Interfacial energy barrier tuning of hierarchical Bi2O3/WO3 heterojunctions for advanced triethylamine sensor

Traditional triethylamine (TEA) sensors suffer from the drawback of serious cross-sensitivity due to the low charge-transfer ability of gas-sensing materials. Herein, an advanced anti-interference TEA sensor is designed by utilizing interfacial energy barriers of hierarchical Bi2O3/WO3 composite. Benefiting from abundant slit-like pores, desirable defect features, and amplification effect of heterojunctions, the sensor based on Bi2O3/WO3 composite with 40% Bi2O3 (0.4-Bi2O3/WO3) demonstrates remarkable performance in terms of faster response/recovery time (1.7-fold/1.2-fold), higher response (2.1-fold), and lower power consumption (30 °C-decrement) as compared with the pristine WO3 sensor. Furthermore, the composite sensor exhibits long-term stability, reproducibility, and negligible response towards interfering molecules, indicating the promising potential of Bi2O3/WO3 heterojunctions in anti-interference detection of low-concentration TEA in real applications. This work not only offers a rational solution to design advanced gas sensors by tuning the interfacial energy barriers of heterojunctions, but also provides a fundamental understanding of hierarchical Bi2O3 structures in the gas-sensing field.

1 Introduction  Triethylamine (TEA), known as one of the most common volatile organic compounds (VOCs), not only easily causes open fire and explosions, but also brings great damage to human dermal, ocular, and respiratory system due to its corrosivity and toxicity [1][2][3]. According to the permissible exposure degrees stipulated by the † Mingxin Zhang and Kai Liu contributed equally to this work. high cost and cumbersome detection procedures [10][11][12]. Therefore, reliable TEA sensors with high response, low cost, and convenient operation are required for promising applications in industrial production, food safety, and human health. In addition, the traditional TEA sensors still suffer from the drawback of serious cross-sensitivity, mainly due to low charge-transfer ability of gas-sensing materials.
Metal oxide semiconductors (MOSs) have been widely used in the gas sensors due to their attractive physical and chemical properties [13][14][15]. Generally, a semiconductor gas sensor is composed of a receptor function and a transducer function, which recognizes the adsorption and reaction of target gases on a sensing layer. The change in the electrical resistance of the sensing materials is recorded as a sensor signal. Among various MOSs, tungsten oxide (WO 3 ) demonstrates high sensitivity, fast response, and long-term stability towards various gas species, owing to its nontoichiometry and chemical diversity [16]. However, the high activation energy for the chemisorption results in relatively high operating temperatures of WO 3 -based sensors. The formation of the heterojunctions and the heterogeneous structures has definitely proved that they can improve the sensing behavior of the MOSs [17][18][19]. As reported in Refs. [20,21], the valence bands of WO 3 and bismuth oxide (Bi 2 O 3 ) are 3.44 and 3.13 eV, respectively. Thus, the holes on the valence bands of both WO 3 and Bi 2 O 3 have a strong oxidative capability that is similar to TiO 2 , making them attractive for photocatalytic applications. Recently, the heterojunction between Bi 2 O 3 and WO 3 has attracted special attention for photodegradation and water splitting. Wei et al. [22] designed the porous WO 3 microspheres embedded with Bi 2 O 3 nanosheets, facilitating the separation and migration of charge carriers for dye photodegradation. Khan et al. [23] synthesized Bi 2 O 3 /h-WO 3 nanocomposite as photoactive materials for water splitting, which exhibited a decreased bandgap and minimized recombination of the charge carriers due to the strain effect of Bi 2 O 3 . As is well known, the adsorption and reaction behavior of the target gases on the gas-sensing materials has relations with the catalytic process. From the viewpoint of the accessible materials, Bi 2 O 3 possesses advantages of tunable electrical conductivity and high electron affinity [24]. Although pure Bi 2 O 3 is rarely used for the gas-sensing purpose due to its slow electronic mobility at low temperatures [25], it was reported that the Bi 2 O 3 /SnO 2 materials are sensitive to CO due to the participation of the Bi 2 Sn 2 O 7 phase [26]. The Bi 2 O 3 /WO 3 heterogeneous materials thus show great potential to achieve reliable gas sensors, and more related research is needed to explore.
In this work, we design an advanced TEA sensor by utilizing the interfacial energy barriers of the hierarchical Bi 2 O 3 /WO 3 composite. The composite was prepared through the hydrothermal-ultrasonic combination procedure followed by thermal treatment. On the basis of the hierarchical Bi 2 O 3 /WO 3 composite, the designed sensor possesses a high response of the TEA of 9.2-50 ppm at a low temperature of 140 ℃. The response and recovery time are 1.7-fold and 1.2-fold, respectively, as compared with those of the pristine WO 3 sensor. Furthermore, the composite sensor exhibits long-term stability, reproducibility, and negligible response towards the interfering molecules, indicating its potential application for anti-interference detection of the lowoncentration TEA. Herein, the superior TEA-sensing performance is attributed to abundant slit-like pores, desirable defect features, and amplification effect of the heterojunctions between WO 3 and Bi 2 O 3 . The presented work offers an easy solution for the rational design of other binary MOS sensors by utilizing the interfacial energy barriers of hierarchical metal oxides.

1 Preparation of hierarchical Bi 2 O 3
In a typical process, Bi(NO 3 ) 3 of 0.97 g was dissolved in a mixture of ethanol of 34 mL and ethylene glycol of 17 mL under magnetic stirring at room temperature to form a uniform transparent solution. After stirring for 30 min, the resulting solution was transferred into a Teflon-lined stainless-steel autoclave of 50 mL and heated at 160 ℃ for 5 h. The white precipitates were collected by centrifugation, washed with ethanol for several cycles, and then dried in an oven at 60 ℃ for 8 h.

4 Fabrication and gas-sensing test of sensors
The fabrication of the gas sensor was divided into several steps. Firstly, the as-prepared samples of 1 mg and terpineol (binding agent) of several microliters were ground in an agate mortar for 5 min to form a uniform slurry. The obtained slurry was slowly and evenly coated on a commercial Ag/Cr interdigital electrode by spinning at 2000 r/min for 15 s. As shown in Fig. S1(a) in the Electronic Supplementary Material (ESM), a successive gas-sensing layer on the surface of the interdigital electrode was formed after annealing at 400 ℃ for 2 h. Finally, the sensor element was aged at 200 ℃ for 10 h to improve the stability and repeatability. The sensor resistance in air (R 0 ) or target gas (R g ) was tested under a direct current (DC) bias voltage of 3 V in a homemade chamber connected with a computer-controlled sourcemeter (2612B, Keithley), as shown in Fig. S1(b) in the ESM. The response value of the gas sensor is calculated by the expression of R 0 /R g for the reducing gas. Moreover, the response and recovery time are defined as the time required for the sensor to reach 90% of the total variation values.

Results and discussion
As shown in Fig. 1(a), the hierarchical WO 3 and Bi 2 O 3 architectures were prepared through the hydrothermal and ultrasonic procedures, respectively, and the hierarchical Bi 2 O 3 /WO 3 composite was obtained after the mixing and thermal treatment. The formation process of the hierarchical WO 3 architectures can be explained by the ultrasonic cavitation and Ostwald ripening [28]. At the initial stage of the reaction, oxalic acid is considered as the chelating agent to control the nucleation rate and slow down the sol-gel process in an aqueous solution. After that, the colorless 2 4 WO  ions are transformed into a yellow H 2 WO 4 precursor.
In the ultrasonic process, the nucleophilic addition reaction between the water molecule and the H 2 WO 4 molecule results in the formation of hexagonal WO(OH) 4 (H 2 O) octahedra. Due to the mono-oxo coordination nature of W 6+ , each W atom is connected by four bridging oxygens (OH) in the XY plane, one terminal oxygen W=O in the Z axis, and one water molecule by hydrogen bonds at the symmetric position of W=O bonds. This octahedral structure easily forms a lamellar structure in the XY plane through a co-angular polycondensation reaction, and water molecules are located between these lamellas. Finally, the WO(OH) 4 (Fig. 1(c)), the sintering growth of the hierarchical WO 3 under the calcination is observed ( Fig. 1(b)). In the 0.4-Bi 2 O 3 /WO 3 composite ( Fig. 1 [31]. For the pure Bi 2 O 3 , the characteristic Raman peaks from 100 to 500 m −1 are in good agreement with monoclinic α-Bi 2 O 3 [32]. All the characteristic peaks of WO 3 and Bi 2 O 3 are obviously present in the Raman spectrum of Bi 2 O 3 /WO 3 composite curves ( Fig. 2(b)), suggesting the formation of the Bi 2 O 3 /WO 3 composite with no impurities. Along with the increase of Bi 2 O 3 content in the composite, the relative intensity of the characteristic peaks from WO 3 decreases, and the relative intensity of the characteristic peaks from Bi 2 O 3 increases (Fig. S6 in the ESM). Herein, the slight variations in the position, width, and relative intensity of these characteristic peaks are observed, which may be ascribed to the slight structural variations and sample shape of the Bi 2 O 3 /WO 3 composite, as compared with those of the pristine Bi 2 O 3 and WO 3 . Figure 2(c) compares the UV-Vis absorption spectra of the as-prepared materials, in which the WO 3 Fig. 2(c)). As compared with those of single components, the decreased band gap of the Bi 2 O 3 /WO 3 composite may be attributed to the formation of defects and impurity energy levels within the forbidden band gap [33].
In order to further investigate the surface chemical compositions and chemical states, the XPS studies were carried out for the obtained samples. As shown in Fig. 2(d), two oxidation states of W 6+ and W 5+ exist in the WO 3 and Bi 2 O 3 /WO 3 samples. The dominant chemical state of W is W 6+ in the samples, which displays the highest binding energy peak [34]. The existence of W 5+ atom proves that there are considerable amounts of defects in the crystal structure of WO 3 . Figure 2 www.springer.com/journal/40145 respectively [35]. In the O 1s spectra (Fig. 2(f)), the binding energies of 530.68, 530.28, and 532.38 eV are attributed to the W-O bond, Bi-O bond, and lattice oxygen, respectively. As compared with those of Bi 2 O 3 and WO 3 , obvious shifts of the XPS spectra are observed for the Bi 2 O 3 /WO 3 composite, reflecting a change in the local environment and electronic properties of surface atoms caused by the heterojunction. This result further demonstrates the co-existence of WO 3 and Bi 2 O 3 in the composite, which is in agreement with the SEM (Fig. 1), XRD (Fig. 2(a)), and Raman ( Fig. 2(b)) results.
For semiconductor materials, when the electrons or defects of the transitions migrate to a higher energy level, and then return to the ground state after illumination excitation, they will emit certain characteristic spectra. Therefore, the PL is an effective means to study the defects of semiconductors [36]. Figure 3(a) shows that all the obtained samples present luminescence in both the ultraviolet and the visible regions, which may be caused by the photoexcited electron-hole pairs or intrinsic defects. Figures 3(b)-3(d) show the defectrelated luminescence of the PL spectra obtained by Gaussian fitting for the WO 3 , Bi 2 O 3 , and Bi 2 O 3 /WO 3 samples. As is well known, near-band-edge (NBE) emission in the ultraviolet region is caused by the inter-band transitions and exciton recombination, while deep level (DL) emission in the visible region is caused by a shift in the subcenter of the laser in the deep energy level. The DL is associated with the defects such as oxygen vacancies and interstitial particles, whose intensity is proportional to defect density. Therefore, the higher ratio (I DL /I NBE ) of DL emission to NBE emission means a higher relative content of defects in the sample. For WO 3 and Bi 2 O 3 , the relative PL ratios of I DL /I NBE were calculated to be about 18.6 and 13.1, respectively. For the Bi 2 O 3 /WO 3 samples, there is no NBE emission lower than 400 nm, showing that more defects are present in the Bi 2 O 3 /WO 3 structure, and more electrons can participate in the redox reaction with the adsorbed oxygen.
The resistance and response of the semiconductor gas sensors are strongly dependent on the operating temperature, which provides heat energy to overcome the activation barriers of chemical adsorption and the reaction between the target gases and the sensor surface. Figure 4(a) shows the temperature-dependent responses of the gas sensors to 50 ppm TEA with the operating temperature ranging from 95 to 200 ℃. For the 0.4-Bi 2 O 3 /WO 3 sensor, the response rapidly increases from 2.1 to 9.2 upon increasing the operating temperature from 95 to 140 ℃. However, its values gradually decrease with the further increase in the operating temperature. Hereinafter, 140 ℃ is selected as the optimum operating temperature for the gas-sensing tests. As compared with the pure WO 3 sensor, whose peak response is 4.3 at 170 ℃, the sensor based on the 0.4-Bi 2 O 3 /WO 3 composite exhibits a much higher response (2.1-fold) to the TEA at a lower operating temperature (30 ℃-decrement), revealing the advantage of the Bi 2 O 3 /WO 3 composite in TEA detection. Herein, the performances of recent TEA sensors based on various metal oxides are compared [37][38][39][40][41][42][43][44][45][46], as summarized in Table 1. There is no doubt that the Bi 2 O 3 /WO 3 sensor possesses the ideal gas-sensing characteristics, including high response and low power consumption. Furthermore, the resistance variations of the 0.4-Bi 2 O 3 /WO 3 sensor towards 50 ppm TEA were measured under different operating temperatures, as shown in Fig. 4(b). The sensor resistance gradually decreases with the increase in the temperature, mainly because the quantity and mobility of the carriers are influenced by the temperature.
For a clear comparison, the dynamic response curves of all the sensors at 140 ℃ towards 50 ppm TEA are shown in Fig. S7 in the ESM, and the response characteristics of all the sensors are summarized in   3 . The electrons from the conduction band of WO 3 are captured by the oxygen molecules adsorbed on the surface of WO 3 to form the chemically adsorbed oxygen and an electron depletion layer. The electron depletion layer will decrease the carrier concentration, increasing the baseline resistance of WO 3 . Moreover, the electron transfer between the heterojunctions is accelerated, facilitating the reaction between the gas species and the gas-sensing materials, thus delivering a fast response and recovery of the sensor. During desorption, the baseline resistance returns to the initial values, indicating that the chemical reaction between the Bi 2 O 3 /WO 3 composite and the TEA is highly reversible. The above phenomenon was also demonstrated by the extended long-term stability and reproducibility tests, as well as the transient plots to different concentrations of the TEA. The resistance value of the composite is gradually decreased as the molar ratio of Bi 2 O 3 increases, mainly because Bi 2 O 3 provides a larger band gap, reducing the number of the electrons in the conduction band of the Bi 2 O 3 /WO 3 composite. The Bi 2 O 3 /WO 3 sensor exhibits no response at higher temperatures, which may be attributed to a combined effect of the instability of the TEA molecules to adsorb on the sensing materials and repulsion of these molecules with previously adsorbed oxygen molecules. In practical applications, both the response value and response/recovery time are crucial parameters. In light of this, transient plots of the gas sensors based on the pure WO 3 and 0.4-Bi 2 O 3 /WO 3 composites were further studied at the optimal working temperature. As shown in Fig. 5(a), the response time of the 0.4-Bi 2 O 3 / WO 3 composite is shortened from 151 to 89 s, and the recovery time is shortened from 194 to 162 s. When the 0.4-Bi 2 O 3 /WO 3 composite is exposed to the TEA, the rise in the response implies that the materials exhibit the characteristics of an n-type semiconductor. When exposed to dry air, the response gradually decreases and eventually recovers to the initial value, indicating that the sensor response is stable. Furthermore, we evaluated the selectivity of the sensors based on the WO 3 , Bi 2 O 3 , and Bi 2 O 3 /WO 3 composites towards 50 ppm of the interfering molecules including methanol (CH 4 O), ethanol (C 2 H 6 O), acetone (C 3 H 6 O), isopropanol (C 3 H 8 O), toluene (C 7 H 8 ), ammonia (NH 3 ), and TEA (C 6 H 15 N). As shown in Fig. 5(b), all the 0.2-Bi 2 O 3 /WO 3 , 0.4-Bi 2 O 3 /WO 3 , and 0.6-Bi 2 O 3 /WO 3 sensors exhibit good selectivity. Especially, the response of the 0.4-Bi 2 O 3 /WO 3 sensor reaches about 9.2 towards the TEA while only 1-3.5 towards other gases. Herein, the high response and good selectivity of the composite sensor to the TEA is mainly attributed to the following two aspects: (1) the intrinsic acidic feature of WO 3 , which is beneficial for the adsorption of the TEA molecule with N atom served as the Lewis basic site; (2) the formation of the heterojunction between Bi 2 O 3 and WO 3 , which promotes the charge transfer. As reported, the bond energy of O-H (methanol, ethanol), C=O (acetone), C-C (isopropanol), C=C (toluene), N-H (ammonia), and C-N (TEA) is 459, 799, 345, 610, 392, and 307 kJ/mol [47], respectively. The lower C-N bond energy facilitates the reaction activity of the TEA molecules among the above gases. As compared with other VOCs, the TEA molecule more easily loses the electron, and its N atom serves as the Lewis basic site, which is easily absorbed on the surface of acidic WO 3 [9].
In order to intuitively highlight the advantages of each sensor in different aspects and obtain the balance between the superior and the optimal working conditions, the performance parameters of S, Q, 1/T, 1/τ res , and 1/τ rec are multidimensionally compared in a wind rose diagram, as shown in Fig. 5(c). It is visible that the area of 0.4-Bi 2 O 3 /WO 3 is larger than the area of others, confirming that the optimal 0.4-Bi 2 O 3 /WO 3 sensor possesses a more balanced sensing performance for the TEA detection. Meanwhile, the response can rapidly restore to its original baseline after the TEA is released. This result manifests that the 0.4-Bi 2 O 3 /WO 3 sensor is reproducible and has widely detection range from 0.5 to 100 ppm. The linear relationship between the sensor response and the gas concentration implies the advantage of the sensor in detecting low concentrations of the TEA (Fig. 5(e)). The detection limit (D L ) of the sensor can be extrapolated from the linear regime of the response values, which should be distinguishably differentiated from the background level. When the criterion for gas detection was set to R 0 /R g > 1.5, the theoretical D L of the 0.4-Bi 2 O 3 /WO 3 sensor was calculated to be 32 ppb, suggesting that the composite materials have the advantage for detecting the low-concentration TEA. With the increase of the TEA concentration, the response and recovery time basically presents a downward trend at the initial stage and remained unchanged thereafter ( Fig. 5(f)), which may be the result of the nonuniform gas concentration after injection at the initial stage.
It is generally known that the gas-sensing performance of the semiconductor materials is critically influenced by relative humidity (RH). Herein, the humiditydependent experiments of the Bi 2 O 3 /WO 3 sensor to 50 ppm TEA were conducted under different RHs in the range of 11%-97% RH (Fig. S8 in the ESM). As summarized in Fig. 6(a), the sensor response slightly declines about 11% along with the increase of the RH from 11% to 97%, which is apparently smaller than those reported in Refs. [15,30,38,42]. In addition, the anti-interference performance of the Bi 2 O 3 /WO 3 sensor was tested under three mixed gases of isopropanol, triethanolamine, and TEA (I-10 ppm: 10 ppm : 10 ppm; II-20 ppm : 20 ppm : 20 ppm; and III-50 ppm : 50 ppm : 50 ppm). As shown in Fig. 6(b), the sensor response decreases negligibly in the presence of the interfering molecules (triethanolamine and isopropanol), demonstrating the superiority of the Bi 2 O 3 /WO 3 sensor in the mixed gas detection. The long-term stability and reproducibility were also measured to evaluate the sensor performance, as shown in Figs. 6(c) and 6(d), respectively. No obvious variations in the response are observed during the test period of nearly 30 d and after running for 10 cycles, suggesting excellent stability and reproducibility of the Bi 2 O 3 /WO 3 gas sensor.
The synergistic effect between WO 3 and Bi 2 O 3 plays a crucial role in improving the performance of the Bi 2 O 3 /WO 3 sensor. Firstly, the conductivity change of the composite upon the oxygen adsorption and gas reaction is believed to be a primary reason for the gas-sensing behavior. The band gaps of WO 3 and Bi 2 O 3 are 2.78 and 2.83 eV, respectively. When they are in contact with each other, free electrons will flow from Bi 2 O 3 to WO 3 to equate the Fermi levels, creating an electron depletion layer and an electron accumulation layer on the Bi 2 O 3 side and WO 3 side, respectively (Fig. 7). When the sensor is exposed to an air atmosphere, the oxygen molecules are easily adsorbed on the surface  Energy-band diagrams and depletion layer changes of sensor based on Bi 2 O 3 /WO 3 composite when exposed to air and TEA. Note: E c , E F , and E v represent the energies at conduction band, Fermi level, and valence band, respectively. of the Bi 2 O 3 /WO 3 composite to generate the oxygenion species (predominantly exist in the form of O − ), which increases the thickness of the depletion layer. Upon exposure to the TEA gas at 140 ℃, the trapped electrons are released to the composite by the reaction between the TEA molecules and chemisorbed oxygen species (C 6 H 15 N + O − → CO 2 + N 2 + H 2 O + e − ), which decreases the electron depletion layer and results in a sharp decline in the resistance as observed. Secondly, the reactive sites are relatively increased by the defects in the composite (Fig. 3), which accelerates the electron transfer and uplifts band bending for the electrons to pass through the electron depletion layer. Additionally, the rational design of a hierarchical structure guarantees abundant channels for the gas adsorption and desorption. The as-formed slit-like pores efficiently increase the contact region, facilitate the gas molecules to diffuse and interact with the composite, and thus advance the sensor performance.

Conclusions
In summary, the advanced Bi 2 O 3 /WO 3 composite has been successfully synthesized through a combination of the hydrothermal-ultrasonic procedure followed by the thermal treatment. Owing to the formation of the heterojunctions, the composite possesses decreased band gap and increased defect content, which is confirmed by the UV-Vis and PL analysis, respectively. Benefiting from the abundant slit-like pores, desirable defect features, and amplification effect of the heterojunctions between WO 3 and Bi 2 O 3 , the composite sensor shows a distinctive response of 0.5-2.8 ppm TEA, which meets up with the threshold value of toxic TEA concentration established by the OSHA. As compared with pristine WO 3 sensor, the composite sensor demonstrates faster response/recovery time (1.7-fold/1.2-fold), higher response (2.1-fold), and lower power consumption (30 ℃decrement). In addition, the composite sensor exhibits the long-term stability, reproducibility, and negligible response towards the interfering molecules, indicating the promising potential of the Bi 2 O 3 /WO 3 heterojunction in the anti-interference detection of the low-concentration TEA in real applications. The novel design of the Bi 2 O 3 / WO 3 composite in this work shows great promise for rationally designing the advanced semiconductor gas sensors by utilizing the interfacial energy barriers of the hierarchical metal oxides.