAs toward 5 ppm acetone as a function of operating temperature (280-380°C). (b) Sensor responses vs. acetone concentration (0.4-20 ppm) at 340°C. The corresponding transient response curves of (c) SnO₂ NSAs and α-Fe₂O₃/SnO₂ HNAs. Resistance curves of (d) SnO₂ NSAs and (e) α-Fe₂O₃/SnO₂ HNAs at 340°C. (f) Selectivity of the sensors to various gases (1 ppm) at 340°C.

Figure 5(b) gives the acetone sensing properties of the above three sensors at the same operating temperature of 340°C. It is clear that the sensor response increases with the acetone concentration ranging from 0.4 to 20 ppm for each sensor. Especially, in the case of α-Fe₂O₃/SnO₂ HNAs, the response increases rapidly over the whole concentration range, which is rather different from the other two sensors. The response values of α-Fe₂O₃/SnO₂ HNAs are 3.25, 4.64, 5.37, 7.68, 9.91, 10.69, 12.34, 16.55, and 21.26 toward 0.4, 0.8, 1, 2, 3, 4, 5, 10, and 20 ppm acetone, respectively. In comparison, the response values of pure SnO₂ NSAs and α-Fe₂O₃ NRs toward acetone can be as low as 1.16 and 1.03, respectively, at a concentration of 0.4 ppm, and their values are still less than 3.1 even toward 20 ppm acetone. Therefore, the α-Fe₂O₃/SnO₂ HNA-based sensor exhibits the highest response values toward acetone in the three sensors, indicating the improvement of sensitivity.

Figure 5(c) plots the corresponding transient response curves of the pure SnO₂ NSA- and α-Fe₂O₃/SnO₂ HNA-based sensors over an acetone concentration range of 0.4 to 20 ppm recorded at 340°C. Obviously, these response curves present a sharp increase upon acetone exposure and can recover to their original values in air. In accordance with these, the sensor resistance curves are shown in Figures 5(d) and 5(e). Amongst them, the α-Fe₂O₃/SnO₂ HNA-based sensor exhibits a higher resistance in air (R_a, 282.683 MΩ, Figure 5(e)) than that of pure SnO₂ NSAs (204.05 kΩ, Figure 5(d)) and α-Fe₂O₃ NRs (132.298 MΩ, ). Upon exposure to acetone gas, the sensor resistance quickly decreases as expected and then recovers to its R_a after being exposed to air. R_g decreases monotonically with the increase of acetone concentration; in other words, the sensor response increases (refer to Figure 5(c) and ).

The response and recovery times (t_res and t_rec) are very important parameters for high-performance gas sensors. The response time t_res (or recovery time t_rec) is defined as the time required to reach 90% resistance change when the sensor is exposed to target gas (or air). As shown in , the α-Fe₂O₃/SnO₂ HNA-based sensor shows a faster t_res (14 s, at 1 ppm) at 340°C compared with that of the SnO₂ NSAs (37 s, at 1 ppm). On the contrary, the t_rec of the α-Fe₂O₃/SnO₂ HNA-based sensor always exceeds one minute (62–159 s, in the range 0.4–20 ppm), which is apparently higher than that of the SnO₂ NSA-based sensor (22–34 s, in the range 0.4–20 ppm). According to the previous studies, the vertically ultrathin SnO₂ NSAs can provide as much surface area as possible to adsorb gas molecules and facilitate the adsorption/desorption of the acetone gas. When the α-Fe₂O₃ NRs were introduced, the branched α-Fe₂O₃ NRs on the surface of SnO₂ NSAs will adsorb more acetone molecules, making the α-Fe₂O₃/SnO₂ HNAs more sensitive and present faster response toward acetone.

To study the selectivity in the above sensors, some interfering gases (formaldehyde, toluene, benzene, and ammonia) were measured at 340°C with a low concentration of 1 ppm. It can be seen in Figure 5(f) that the α-Fe₂O₃/SnO₂ HNA-based sensor exhibits higher responses toward all gases than those of pure SnO₂ NSAs and α-Fe₂O₃ NRs. Especially, all the sensors obtain their highest responses toward acetone compared with other interfering gases. In the case of the α-Fe₂O₃/SnO₂ HNA-based sensor, it shows the highest response toward acetone (5.37), then toward formaldehyde (1.23) and toluene (1.16) and is almost insensitive toward ammonia (1.11) and benzene (1.09). On the other hand, the corresponding responses of pure SnO₂ NSAs toward above gases are 1.33, 1.02, 1.02, 1.02, and 1.01 in turn (for α-Fe₂O₃ NRs: 1.30, 1.03, 1.02, 1.03, and 1.03). These results suggest that the α-Fe₂O₃ NRs, indeed, have a significant impact on the selectivity of the SnO₂ NSAs toward acetone.

The reproducibility of the sensors at 340°C has been investigated by continuously testing the sensors to 5 ppm and 1 ppm acetone with 5 cycles for each. As shown in Figures 6(a)–6(c), all the sensors maintain their response values without obvious variation (less than 4%) during the cyclic testing, indicating excellent reproducibility of our devices. By comparison, Figure 6(d) illustrates the statistical analysis of the results of sensor responses (for SnO₂ NSAs sensor: 1.32 ± 0.02 at 1 ppm and 2.00 ± 0.01 at 5 ppm; α-Fe₂O₃ NRs sensor: 1.33 ± 0.03 at 1 ppm and 2.02 ± 0.06 at 5 ppm; α-Fe₂O₃/SnO₂ HNAs sensor: 5.05 ± 0.11 at 1 ppm and 11.80 ± 0.29 at 5 ppm), further demonstrating their robustness as acetone sensors.

Figure 6.

Reproducibility of (a) SnO₂ NSAs, (b) α-Fe₂O₃ NRs, and (c) α-Fe₂O₃/SnO₂ HNA-based sensors toward acetone (5 ppm and 1 ppm, each of 5 cycles) at 340°C; (d) is the corresponding comparison of the sensor responses. (e) Stability of α-Fe₂O₃/SnO₂ HNA-based sensor toward 5 ppm acetone at 340°C for 93 days; the inset is the SEM image of α-Fe₂O₃/SnO₂ HNAs taken after 93 days of gas sensing test. (f) Response of α-Fe₂O₃/SnO₂ HNA-based sensor to 5 ppm acetone at 340°C under different relative humidity (20–90% RH).

To assess the long-term stability of our sensor, we tested the α-Fe₂O₃/SnO₂ HNA-based sensor for 93 days toward 5 ppm acetone at 340°C. The mean response of the α-Fe₂O₃/SnO₂ HNA-based sensor is 11.62 with a standard deviation estimated to be 0.93 during the whole period, suggesting its stability for acetone detection over a long period. Furthermore, it can be clearly seen that there is no obvious change between the nanostructures of α-Fe₂O₃/SnO₂ HNAs before (Figure 2(c)) and after (inset of Figure 6(e)) a number of gas sensing tests. This observation is consistent with the good long-term stability of the α-Fe₂O₃/SnO₂ HNA-based sensor.

It is well-known that human exhaled breath is highly humid (RH ≥ 80%) and the existence of water vapor has a significant influence on the gas sensing performance for MOX-based gas sensors. As shown in Figure 6(f), the response of α-Fe₂O₃ NRs to 5 ppm acetone was measured as a function of relative humidity (20%–90% RH). The responses of α-Fe₂O₃/SnO₂ HNAs under 20%, 40%, 60%, 80%, and 90% RH were 12.34, 6.02, 4.69, 3.86, and 3.62, respectively. Obviously, the response of α-Fe₂O₃/SnO₂ HNAs is highly dependent on relative humidity, and some available approaches (such as employing water filtering membranes) are needed to eliminate the influence of water vapor.

Considering the previous reports in Table 1 [11, 35–38], the α-Fe₂O₃/SnO₂ HNA-based sensor in this work possesses relatively medium sensitivity (or operating temperature). We can conclude that the acetone sensing properties of MOXs can be further enhanced by constructing heterostructures or modifying with noble metals. As mentioned before, the acetone detection capability (or resolution) for the diagnosis of diabetes mellitus should be as low as sub-ppm, all of which need sufficient and reliable sensors for acetone. In this sense, the high sensitivity, good selectivity, and excellent reproducibility of the α-Fe₂O₃/SnO₂ HNA-based sensor imply that it can potentially be used for breath acetone analysis.

Table 1.

Comparison of the acetone sensing properties of MOX-based sensors.

Materials	Temperature (°C)	RH (%)	Detection range (ppm)	Response (Ra/Rg)	t res/trec (s/s)	Ref.
α-Fe2O3/SnO2 HNAs	340	20	0.4–20	5.37@1 ppm	14/70	This work
Pt-SnO2 fibers	300	80	0.12–3	3.47@3 ppm	15/6	35
ZnO@ZIF-CoZn nanowire arrays	260	0–90	10–2000	2.3@10 ppm	43/61	36
PdO-Co3O4 hollow nanocages	350	90	0.4–5	1.52∗@1 ppm	—	37
NiO/ZnO hollow spheres	275	30	0.8–100	1.6@0.8 ppm	1/20	38
ZnO/CuO inverse opals	310	93.5	0.2–50	1.8@1 ppm	7/13	11

^∗ R _g/R_a.

2.3. Sensing Mechanism

For n-type MOXs (SnO₂ and α-Fe₂O₃), their acetone sensing mechanisms can be briefly understood as the reaction between the adsorbed oxygen species and acetone molecules on the active sites of sensitive materials. [24] In general, an electron depletion layer (EDL) can be formed on the near surface of SnO₂ nanosheets (Figure 7(a)) owing to the adsorbed oxygen species (O^–, O₂^–, and O^2–) after exposing to air and makes the absorbed oxygen species capture free electrons from the conduction band of SnO₂. This results in a decrease of electron concentration (or width of EDL) and thus a relatively high resistance in air atmosphere. The generation and transformation processes of the oxygen species at different operating temperatures have the following expressions [39]:

$$\begin{array}{c}{\text{O}}_2\left(\text{gas}\right)⟷{\text{O}}_2\left(\text{ads}\right)\\ {\text{O}}_2+{\text{e}}^{-}⟷{{\text{O}}_2}^{-}\\ {{\text{O}}_2}^{-}+{\text{e}}^{-}⟷2{\text{O}}^{-}\\ {\text{O}}^{-}+{\text{e}}^{-}⟷{\text{O}}^{2-}\end{array}$$ (1)

Figure 7.

Schematic illustration of the acetone sensing mechanism of (a) the pure SnO₂ NSAs and (b) α-Fe₂O₃/SnO₂ HNAs (not drawn to scale). (c) Energy band diagrams of α-Fe₂O₃/SnO₂ system before and after equilibrium. E_VAC: the vacuum level; E_F: Fermi level; E_CB: the bottom of conduction band; E_VB: the top of valence band; E_g: band gap.

On the contrary, upon exposure to reducing gases such as acetone, acetone molecules will react with the absorbed oxygen species, as expressed by Equation (2) and release free electrons back to the SnO₂ nanosheets. Hence, the electron concentration increase will cause an increase in conductivity (or a decrease in sensor resistance), and the width of EDL also becomes broader. According to the SEM and TEM observations, the vertically distributed SnO₂ nanosheets connect with each other to construct an interlaced electron transport network on the substrate. However, the modulation mechanism of this type of transport network is not efficient because of the same energy band structure in the pure SnO₂ NSAs. In other words, the R_a of the pure SnO₂ NSA-based sensor is rather low (Figure 5(d)), which is too difficult to obtain dramatic change, especially at low acetone concentrations.

$$\begin{array}{c}{\text{CH}}_3{\text{COCH}}_3+8{\text{O}}^{-}⟶3{\text{CO}}_2+3{\text{H}}_2\text{O}+8{\text{e}}^{-}\\ {\text{CH}}_3{\text{COCH}}_3+8{\text{O}}^{2-}⟶3{\text{CO}}_2+3{\text{H}}_2\text{O}+16{\text{e}}^{-}\end{array}$$ (2)

In the case of α-Fe₂O₃/SnO₂ HNAs, the sensor exhibits enhanced sensitivity toward acetone, this may be attributed to the following reasons: (1) In the formation of α-Fe₂O₃–SnO₂ n–n heterostructures, by combining these two MOXs with different work functions (SnO₂: 𝛷 = 4.9 eV; α-Fe₂O₃: 𝛷 = 5.88 eV) [40, 41], the free electrons tend to transfer from the higher side to the lower side, until the equilibrium Fermi level is reached (Figure 7(c)) [26, 42]. In this process, the SnO₂ nanosheet near the heterostructure interface will lose more electrons, which leads to a broader conduction region in air (Figure 7(b)) [43]. Similar to other reports, the R_a of the α-Fe₂O₃/SnO₂ HNAs in this work is much higher than that of the pure SnO₂ NSAs.

(2) In the novel 1D/2D α-Fe₂O₃/SnO₂ hybrid architectures, when the sensor is exposed to acetone, the stretched-out α-Fe₂O₃ NRs provide an extra surface area and active sites for the gas adsorption. Thus, more oxygen species and acetone molecules can be adsorbed on the surface of α-Fe₂O₃/SnO₂ HNAs (Figure 7(b)), which provides more opportunities for Equation (2). The conduction region in SnO₂ nanosheets will be broadened as well as a decrease in R_g. On the other hand, the free electrons generated on the surface of α-Fe₂O₃ NRs will flow to SnO₂ NSAs and allow a dramatic decrease in the width of the electron depletion region at the interface of the α-Fe₂O₃/SnO₂ heterostructure. It may further result in an increase of sensor response toward acetone. So the modulation mechanism of α-Fe₂O₃/SnO₂ HNAs becomes more efficient than that of pure SnO₂ NSAs. Additionally, much hard work is still needed to study the influence of ambient humidity, filter units, and clinical tests, making it more suitable for breath acetone analysis.

3. Conclusion

The α-Fe₂O₃/SnO₂ HNA-based acetone sensor has been fabricated via a facile two-step on-chip growth (or CBD method) process. The results indicate that the as-prepared sensor presents a well-defined 1D/2D hybrid architecture, where the ultrathin α-Fe₂O₃ NRs (an average diameter ~12.7 nm) are distributed among the interconnected SnO₂ NSAs. Gas sensing measurements show that the α-Fe₂O₃/SnO₂HNA-based sensor exhibits superior acetone sensing properties (high sensitivity, good reproducibility, and selectivity), even at a sub-ppm level, compared with those of the pure SnO₂ NSA- and α-Fe₂O₃ NR-based sensors. The improved acetone sensing performance may be due to the formed α-Fe₂O₃–SnO₂ heterostructures and their unique hybrid nanostructures. Our work suggests that the α-Fe₂O₃/SnO₂ HNAs can be a promising candidate for sub-ppm acetone detection in breath analysis.

4. Materials and Methods

4.1. Preparation of α-Fe₂O₃/SnO₂ HNAs

In brief, 0.6 mmol SnCl₂·2H₂O and 0.8 mmol CO(NH₂)₂ were dissolved into 20 mL deionized water and stirred for 15 min at room temperature. Then a piece of chip with several Au electrodes was washed with acetone and ethanol and deionized water for several times, which was afterwards vertically dipped into the above solution and maintained at 95°C for 8 h. After washing and drying at 60°C in an oven, the prefabricated SnO₂ NSAs were immersed in an aqueous solution (containing 0.1 M FeSO₄·7H₂O and 1.0 M CO(NH₂)₂) and kept at 80°C for 1 h. Similarly, the chip was washed and dried again at 60°C. The final chip was annealed at 400°C for 3 h in air to achieve α-Fe₂O₃/SnO₂ HNAs.

In addition, the pure SnO₂ NSAs were also annealed under the same conditions. For a pure α-Fe₂O₃ NR-based sensor, 0.1 M FeSO₄·7H₂O and 1.0 M CO(NH₂)₂ were mixed and maintained at 80°C for 1 h, and the collected precipitate was dip-coated on the Au electrodes and then annealed at 400°C in air for 3 h.

4.2. Characterization and Gas Sensing Measurements

The morphologies and compositions of as-prepared products were investigated by a scanning electron microscope (SEM, Zeiss Gemini 300) equipped with energy dispersive X-ray (EDX) spectroscope and a high-resolution transmission electron microscope (HRTEM, FEI Tecnai G2 F30). The chemical states of the surface species were determined by using X-ray photoelectron spectroscopy (XPS, ESCALB 250Xi). The gas sensing properties of sensors were performed on a commercial CGS-4TPs system (Beijing Elite Tech Co., Ltd., China). Gaseous acetone diluted with dry air was injected by a syringe. The operating temperature ranges from 280 to 380°C with a relative humidity around 20%.

Conflicts of Interest

All authors declare that there is no competing financial interest.

Authors' Contributions

Huimin Gong, Changhui Zhao, and Fei Wang designed the experiments and contributed to the manuscript. Gaoqiang Niu and Wei Zhang carried out the material characterizations. The initial draft was edited by Changhui Zhao and Fei Wang. All authors reviewed the final version.

Supplementary Materials

Supplementary Materials

Figure S1: schematic drawing of the gas sensing measurement systems. Figure S2: SEM images of pure α-Fe₂O₃ NRs. Figure S3: transient response curve of α-Fe₂O₃ NRs toward different acetone concentrations at 340°C and the corresponding resistance curve. Figure S4: response/recovery time vs. acetone concentration (0.4-20 ppm) with SnO₂ NSAs and α-Fe₂O₃/SnO₂ HNAs at 340°C. Table S1: brief summary of results reported on exhaled breath analysis. Table S2: gas sensing properties of Fe₂O₃–SnO₂ systems toward various gases.