Facile Synthesis of Highly Dispersed Co3O4 Nanoparticles on

Jul 18, 2018 - Facile Synthesis of Highly Dispersed Co3O4 Nanoparticles on Expanded, Thin Black Phosphorus for a ppb-Level NOx Gas Sensor. Yang Liu ...
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Facile Synthesis of Highly Dispersed Co3O4 Nanoparticles on Expanded, Thin Black Phosphorus for a ppb-Level NOx Gas Sensor Yang Liu, Yang Wang, Muhammad Ikram, He Lv, Jingbo Chang, Zhengkang Li, Laifeng Ma, Afrasiab Ur Rehman, Ganhua Lu, Junhong Chen, and Keying Shi ACS Sens., Just Accepted Manuscript • DOI: 10.1021/acssensors.8b00397 • Publication Date (Web): 18 Jul 2018 Downloaded from http://pubs.acs.org on July 19, 2018

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Facile Synthesis of Highly Dispersed Co3O4 Nanoparticles on Expanded, Thin Black Phosphorus for a ppb-Level NOx Gas Sensor Yang Liua§, Yang Wanga§, Muhammad Ikrama, He Lva, Jingbo Changb, Zhengkang Lia, Laifeng Maa, Afrasiab Ur Rehmana, Ganhua Lub, Junhong Chenb*, Keying Shia* a

Key Laboratory of Functional Inorganic Material Chemistry, Ministry of Education. School of Chemistry and Material Science, Heilongjiang University, Harbin, 150080, P. R. China.

b

Department of Mechanical Engineering, University of Wisconsin-Milwaukee, 3200 North Cramer Street, Milwaukee, Wisconsin 53211, United States. KEYWORDS: Expanded black phosphorus, few-layer black phosphorus, Co3O4@BP-PEI composite, highly dispersed Co3O4 nanoparticles, NOx, gas-sensing

ABSTRACT: Expanded few-layer black phosphorus nanosheets (FL-BP NSs) were functionalized by branched polyethylenimine (PEI) using a simple noncovalent assembly to form air-stable overlayers (BP-PEI), and a Co3O4@BP-PEI composite was designed and synthesized using a hydrothermal method. The size of the highly dispersed Co3O4 nanoparticles (NPs) on the FL-BP NSs can be controlled. The BP-C5 (190 for 5 h) sensor, with 4- to 6-nm Co3O4 NPs on the FL-BP NSs, exhibited an ultrahigh sensitivity of 8.38 and a fast response of 0.67 s to 100 ppm NOx at room temperature in air, which is four times faster than the response of the FL-BP NS sensor, and the lower detection limit reached 10 ppb. This study points to a promising method for tuning properties of BP-based composites by forming air-stable overlayers and highly dispersed metal oxide NPs for use in high-performance gas sensors.

NOx gases (e.g., NO and NO2) are common gases that are byproducts of industrial production and automobile combustion. According to the U.S. Environmental Protection Agency, exposure to NO2 concentrations greater than 53 ppb can cause potential health problems.1 Therefore, developing a better gas sensor for detecting NO2 in a wide range of applications is highly important. Black phosphorus (BP) is a novel two-dimensional (2D) material that has individual atomic layers stacked together through the van der Waals interaction.2 Researchers have synthesized few-layer BP nanosheets (NSs) using a liquid exfoliation method.3-7 BP thin films show a high mobility, greater than 600 cm2 Vs-1 at room temperature, and the unique anisotropic nature within the planes of the layers may allow for the realization of novel electronic and nano-mechanical devices.8-12 BP has shown potential for gas-sensing applications1 because of its rapid chemical activity and higher surfaceto-volume ratio, and its outstanding gas-sensing performance is even better than that of graphene and MoS2.13-15 Zhou,1 Cui,14 and Martin et al.15 used BP as a gas sensor, and Cui et al.14 reported the fabrication of a field-effect transistor (FET) sensor to detect NO2 based on multilayers of BP obtained by mechanical exfoliation; and the results showed that the sensors could detect NO2 concentrations as low as 20 ppb, demonstrating a high sensitivity. Zhou et al.1 fabricated BP FET sensors that showed

excellent sensitivity for detecting NO2 at lower concentrations (5 ppb) compared with that of other 2D materialbased sensors.1,14,15,16 However, except for BP FET sensors, the reported conventional BP-based gas sensors for detecting NOx at ppb levels are difficult to realize. It is well known that thin BP NSs are sensitive to environmental molecules, and thus researchers face experimental challenges when fabricating and measuring BPbased sensors in air. As such, current BP-based gassensing studies have focused on protecting the sensor surface by encapsulating the surface with graphene/MoS217,18 or oxides19 to improve their structural stability. In addition, cobalt oxide (Co3O4), an important ptype20,21 semiconductor with potential applications as a heterogeneous catalyst, is also of interest for gas-sensing because of its good magnetic properties, use as a reagent in electrochromic devices, and rapid response for gassensing.22,23 Xie et al.24 reported that Co3O4 nanorods not only catalyze CO oxidation at much lower temperatures, e.g., –77 , but also remain stable in the moist atmosphere of a normal feed gas. Co3O4 semiconductor materials have good development prospects as gas sensors; therefore, forming the air-stable overlayers and highly dispersed Co3O4 nanoparticles in BP NSs is a promising method to significantly enhance the sensing response of BP-based gas sensors at room temperature.

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In this research we exfoliated bulk BP using ultrasonic energy (Scheme 1) to break down the BP and create expanded layers (Figure S3). The expanded few-layer BP nanosheets (FL-BP NSs) contained a number of defects, which were then functionalized by branched PEI using a simple noncovalent assembly to form air-stable overlayers. We then used inexpensive Co3O4 as an additive to modify the BP-PEI to achieve an appropriate electrical conductivity by controlling the hydrothermal time. The synergistic effect between the FL-BP NSs and Co3O4 NPs is beneficial for NOx gas-sensing, and the BP-C5 sensor exhibited excellent sensitivity for NOx.

Scheme 1. Designed strategy for the formation of Co3O4@BP-PEI composite. Methods Section Preparation of the few-layer BP NSs. BP was lightly ground with a mortar and pestle, and 20 mg of the obtained bulk BP was dispersed in 50 mL of solvent, such as deionized water, dimethyl formamide, or diethylene glycol dimethyl ether. The mixture solution was then sonicated at a 60% amplitude with a horn-probe sonic tip (VibraCell KBS-150, 150 W) in ice water for 3 h. Afterward, a stable dispersion was obtained by removing the unexfoliated material by centrifugation at 2,000 rpm for 30 min. Synthesis of the Co3O4@BP-PEI composite. First, 2.4 mg of the FL-BP NSs was exfoliated with diethylene glycol dimethyl ether. Then, branched PEI (4 mg) with a MW = 600 (Sigma-Aldrich) was added into 100 mL of deionized water, uniformly mixed and magnetically stirred. The pH of the mixture was adjusted to approximately 9.0 and maintained for 1 h. Then, 20 mL of a 2-mg mL-1 Co(NO3)2 solution was added into the above solution. During the mixing process, the pH was adjusted to remain below 9.0. Next, a 90-mg mL-1 NaOH concentrated solution was gently added dropwise into the mixed solution until the pH was nearly 12.0. Meanwhile, the solution was subjected to a 40-mL min-1 airflow for 2 h. Then, the precursor was maintained for 12 h at ambient temperature. The deposits were filtered and washed with deionized water (DI). The obtained deposits were placed in a Teflon-lined stainlesssteel autoclave with deionized water. The hydrothermal synthesis was carried out at 190 °C, and the time durations were 3, 5, and 7 h. The as-prepared Co3O4@BP-PEI composites were named BP-C3, BP-C5, and BP-C7, respectively. (Note: ‘C’ stands for cobaltosic oxide (Co3O4).)

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Characterization. The crystal phases of the samples were characterized by XRD (D/max-III-B, Rigaku, Japan) with following experimental conditions: Cu Kα radiation; wavelength λ = 1.5406 Å; and graphite monochromator 40 kV. Raman spectroscopy characterizations were obtained on a Jobin Yvon HR 800 micro-Raman spectrometer at room temperature. To observe the structural and morphological changes in the composites, scanning electron microscopy (HITACHI S-4800) and transmission electron microscopy (JEOL-2100) were used. To determine the thickness of the exfoliated FL-BP NSs, AFM was performed on an Agilent Technology 5100 AFM system. XPS data were obtained with an AXIS ULPRA DLD instrument (Shimadzu Company). Gas-sensing tests. An interdigitated Au electrode was selected for the gas-sensing tests; the electrode spacing was 2 μm. A certain amount of BP or the Co3O4@BP-PEI composite was ultrasonically dispersed in 1.0 mL of ethanol. Then, 0.05 mL of the composite solution was gently dropped onto the interdigital electrode to form a uniform film, and then dried at 60 °C for 5 h to evaporate the solvent. As a result, the gas sensor was obtained. The sensor was mounted in a test chamber with an entrance and an exit. The electrical resistance measurements of the sensor were carried out at room temperature with a relative humidity of 24%, which was controlled by a humidity sensor. Then, NO gas was introduced into the chamber using a microsyringe, and the sensor response was observed on a computer. Note that since NO reacts easily with O2 the sensor response is indeed to the mixture of NO2 and NO, which we denoted as NOx in this work. The concentration of the NOx gas decreased in the order of 100, 50, 30, 10, 5, 3, 1, 0.5, 0.05, and 0.01 ppm. The NOx concentration was verified using a nitrogen oxide detector (JK40-NOx Shenzhen Jishunan Technology Co. Ltd). The value of R (gassensor response) was defined as Ra/Rg, where Ra is the resistance of the thin film measured in air, and Rg is the resistance in the target gas. The sensing tests were carried out in the ambient environment at room temperature, and the repeatability of the sensor was studied using four sensors for each sensing test. The response time was defined as the time required for the variation in resistance to reach 80% of the equilibrium value after a test gas was injected. Results and discussion Morphological and structural characterizations of the Co3O4@BP-PEI composite Figure 1a shows the dominant diffraction peaks at approximately 2θ = 16.9°, 34.2°, and 52.4°, which were assigned to the BP (020), (040), and (060) planes corresponding with d spacings of 5.24, 2.62, and 1.75 Ǻ, respectively (JCPDS no. 73-1358).4 The XRD pattern of the bulk BP shows the crystalline phase of BP. As shown in Figure 1c, the Raman spectra of the bulk BP and exfoliated FL-BP NSs show nearly identical peaks at approximately 363, 439, and 468 cm−1, which can be assigned to the Ag1, B2g, and Ag2 modes of BP, respectively.

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Figure 1. XRD diffraction pattern, Raman spectra, AFM images, and XPS spectra of the samples. (a) XRD pattern of the bulk BP. (b) AFM image of the FL-BP NSs exfoliated by diethylene glycol dimethyl ether. (c) Raman spectra of bulk BP and the FL-BP NSs. (d) Corresponding height profiles of the FL-BP NSs in (b). (e), (f) P 2p of the FL-BP NSs and BP-C5. (g), (h) O 1s of BP and BP-C5. (i) Co 2p of BP-C5. (j) N 1s of BP-C5. Compared with the bulk BP, the Raman spectrum of the FL-BP NSs shows slight shifts toward higher wavenumbers.4 AFM was used to characterize the thickness of the FL-BP NSs, as shown in Figure 1(b,d). The thickness of the BP nanosheet is in the range of 5–16.3 nm, which suggested that our FL-BP sample is multilayered.25 Figure 1(e-j) shows the XPS spectra of the FL-BP NSs and BP-C5. Further information about the chemical state and elemental analysis was obtained from the highresolution XPS spectra for O 1s, N 1s, and Co 2p, as shown in Figure 1(g-j). For the P 2p region of the FL-BP NSs and BP-C5 spectra, five peaks were observed at 129.1 (~129.4), 130.0 (~130.3), 131.9, 132.9, and 133.7 (~133.9) eV26. The peaks located at 129.1 and 130.3 eV can be attributed to P 2p3/2 and P 2p1/2,9,27 which correspond with the phosphorus-phosphorus bonds in BP. We assigned the peaks located at 131.9, 132.9, and 133.7 eV to oxidized P species, which may be present as a stable sub-nanometer layer of P2O5 or an intermediate stable oxide, p-P4O2 (POx), on the BP surface, resembling bridging (P−O−P) and dangling (O−P=O) bonding environments.9,26 For the BP-C5 composite, the peaks at 530.2 and 531.2 eV can be attributed to the lattice oxygen and oxide defect states of Co3O4,28 and the peak at 532.5 eV is due to chemisorbed oxygen (see Figure 1h). The oxygen content estimates from the XPS measurements are 27.5% (lattice oxygen), 45.4% (oxide defect states), and 27.1% (chemisorbed oxygen) in the BP-C5. The O 1s peaks at 531.4 and 532.5 eV might be due to sub-nanometer P=O and P−O on the BP surface. In the Co 2p range, the three main peaks in the XPS spectra (Figure 1i) at 779.7, 781.4, and 785.6 eV were assigned to Co-P,29-31 Co-O(Co-N),31-33 and Co salts,34 respectively. Hence, some defects were present on the BP-

C5 composite due to Co-N and Co-P. Cobalt (in Co-P, CoN) serves as an active center and facilitates the catalysis reaction;31, 35 therefore, the Co-N and Co-P structures are beneficial for gas detection and can enhance the sensing property of an NOx sensor with their good catalytic activity. In Figure 1j, the appearance of N 1s peaks at 401.3 and 399.4 eV in the XPS spectra shows that two types of nitrogen atoms are present, which are from the polyelectrolyte PEI.36 The TEM/HRTEM images of the exfoliated (expanded) FL-BP NSs are shown in Figures S1-S4 and Figure S7 (Supporting Information). Figure S7(a,b) and Figures S3-S4 present low-magnification TEM images of the exfoliated BP. Figure S4 and Figure S7c (Supporting Information) are the HRTEM images of exfoliated BP. The lattice plane spacings of BP were 0.164, 0.250 (~0.259), 0.264 (~0.265), 0.337, and 0.528 nm, which represent the d spacings of the (200), (111), (004), (021), and (002) planes of BP, respectively. Furthermore, the selective area electron diffraction (SAED) pattern in Figure S3 (Supporting Information) shows that the BP nanosheets are polycrystalline. Additional observations from the TEM images show that defects appear on the surface of the exfoliated BP. The expanded BP boundaries are observed in some of the images (Figure S7(b,c) and Figure S3 (Supporting Information)), suggesting that the expanded BP was intercalated with diethylene glycol dimethyl ether molecules during the exfoliation. The interlamellar spacing reached 7~8 nm (Figure S7b), and the thin, expanded BP NSs still retain their highly crystalline characteristics.37 In our present work, a hydrothermal process at 190 °C for 3, 5, and 7 h was used to obtain the Co3O4@BP-PEI composites, BP-C3, BP-C5, and BP-C7, respectively.

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Figure 2. Low-magnification TEM (a, b) and HRTEM (c, d) images of the BP-C5 nanosheet. (BP exfoliated by diethylene glycol dimethyl ether). The TEM images of the BP-C3 and BP-C7 samples are shown in Figure S5-S6 and S10 (Supporting Information), respectively. The TEM images of BP-C5 are displayed in Figure 2(a-d) and Figure S7-S9 (Supporting Information). In BP-C5, the size of the Co3O4 NPs is approximately 3-6 nm (see Figure 2 and Figure S8, Supporting Information), and for BP-C3 and BP-C7, the size of the Co3O4 NPs is approximately 3-4 and 7-8 nm, respectively (Figure S5, Figure S10, Supporting Information). The TEM images in Figure 2(a,b) reveal that the Co3O4 NPs are uniformly dispersed on the surfaces of the BP NSs. The measured interplanar spacings of BP-C5 were 0.204 (~0.206), 0.285 (~0.289), and 0.247 nm, which correspond to the d spacings of the (400), (220) and (311) planes of Co3O4, respectively (Figure 2(c,d)). Similarly, the measured interplanar spacings of BP were 0.167, 0.208, 0.224, and 0.256 nm, which correspond to the d spacings of the (200), (022), (014) and (111) planes of BP (Figure 2d), respectively. The HRTEM image also clearly shows that the Co3O4 NPs are uniformly distributed on the surface of BP (Figure 2d) and connect with each other to form junction structures. Moreover, defects exist in the interfaces between the Co3O4 NPs and BP NSs (Figure 2d, Figure S7S9 (Supporting Information)). Polycrystalline diffraction rings were clearly observed from the outside to inside and correspond to the (211), (200), (021), and (111) planes of BP and the (220) and (111) planes of Co3O4 (the inset of Figure S7f). The size and uniform distribution of Co3O4 on the surface of BP in the Co3O4@BP-PEI composites were controlled by changing the hydrothermal reaction time. The Co3O4 particles collapsed with the defects on the BP (or

BP-PEI) surface, which were confirmed by the TEM images. The XRD diffraction patterns of BP-C5 and BP-C7 are shown in Figure S9c-10c (Supporting Information). Figure S11a (Supporting Information) is the STEM image of the BP-C5 composite, and Figure S11(b-e) shows that the composite contains P, O, Co, and N. The STEMEDS mapping shows the Co3O4@BP-PEI composite contains P, O, Co, and N. Figure S11 (Supporting Information) shows that the presence of P, O, Co, and N is uniform on the composite. Co3O4@BP-PEI composite formation mechanism

Scheme 2. The illustrated Co3O4@BP-PEI composite formation process.

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Figure 3. The NOx gas detection results of the FL-BP NS and BP-C5 thin film sensors at room temperature in air. (a) Dynamic response of BP-C5 to 100 ppm-10 ppb NOx. (b) Response and response time of BP-C5 to 100 ppm-10 ppb NOx. (c) Selectivity in the presence of 100 ppm of various gases. (d) The calibration curve of BP-C5 for 0.03-100 ppm NOx. (e) The response of BP-C5 to 3 ppm NOx at room temperature for 11 days. (f) The reproducibility of the BP-C5 sensor with consecutive exposure to 1 ppm NOx for 8 cycles. (relative humidity: 24%). A simple formation mechanism model of the Co3O4@BP-PEI composite is proposed in Scheme 2. Branched PEI has a high concentration of polar groups. When the as-exfoliated FL-BP NSs were dispersed in a PEI aqueous solution at a concentration of 0.03 mass% at pH 9.0, the surfaces of the BP NSs adsorbed PEI, which resulted in PEI-modified BP NSs (BP-PEI). Subsequently, an NaOH solution with a concentration of approximately 90 mg mL-1 was used to change the pH of the solution to 12, and the surface of the BP-PEI became negatively charged. The high-density amino group on the surface of the BPPEI served as a primer to adsorb Co2+ cations in the solution.28 NaOH was chosen for the uniform precipitation of Co(OH)2 NPs on the surface of the BP-PEI. The pH 12 solution was contacted with air to further oxidize the Co(OH)2 to Co(OH)3.28 After the precursor solution was allowed to sit for approximately 24 h at ambient temperature, the Co(OH)3 deposits transformed into CoOOH. Then, the solution was hydrothermally treated at 190 °C for 3–7 h, which resulted in the Co3O4@BP-PEI composite. In the Co3O4@BP-PEI composite, the thin, expanded BP NSs still retained their highly crystalline characteristics. However, the FL-BP NSs predictably introduced defects and dangling bonds (P=O and P−O), which have a high activity and significantly increase the interface interaction ability for the construction of the Co3O4@BP-PEI composite. PEI forms air-stable overlayers on the surface of BP and provides high-density, homogeneous functional groups on the BP surface to bind the Co3O4 NPs (Scheme 2). Moreover, PEI plays an important role in the high-

density scatter of the Co3O4 NPs and in strengthening the interaction between the Co3O4 NPs and BP. Gas sensor performance In our study the measured thickness of the expanded FL-BP NSs ranged from 5 to 16.3 nm. Considering the random distribution of the thickness of BP created from a liquid-phase exfoliation with diethylene glycol dimethyl ether, we fabricated a series of sensors with a channel length of 2 μm using FL-BP, Co3O4@BP-PEI, BP-PEI, Co3O4, and CoP thin films (Tables S1-S2, Figure S13, Supporting Information). Sensors based on the FL-BP NSs and Co3O4@BP-PEI were simultaneously fabricated, as shown in Figure 3, for comparison. Figure 3a shows the dynamic response and response time of the BP-C5 sensor for different concentrations of NOx at room temperature. The response of BP-C5 decreased with a decrease in the NOx concentration. In addition, the minimum detection concentration of the BP-C5 sensor was 0.01 ppm (10 ppb), and it had the highest response and fastest response speed over the entire range of NOx concentrations. The responses of FL-BP and Co3O4@BP-PEI to 100 ppm NOx were 8.38 and 2.09, respectively (Figure 3a, Figure S15c). The response of the BP-C5 sensor was 4, 3.2, 4.3, 7.5, and 7.4 times higher than the responses of the exfoliated FL-BP, BP-C3, BP-C7, BP-PEI, pure Co3O4, and CoP sensors, respectively (Figure S15(a,b)), Tables S1-S2, Figure S13, Supporting Information). Most importantly, the BPC5 sensor has the shortest response time (0.67 s) for 100 ppm NOx, and the response time of the BP-C5 sensor

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Scheme 3. The Co3O4@BP-PEI sensor gas sensing mechanism. (a) HRTEM image of BP-C5 showing the defects. (b) Schematic of the Co3O4@BP-PEI sensor and the charge states of the composite formed with the small Co3O4 NPs. (c) Gassensing reactions. (d) Model of the Co3O4@BP-PEI composite; the oxygen molecules capture the electrons to form chemisorbed oxygen and the NOx gas-sensing response. was always less than 11 s over the entire NOx concentration range from 100 ppm to 0.01 ppm, which was faster than the response time of the exfoliated BP sensor. Selectivity is an important aspect in evaluating the properties of a gas sensor for real applications, and thus the sensor’s response to gases, such as H2, CH4, CO, and NH3, was also surveyed at room temperature. Figure 3c shows that BP-C5 exhibited a higher selectivity for NOx and almost no response to 100 ppm CH4, CO, and NH3. In addition, we used BP-C5 sensors to detect pure NO2 (Figure S14) and observed a response of ~8.4 for 100 ppm NO2, which is slightly higher than that of 100 ppm NOx (mixture of NO and NO2) (Figure 3c). The response times toward NO2 were generally similar with or slightly shorter than those toward NOx. The higher response and shorter response time toward NO2 could be due to the lower binding energy of NO2 than that of NO,38 which could facilitate faster and more effective desorption of NO2 from the BP-C5 surface. As shown in Figure 3f, a sensing signal with steady response was observed at approximately 2.45 (~2.47), and the BP-C5 sensor displayed a reversible response signal for both the adsorption and desorption processes of NOx gas, even when repeated for eight consecutive cycles at 1 ppm NOx. Figure 3d shows the linear relationship of the sensor response to 0.03–100 ppm NOx. The linear relationship was formed by plotting log R versus log [NOx] for the BP-C5 sensor calibration curve, and R2 was nearly 0.974. Furthermore, BP-C5 was nearly stable for sensing 3 ppm NOx at room temperature, as shown in Figure 3e, and the response and response time to 3 ppm NOx remained constant for approximately 11 days at room temperature.

Therefore, the results show that the BP-C5 sensor has good stability. The above analyses prove that the measured response of the BP-C5 sensor to NOx is higher than that of all the other sensors, indicating that FL-BP NSs have the advantage of excellent electrical conductivity. The highly dispersed Co3O4 NPs attached to the FL-BP NSs increase the charge modulation efficiency. For a fast response, sensor materials must have a high resistance and efficient charge modulation. Gas-sensing mechanism The surface charge model, which refers to the change in sensor resistance due to different gases, was followed to explain the gas-sensing mechanism of the Co3O4@BP-PEI sensors.39 In the Co3O4@BP-PEI composite, Co3O4 is typical p-type semiconductor; its “true” energy gap corresponds to the O2- valence band (VB), and the Co2+ conduction band (CB) is 2.0 eV.40 BP is a p-type semiconducting material with a direct band gap of 0.3~2.0 eV.2,41,42 When Co3O4 NPs are anchored onto the FL-BP NSs, the electrons from the CB of BP shift to the CB of Co3O4, and the holes migrate in the opposite direction, i.e., from the VB of Co3O4 to the VB of BP. Under these circumstances, the energy band twists in the layer where the holes accumulate, and the Fermi level is nearly smooth. Unlike BP and Co3O4, PEI is an n-type semiconductor, and the same process will lead to the formation of p-p (pn-p) heterojunction structures in the interface. Incorporating BP NSs with an appropriate thickness can form a cascading band structure in devices, which facilitates electron transport43 and enhances gas-sensing (Figure 3).44 Upon exposing the Co3O4@BP-PEI gas sensor to air (Scheme 3), the Co3O4@BP-PEI NS surfaces readily adsorb

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oxygen molecules and gain electrons from the CB of Co3O4@BP-PEI to construct adsorbed oxygen (O2-) at room temperature, as shown in Equation (1)13 in Scheme 3. The sample is a p-type semiconductor with hole carriers, and the oxygen adsorption (O2-) increases the density of the holes in the VB. As a result, a hole-accumulation layer formed due to the loss of electrons (Scheme 3b). When the material is exposed to NOx, BP can adsorb NOx (the adsorption density is ~1015 cm2 for a 4.8 nm-thick BP layer exposed to 20 ppb NO2 at 300 K);14 therefore, the higher NOx adsorption is beneficial for the migration of NOx from the BP to the adjacent Co3O4 NPs in Co3O4@BP-PEI, and Co3O4@BP-PEI with a highly dispersed, small Co3O4 active phase can facilitate the efficient oxidation of NOx to NOx- (Scheme 3c).45-48 Here, small Co3O4 NPs (≤6 nm) on BP-C5 may have already been depleted under the standby O2 adsorption conditions, and the overlapped parts of the Co3O4 NPs contain a high density of defects. Meanwhile, Co-P and chemisorbed oxygen exist in n-p or p-p heterostructures of the BP-C5 composite. As a result, small Co3O4 NPs have a high density of defects, which provide active detecting sites and accessibility to active centers for target gases. Especially in nanohybrid structures, the depletion region in the Co3O4 NPs is extended into the BP and charge modulation occurs. The incorporation of Co3O4 NPs and FL-BP facilitate the reduction of Co3+-Co2+ and oxidize NOx to NOx-.39-40 Thus, a sensor material possessing high resistance and charge modulation ability is more favorable for a better sensor response. Conclusion In the present work, we reported gas sensors for NOx using BP FL-BP NSs and a Co3O4@BP-PEI composite, which were synthesized by a hydrothermal method using exfoliated FL-BP with branched PEI and simultaneous exfoliation to further expand the BP. The BP-C5 sensor exhibited excellent sensitivity and could detect 10 ppb of NOx. The BP-C5 sensor exhibited a high sensitivity of 8.38 and a fast response time of 0.67 s at 100 ppm NOx. The increases in the sensing properties are attributed to the synergy between the superior conductivity of FL-BP and the heterostructure of the Co3O4@BP-PEI composite. Strategies for fabricating different metal oxides and nanoscale building units for 2D BP-based heterostructures will create new opportunities for designing multifunctional BP-based nanocomposites. ASSOCIATED CONTENT Supporting Information. The Supporting Information is available free of charge. Formation Mechanism of Co3O4@BP-PEI, TEM, HRTEM, SEAD analysis of BP, Co3O4@BP-PEI, CoP, Co3O4, STEM image of Co3O4@BP-PEI, XRD of Co3O4@BP-PEI, gas selectivity test, Figure S1-S18, and Table S1-S3 AUTHOR INFORMATION Corresponding Author *E-mail: [email protected]. *E-mail: [email protected].

Author Contributions §

These authors contributed equally to this work.

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